Multi-principal element alloy, method for preparing the same and uses thereof

Abstract

The present invention relates to a multi-principal element alloy (MPEA) which consists of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloy: 30-50 wt.% Ni, 10-30 wt.% Mn, 10-20 wt.% Cu and 10-30 wt.% Cr, the balance being Fe and impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%. The present invention also relates to a process for the preparation of these MPEAs, as well as to MPEAs obtained or obtainable according to this process. The invention also relates to use of an MPEA according to the invention, more particularly an MPEA as cast or a heat-treated MPEA, as material for hydrogen storage.

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention belongs to the sector of metallurgical industry. More particularly, it relates to new multi-principal element alloys, in particular to multi-principal element alloys as cast and heat-treated multi-principal element alloys, as well as to their preparation. These alloys have particular crystallinity properties, such as a dual microstructure which can be modified depending on the use of different cooling rates or a thermal treatment during its preparation. They also have specific mechanical properties, such as intermediate Vickers hardness, high yield and high tensile strength, among others, which would make them particularly suitable for their use as material for hydrogen storage, to which the present invention also relates.BACKGROUND OF THE INVENTION

[0002] Hydrogen is considered as a green source of energy. It is storable, transportable, and may effectively act as clean energy carrier.

[0003] Hydrogen storage requires the use of materials which do not undergo hydrogen embrittlement (HE), which is a reduction in elongation and toughness caused by exposure to hydrogenated environments, more particularly, due to hydrogen dissociation and absorption occurring in the storage material. Furthermore, while hydrogen can be stored in different forms, it is mostly stored in gas or liquid condition, in both cases requiring significantly low-temperature and highpressure conditions. Such low temperatures are also known to require the use of materials with high mechanical resistance, among other particular properties.

[0004] While certain austenitic stainless steels (e.g., type 316L austenitic stainless steel) or aluminium alloys may be used for storing hydrogen due to having high resistance to hydrogen embrittlement, they typically have reduced mechanical strength while, at the same time, they are significantly more expensive compared to other steels such as Cr-Mo or Ni-Cr-Mo steels. In parallel, other known steels employed in the construction of structural elements, such as the 42CrMo4 steel, are not adequate for hydrogen storage due to being susceptible to hydrogen embrittlement.

[0005] Multi-principal element alloys (MPEAs), including high-entropy alloys (HEAs), are well known alloys with a solid solution hardening mechanism of different elements in the range of near to equiatomic proportion which minimizes the quantity of different phases. There exist evidence of some MPEAs with a major face-centered cubic (FCC) microstructure, such a CoNiV alloys, presenting slightly improved mechanical properties at low temperature in the presence of hydrogen. However, the presence of Co and V in those alloys dramatically increases the cost of the material due to their scarce availability, therefore hindering their use as hydrogen storage solutions at industrial scale.

[0006] In this context, it becomes apparent that there exists the need for new metallic materials which offer increased yield strength and tensile strength compared to other types of crystalline materials, without significantly comprising the cost of their use at industrial scale. It is also of particular interest for the industry that these new materials may also potentially provide advantages in properties such as hydrogen embrittlement resistance, so that they can be incorporated into hydrogen storage solutions.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 -Light Microscope (LM) image of an exemplary MPEA as cast of the present invention, prepared as described in Example 1, showing its microstructure. Scale bar 10 µm (right bottom of figure). FIG. 2 -Light Microscope (LM) image of an exemplary MPEA(T1+T2) of the present invention, prepared as described in Example 2, showing its microstructure. Scale bar 10 µm (right bottom of figure). FIG. 3 -Transmission Electron Microscopy (TEM) image of the chrome phases present in an exemplary MPEA as cast of the present invention, prepared as described in Example 1, showing its microstructure. Scale bar 1 µm (left bottom of figure). FIG. 4 -Energy Dispersive Spectroscopy (EDS) composition analysis using line scan of the segregation in an exemplary MPEA(T1+T2) according to the invention, prepared according to Example 2, corresponding to light and dark phases corresponding to the matrix. Scale bar 400 µm (left bottom of figure). FIG. 5 -Light Microscope (LM) image of chrome-phase evolution from an exemplary MPEA as cast of the invention, to an MPEA of the invention wherein cooling has been performed with water (high cooling rate), as described in Example 4. Scale bar 40 µm (right bottom of figure). FIG. 6 -X-Ray Diffraction (DRX) of main phases present in an exemplary MPEA(T1+T2) of the invention, prepared as described in Example 2. FIG. 7 -X-Ray Diffraction (DRX) of main phases present in an exemplary MPEA as cast of the invention, prepared as described in Example 4 with high cooling rate. FIG. 8 -SEM images of Secondary Dendrite Arm Spacing (SDAS) analyses carried out with three exemplary MPEAs of the invention, produced employing different cooling conditions. From top to bottom: exemplary MPEA according to Example 1, cooled down in silica mould; exemplary MPEA prepared according to Example 1 and poured in conditions according to Example 3, cooled down in chromite sand mould; exemplary MPEA according to Example 4, cooled down in silica mould with water spraying. Scale bar 400 µm (right bottom of each one of the 3 images in the figure). DESCRIPTION OF THE INVENTION

[0008] The present invention provides new multi-principal element alloys (MPEAs), which are equivalent in microstructural terms as HEAs, together with methods for their preparation and their use as defined in the appended claims.

[0009] In a first aspect, the present invention relates to a multi-principal element alloy (MPEA) according to claim 1.

[0010] More particularly, according to a first aspect of the present invention, it is provided a multi-principal element alloy which consists of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloy: Ni:30 - 50 wt.%,Mn:10 - 30 wt.%,Cu:10 - 30 wt. %, andCr:10 - 30 wt.%, the balance being Fe and impurities, and wherein the content of Fe and impurities is equal to or less than 1 wt. %. Examples of those impurities include carbon (C) which, when present, shall not exceed 0.1 wt.% of the total weight of the alloy.

[0011] Preferably, the balance consists of Fe and impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%, and wherein those impurities are unavoidable impurities. Those unavoidable impurities may include carbon (C) though, when present, its concentration shall be preferably limited to 0.1 wt.% or less with respect to the total weight of the MPEA, and / or silicon (Si). In an embodiment, the balance consists of Fe and impurities, wherein the content of Fe and unavoidable impurities may be equal to or less than 0.8 wt.%, equal to or less than 0.6 wt.%, or equal to or less than 0.5 wt.%. In an embodiment, the balance consists of Fe and unavoidable impurities, wherein the content of Fe and unavoidable impurities may be equal to or less than 1.0 wt.%, equal to or less than 0.8 wt.%, equal to or less than 0.6 wt.%, or equal to or less than 0.5 wt.%, and wherein unavoidable impurities comprise C and / or Si.

[0012] By limiting the concentration of impurities included in the alloy structure, homogeneity and mechanical strength can be further increased regardless of the shape dimension of the structure.

[0013] The multi-principal element alloy according to the first aspect of the invention may be a multi-principal element alloy as cast (herein also referred to as "MPEA as cast" or "MPEA(CAST)") or a multi-principal element alloy after its heat treatment (herein also referred to as "heat-treated multi-principal element alloy", "heat-treated MPEA" or "MPEA(T1+T2)").

[0014] In particular, the inventors have found that the MPEA as cast of the present invention can be submitted to a heat treatment rendering a modified MPEA - which is the heat-treated MPEA -, which presents a more homogeneous hardness compared to the corresponding MPEA as cast, as illustrated in Table 2 of the Examples. Heat-treated MPEAs according to the present invention have also been found to present slightly lower hardness than the MPEAs as cast samples consisting of the same chemical composition, as illustrated in the Examples herein provided, particularly in Table 2 therefrom.

[0015] Unless indicated otherwise, all percentages provided throughout the specification are weight percentages. Furthermore, in the context of the present invention, and unless otherwise stated, Vickers hardness values correspond to surface or nucleus hardness values, which have been measured in accordance with UNE EN-ISO6507-1:2015, as described in the Examples.

[0016] In an embodiment, according to a first aspect of the present invention, it is provided a multi-principal element alloy as cast which consists of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloy: Ni:30 - 50 wt.%,Mn:10 - 30 wt.%,Cu:10 - 30 wt. %, andCr:10 - 30 wt.%, the balance being Fe and impurities, and wherein the content of Fe and impurities is equal to or less than 1 wt. %.

[0017] In another embodiment, according to a first aspect of the present invention, it is provided a heat-treated multi-principal element alloy which consists of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloy: Ni:30 - 50 wt.%,Mn:10 - 30 wt.%,Cu:10 - 30 wt. %, andCr:10 - 30 wt.%, the balance being Fe and impurities, and wherein the content of Fe and impurities is equal to or less than 1 wt. %.

[0018] The terms "about" or "around" when used throughout the specification preceding a number and referring to it, are to be understood as disclosing the particular value and designating any value lying within the range defined by the number ±5%, more preferably a range defined by the number ±2%. For example, the expressions "about 1" or "around 1" shall be construed as "within the range of 0.95 to 1.05", preferably as "within the range of 0.98 to 1.02", with both endpoints included within the scope of the interval.

[0019] According to a particular embodiment, the Ni amount in the MPEA, which can be an MPEA as cast or a heat-treated MPEA, is comprised between 32 and 48 wt.%, between 35 and 45 wt.%, or between 38 and 45 wt.%. In some embodiments, the Ni amount in the MPEA is comprised between 30 and 47 wt.%, between 33 and 48 wt.%, between 36 and 48 wt.%, between 40 and 50 wt.%, between 40 and 45 wt.% or between 32 and 40 wt.%.

[0020] The Mn amount in the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may be particularly comprised between 12 and 28 wt.%, between 15 and 30 wt.%, or between 15 and 25 wt.%. In some embodiments, the Mn amount in the MPEA is comprised between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%.

[0021] In particular, the Cu amount in the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may be comprised between 12 and 28 wt.%, or between 15 and 30 wt.%, or between 15 and 25 wt.%. In some embodiments, the Cu amount in the MPEA, which can be an MPEA as cast or a heat-treated MPEA, is comprised between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%.

[0022] According to another particular embodiment, the Cr amount in the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may be comprised between 12 and 28 wt.%, or between 15 and 30 wt.%, or between 15 and 25 wt.%. In some embodiments, the Cr amount in the MPEA is comprised between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%.

[0023] The skilled person can readily understand that all the above disclosed ranges and subranges of each Ni, Mn, Cu and Cr can be combined, and that the resulting combinations are intended to be included in the scope of the present invention, in particular for the MPEAs according to the first aspect of the invention and / or the third aspect of the invention, more particularly, for any MPEA as cast and / or any heat-treated MPEA according to the first aspect of the invention and / or the third aspect of the invention.

[0024] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 32 to 48 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% Cu, and 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0025] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 32 to 48 wt.% Ni, 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Mn, 12 to 28 wt.% Cu and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0026] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 32 to 48 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Cu, and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0027] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 32 to 48 wt.% or 35 to 45 wt.% or 38 to 45 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% Cu, and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0028] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 35 to 45 wt.% Ni, 15 to 30 wt.% Mn, 15 to 30 wt.% Cu, and 15 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0029] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 38 to 45 wt.% Ni, 15 to 30 wt.% Mn, 15 to 30 wt.% Cu, and 15 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0030] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 38 to 45 wt.% Ni, 12 to 28 wt.% Mn, 15 to 30 wt.% Cu, and 15 to 25 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0031] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 35 to 45 wt.% Ni, 15 to 25 wt.% Mn, 12 to 28 wt.% Cu, and 15 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0032] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 36 to 48 wt.% Ni, 10 to 27 wt.% Mn, 15 to 25 wt.% Cu, and 10 to 27 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0033] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 30 to 47 wt.% Ni, 12 to 25 wt.% Mn, 15 to 20 wt.% Cu, and 21 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0034] The MPEAs according to the invention have been surprisingly found to have good machinability, intermediate Vickers hardness, high yield and high tensile strength, as shown in the Examples, in particular in Tables 2 and 3 thereof. In addition to this, it has been found that the chemical composition of these alloys according to the invention, together with their microstructure, advantageously results in specific performance properties such as high young modulus and high tensile strength compared to other types of crystalline materials.

[0035] Mechanical testing of the MPEAs of the invention has been carried out in accordance with UNE EN ISO 6892-1 2020, more particularly, UNE EN ISO 6892-1 2020 A224. Such mechanical testing evidenced coherent mechanical results with remarkably high yield strength and tensile strength in heat-treated MPEAs according to the invention.

[0036] All these chemical and mechanical properties would make the MPEAs according to the invention particularly suitable for hydrogen storage, since they would be significantly less prone to hydrogen embrittlement.

[0037] Further advantageously, the MPEAs according to the invention do not include critical raw materials of high scarcity in nature and / or which are excessively costly, compared to existing alloys typically employed in hydrogen storage applications which may include Co and / or V, among other elements.

