Shape memory alloy and process for production thereof

Abstract

A shape memory alloy based on Fe-Mn-Si contains (in % by weight): Mn: 15 – 20%, Si: 4 - 9%, Cr: 11 – 18%, Ni: 2.5 - 7%, V: 0.3 – 0.9%, C: 0.03 - 0.3%, N: 0.005 – 0.5%, where Cr + Si: 15 – 22%, and the optional elements Al: < 0.1%, P: < 0.1%, S: < 0.05%, Mo: < 3%, Co: < 1%, Nb: < 1%, rare earth metals < 0.3% in total, the balance being iron and unavoidable impurities.

Description

[0001] SHAPE MEMORY ALLOY AND METHOD FOR PRODUCING A

[0002] SUCH

[0003] The invention relates to a shape memory alloy based on Fe-Mn-Si and a method for producing such a shape memory alloy based on Fe-Mn-Si.

[0004] Shape memory alloys are characterized by exhibiting a shape memory effect (also known as SME). This effect is based on a reversible, diffusionless shearing movement of the metal lattice from γ-austenite (face-centered cubic, fcc) to metastable γ-martensite (hexagonal close-packed, hcp).

[0005] The original microstructure of an undeformed Fe-Mn-Si shape memory alloy (SMA) is γ-austenite. Deformation of the alloy can cause the stress generated during deformation to promote the transformation of γ-austenite into γ-martensite, a process known as "martensitic transformation" or "forward martensitic phase transformation." After deformation, the stress-induced γ-martensite can revert to γ-austenite upon activation (e.g., by heating). This allows the original shape of an Fe-Mn-Si shape memory alloy to be restored to a certain extent. This process is also known as "austenitic transformation" or "reverse martensitic phase transformation."

[0006] In other words, SME is characterized by a transformation into metastable s-martensite induced by strain, as well as a reconversion to y-austenite caused by heating. During the reconversion, a contraction occurs, which can build up stress in the shape memory alloy, provided it is mechanically fixed (whereby the mechanical fixation prevents contraction).

[0007] In many applications of Fe-Mn-Si-based shape memory alloys, improved corrosion resistance is desirable. This is particularly true when the shape memory alloy is used, for example, as reinforcement or a structural element in construction (e.g., concrete structures). The increased corrosion resistance should not compromise the shape memory alloy's inherent properties.

[0008] Furthermore, an increase or at least no deterioration of the yield strength and thus the re-transformation stress of the shape memory alloy is desired in order to achieve sufficiently high stresses in practical applications (e.g. concrete construction, bridge construction, etc.).

[0009] Another aspect – especially in applications in the construction sector – is that the shape memory alloy should have high strength and / or the lowest possible activation temperature to avoid damaging, for example, the concrete.

[0010] One of the problems addressed by the invention is to create a shape memory alloy with high corrosion resistance combined with high reverse transformation stress and strength. Furthermore, the invention aims to provide a method for producing a shape memory alloy with these properties.

[0011] The problem addressed by the invention is solved by the features of the independent claims. Examples and further developments are the subject of the pending claims.

[0012] Accordingly, a shape memory alloy based on Fe-Mn-Si contains (in wt.%) Mn: 15 - 20%, Si: 4 - 9%, Cr: 11 - 18%,

[0013] Ni: 2.5 - 7%, V: 0.3 - 0.9%, C: 0.03 - 0.3%, N: 0.005 - 0.5%, where Cr + Si: 15 - 22%, as well as the optional elements Al: < 0.1%, P: < 0.1%, S: < 0.05%, Mo: < 3%, Co: < 1%, Nb: < 1%, rare earth metals in total < 0.3%, the remainder iron and unavoidable impurities.