[0038] Examples of the microstructure of multi-principal element alloys according to the invention, more particularly of the microstructure of an MPEA as cast and a heat-treated MPEA, are presented in Figure 1 and Figure 2, respectively.

[0039] MPEAs as cast according to the invention, in particular when solidified in a silica mould, have been found to comprise a microstructure which is composed of a first phase which has a face-centered cubic crystalline structure (FCC), and a second phase which has a body-centered cubic crystalline structure (BCC) which is present in less quantity than the first phase.

[0040] Figure 1 corresponds to the light microscope (LM) image of an exemplary MPEA as cast according to the invention, prepared as described in Example 1 with a chemical composition corresponding to a molar formulation equivalent to Ni 2 MnCrCu, so as to ensure that the atomic percentage of each alloying element is kept in similar values for ensuring a high disorder level or what is known as cocktail effect. It can be therein observed that such exemplary MPEA as cast according to the invention, which has been solidified in a silica mould as described in Example 1, comprises a microstructure which is composed of a principal FCC phase and a secondary BCC phase which is present in less quantity than the FCC one. When a heat treatment is carried out, some of the chemical elements segregated in interdendritic spaces are homogenised and differences in chemical composition between phases are therefore reduced as shown in Figure 2. However, when a faster cooling rate is employed, basically due to mould dissolving with water cooling, it can then be observed how Cr particles do not appear, as shown in Figure 5.

[0041] The alloy phase identification, as well as the chemical elements distribution analysis of the MPEAs of the invention has been conducted with an element mapping method using transmission electron microscopy (TEM). In particular, MPEAs as cast have been found to comprise a microstructure conformed by a principal FCC phase that presents a darker or lighter grey colour in TEM images depending on the segregation present, together with precipitated particles made basically of Cr that can be assimilated to a BCC phase. Figure 3 shows a TEM image of an exemplary MPEA as cast according to the invention, wherein the darkest areas correspond to particles enriched in Cr resulting from segregation, while the other chemical elements remain homogeneously dispersed in the matrix.

[0042] However, where an MPEA according to the invention has been produced using a rapid cooling rate, e.g., by using water as cooling medium, differences in the crystallographic structure are observed compared to an MPEA as cast according to the invention with the same elemental composition. More specifically, when water was employed for cooling down an MPEA according to the invention, the BCC phase corresponding to the Cr-rich matrix would no longer be observed, as illustrated by Figure 7 corresponding to an exemplary MPEA according to the invention.

[0043] The chemical composition analysis carried out by Energy Dispersive Spectroscopy (EDS) with line scan has shown that segregation of different chemical elements in Secondary Dendrite Arm Spacing (SDAS) may exist. By way of illustration, Figure 4 corresponds to an EDS composition analysis using line scan of the segregation in an exemplary MPEA(T1+T2) according to the invention. The darker grey matrix presents higher Cr content than the light grey matrix, whereas the lighter grey matrix presents higher content of Cu than the darker grey matrix.

[0044] Furthermore, Figure 8 illustrates the fact that the Secondary Dendrite Arm Spacing (SDAS) values of the MPEAs according to the invention have been found to change depending on the conditions employed for cooling down the alloy: SDAS around 50 µm in the case of silica mould sand, around 17 µm in the case of chromite sand and around 12 µm in the case of water cooled down sample.

[0045] Without wishing to be bound by theory, it is postulated that MPEAs which have undergone a heat treatment may reach a thermodynamic equilibrium based on a two-phase microstructure and therefore provide excellent properties due to their high chemical homogeneity (compared to an MPEA as cast having the same elemental composition), which may also be achieved by using a relatively low cooling rate during the solidification step in the alloy production. In contrast, MPEAs which have undergone a very rapid cooling during the solidification step of the alloy production were found to have a microstructure with an FCC phase as single or almost single phase (e.g., at least 90% of FCC phase, with equal to or less than 10% of a BCC phase), which is postulated to derive from the existence of a metastable equilibrium. Such relatively rapid cooling was found to advantageously provide enhanced chemical and phase homogeneities.

[0046] In an embodiment, the MPEA according to the invention comprises a microstructure which is composed of at least 80% of a first phase which has an FCC crystalline structure, and equal to or less than 20% of a second phase which has a BCC crystalline structure, as determined by SEM. Preferably, the MPEA according to the invention comprises a microstructure which is composed of at least 90% of a first phase which has an FCC crystalline structure, and equal to or less than 10% of a second phase which has a BCC crystalline structure, as determined by SEM. Any percentage value of any of the phases in the microstructure, in particular either the first phase or the second phase of the microstructure comprised by the MPEAs according to the invention, shall be understood as an area percentage calculated in respect of the total area of the microstructure. By way of illustration, when the microstructure comprised by the MPEAs according to the invention is composed of "at least 80%" of a first phase, it shall be understood that the first phase corresponds to at least 80%, in area units, of the total area of the microstructure.

[0047] In an embodiment, the MPEA according to the invention comprises a microstructure which is composed of at least 98% of a first phase which has an FCC crystalline structure, and equal to or less than 2% of a second phase which has a BCC crystalline structure, as determined by SEM. The MPEA according to the invention may comprise a microstructure which is composed of at least 99% of a first phase which has an FCC crystalline structure, and equal to or less than 1% of a second phase which has a BCC structure, as determined by SEM. In another embodiment, The MPEA according to the invention may comprise a microstructure which is composed of at least 99.5% of a first phase which has an FCC crystalline structure, and equal to or less than 0.5% of a second phase which has a BCC crystalline structure, as determined by SEM.

[0048] The MPEA according to the invention may be an MPEA as cast which comprises a microstructure which is composed of 50-70 % a first phase which has an FCC crystalline structure, and 30-50 % of a second phase which has a BCC crystalline structure, as determined by SEM.

[0049] The MPEAs according to the invention may be a heat-treated MPEA which comprises a microstructure which is composed of at least 80%, 85%, 90% or 95% of a first phase which has an FCC crystalline structure, and equal to or less than 20%, 15%, 10% or 5%, respectively of a second phase which has a BCC crystalline structure, as determined by SEM.

[0050] In an embodiment, the MPEAs according to the invention, in particular those MPEAs which have been subject to a rapid cooling rate, comprise a microstructure which is composed of at least 95% of a first phase, and equal to or less than 5% of a second phase, as determined by SEM, wherein the first phase has a metastable FCC crystalline structure, and the second phase has a BCC crystalline structure. In a preferred embodiment, the MPEAs according to the invention, in particular those MPEAs which have been subject to a rapid cooling rate, comprise a microstructure which is composed of at least 98% of a first phase, and equal to or less than 2% of a second phase, as determined by SEM, wherein the first phase has a metastable FCC crystalline structure, and the second phase has a BCC crystalline structure. In still another preferred embodiment, the MPEAs according to the invention, in particular those MPEAs which have been subject to a rapid cooling rate, comprise a microstructure which is composed of at least 99% of a first phase, and equal to or less than 1% of a second phase, as determined by SEM, wherein the first phase has a metastable FCC crystalline structure, and the second phase has a BCC crystalline structure.

[0051] In a second aspect, the present invention provides a process for the preparation of a multi-principal element alloy, characterized by comprising the following steps: a) Introducing alloying elements of the alloy which is to be prepared in a furnace for forming an alloying mixture; b) heating the alloying mixture at a temperature in the range from 1200°C to 1700°C for forming a molten mixture; c) optionally removing oxygen and / or removing hydrogen while the mixture is in molten state, said oxygen removal step comprising the introduction of a deoxidizing agent; d) removing slag, optionally aided by flux addition , e) maintaining the mixture in molten state for homogenization; f) pouring of the molten mixture in a mould; and g) solidifying the molten mixture to obtain the multi-principal element alloy.

[0052] The furnace according to a particular embodiment is a conventional furnace such as an induction melting furnace.

[0053] Preferably, the alloying elements introduced in step a) of the process according to the second aspect of the invention consist of Ni, Mn, Cu, Cr and Fe in order to obtain the desired multi-principal element alloy compositions of the present invention, wherein each alloying element may be introduced in step a) in substantially pure form.

[0054] The expression "substantially pure form" as herein used in reference to an alloying element shall be understood as meaning that an alloying element has a purity equal to or greater than 99 wt.% (i.e., 99-100 wt.%), preferably a purity equal to or greater than 99.5 wt.%, and more preferably a purity equal to or greater than 99.9 wt.%.

[0055] Still more preferably, the alloying elements introduced in step a) of the process according to the second aspect of the invention consist of Ni, Mn, Cu, Cr and Fe, and each one of them is introduced in substantially pure form. Even more preferably, the alloying elements introduced in step a) of the process according to the second aspect of the invention consist of Ni, Mn, Cu, Cr and Fe, wherein each one of them is introduced in the following form: ∘ Substantially pure Cu: Cu having 99-100 wt.% purity; ∘ Substantially pure Mn: Mn having 99-100 wt.% purity; ∘ Substantially pure Cr: Cr having 99-100 wt.% purity; and ∘ Substantially pure Ni: Ni having 99-100 wt.% purity.

[0056] The alloying mixture formed in step a) preferably consists of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: Ni:30 - 50 wt.%,Mn:10 - 30 wt.%,Cu:10 - 30 wt. %, andCr:10 - 30 wt.%, the balance being Fe and impurities, and wherein the content of Fe and impurities is equal to or less than 1 wt. %. Examples of those impurities include carbon (C) which, when present, shall not exceed 0.1 wt.% of the total weight of the alloying mixture. Preferably, the balance consists of Fe and impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%, equal to or less than 0.8 wt.%, equal to or less than 0.6 wt.%, or equal to or less than 0.5 wt.%, and wherein those impurities are unavoidable impurities. Those unavoidable impurities may include carbon (C) though, when present, its concentration shall be preferably limited to 0.1 wt.% or less of the total weight of the alloying mixture. In an embodiment, the balance consists of Fe and unavoidable impurities, wherein the content of Fe and unavoidable impurities may be equal to or less than 1.0 wt.%, equal to or less than 0.8 wt.%, equal to or less than 0.6 wt.%, or equal to or less than 0.5 wt.%, and wherein unavoidable impurities comprise C and / or Si.

[0057] This process according to the invention is particularly suitable for producing the metal-principal element alloys of the present invention, and any adjustment of the necessary quantities of starting materials or reagents for producing an MPEA according to the invention with a desired composition can be readily carried out by a skilled person in the art. In particular, the process of the invention is suitable for producing the MPEA as cast according to the invention, as well as the heat-treated MPEA according to the invention, since it can be easily tailored depending on the characteristics of the metal-principal element alloy which is to be manufactured by adjusting, for example, the amounts of alloying elements employed as raw materials, the melting temperature or the temperature and cooling rate during solidification of the molten mixture.

[0058] According to a particular embodiment, the Ni amount in the alloying mixture formed in step a) may be comprised between 32 and 48 wt.%, between 35 and 45 wt.%, between 38 and 45 wt, between 30 and 47 wt.%, between 33 and 48 wt.%, between 36 and 48 wt.%, between 40 and 50 wt.%, between 40 and 45 wt.% or between 32 and 40 wt.%, based on the total weight of the alloying mixture.

[0059] In particular, the Mn amount in the alloying mixture formed in step a) may be comprised between 12 and 28 wt.%, between 15 and 30 wt.%, between 15 and 25 wt.%, between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%, based on the total weight of the alloying mixture.

[0060] According to another embodiment, the Cu amount in the alloying mixture formed in step a) may be comprised between 12 and 28 wt.%, between 15 and 30 wt.%, between 15 and 25 wt.%, between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%, based on the total weight of the alloying mixture.

[0061] In another embodiment, the Cr amount in the alloying mixture formed in step a) may be comprised between 12 and 28 wt.%, between 15 and 30 wt.%, between 15 and 25 wt.%, between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%, based on the total weight of the alloying mixture.

[0062] The alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 32 to 48 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% Cu, and 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0063] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 32 to 48 wt.% Ni, 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Mn, 12 to 28 wt.% Cu and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0064] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 32 to 48 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Cu, and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0065] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 32 to 48 wt.% or 35 to 45 wt.% or 38 to 45 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% Cu, and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0066] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 35 to 45 wt.% Ni, 15 to 30 wt.% Mn, 15 to 30 wt.% Cu, and 15 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0067] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 38 to 45 wt.% Ni, 12 to 28 wt.% Mn, 15 to 30 wt.% Cu, and 15 to 25 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0068] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 35 to 45 wt.% Ni, 15 to 25 wt.% Mn, 12 to 28 wt.% Cu, and 15 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0069] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 36 to 48 wt.% Ni, 10 to 27 wt.% Mn, 15 to 25 wt.% Cu, and 10 to 27 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0070] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 30 to 47 wt.% Ni, 12 to 25 wt.% Mn, 15 to 20 wt.% Cu, and 21 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0071] Step b) of the process of the present invention may preferably be carried out at a temperature in the range from 1300°C to 1700°C for forming the molten mixture.

[0072] During the process of the invention, it is preferable to protect the surface of the melt mixture in the furnace with a protective product (generally known as "slag coagulation flux") to prevent oxidation losses of alloying elements and gas pick-up from the atmosphere. Further, during the process the mixture can be covered with a ceramic fibre blanket in order to prevent that hydrogen from the atmosphere dissolves in the molten metal and that also metal oxidations take place.

[0073] During the melting process, oxygen and hydrogen may dissolve in the melt due to generated turbulence, which can cause porosity and inclusions. If those gases were not removed, they would accumulate, causing holes to appear in the last solidification step h). In order to avoid such occurrence, preferably step c) is carried out, said step c) comprising removing oxygen and / or hydrogen while the mixture is in molten state, said oxygen removal step comprising the introduction of a deoxidizing agent, such as ferro silicon calcium. The deoxidizing agent may be preferably introduced in a fixed proportion (e.g., 0.10 wt.% based on the total weight of molten metal) in the furnace once all alloying elements have been loaded and molten. The hydrogen removal is preferably carried out by nitrogen or argon bubbling, from the bottom of the furnace, by means of a porous plug.

[0074] Once the molten mixture if formed, reaction products (known as "slag") are removed from the top of the surface in the furnace in step d). Such removal may be preferably carried out by adding slag coagulation flux until the slag is thick enough to be removed with a metal stick. The furnace must be switched off to stop turbulence and allow slag floating in induction furnaces.

[0075] Step e) is necessary to allow sufficient time to achieve a correct homogenization of the alloying elements in the melt before it is poured into a mould in step f). Preferably, in step e), the mixture is maintained in molten state for not less than 10 minutes. After step e), the melt is typically transferred to a pouring ladle, and then poured in a mould.

[0076] The temperatures which are preferably employed in steps b) (i.e. melting temperature) and f) (i.e. pouring temperature) are variable depending for example on the thickness of the part to be cast. The temperature employed in step b) and / or step f) is preferably in the range of around 1200°C-1700°C, preferably around 1400°C-1600°C, more preferably in the range of 1400°C-1600°C, still more preferably in the range of 1450°C-1600°C, even more preferably in the range of 1500°C-1600°C, or for example for example 1600°C. By way of illustration, temperatures in the range of 1400°C-1600°C, for example 1600°C, are usually employed for parts with a thickness of less than 38 mm, and around 1400°C for parts having more than 38 mm thickness.

[0077] As regards the solidification step g) of the process, the cooling rate parameter has been found to be of particular interest for producing MPEAs of the invention with different microstructures. This parameter may be modified, for example, by choosing a mould made of a particular material, wherein the molten mixture is poured. Thus, for example, a cooling rate in the range from 0.05°C / s to 0.1°C / s can be achieved by using a silica sand mould, while cooling rates in the range from 0.1 °C / s to 0.2°C / s or from 2°C / s to 4°C / s can be achieved by using a chromite sand mould or a water-soluble silica sand mould made of inorganic colloidal silica, respectively.

[0078] In an embodiment, step g) of the process of the invention comprises cooling the molten mixture down at a cooling rate in the range from 0.5°C / s to 4.0°C / s, in particular at a cooling rate in the range of 0.8-3.5°C / s, in the range of 1.0-2.0°C / s or in the range of 2.0-4.0°C / s. These rapid cooling rates were found to favour the obtention of MPEAs with microstructures having an FCC phase as either single or almost single phase, which is postulated to correspond to a metastable equilibrium.

[0079] In another embodiment, step g) of the process of the invention comprises cooling the molten mixture down at a cooling rate in the range from 0.05°C / s to less than 0.5°C / s, in particular at a cooling rate in the range of 0.05-0.4°C / s or in the range of 0.1-0.4°C / s. These slower cooling rates were found to favour the formation of two-phase microstructures in thermodynamic equilibrium, comprising BCC and FCC phases with higher homogeneity in hardness.

[0080] The process according to the second aspect of the invention may further comprise, subsequently to step g), a thermal treatment step so as to obtain a heat treated MPEA or MPEA(T1+T2). This thermal treatment step can be carried out by any conventional manner known by the person skilled in the art, and is typically performed in a furnace, such as an electrical resistance non-controlled atmosphere thermal treatment furnace. The thermal treatment step comprises subjecting the MPEA of the invention, in particular the MPEA as cast, to a first thermal treatment (i.e., solubilisation thermal treatment or solubilisation heat treatment) at temperature of 500-1200°C, followed by a second thermal treatment (i.e., precipitation thermal treatment or precipitation heat treatment) at a temperature of 500-800°C, with an optional cooling down step in between both thermal treatments (i.e., the first thermal treatment and the second thermal treatment) in a cooling medium. The MPEA as cast resulting from the first thermal treatment, and before subjecting it to a second thermal treatment, shall be hereinafter referred to as "MPEA(T1)".

[0081] The time the alloy (in particular, as cast) may be subject to the optional thermal treatment step subsequent to step g) varies depending on factors such as the size of the part which is to be produced or the selected temperature, and is typically from 4 to 60 hours. Cooling media employed in the optional cooling down step can also vary depending on the cooling rate required, and suitable examples thereof include, in a non-limiting way, cooling inside the furnace, calm-air cooling, forced-air cooling, cooling in oil, or cooling in water. The cooling down temperature in the optional cooling down step may preferably be in the range from 500°C to 700°C, while the duration of the cooling down step may vary between 1 and 10 hours.

[0082] In the context of the present invention, the term "calm air" or "calm-air" shall be understood as referring to an environment in absence or nearly absence of air currents and / or wind, i.e., wind and / or air currents moving at a speed below 1 knot, or "zero" (0) on the international scale of wind force known as "Beaufort scale". The temperature in "calm air" or "calm-air" conditions shall be understood as ambient or room temperature, taken in the broad sense, i.e., generally between 15°C and 30°C, more specifically between 20°C and 27°C, between 20°C and 25°C, or between 20°C and 22°C.

[0083] Inventors have found that temperatures around 950°C in the first treatment step following step g) were particularly effective for the MPEAs as cast to reach a new thermodynamic equilibrium, wherein segregation is reduced and homogenization in hardness takes place.

[0084] Preferably, the process of the present invention may further comprise heating the solidified molten mixture resulting from step g) at a temperature in the range from 500°C to 1200°C, preferably from 700°C to 1100°C, more preferably from 800°C to 1000°C, even more preferably from 900°C to 1050°C or still even more preferably at a temperature of about 950°C, in a first thermal treatment step. The process according to the invention may optionally further comprise a second thermal treatment step, taking place after the first thermal treatment step, which comprises heating the molten solidified mixture resulting from the first thermal treatment step at a temperature in the range from 500°C to 800°C, more preferably from 600°C to 760°C, even more preferably from 650°C to 800°C and most preferably at about 760°C. The process of the invention may preferably comprise the cooling down step between the first and second thermal treatment steps in a cooling medium, as described above, and / or an additional cooling step after the second thermal treatment has taken place. The additional cooling down step may comprise subjecting the solidified molten mixture resulting from the second thermal treatment to an intermediate lower temperature which is in the range of 500-700°C, preferably in the range of 550-650°C and most preferably a temperature of about 650°C, followed by further cooling down, for example, by cooling inside the furnace, calm-air cooling, forced-air cooling, cooling in oil, or cooling in water. In fact, inventors have found that the use of temperatures about 650°C during the cooling down step were particularly effective as they promoted the hardening of the alloy due to the precipitation of Cr phases.

[0085] In a preferred embodiment, the molten mixture is solidified in step g) by cooling it down at a cooling rate in the range from 0.05°C / s to less than 0.5°C / s, in particular at a cooling rate in the range of 0.05-0.4°C / s or in the range of 0.1-0.4°C / s, followed by a subsequent additional step, which is a first thermal treatment step wherein the resulting solidified molten mixture is heated at a temperature in the range from 500°C to 1200°C, preferably from 700°C to 1100°C, more preferably from 800°C to 1000°C, said first thermal treatment being optionally followed by a second thermal treatment step, which comprises heating the molten solidified mixture resulting from the first thermal treatment step at a temperature in the range from 500°C to 800°C.

[0086] In a third aspect of the invention, it is provided a multi-principal element alloy which is obtained or obtainable by the process according to the second aspect of the invention. This MPEA may have the same elemental composition as the MPEAs defined according to the first aspect of the invention.

[0087] In particular, the MPEAs according to the first of the third aspect of the invention may have a Vickers hardness (HV) in the range of 280-290 kgf / mm 2< (i.e., 2746 - 2844 MPa in SI units), as determined in accordance with UNE EN-ISO6507-1:2015.

[0088] Preferably, the MPEA obtainable or obtained by the process according to the second aspect of the invention comprises a microstructure which is composed of at least 80% of a first phase which has an FCC crystalline structure, and equal to or less than 20% of a second phase which has a BCC crystalline structure, as determined by SEM. More preferably, the MPEA obtainable or obtained by the process of the invention comprises a microstructure which is composed of at least 90% of a first phase which has an FCC crystalline structure, and equal to or less than 10% of a second phase which has a BCC crystalline structure, as determined by SEM.

[0089] In a preferred embodiment, the MPEA according to the invention comprises a microstructure which is composed of at least 90% of a first phase which has an FCC crystalline structure, and equal to or less than 10% of a second phase which has a BCC crystalline structure, as determined by SEM, said microstructure having the following characteristic peaks depending on the cooling rate employed during step h) of the process: • Cooling rate 0.05-below 0.5 °C / s: Position (±0.2° °2Th.)Relative intensity (%)Crystalline phase43.222892.92FCC44.319524.97BCC50.0739100.00FCC73.63827.28FCC81,770311.74BCC • Cooling rate 0.5-4.0 °C / s: Position (±0.2° °2Th.)Relative intensity (%)Crystalline phase43.150884.85FCC50.246960.34FCC73.951879.41FCC89.7248100.00FCC

[0090] In another preferred embodiment, the MPEA obtainable or obtained by the process according to the second aspect of the invention comprises a microstructure which is composed of at least 90% of a first phase having a metastable FCC crystalline structure, and equal to or less than 10% of a second phase which has a BCC crystalline structure, as determined by SEM. More preferably, the MPEA obtainable or obtained by the process of the invention comprises a microstructure which is composed of at least 95% of a first phase having a metastable FCC crystalline structure, and equal to or less than 5% of a second phase which has a BCC crystalline structure, as determined by SEM.

[0091] In a fourth aspect of the invention, it is provided the use of a multi-principal element alloy according to the invention, more particularly according to the first or third aspect of the invention, as material for hydrogen storage.

[0092] Preferably, the use according to the fourth aspect of the invention is the use of an MPEA as cast or a heat-treated MPEA of the present invention, as material for hydrogen storage.

[0093] Throughout the description and the claims, the word "comprise" and variations thereof are not intended to exclude other technical features, ingredients or steps. Additional advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention without undue burden.EXAMPLES

[0094] The following examples are provided by way of illustration and shall not be construed as limiting the invention. Numerous variations are possible, and it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described in the following examples.

[0095] For the tests carried out in relation to the present invention, keel blocks Y2 were prepared using chemically bonded silica sand, chromite sand or inorganic colloidal silica, in all three cases following the standard norm UNE-EN 1563:1998. Keel blocks Y2 were then extracted from the moulds and cleaned by shot blasting.

[0096] Keel blocks were then cut and test samples for micrographic inspection of rectangular dimensions of approximately 30 x 30 x 10 mm were prepared by surface polishing; their microstructures were then analysed with a scanning electron microscope (SEM) (JEOL JSM-6400).

[0097] An Element Mapping Method using transmission electron microscopy (TEM) was employed for identifying the phases in the alloys, as well as their chemical elements distribution. The TEM equipment was a Talos F200i a 200 kV in mode STEM (Scanning Transmission Electron Microscopy). The element mapping was carried out using the Energy Dispersive X-Ray Spectroscopy (EDX) Seper-X G2 equipment that this microscopy incorporated.

[0098] Vickers hardness was determined using test samples of rectangular dimensions of approximately 30 x 30 x 10 mm using an INSTROM WOLPER equipment model TESTOR 971 / 3000. Hardness was determined preferably in the centre of the obtained samples and always to more than 5 mm from any surface. Measurements were made according to the method described under the Examples, in accordance with UNE EN-ISO6507-1:2015.

[0099] An Energy Dispersive Spectrometer (EDA) INCA X-sight Series Si(Li) pentaFET (Oxford) was used for quantitative chemical analysis of the metal-principal element alloys.