[0014] Furthermore, a process for producing a shape memory alloy based on Fe-Mn-Si comprises the following steps: Melting a metal melt containing (in wt.%) Mn: 15-20%, Si: 4-9%, Cr: 11-18%, Ni: 2.5-7%, V: 0.3-0.9%,

[0015] C: 0.03 - 0.3%, N: 0.005 - 0.5%, where Cr + Si: 15 - 22%, as well as the optional elements Al: < 0.1%, P: < 0.1%, S: < 0.05%, Mo: < 3%, Co: < 1%, Nb: < 1%, rare earth metals in total < 0.3%, the remainder iron and unavoidable impurities; casting of the molten metal into a semi-finished product; hot forming of the semi-finished product into a formed semi-finished product; and solution annealing of the formed semi-finished product at an annealing temperature between 1000°C and 1200°C for an annealing time of 0.5 to 10 hours.

[0016] The comparatively high chromium content results in high corrosion resistance for the shape memory alloy. The other alloying elements were selected to ensure that the shape memory alloy retains good shape memory properties. Furthermore, it has been found that the disclosed shape memory alloy can be produced with a high ferrite content, resulting in high strength. The alloying elements and their effects are described below. Percentage values ​​for alloying elements are given in wt.%.

[0017] Manganese (Mn)

[0018] Mn serves to stabilize the Y _Austenite (hereinafter referred to as austenite) is present in the microstructure and particularly influences the switching behavior (phase transition between austenite and e-martensite) in shape memory alloys. Below a Mn content of 15%, the austenite-stabilizing effect is absent. A Mn content above 20% has adverse effects on martensite formation (unless otherwise specified, martensite in the following always refers to e-martensite).

[0019] Silicon (Si)

[0020] Silicon (Si) ensures the reversibility of the phase transformation from martensite to austenite. It also increases oxidation resistance. Below 4%, Si reduces the SME (synthetic mineralization). At silicon contents above 9%, embrittlement of the material can be observed.

[0021] Silicon reduces the stacking fault energy, thereby accelerating the transformation from austenite to martensite. Furthermore, silicon acts as a ferrite stabilizer.

[0022] Chromium (Cr)

[0023] Cr increases corrosion resistance. In addition, Cr is a ferrite stabilizer.

[0024] To effectively increase corrosion resistance, a Cr content of at least 11%, preferably at least 12%, is required. A Cr content above 18% results in more than 50% ferrite being present in the microstructure. Since only austenite contributes to the SME (small mineral aggregation), an excessively high ferrite content is detrimental to the SME. Furthermore, chromium increases the stacking fault energy (YSF, hereinafter referred to as SFE), which should be kept as low as possible to preferentially obtain martensite. If the SFE is too high, the SME disappears, as only twins then form in the austenite.

[0025] Nickel (Ni)

[0026] Ni serves to stabilize the austenitic structure and also improves the formability of the shape memory alloy.

[0027] Ni contents above 7% have no positive effect on the material properties and are also avoided for cost reasons. At Ni contents above 7%, there is a risk that the austenite will be stabilized to such an extent that the martensitic phase transformation required for SME (small-scale metal alloying) is hindered and no longer occurs sufficiently during the necessary pre-deformation. Therefore, Ni contents of a maximum of 7% are preferred.

[0028] Vanadium (V)

[0029] V has a higher affinity for C and N than Cr. V can therefore ensure that Cr remains dissolved in the microstructure, thus increasing corrosion resistance.

[0030] Another advantageous effect of V is that during aging treatment it forms fine vanadium carbonitrides V(C,N) (diameter < 10 pm) (so-called vanadium carbonitride precipitates), which are homogeneously distributed throughout the microstructure of the solid shape memory alloy. The vanadium carbonitrides V(C,N) result in an increase in strength. That is, V, in the presence of C > 0.1%, enables the "growth" of [missing information] during an (optional) aging treatment.

[0031] V(C,N) precipitates, which are finely dispersed as V(C,N) particles in the microstructure of the shape memory alloy after aging treatment. These V(0,N) precipitates have a positive effect on the SME.

[0032] The minimum V content is therefore 0.3%. The maximum V content is 0.9%. At a V content greater than approximately 0.9%, unfavorable excretion kinetics with respect to V(C,N) excretion may occur.