[0100] X-Ray Diffraction (XRD) analyses were performed using a PANalytical Xpert PRO diffractometer, equipped with a goniometric vertical copper tube (Bragg-Brentano geometry). The measuring conditions were as follows: Current type: 40 KV y 40 mA Angular sweep: 10-90° 2Θ

[0101] Light Microscopy (LM) analysis was carried out with a Leica DM4000M equipment with digital camera ProgRes C7 from Jenoptik, with 50X magnification. Samples were etched using Kalling's No. 2. This technique was used to identify differences in secondary Dendrite Arm Spacing (SDAS) and segregations visually by comparison of different pictures.EXAMPLE 1 - Manufacturing of an MPEA as cast according to the invention

[0102] Different substantially pure elements (Ni, Mn, Cr, Cu) of purity certified as higher than 99.0 wt.% were selected to obtain the exemplary chemical composition shown below in Table 1: Table 1 Alloy compositionNi (wt. %)Mn (wt.%)Cr (wt.%)Cu (wt.%)Balance: Fe and unavoidable impurities (wt.%)MPEA as cast*42.117.318.222.00.4

[0103] These elements were introduced in the adequate proportion in an induction furnace of 100-kg capacity to form the alloying mixture.

[0104] Melting of the alloying mixture was carried out when a temperature of 1560°C was reached. Argon purging was performed by the bottom of the furnace to eliminate hydrogen with a porous plug, and slag was then removed from the top of the furnace.

[0105] Once the alloying mixture was molten, and the slag was removed from the top of the furnace, the mixture was maintained in molten state for adequate mixing-up (i.e., homogenization) of the elements. The metal was then transferred to a ladle, and then pouring of the molten mixture was performed into two keel blocks Y2 of chemically-bonded silica sand moulds in order to obtain test specimens for later-on testing. The cooling rate in the chemically-bonded silica sand mould was estimated to be 0.05-0.5°C / sec

[0106] The general microstructure of this alloy is shown on Figure 1, and the specific nature of Cr particles on Figure 3.

[0107] Figure 1 is a light microscopy (LM) image which shows the dual microstructure of this exemplary MPEA, which is composed of a principal FCC phase and a secondary BCC phase which is present in less quantity than the first one. Figure 3 shows a TEM image of this exemplary MPEA, wherein the darkest areas correspond to particles enriched in Cr resulting from segregation, while the other chemical elements remain homogeneously dispersed in the matrix.EXAMPLE 2 - Manufacturing of an MPEA(T1+T2) according to the invention (low cooling rate)

[0108] A sample of an MPEA as cast of the present invention, produced as described in Example 1 and having the elemental composition therein indicated, has been processed in a heat treatment furnace using electric resistance as heating media, with no protective atmosphere.

[0109] The samples have been heated up to 950°C, maintained for 1 hour and cooled down in water. The next step has comprised the heating up at 760°C, maintaining the sample at this temperature for 5 hours, cooling down at 650°C in two hours and maintaining the sample at this temperature for 1 hour followed by cooling it down in calm air.

[0110] The results of hardness and mechanical tests after the different heat treatments are respectively gathered in Tables 2 and 3 below: Table 2 shows the hardness evolution of MPEAs of the invention, from an MPEA as cast according to Example 1 (MPEA(CAST), first entry in Table 2) to the heat-treated MPEA of the invention prepared in the present example (MPEA(T1+T2), third entry in Table 2). For comparative purposes, Table 2 also illustrates the experimental results retrieved from samples of the MPEA as cast of Example 1 when it had undergone the first thermal treatment, but not the second one yet, thus herein referred to as MPEA(T1) (second entry in Table 2) to distinguish it from the MPEA(T1+T2) samples which are heat-treated with a first thermal treatment and second thermal treatment as explained above. In all three examples (i.e., MPEA(CAST), MPEA(T1) and MPEA(T1+T2)), five different measurements were made from different points of the material, in order to assess their homogeneity. Table 2 Vickers hardness (HV) Measure 1 Measure 2 Measure 3 Measure 4 Measure 5 Average value Variation range MPEA(CAST) 274.9310.5332.3319.8323.6312.2257.4MPEA(T1) 270.9262.9262.4269.5266266.348.5MPEA(T1+T2) 282.4279.6282.9285.4280.1282.085.8

[0111] Results showed in Table 2 evidenced the fact that thermally treating the MPEAs of the invention advantageously resulted in enhanced homogeneity, in terms of hardness, throughout the material as a whole. In parallel, it was found that heat-treated MPEA samples presented slightly lower hardness than the MPEAs as cast having the same chemical composition.

[0112] Table 3 illustrates the mechanical results obtained with the heat-treated MPEA obtained as described above in this Example. These results serve to illustrate the fact that MPEAs of the invention, particularly heat-treated MPEAs according to the invention, have remarkably high yield strength (Y.S.) and tensile strength (T.S.). Table 3 Tensile test Diameter (mm) Y.S. 0.2% offset (MPa) T.S. (MPa) Elong. (L o =5d) (%) Area reduction (%) MPEA(T1+T2) 6.056057093.911

[0113] The values of the mechanical properties shown in Table 3 are advantageous in terms of very high value of yield, near the tensile strength that ensures a good dimensional stability once subjected to high pressures when containing gases such as hydrogen.

[0114] The microstructure of this heat-treated MPEA (i.e., MPEA(T1+T2)) is shown on Figure 2. In this figure, reduction of segregation is appreciated due the attenuation of the colour contrast in the interdendritic spaces.

[0115] The particular chemical composition of the phases is shown on Figure 4, which corresponds to an EDA composition analysis using line scan, from which segregation could be qualitatively and quantitatively analysed, as shown in Table 4: Table 4 C Si Cr Mn Fe Ni Cu Dark grey area 2.60.431.310.10.746.78.3Light grey area 3.90.615.318.00.441.420.4Light area with Cr particles 2.80.45.520.20.327.743.0

[0116] The darker grey matrix presented higher Cr content than the light grey matrix, whereas the lighter grey matrix presented higher content of Cu than the darker grey matrix, and the light lighter grey matrix also incorporates in some cases Cr particles.EXAMPLE 3 - Manufacturing of an MPEA as cast according to the invention (intermediate cooling rate)

[0117] In order to evaluate the effect of cooling rate in the microstructure of the MPEAs as cast of the present invention, keel blocks Y2 were prepared using chemically bonded chromite sand moulds. Those mould were dimensioned following the standard norm UNE-EN 1563:1998. The keel blocks Y2 were then extracted from the moulds and cleaned by shot blasting.

[0118] An MPEA according to the invention, having the same elemental composition as that of Example 1, was prepared under the same process conditions as those described in Example 1, the difference being that the molten mixture was poured in a chromite sand mould. The cooling rate in the chromite sand mould was estimated to be of 0.5-1°C / sec.

[0119] Chromite was found to present a higher cooling-rate potential than the silica sand moulds employed in Example 1, and allowed to obtain finer microstructures with lower segregation and particular crystallinity.

[0120] The SEM images in Figure 8 show a reduction in SDAS in the case of MPEA samples manufactured using chromite sand moulds (central image of Figure 8) compared with MPEA samples manufactured with silica sand moulds (top image of Figure 8).EXAMPLE 4 - Manufacturing of an MPEA as cast according to the invention (high cooling rate)

[0121] In order to further evaluate the effect of cooling rate in the microstructure of the MPEAs as cast of the present invention, test bar moulds were prepared using special inorganic colloidal silica. After pouring the molten mixture in the mould, water spray was applied over the mould, dissolving the mould and achieving a higher cooling rate.

[0122] Water cooling was found to present a higher cooling-rate potential than chromite sand moulds, and a much higher cooling-rate potential than silica sand moulds, allowing the obtention of even finer microstructures with lower segregation and particularly crystallinity compared to processes carried out with solidification at lower cooling rates.

[0123] Light optical microscopy in Figure 5 and DRX results in Figure 7 of this MPEA as cast according to the invention show the reduction of segregation and presence of chrome phases.

[0124] SEM images in Figure 8 show a reduction in SDAS depending on the mould material, with the lowest value corresponding to cooling carried out in the presence of water: around 50 µm in the case of a silica sand mould (Example 1), around 17 µm in the case of a chromite sand (Example 3) and around 12 µm in the case of a water cooled down sample (present Example).

Examples

example 1-manufacturing

EXAMPLE 1 - Manufacturing of an MPEA as cast according to the invention

[0102]Different substantially pure elements (Ni, Mn, Cr, Cu) of purity certified as higher than 99.0 wt.% were selected to obtain the exemplary chemical composition shown below in Table 1:

Table 1

Alloy compositionNi (wt. %)Mn (wt.%)Cr (wt.%)Cu (wt.%)Balance: Fe and unavoidable impurities (wt.%)

MPEA as cast*42.117.318.222.00.4

[0103]These elements were introduced in the adequate proportion in an induction furnace of 100-kg capacity to form the alloying mixture.

[0104]Melting of the alloying mixture was carried out when a temperature of 1560°C was reached. Argon purging was performed by the bottom of the furnace to eliminate hydrogen with a porous plug, and slag was then removed from the top of the furnace.

[0105]Once the alloying mixture was molten, and the slag was removed from the top of the furnace, the mixture was maintained in molten state for adequate mixing-up (i.e., homogenization) of the elements. The...

example 2-manufacturing

EXAMPLE 2 - Manufacturing of an MPEA(T1+T2) according to the invention (low cooling rate)

[0108]A sample of an MPEA as cast of the present invention, produced as described in Example 1 and having the elemental composition therein indicated, has been processed in a heat treatment furnace using electric resistance as heating media, with no protective atmosphere.

[0109]The samples have been heated up to 950°C, maintained for 1 hour and cooled down in water. The next step has comprised the heating up at 760°C, maintaining the sample at this temperature for 5 hours, cooling down at 650°C in two hours and maintaining the sample at this temperature for 1 hour followed by cooling it down in calm air.

[0110]The results of hardness and mechanical tests after the different heat treatments are respectively gathered in Tables 2 and 3 below: Table 2 shows the hardness evolution of MPEAs of the invention, from an MPEA as cast according to Example 1 (MPEA(CAST), first entry in Table 2) to the heat-tre...

example 3-manufacturing

EXAMPLE 3 - Manufacturing of an MPEA as cast according to the invention (intermediate cooling rate)

[0117]In order to evaluate the effect of cooling rate in the microstructure of the MPEAs as cast of the present invention, keel blocks Y2 were prepared using chemically bonded chromite sand moulds. Those mould were dimensioned following the standard norm UNE-EN 1563:1998. The keel blocks Y2 were then extracted from the moulds and cleaned by shot blasting.

[0118]An MPEA according to the invention, having the same elemental composition as that of Example 1, was prepared under the same process conditions as those described in Example 1, the difference being that the molten mixture was poured in a chromite sand mould. The cooling rate in the chromite sand mould was estimated to be of 0.5-1°C / sec.

[0119]Chromite was found to present a higher cooling-rate potential than the silica sand moulds employed in Example 1, and allowed to obtain finer microstructures with lower segregation and particul...

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention belongs to the sector of metallurgical industry. More particularly, it relates to new multi-principal element alloys, in particular to multi-principal element alloys as cast and heat-treated multi-principal element alloys, as well as to their preparation. These alloys have particular crystallinity properties, such as a dual microstructure which can be modified depending on the use of different cooling rates or a thermal treatment during its preparation. They also have specific mechanical properties, such as intermediate Vickers hardness, high yield and high tensile strength, among others, which would make them particularly suitable for their use as material for hydrogen storage, to which the present invention also relates.BACKGROUND OF THE INVENTION

[0002] Hydrogen is considered as a green source of energy. It is storable, transportable, and may effectively act as clean energy carrier.

[0003] Hydrogen storage requires the use of materials which do not undergo hydrogen embrittlement (HE), which is a reduction in elongation and toughness caused by exposure to hydrogenated environments, more particularly, due to hydrogen dissociation and absorption occurring in the storage material. Furthermore, while hydrogen can be stored in different forms, it is mostly stored in gas or liquid condition, in both cases requiring significantly low-temperature and highpressure conditions. Such low temperatures are also known to require the use of materials with high mechanical resistance, among other particular properties.

[0004] While certain austenitic stainless steels (e.g., type 316L austenitic stainless steel) or aluminium alloys may be used for storing hydrogen due to having high resistance to hydrogen embrittlement, they typically have reduced mechanical strength while, at the same time, they are significantly more expensive compared to other steels such as Cr-Mo or Ni-Cr-Mo steels. In parallel, other known steels employed in the construction of structural elements, such as the 42CrMo4 steel, are not adequate for hydrogen storage due to being susceptible to hydrogen embrittlement.

[0005] Multi-principal element alloys (MPEAs), including high-entropy alloys (HEAs), are well known alloys with a solid solution hardening mechanism of different elements in the range of near to equiatomic proportion which minimizes the quantity of different phases. There exist evidence of some MPEAs with a major face-centered cubic (FCC) microstructure, such a CoNiV alloys, presenting slightly improved mechanical properties at low temperature in the presence of hydrogen. However, the presence of Co and V in those alloys dramatically increases the cost of the material due to their scarce availability, therefore hindering their use as hydrogen storage solutions at industrial scale.

[0006] In this context, it becomes apparent that there exists the need for new metallic materials which offer increased yield strength and tensile strength compared to other types of crystalline materials, without significantly comprising the cost of their use at industrial scale. It is also of particular interest for the industry that these new materials may also potentially provide advantages in properties such as hydrogen embrittlement resistance, so that they can be incorporated into hydrogen storage solutions.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 -Light Microscope (LM) image of an exemplary MPEA as cast of the present invention, prepared as described in Example 1, showing its microstructure. Scale bar 10 µm (right bottom of figure). FIG. 2 -Light Microscope (LM) image of an exemplary MPEA(T1+T2) of the present invention, prepared as described in Example 2, showing its microstructure. Scale bar 10 µm (right bottom of figure). FIG. 3 -Transmission Electron Microscopy (TEM) image of the chrome phases present in an exemplary MPEA as cast of the present invention, prepared as described in Example 1, showing its microstructure. Scale bar 1 µm (left bottom of figure). FIG. 4 -Energy Dispersive Spectroscopy (EDS) composition analysis using line scan of the segregation in an exemplary MPEA(T1+T2) according to the invention, prepared according to Example 2, corresponding to light and dark phases corresponding to the matrix. Scale bar 400 µm (left bottom of figure). FIG. 5 -Light Microscope (LM) image of chrome-phase evolution from an exemplary MPEA as cast of the invention, to an MPEA of the invention wherein cooling has been performed with water (high cooling rate), as described in Example 4. Scale bar 40 µm (right bottom of figure). FIG. 6 -X-Ray Diffraction (DRX) of main phases present in an exemplary MPEA(T1+T2) of the invention, prepared as described in Example 2. FIG. 7 -X-Ray Diffraction (DRX) of main phases present in an exemplary MPEA as cast of the invention, prepared as described in Example 4 with high cooling rate. FIG. 8 -SEM images of Secondary Dendrite Arm Spacing (SDAS) analyses carried out with three exemplary MPEAs of the invention, produced employing different cooling conditions. From top to bottom: exemplary MPEA according to Example 1, cooled down in silica mould; exemplary MPEA prepared according to Example 1 and poured in conditions according to Example 3, cooled down in chromite sand mould; exemplary MPEA according to Example 4, cooled down in silica mould with water spraying. Scale bar 400 µm (right bottom of each one of the 3 images in the figure). DESCRIPTION OF THE INVENTION

[0008] The present invention provides new multi-principal element alloys (MPEAs), which are equivalent in microstructural terms as HEAs, together with methods for their preparation and their use as defined in the appended claims.

[0009] In a first aspect, the present invention relates to a multi-principal element alloy (MPEA) according to claim 1.

[0010] More particularly, according to a first aspect of the present invention, it is provided a multi-principal element alloy which consists of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloy: Ni:30 - 50 wt.%,Mn:10 - 30 wt.%,Cu:10 - 30 wt. %, andCr:10 - 30 wt.%, the balance being Fe and impurities, and wherein the content of Fe and impurities is equal to or less than 1 wt. %. Examples of those impurities include carbon (C) which, when present, shall not exceed 0.1 wt.% of the total weight of the alloy.

[0011] Preferably, the balance consists of Fe and impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%, and wherein those impurities are unavoidable impurities. Those unavoidable impurities may include carbon (C) though, when present, its concentration shall be preferably limited to 0.1 wt.% or less with respect to the total weight of the MPEA, and / or silicon (Si). In an embodiment, the balance consists of Fe and impurities, wherein the content of Fe and unavoidable impurities may be equal to or less than 0.8 wt.%, equal to or less than 0.6 wt.%, or equal to or less than 0.5 wt.%. In an embodiment, the balance consists of Fe and unavoidable impurities, wherein the content of Fe and unavoidable impurities may be equal to or less than 1.0 wt.%, equal to or less than 0.8 wt.%, equal to or less than 0.6 wt.%, or equal to or less than 0.5 wt.%, and wherein unavoidable impurities comprise C and / or Si.

[0012] By limiting the concentration of impurities included in the alloy structure, homogeneity and mechanical strength can be further increased regardless of the shape dimension of the structure.

[0013] The multi-principal element alloy according to the first aspect of the invention may be a multi-principal element alloy as cast (herein also referred to as "MPEA as cast" or "MPEA(CAST)") or a multi-principal element alloy after its heat treatment (herein also referred to as "heat-treated multi-principal element alloy", "heat-treated MPEA" or "MPEA(T1+T2)").

[0014] In particular, the inventors have found that the MPEA as cast of the present invention can be submitted to a heat treatment rendering a modified MPEA - which is the heat-treated MPEA -, which presents a more homogeneous hardness compared to the corresponding MPEA as cast, as illustrated in Table 2 of the Examples. Heat-treated MPEAs according to the present invention have also been found to present slightly lower hardness than the MPEAs as cast samples consisting of the same chemical composition, as illustrated in the Examples herein provided, particularly in Table 2 therefrom.

[0015] Unless indicated otherwise, all percentages provided throughout the specification are weight percentages. Furthermore, in the context of the present invention, and unless otherwise stated, Vickers hardness values correspond to surface or nucleus hardness values, which have been measured in accordance with UNE EN-ISO6507-1:2015, as described in the Examples.

[0016] In an embodiment, according to a first aspect of the present invention, it is provided a multi-principal element alloy as cast which consists of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloy: Ni:30 - 50 wt.%,Mn:10 - 30 wt.%,Cu:10 - 30 wt. %, andCr:10 - 30 wt.%, the balance being Fe and impurities, and wherein the content of Fe and impurities is equal to or less than 1 wt. %.

[0017] In another embodiment, according to a first aspect of the present invention, it is provided a heat-treated multi-principal element alloy which consists of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloy: Ni:30 - 50 wt.%,Mn:10 - 30 wt.%,Cu:10 - 30 wt. %, andCr:10 - 30 wt.%, the balance being Fe and impurities, and wherein the content of Fe and impurities is equal to or less than 1 wt. %.

[0018] The terms "about" or "around" when used throughout the specification preceding a number and referring to it, are to be understood as disclosing the particular value and designating any value lying within the range defined by the number ±5%, more preferably a range defined by the number ±2%. For example, the expressions "about 1" or "around 1" shall be construed as "within the range of 0.95 to 1.05", preferably as "within the range of 0.98 to 1.02", with both endpoints included within the scope of the interval.

[0019] According to a particular embodiment, the Ni amount in the MPEA, which can be an MPEA as cast or a heat-treated MPEA, is comprised between 32 and 48 wt.%, between 35 and 45 wt.%, or between 38 and 45 wt.%. In some embodiments, the Ni amount in the MPEA is comprised between 30 and 47 wt.%, between 33 and 48 wt.%, between 36 and 48 wt.%, between 40 and 50 wt.%, between 40 and 45 wt.% or between 32 and 40 wt.%.

[0020] The Mn amount in the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may be particularly comprised between 12 and 28 wt.%, between 15 and 30 wt.%, or between 15 and 25 wt.%. In some embodiments, the Mn amount in the MPEA is comprised between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%.

[0021] In particular, the Cu amount in the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may be comprised between 12 and 28 wt.%, or between 15 and 30 wt.%, or between 15 and 25 wt.%. In some embodiments, the Cu amount in the MPEA, which can be an MPEA as cast or a heat-treated MPEA, is comprised between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%.

[0022] According to another particular embodiment, the Cr amount in the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may be comprised between 12 and 28 wt.%, or between 15 and 30 wt.%, or between 15 and 25 wt.%. In some embodiments, the Cr amount in the MPEA is comprised between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%.

[0023] The skilled person can readily understand that all the above disclosed ranges and subranges of each Ni, Mn, Cu and Cr can be combined, and that the resulting combinations are intended to be included in the scope of the present invention, in particular for the MPEAs according to the first aspect of the invention and / or the third aspect of the invention, more particularly, for any MPEA as cast and / or any heat-treated MPEA according to the first aspect of the invention and / or the third aspect of the invention.

[0024] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 32 to 48 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% Cu, and 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0025] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 32 to 48 wt.% Ni, 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Mn, 12 to 28 wt.% Cu and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0026] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 32 to 48 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Cu, and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0027] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 32 to 48 wt.% or 35 to 45 wt.% or 38 to 45 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% Cu, and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0028] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 35 to 45 wt.% Ni, 15 to 30 wt.% Mn, 15 to 30 wt.% Cu, and 15 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0029] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 38 to 45 wt.% Ni, 15 to 30 wt.% Mn, 15 to 30 wt.% Cu, and 15 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0030] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 38 to 45 wt.% Ni, 12 to 28 wt.% Mn, 15 to 30 wt.% Cu, and 15 to 25 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0031] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 35 to 45 wt.% Ni, 15 to 25 wt.% Mn, 12 to 28 wt.% Cu, and 15 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0032] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 36 to 48 wt.% Ni, 10 to 27 wt.% Mn, 15 to 25 wt.% Cu, and 10 to 27 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0033] In some embodiments, the MPEA, which can be an MPEA as cast or a heat-treated MPEA, may consist of 30 to 47 wt.% Ni, 12 to 25 wt.% Mn, 15 to 20 wt.% Cu, and 21 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0034] The MPEAs according to the invention have been surprisingly found to have good machinability, intermediate Vickers hardness, high yield and high tensile strength, as shown in the Examples, in particular in Tables 2 and 3 thereof. In addition to this, it has been found that the chemical composition of these alloys according to the invention, together with their microstructure, advantageously results in specific performance properties such as high young modulus and high tensile strength compared to other types of crystalline materials.

[0035] Mechanical testing of the MPEAs of the invention has been carried out in accordance with UNE EN ISO 6892-1 2020, more particularly, UNE EN ISO 6892-1 2020 A224. Such mechanical testing evidenced coherent mechanical results with remarkably high yield strength and tensile strength in heat-treated MPEAs according to the invention.

[0036] All these chemical and mechanical properties would make the MPEAs according to the invention particularly suitable for hydrogen storage, since they would be significantly less prone to hydrogen embrittlement.

[0037] Further advantageously, the MPEAs according to the invention do not include critical raw materials of high scarcity in nature and / or which are excessively costly, compared to existing alloys typically employed in hydrogen storage applications which may include Co and / or V, among other elements.

[0038] Examples of the microstructure of multi-principal element alloys according to the invention, more particularly of the microstructure of an MPEA as cast and a heat-treated MPEA, are presented in Figure 1 and Figure 2, respectively.

[0039] MPEAs as cast according to the invention, in particular when solidified in a silica mould, have been found to comprise a microstructure which is composed of a first phase which has a face-centered cubic crystalline structure (FCC), and a second phase which has a body-centered cubic crystalline structure (BCC) which is present in less quantity than the first phase.

[0040] Figure 1 corresponds to the light microscope (LM) image of an exemplary MPEA as cast according to the invention, prepared as described in Example 1 with a chemical composition corresponding to a molar formulation equivalent to Ni 2 MnCrCu, so as to ensure that the atomic percentage of each alloying element is kept in similar values for ensuring a high disorder level or what is known as cocktail effect. It can be therein observed that such exemplary MPEA as cast according to the invention, which has been solidified in a silica mould as described in Example 1, comprises a microstructure which is composed of a principal FCC phase and a secondary BCC phase which is present in less quantity than the FCC one. When a heat treatment is carried out, some of the chemical elements segregated in interdendritic spaces are homogenised and differences in chemical composition between phases are therefore reduced as shown in Figure 2. However, when a faster cooling rate is employed, basically due to mould dissolving with water cooling, it can then be observed how Cr particles do not appear, as shown in Figure 5.

[0041] The alloy phase identification, as well as the chemical elements distribution analysis of the MPEAs of the invention has been conducted with an element mapping method using transmission electron microscopy (TEM). In particular, MPEAs as cast have been found to comprise a microstructure conformed by a principal FCC phase that presents a darker or lighter grey colour in TEM images depending on the segregation present, together with precipitated particles made basically of Cr that can be assimilated to a BCC phase. Figure 3 shows a TEM image of an exemplary MPEA as cast according to the invention, wherein the darkest areas correspond to particles enriched in Cr resulting from segregation, while the other chemical elements remain homogeneously dispersed in the matrix.

[0042] However, where an MPEA according to the invention has been produced using a rapid cooling rate, e.g., by using water as cooling medium, differences in the crystallographic structure are observed compared to an MPEA as cast according to the invention with the same elemental composition. More specifically, when water was employed for cooling down an MPEA according to the invention, the BCC phase corresponding to the Cr-rich matrix would no longer be observed, as illustrated by Figure 7 corresponding to an exemplary MPEA according to the invention.

[0043] The chemical composition analysis carried out by Energy Dispersive Spectroscopy (EDS) with line scan has shown that segregation of different chemical elements in Secondary Dendrite Arm Spacing (SDAS) may exist. By way of illustration, Figure 4 corresponds to an EDS composition analysis using line scan of the segregation in an exemplary MPEA(T1+T2) according to the invention. The darker grey matrix presents higher Cr content than the light grey matrix, whereas the lighter grey matrix presents higher content of Cu than the darker grey matrix.

[0044] Furthermore, Figure 8 illustrates the fact that the Secondary Dendrite Arm Spacing (SDAS) values of the MPEAs according to the invention have been found to change depending on the conditions employed for cooling down the alloy: SDAS around 50 µm in the case of silica mould sand, around 17 µm in the case of chromite sand and around 12 µm in the case of water cooled down sample.

[0045] Without wishing to be bound by theory, it is postulated that MPEAs which have undergone a heat treatment may reach a thermodynamic equilibrium based on a two-phase microstructure and therefore provide excellent properties due to their high chemical homogeneity (compared to an MPEA as cast having the same elemental composition), which may also be achieved by using a relatively low cooling rate during the solidification step in the alloy production. In contrast, MPEAs which have undergone a very rapid cooling during the solidification step of the alloy production were found to have a microstructure with an FCC phase as single or almost single phase (e.g., at least 90% of FCC phase, with equal to or less than 10% of a BCC phase), which is postulated to derive from the existence of a metastable equilibrium. Such relatively rapid cooling was found to advantageously provide enhanced chemical and phase homogeneities.

[0046] In an embodiment, the MPEA according to the invention comprises a microstructure which is composed of at least 80% of a first phase which has an FCC crystalline structure, and equal to or less than 20% of a second phase which has a BCC crystalline structure, as determined by SEM. Preferably, the MPEA according to the invention comprises a microstructure which is composed of at least 90% of a first phase which has an FCC crystalline structure, and equal to or less than 10% of a second phase which has a BCC crystalline structure, as determined by SEM. Any percentage value of any of the phases in the microstructure, in particular either the first phase or the second phase of the microstructure comprised by the MPEAs according to the invention, shall be understood as an area percentage calculated in respect of the total area of the microstructure. By way of illustration, when the microstructure comprised by the MPEAs according to the invention is composed of "at least 80%" of a first phase, it shall be understood that the first phase corresponds to at least 80%, in area units, of the total area of the microstructure.

[0047] In an embodiment, the MPEA according to the invention comprises a microstructure which is composed of at least 98% of a first phase which has an FCC crystalline structure, and equal to or less than 2% of a second phase which has a BCC crystalline structure, as determined by SEM. The MPEA according to the invention may comprise a microstructure which is composed of at least 99% of a first phase which has an FCC crystalline structure, and equal to or less than 1% of a second phase which has a BCC structure, as determined by SEM. In another embodiment, The MPEA according to the invention may comprise a microstructure which is composed of at least 99.5% of a first phase which has an FCC crystalline structure, and equal to or less than 0.5% of a second phase which has a BCC crystalline structure, as determined by SEM.

[0048] The MPEA according to the invention may be an MPEA as cast which comprises a microstructure which is composed of 50-70 % a first phase which has an FCC crystalline structure, and 30-50 % of a second phase which has a BCC crystalline structure, as determined by SEM.

[0049] The MPEAs according to the invention may be a heat-treated MPEA which comprises a microstructure which is composed of at least 80%, 85%, 90% or 95% of a first phase which has an FCC crystalline structure, and equal to or less than 20%, 15%, 10% or 5%, respectively of a second phase which has a BCC crystalline structure, as determined by SEM.

[0050] In an embodiment, the MPEAs according to the invention, in particular those MPEAs which have been subject to a rapid cooling rate, comprise a microstructure which is composed of at least 95% of a first phase, and equal to or less than 5% of a second phase, as determined by SEM, wherein the first phase has a metastable FCC crystalline structure, and the second phase has a BCC crystalline structure. In a preferred embodiment, the MPEAs according to the invention, in particular those MPEAs which have been subject to a rapid cooling rate, comprise a microstructure which is composed of at least 98% of a first phase, and equal to or less than 2% of a second phase, as determined by SEM, wherein the first phase has a metastable FCC crystalline structure, and the second phase has a BCC crystalline structure. In still another preferred embodiment, the MPEAs according to the invention, in particular those MPEAs which have been subject to a rapid cooling rate, comprise a microstructure which is composed of at least 99% of a first phase, and equal to or less than 1% of a second phase, as determined by SEM, wherein the first phase has a metastable FCC crystalline structure, and the second phase has a BCC crystalline structure.

[0051] In a second aspect, the present invention provides a process for the preparation of a multi-principal element alloy, characterized by comprising the following steps: a) Introducing alloying elements of the alloy which is to be prepared in a furnace for forming an alloying mixture; b) heating the alloying mixture at a temperature in the range from 1200°C to 1700°C for forming a molten mixture; c) optionally removing oxygen and / or removing hydrogen while the mixture is in molten state, said oxygen removal step comprising the introduction of a deoxidizing agent; d) removing slag, optionally aided by flux addition , e) maintaining the mixture in molten state for homogenization; f) pouring of the molten mixture in a mould; and g) solidifying the molten mixture to obtain the multi-principal element alloy.

[0052] The furnace according to a particular embodiment is a conventional furnace such as an induction melting furnace.

[0053] Preferably, the alloying elements introduced in step a) of the process according to the second aspect of the invention consist of Ni, Mn, Cu, Cr and Fe in order to obtain the desired multi-principal element alloy compositions of the present invention, wherein each alloying element may be introduced in step a) in substantially pure form.

[0054] The expression "substantially pure form" as herein used in reference to an alloying element shall be understood as meaning that an alloying element has a purity equal to or greater than 99 wt.% (i.e., 99-100 wt.%), preferably a purity equal to or greater than 99.5 wt.%, and more preferably a purity equal to or greater than 99.9 wt.%.

[0055] Still more preferably, the alloying elements introduced in step a) of the process according to the second aspect of the invention consist of Ni, Mn, Cu, Cr and Fe, and each one of them is introduced in substantially pure form. Even more preferably, the alloying elements introduced in step a) of the process according to the second aspect of the invention consist of Ni, Mn, Cu, Cr and Fe, wherein each one of them is introduced in the following form: ∘ Substantially pure Cu: Cu having 99-100 wt.% purity; ∘ Substantially pure Mn: Mn having 99-100 wt.% purity; ∘ Substantially pure Cr: Cr having 99-100 wt.% purity; and ∘ Substantially pure Ni: Ni having 99-100 wt.% purity.

[0056] The alloying mixture formed in step a) preferably consists of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: Ni:30 - 50 wt.%,Mn:10 - 30 wt.%,Cu:10 - 30 wt. %, andCr:10 - 30 wt.%, the balance being Fe and impurities, and wherein the content of Fe and impurities is equal to or less than 1 wt. %. Examples of those impurities include carbon (C) which, when present, shall not exceed 0.1 wt.% of the total weight of the alloying mixture. Preferably, the balance consists of Fe and impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%, equal to or less than 0.8 wt.%, equal to or less than 0.6 wt.%, or equal to or less than 0.5 wt.%, and wherein those impurities are unavoidable impurities. Those unavoidable impurities may include carbon (C) though, when present, its concentration shall be preferably limited to 0.1 wt.% or less of the total weight of the alloying mixture. In an embodiment, the balance consists of Fe and unavoidable impurities, wherein the content of Fe and unavoidable impurities may be equal to or less than 1.0 wt.%, equal to or less than 0.8 wt.%, equal to or less than 0.6 wt.%, or equal to or less than 0.5 wt.%, and wherein unavoidable impurities comprise C and / or Si.

[0057] This process according to the invention is particularly suitable for producing the metal-principal element alloys of the present invention, and any adjustment of the necessary quantities of starting materials or reagents for producing an MPEA according to the invention with a desired composition can be readily carried out by a skilled person in the art. In particular, the process of the invention is suitable for producing the MPEA as cast according to the invention, as well as the heat-treated MPEA according to the invention, since it can be easily tailored depending on the characteristics of the metal-principal element alloy which is to be manufactured by adjusting, for example, the amounts of alloying elements employed as raw materials, the melting temperature or the temperature and cooling rate during solidification of the molten mixture.

[0058] According to a particular embodiment, the Ni amount in the alloying mixture formed in step a) may be comprised between 32 and 48 wt.%, between 35 and 45 wt.%, between 38 and 45 wt, between 30 and 47 wt.%, between 33 and 48 wt.%, between 36 and 48 wt.%, between 40 and 50 wt.%, between 40 and 45 wt.% or between 32 and 40 wt.%, based on the total weight of the alloying mixture.

[0059] In particular, the Mn amount in the alloying mixture formed in step a) may be comprised between 12 and 28 wt.%, between 15 and 30 wt.%, between 15 and 25 wt.%, between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%, based on the total weight of the alloying mixture.

[0060] According to another embodiment, the Cu amount in the alloying mixture formed in step a) may be comprised between 12 and 28 wt.%, between 15 and 30 wt.%, between 15 and 25 wt.%, between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%, based on the total weight of the alloying mixture.

[0061] In another embodiment, the Cr amount in the alloying mixture formed in step a) may be comprised between 12 and 28 wt.%, between 15 and 30 wt.%, between 15 and 25 wt.%, between 10 and 27 wt.%, between 10 and 15 wt.%, between 12 and 25 wt.%, between 15 and 20 wt.%, between 12 and 20 wt.% or between 21 and 30 wt.%, based on the total weight of the alloying mixture.

[0062] The alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 32 to 48 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% Cu, and 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0063] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 32 to 48 wt.% Ni, 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Mn, 12 to 28 wt.% Cu and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0064] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 32 to 48 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% or 15 to 30 wt.% or 15 to 25 wt.% Cu, and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0065] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 32 to 48 wt.% or 35 to 45 wt.% or 38 to 45 wt.% Ni, 12 to 28 wt.% Mn, 12 to 28 wt.% Cu, and 12 to 28 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0066] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 35 to 45 wt.% Ni, 15 to 30 wt.% Mn, 15 to 30 wt.% Cu, and 15 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0067] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 38 to 45 wt.% Ni, 12 to 28 wt.% Mn, 15 to 30 wt.% Cu, and 15 to 25 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0068] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 35 to 45 wt.% Ni, 15 to 25 wt.% Mn, 12 to 28 wt.% Cu, and 15 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0069] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 36 to 48 wt.% Ni, 10 to 27 wt.% Mn, 15 to 25 wt.% Cu, and 10 to 27 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0070] In another embodiment, the alloying mixture in step a) may consist of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloying mixture: 30 to 47 wt.% Ni, 12 to 25 wt.% Mn, 15 to 20 wt.% Cu, and 21 to 30 wt.% Cr, the balance being Fe and impurities, said impurities preferably being unavoidable impurities, wherein the content of Fe and impurities is equal to or less than 1 wt.%.

[0071] Step b) of the process of the present invention may preferably be carried out at a temperature in the range from 1300°C to 1700°C for forming the molten mixture.

[0072] During the process of the invention, it is preferable to protect the surface of the melt mixture in the furnace with a protective product (generally known as "slag coagulation flux") to prevent oxidation losses of alloying elements and gas pick-up from the atmosphere. Further, during the process the mixture can be covered with a ceramic fibre blanket in order to prevent that hydrogen from the atmosphere dissolves in the molten metal and that also metal oxidations take place.

[0073] During the melting process, oxygen and hydrogen may dissolve in the melt due to generated turbulence, which can cause porosity and inclusions. If those gases were not removed, they would accumulate, causing holes to appear in the last solidification step h). In order to avoid such occurrence, preferably step c) is carried out, said step c) comprising removing oxygen and / or hydrogen while the mixture is in molten state, said oxygen removal step comprising the introduction of a deoxidizing agent, such as ferro silicon calcium. The deoxidizing agent may be preferably introduced in a fixed proportion (e.g., 0.10 wt.% based on the total weight of molten metal) in the furnace once all alloying elements have been loaded and molten. The hydrogen removal is preferably carried out by nitrogen or argon bubbling, from the bottom of the furnace, by means of a porous plug.

[0074] Once the molten mixture if formed, reaction products (known as "slag") are removed from the top of the surface in the furnace in step d). Such removal may be preferably carried out by adding slag coagulation flux until the slag is thick enough to be removed with a metal stick. The furnace must be switched off to stop turbulence and allow slag floating in induction furnaces.

[0075] Step e) is necessary to allow sufficient time to achieve a correct homogenization of the alloying elements in the melt before it is poured into a mould in step f). Preferably, in step e), the mixture is maintained in molten state for not less than 10 minutes. After step e), the melt is typically transferred to a pouring ladle, and then poured in a mould.

[0076] The temperatures which are preferably employed in steps b) (i.e. melting temperature) and f) (i.e. pouring temperature) are variable depending for example on the thickness of the part to be cast. The temperature employed in step b) and / or step f) is preferably in the range of around 1200°C-1700°C, preferably around 1400°C-1600°C, more preferably in the range of 1400°C-1600°C, still more preferably in the range of 1450°C-1600°C, even more preferably in the range of 1500°C-1600°C, or for example for example 1600°C. By way of illustration, temperatures in the range of 1400°C-1600°C, for example 1600°C, are usually employed for parts with a thickness of less than 38 mm, and around 1400°C for parts having more than 38 mm thickness.

[0077] As regards the solidification step g) of the process, the cooling rate parameter has been found to be of particular interest for producing MPEAs of the invention with different microstructures. This parameter may be modified, for example, by choosing a mould made of a particular material, wherein the molten mixture is poured. Thus, for example, a cooling rate in the range from 0.05°C / s to 0.1°C / s can be achieved by using a silica sand mould, while cooling rates in the range from 0.1 °C / s to 0.2°C / s or from 2°C / s to 4°C / s can be achieved by using a chromite sand mould or a water-soluble silica sand mould made of inorganic colloidal silica, respectively.

[0078] In an embodiment, step g) of the process of the invention comprises cooling the molten mixture down at a cooling rate in the range from 0.5°C / s to 4.0°C / s, in particular at a cooling rate in the range of 0.8-3.5°C / s, in the range of 1.0-2.0°C / s or in the range of 2.0-4.0°C / s. These rapid cooling rates were found to favour the obtention of MPEAs with microstructures having an FCC phase as either single or almost single phase, which is postulated to correspond to a metastable equilibrium.

[0079] In another embodiment, step g) of the process of the invention comprises cooling the molten mixture down at a cooling rate in the range from 0.05°C / s to less than 0.5°C / s, in particular at a cooling rate in the range of 0.05-0.4°C / s or in the range of 0.1-0.4°C / s. These slower cooling rates were found to favour the formation of two-phase microstructures in thermodynamic equilibrium, comprising BCC and FCC phases with higher homogeneity in hardness.

[0080] The process according to the second aspect of the invention may further comprise, subsequently to step g), a thermal treatment step so as to obtain a heat treated MPEA or MPEA(T1+T2). This thermal treatment step can be carried out by any conventional manner known by the person skilled in the art, and is typically performed in a furnace, such as an electrical resistance non-controlled atmosphere thermal treatment furnace. The thermal treatment step comprises subjecting the MPEA of the invention, in particular the MPEA as cast, to a first thermal treatment (i.e., solubilisation thermal treatment or solubilisation heat treatment) at temperature of 500-1200°C, followed by a second thermal treatment (i.e., precipitation thermal treatment or precipitation heat treatment) at a temperature of 500-800°C, with an optional cooling down step in between both thermal treatments (i.e., the first thermal treatment and the second thermal treatment) in a cooling medium. The MPEA as cast resulting from the first thermal treatment, and before subjecting it to a second thermal treatment, shall be hereinafter referred to as "MPEA(T1)".

[0081] The time the alloy (in particular, as cast) may be subject to the optional thermal treatment step subsequent to step g) varies depending on factors such as the size of the part which is to be produced or the selected temperature, and is typically from 4 to 60 hours. Cooling media employed in the optional cooling down step can also vary depending on the cooling rate required, and suitable examples thereof include, in a non-limiting way, cooling inside the furnace, calm-air cooling, forced-air cooling, cooling in oil, or cooling in water. The cooling down temperature in the optional cooling down step may preferably be in the range from 500°C to 700°C, while the duration of the cooling down step may vary between 1 and 10 hours.

[0082] In the context of the present invention, the term "calm air" or "calm-air" shall be understood as referring to an environment in absence or nearly absence of air currents and / or wind, i.e., wind and / or air currents moving at a speed below 1 knot, or "zero" (0) on the international scale of wind force known as "Beaufort scale". The temperature in "calm air" or "calm-air" conditions shall be understood as ambient or room temperature, taken in the broad sense, i.e., generally between 15°C and 30°C, more specifically between 20°C and 27°C, between 20°C and 25°C, or between 20°C and 22°C.

[0083] Inventors have found that temperatures around 950°C in the first treatment step following step g) were particularly effective for the MPEAs as cast to reach a new thermodynamic equilibrium, wherein segregation is reduced and homogenization in hardness takes place.

[0084] Preferably, the process of the present invention may further comprise heating the solidified molten mixture resulting from step g) at a temperature in the range from 500°C to 1200°C, preferably from 700°C to 1100°C, more preferably from 800°C to 1000°C, even more preferably from 900°C to 1050°C or still even more preferably at a temperature of about 950°C, in a first thermal treatment step. The process according to the invention may optionally further comprise a second thermal treatment step, taking place after the first thermal treatment step, which comprises heating the molten solidified mixture resulting from the first thermal treatment step at a temperature in the range from 500°C to 800°C, more preferably from 600°C to 760°C, even more preferably from 650°C to 800°C and most preferably at about 760°C. The process of the invention may preferably comprise the cooling down step between the first and second thermal treatment steps in a cooling medium, as described above, and / or an additional cooling step after the second thermal treatment has taken place. The additional cooling down step may comprise subjecting the solidified molten mixture resulting from the second thermal treatment to an intermediate lower temperature which is in the range of 500-700°C, preferably in the range of 550-650°C and most preferably a temperature of about 650°C, followed by further cooling down, for example, by cooling inside the furnace, calm-air cooling, forced-air cooling, cooling in oil, or cooling in water. In fact, inventors have found that the use of temperatures about 650°C during the cooling down step were particularly effective as they promoted the hardening of the alloy due to the precipitation of Cr phases.

[0085] In a preferred embodiment, the molten mixture is solidified in step g) by cooling it down at a cooling rate in the range from 0.05°C / s to less than 0.5°C / s, in particular at a cooling rate in the range of 0.05-0.4°C / s or in the range of 0.1-0.4°C / s, followed by a subsequent additional step, which is a first thermal treatment step wherein the resulting solidified molten mixture is heated at a temperature in the range from 500°C to 1200°C, preferably from 700°C to 1100°C, more preferably from 800°C to 1000°C, said first thermal treatment being optionally followed by a second thermal treatment step, which comprises heating the molten solidified mixture resulting from the first thermal treatment step at a temperature in the range from 500°C to 800°C.

[0086] In a third aspect of the invention, it is provided a multi-principal element alloy which is obtained or obtainable by the process according to the second aspect of the invention. This MPEA may have the same elemental composition as the MPEAs defined according to the first aspect of the invention.

[0087] In particular, the MPEAs according to the first of the third aspect of the invention may have a Vickers hardness (HV) in the range of 280-290 kgf / mm 2< (i.e., 2746 - 2844 MPa in SI units), as determined in accordance with UNE EN-ISO6507-1:2015.

[0088] Preferably, the MPEA obtainable or obtained by the process according to the second aspect of the invention comprises a microstructure which is composed of at least 80% of a first phase which has an FCC crystalline structure, and equal to or less than 20% of a second phase which has a BCC crystalline structure, as determined by SEM. More preferably, the MPEA obtainable or obtained by the process of the invention comprises a microstructure which is composed of at least 90% of a first phase which has an FCC crystalline structure, and equal to or less than 10% of a second phase which has a BCC crystalline structure, as determined by SEM.

[0089] In a preferred embodiment, the MPEA according to the invention comprises a microstructure which is composed of at least 90% of a first phase which has an FCC crystalline structure, and equal to or less than 10% of a second phase which has a BCC crystalline structure, as determined by SEM, said microstructure having the following characteristic peaks depending on the cooling rate employed during step h) of the process: • Cooling rate 0.05-below 0.5 °C / s: Position (±0.2° °2Th.)Relative intensity (%)Crystalline phase43.222892.92FCC44.319524.97BCC50.0739100.00FCC73.63827.28FCC81,770311.74BCC • Cooling rate 0.5-4.0 °C / s: Position (±0.2° °2Th.)Relative intensity (%)Crystalline phase43.150884.85FCC50.246960.34FCC73.951879.41FCC89.7248100.00FCC

[0090] In another preferred embodiment, the MPEA obtainable or obtained by the process according to the second aspect of the invention comprises a microstructure which is composed of at least 90% of a first phase having a metastable FCC crystalline structure, and equal to or less than 10% of a second phase which has a BCC crystalline structure, as determined by SEM. More preferably, the MPEA obtainable or obtained by the process of the invention comprises a microstructure which is composed of at least 95% of a first phase having a metastable FCC crystalline structure, and equal to or less than 5% of a second phase which has a BCC crystalline structure, as determined by SEM.

[0091] In a fourth aspect of the invention, it is provided the use of a multi-principal element alloy according to the invention, more particularly according to the first or third aspect of the invention, as material for hydrogen storage.

[0092] Preferably, the use according to the fourth aspect of the invention is the use of an MPEA as cast or a heat-treated MPEA of the present invention, as material for hydrogen storage.

[0093] Throughout the description and the claims, the word "comprise" and variations thereof are not intended to exclude other technical features, ingredients or steps. Additional advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention without undue burden.EXAMPLES

[0094] The following examples are provided by way of illustration and shall not be construed as limiting the invention. Numerous variations are possible, and it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described in the following examples.

[0095] For the tests carried out in relation to the present invention, keel blocks Y2 were prepared using chemically bonded silica sand, chromite sand or inorganic colloidal silica, in all three cases following the standard norm UNE-EN 1563:1998. Keel blocks Y2 were then extracted from the moulds and cleaned by shot blasting.

[0096] Keel blocks were then cut and test samples for micrographic inspection of rectangular dimensions of approximately 30 x 30 x 10 mm were prepared by surface polishing; their microstructures were then analysed with a scanning electron microscope (SEM) (JEOL JSM-6400).

[0097] An Element Mapping Method using transmission electron microscopy (TEM) was employed for identifying the phases in the alloys, as well as their chemical elements distribution. The TEM equipment was a Talos F200i a 200 kV in mode STEM (Scanning Transmission Electron Microscopy). The element mapping was carried out using the Energy Dispersive X-Ray Spectroscopy (EDX) Seper-X G2 equipment that this microscopy incorporated.

[0098] Vickers hardness was determined using test samples of rectangular dimensions of approximately 30 x 30 x 10 mm using an INSTROM WOLPER equipment model TESTOR 971 / 3000. Hardness was determined preferably in the centre of the obtained samples and always to more than 5 mm from any surface. Measurements were made according to the method described under the Examples, in accordance with UNE EN-ISO6507-1:2015.

[0099] An Energy Dispersive Spectrometer (EDA) INCA X-sight Series Si(Li) pentaFET (Oxford) was used for quantitative chemical analysis of the metal-principal element alloys.

[0100] X-Ray Diffraction (XRD) analyses were performed using a PANalytical Xpert PRO diffractometer, equipped with a goniometric vertical copper tube (Bragg-Brentano geometry). The measuring conditions were as follows: Current type: 40 KV y 40 mA Angular sweep: 10-90° 2Θ

[0101] Light Microscopy (LM) analysis was carried out with a Leica DM4000M equipment with digital camera ProgRes C7 from Jenoptik, with 50X magnification. Samples were etched using Kalling's No. 2. This technique was used to identify differences in secondary Dendrite Arm Spacing (SDAS) and segregations visually by comparison of different pictures.EXAMPLE 1 - Manufacturing of an MPEA as cast according to the invention

[0102] Different substantially pure elements (Ni, Mn, Cr, Cu) of purity certified as higher than 99.0 wt.% were selected to obtain the exemplary chemical composition shown below in Table 1: Table 1 Alloy compositionNi (wt. %)Mn (wt.%)Cr (wt.%)Cu (wt.%)Balance: Fe and unavoidable impurities (wt.%)MPEA as cast*42.117.318.222.00.4

[0103] These elements were introduced in the adequate proportion in an induction furnace of 100-kg capacity to form the alloying mixture.

[0104] Melting of the alloying mixture was carried out when a temperature of 1560°C was reached. Argon purging was performed by the bottom of the furnace to eliminate hydrogen with a porous plug, and slag was then removed from the top of the furnace.

[0105] Once the alloying mixture was molten, and the slag was removed from the top of the furnace, the mixture was maintained in molten state for adequate mixing-up (i.e., homogenization) of the elements. The metal was then transferred to a ladle, and then pouring of the molten mixture was performed into two keel blocks Y2 of chemically-bonded silica sand moulds in order to obtain test specimens for later-on testing. The cooling rate in the chemically-bonded silica sand mould was estimated to be 0.05-0.5°C / sec

[0106] The general microstructure of this alloy is shown on Figure 1, and the specific nature of Cr particles on Figure 3.

[0107] Figure 1 is a light microscopy (LM) image which shows the dual microstructure of this exemplary MPEA, which is composed of a principal FCC phase and a secondary BCC phase which is present in less quantity than the first one. Figure 3 shows a TEM image of this exemplary MPEA, wherein the darkest areas correspond to particles enriched in Cr resulting from segregation, while the other chemical elements remain homogeneously dispersed in the matrix.EXAMPLE 2 - Manufacturing of an MPEA(T1+T2) according to the invention (low cooling rate)

[0108] A sample of an MPEA as cast of the present invention, produced as described in Example 1 and having the elemental composition therein indicated, has been processed in a heat treatment furnace using electric resistance as heating media, with no protective atmosphere.

[0109] The samples have been heated up to 950°C, maintained for 1 hour and cooled down in water. The next step has comprised the heating up at 760°C, maintaining the sample at this temperature for 5 hours, cooling down at 650°C in two hours and maintaining the sample at this temperature for 1 hour followed by cooling it down in calm air.

[0110] The results of hardness and mechanical tests after the different heat treatments are respectively gathered in Tables 2 and 3 below: Table 2 shows the hardness evolution of MPEAs of the invention, from an MPEA as cast according to Example 1 (MPEA(CAST), first entry in Table 2) to the heat-treated MPEA of the invention prepared in the present example (MPEA(T1+T2), third entry in Table 2). For comparative purposes, Table 2 also illustrates the experimental results retrieved from samples of the MPEA as cast of Example 1 when it had undergone the first thermal treatment, but not the second one yet, thus herein referred to as MPEA(T1) (second entry in Table 2) to distinguish it from the MPEA(T1+T2) samples which are heat-treated with a first thermal treatment and second thermal treatment as explained above. In all three examples (i.e., MPEA(CAST), MPEA(T1) and MPEA(T1+T2)), five different measurements were made from different points of the material, in order to assess their homogeneity. Table 2 Vickers hardness (HV) Measure 1 Measure 2 Measure 3 Measure 4 Measure 5 Average value Variation range MPEA(CAST) 274.9310.5332.3319.8323.6312.2257.4MPEA(T1) 270.9262.9262.4269.5266266.348.5MPEA(T1+T2) 282.4279.6282.9285.4280.1282.085.8

[0111] Results showed in Table 2 evidenced the fact that thermally treating the MPEAs of the invention advantageously resulted in enhanced homogeneity, in terms of hardness, throughout the material as a whole. In parallel, it was found that heat-treated MPEA samples presented slightly lower hardness than the MPEAs as cast having the same chemical composition.

[0112] Table 3 illustrates the mechanical results obtained with the heat-treated MPEA obtained as described above in this Example. These results serve to illustrate the fact that MPEAs of the invention, particularly heat-treated MPEAs according to the invention, have remarkably high yield strength (Y.S.) and tensile strength (T.S.). Table 3 Tensile test Diameter (mm) Y.S. 0.2% offset (MPa) T.S. (MPa) Elong. (L o =5d) (%) Area reduction (%) MPEA(T1+T2) 6.056057093.911

[0113] The values of the mechanical properties shown in Table 3 are advantageous in terms of very high value of yield, near the tensile strength that ensures a good dimensional stability once subjected to high pressures when containing gases such as hydrogen.

[0114] The microstructure of this heat-treated MPEA (i.e., MPEA(T1+T2)) is shown on Figure 2. In this figure, reduction of segregation is appreciated due the attenuation of the colour contrast in the interdendritic spaces.

[0115] The particular chemical composition of the phases is shown on Figure 4, which corresponds to an EDA composition analysis using line scan, from which segregation could be qualitatively and quantitatively analysed, as shown in Table 4: Table 4 C Si Cr Mn Fe Ni Cu Dark grey area 2.60.431.310.10.746.78.3Light grey area 3.90.615.318.00.441.420.4Light area with Cr particles 2.80.45.520.20.327.743.0

[0116] The darker grey matrix presented higher Cr content than the light grey matrix, whereas the lighter grey matrix presented higher content of Cu than the darker grey matrix, and the light lighter grey matrix also incorporates in some cases Cr particles.EXAMPLE 3 - Manufacturing of an MPEA as cast according to the invention (intermediate cooling rate)

[0117] In order to evaluate the effect of cooling rate in the microstructure of the MPEAs as cast of the present invention, keel blocks Y2 were prepared using chemically bonded chromite sand moulds. Those mould were dimensioned following the standard norm UNE-EN 1563:1998. The keel blocks Y2 were then extracted from the moulds and cleaned by shot blasting.

[0118] An MPEA according to the invention, having the same elemental composition as that of Example 1, was prepared under the same process conditions as those described in Example 1, the difference being that the molten mixture was poured in a chromite sand mould. The cooling rate in the chromite sand mould was estimated to be of 0.5-1°C / sec.

[0119] Chromite was found to present a higher cooling-rate potential than the silica sand moulds employed in Example 1, and allowed to obtain finer microstructures with lower segregation and particular crystallinity.

[0120] The SEM images in Figure 8 show a reduction in SDAS in the case of MPEA samples manufactured using chromite sand moulds (central image of Figure 8) compared with MPEA samples manufactured with silica sand moulds (top image of Figure 8).EXAMPLE 4 - Manufacturing of an MPEA as cast according to the invention (high cooling rate)

[0121] In order to further evaluate the effect of cooling rate in the microstructure of the MPEAs as cast of the present invention, test bar moulds were prepared using special inorganic colloidal silica. After pouring the molten mixture in the mould, water spray was applied over the mould, dissolving the mould and achieving a higher cooling rate.

[0122] Water cooling was found to present a higher cooling-rate potential than chromite sand moulds, and a much higher cooling-rate potential than silica sand moulds, allowing the obtention of even finer microstructures with lower segregation and particularly crystallinity compared to processes carried out with solidification at lower cooling rates.

[0123] Light optical microscopy in Figure 5 and DRX results in Figure 7 of this MPEA as cast according to the invention show the reduction of segregation and presence of chrome phases.

[0124] SEM images in Figure 8 show a reduction in SDAS depending on the mould material, with the lowest value corresponding to cooling carried out in the presence of water: around 50 µm in the case of a silica sand mould (Example 1), around 17 µm in the case of a chromite sand (Example 3) and around 12 µm in the case of a water cooled down sample (present Example).

Examples

example 1-manufacturing

EXAMPLE 1 - Manufacturing of an MPEA as cast according to the invention

[0102]Different substantially pure elements (Ni, Mn, Cr, Cu) of purity certified as higher than 99.0 wt.% were selected to obtain the exemplary chemical composition shown below in Table 1:

Table 1

Alloy compositionNi (wt. %)Mn (wt.%)Cr (wt.%)Cu (wt.%)Balance: Fe and unavoidable impurities (wt.%)

MPEA as cast*42.117.318.222.00.4

[0103]These elements were introduced in the adequate proportion in an induction furnace of 100-kg capacity to form the alloying mixture.

[0104]Melting of the alloying mixture was carried out when a temperature of 1560°C was reached. Argon purging was performed by the bottom of the furnace to eliminate hydrogen with a porous plug, and slag was then removed from the top of the furnace.

[0105]Once the alloying mixture was molten, and the slag was removed from the top of the furnace, the mixture was maintained in molten state for adequate mixing-up (i.e., homogenization) of the elements. The...

example 2-manufacturing

EXAMPLE 2 - Manufacturing of an MPEA(T1+T2) according to the invention (low cooling rate)

[0108]A sample of an MPEA as cast of the present invention, produced as described in Example 1 and having the elemental composition therein indicated, has been processed in a heat treatment furnace using electric resistance as heating media, with no protective atmosphere.

[0109]The samples have been heated up to 950°C, maintained for 1 hour and cooled down in water. The next step has comprised the heating up at 760°C, maintaining the sample at this temperature for 5 hours, cooling down at 650°C in two hours and maintaining the sample at this temperature for 1 hour followed by cooling it down in calm air.

[0110]The results of hardness and mechanical tests after the different heat treatments are respectively gathered in Tables 2 and 3 below: Table 2 shows the hardness evolution of MPEAs of the invention, from an MPEA as cast according to Example 1 (MPEA(CAST), first entry in Table 2) to the heat-tre...

example 3-manufacturing

EXAMPLE 3 - Manufacturing of an MPEA as cast according to the invention (intermediate cooling rate)

[0117]In order to evaluate the effect of cooling rate in the microstructure of the MPEAs as cast of the present invention, keel blocks Y2 were prepared using chemically bonded chromite sand moulds. Those mould were dimensioned following the standard norm UNE-EN 1563:1998. The keel blocks Y2 were then extracted from the moulds and cleaned by shot blasting.

[0118]An MPEA according to the invention, having the same elemental composition as that of Example 1, was prepared under the same process conditions as those described in Example 1, the difference being that the molten mixture was poured in a chromite sand mould. The cooling rate in the chromite sand mould was estimated to be of 0.5-1°C / sec.

[0119]Chromite was found to present a higher cooling-rate potential than the silica sand moulds employed in Example 1, and allowed to obtain finer microstructures with lower segregation and particul...

Claims

1. A multi-principal element alloy, characterized by consisting of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloy: Ni:30 - 50 wt.%,Mn:10 - 30 wt.%,Cu:10 - 30 wt. %, andCr:10 - 30 wt.%, the balance being Fe and impurities; and wherein the content of Fe and impurities is equal to or less than 1 wt. %.

2. The multi-principal element alloy according to claim 1, wherein the Ni amount is comprised between 32 and 48 wt.%, or between 35 and 45 wt.%, or between 38 and 45 wt.%.

3. The multi-principal element alloy according to claim 1 or 2, wherein the Mn amount is comprised between 12 and 28 wt.%, or between 15 and 30 wt.%, or between 15 and 25 wt.%.

4. The multi-principal element alloy according to any preceding claim, wherein the Cu amount is comprised between 12 and 28 wt.%, or between 15 and 30 wt.%, or between 15 and 25 wt.%.

5. The multi-principal element alloy according to any preceding claim, wherein the Cr amount is comprised between 12 and 28 wt.%, or between 15 and 30 wt.%, or between 15 and 25 wt.%.

6. The multi-principal element alloy according to any preceding claim, comprising a microstructure which is composed of at least 90% of a first phase which has a face-centered cubic crystalline structure, and equal to or less than 10% of a second phase which has a body-centered cubic crystalline structure, as determined by SEM.

7. The multi-principal element alloy according to claim 6, wherein the microstructure is composed of at least 95% of the first phase, and equal to or less than 5% of the second phase, as determined by SEM, wherein the first phase has a metastable face-centered cubic crystalline structure.

8. A process for the preparation of a multi-principal element alloy according to any preceding claim, characterized by comprising the following steps: a) Introducing alloying elements of the alloy which is to be prepared in a furnace for forming an alloying mixture; b) heating the alloying mixture at a temperature in the range from 1200°C to 1700°C for forming a molten mixture; c) optionally removing oxygen and / or removing hydrogen while the mixture is in molten state, said oxygen removal step comprising the introduction of a deoxidizing agent; d) removing slag, optionally aided by flux addition , e) maintaining the mixture in molten state for a period of at least 10 minutes for homogenization; f) pouring of the molten mixture in a mould; and g) solidifying the molten mixture to obtain the multi-principal element alloy.

9. The process according to claim 8, wherein the alloying elements consist of Ni, Mn, Cu, Cr and Fe, and / or each alloying element is introduced in step a) in substantially pure form.

10. The process according to claim 8 or 9, wherein the alloying mixture in step a) consists of the following elemental composition, wherein the percentages are expressed by weight with respect to the total weight of the alloy mixture: Ni:30 - 50 wt.%,Mn:10 - 30 wt.%,Cu:10 - 30 wt. %, andCr:10 - 30 wt.%, the balance being Fe and impurities; and wherein the content of Fe and impurities is equal to or less than 1 wt. %;11. The process according to any one of claims 8-10, wherein step g) comprises cooling the molten mixture at a cooling rate in the range from 0. 5 °C / sec to 4.0 °C / sec.

12. The process according to any one of claims 8-10 wherein, subsequently to step g), the process further comprises a first thermal treatment step which comprises heating the solidified molten mixture at a temperature in the range from 500°C to 1200°C, preferably from 700°C to 1100°C, more preferably from 800°C to 1000°C; optionally further comprising a second thermal treatment step, after the first thermal treatment step, which comprises heating the solidified molten mixture resulting from the first thermal treatment step at a temperature in the range from 500°C to 800°C.

13. A multi-principal element alloy obtainable by the process according to any of claims 10-12, which presents a Vickers hardness (HV) in the range of 2746 to 2844 MPa, as determined in accordance with UNE EN-ISO6507-1:2015.

14. The multi-principal element alloy according to claim 13, comprising a microstructure which is composed of at least 90% of a first phase which has a face-centered cubic crystalline structure, and equal to or less than 10% of a second phase which has a body-centered cubic crystalline structure, as determined by SEM.

15. Use of a multi-principal element alloy according to any one of claims 1-7, 13 or 14, as material for hydrogen storage.

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