[0033] Carbon (C)

[0034] Carbon is an austenite stabilizer. Furthermore, carbon improves the SME (synthetic mineral density). Therefore, a minimum carbon content of 0.03% was chosen.

[0035] In addition to its effect as an austenite stabilizer, C acts as an interstitial element and increases the strength of the alloy.

[0036] If the carbon content exceeds 1%, ductility begins to decrease significantly. Furthermore, an excessively high carbon content alters the transformation behavior and, during quenching hardening, leads to the formation of the martensitic a' phase (which, unlike the martensitic s phase, is undesirable). Therefore, the maximum carbon content is 0.3%.

[0037] Nitrogen (N)

[0038] Nitrogen (N) acts (like carbon) as an interstitial element and austenite stabilizer. Furthermore, N increases the strength of the alloy. As with carbon, an excessively high N content alters the transformation behavior and, during quench hardening, leads to the formation of the (undesirable) martensitic oc' phase. Therefore, the maximum N content is 0.5%. Molybdenum (Mo)

[0039] Mo reduces stacking fault energy and improves high-temperature oxidation resistance. At a molybdenum content of less than 0.1%, the effects are negligible. At a molybdenum content of more than 3%, the alloy's shape memory and hot-work properties deteriorate.

[0040] Cobalt (Co)

[0041] Co-injected carbon improves the SME (synthetic material removal rate) and hot-working properties of the alloy. At co-contents below 0.1%, the effects are negligible, while at co-contents above 1%, no further improvements are achieved.

[0042] Niobium (Nb)

[0043] Nb (like V) has a higher affinity for carbon and nitrogen than Cr. Therefore, Nb can be added up to a maximum limit of 1%.

[0044] Rare earth metals

[0045] Rare earth metals, especially Sc, Y, La or Ce, improve corrosion resistance and can be added in total up to, for example, 0.3%.

[0046] Examples and embodiments of the invention are explained in more detail below with reference to the drawings.

[0047] Figure 1 shows a schematic representation of an example of a process sequence (manufacturing route) for the production of a shape memory alloy. Figure 2 shows a representation of the phases of the microstructure of a shape memory alloy generated by electron backscatter diffraction (EBSD).

[0048] Figure 3 illustrates the dependence of martensite formation during pre-stretching (martensite transformation) on the stacking fault energy (SFE).

[0049] Figure 4 shows stress-strain curves for three examples SMA_1, SMA 2, SMA 3 of shape memory alloys according to the disclosure and a reference shape memory alloy SMA_R during pre-strain (martensitic transformation).

[0050] Figure 5 shows stress-temperature curves for three examples SMA_1, SMA_2, SMA_3 of shape memory alloys according to the disclosure and a reference shape memory alloy SMA_R during activation (austenite conversion) and subsequent cooling.

[0051] Figure 6 shows microstructures of the shape memory alloys SMA_1, SMA 2, SMA 3 and of shape memory alloys from the prior art in a phase diagram (so-called Schaeffler diagram) of Ni and Cr equivalents.

[0052] The process steps described below with reference to Figure 1 are exemplary and can be replaced or supplemented by other or similar process steps. In particular, further processes may be included between the process steps described below, which are not discussed in detail here.

[0053] The starting point for alloy production is a melting process 1, in which a molten metal is melted. This process can be carried out, for example, in an electric arc furnace (EAF), in an induction furnace (e.g., a vacuum induction degassing furnace VID or by VIM (vacuum induction melting)).

[0054] Reference numeral 2 denotes a secondary metallurgical treatment (secondary metallurgy). In this process, the alloy composition is adjusted within the specified ranges. For this purpose, an AOD (Argon Oxygen Decarburization) converter, a VOD (Vacuum Oxygen Decarburization) plant, and / or a ladle furnace (LE) can be used, for example.

[0055] The molten metal (shape memory alloy) is then cast into a semi-finished product at reference numeral 3. For example, ingot casting or bar casting can be used.

[0056] In a later process step (reference numeral 4), the semi-finished product undergoes hot forming to produce a formed semi-finished product. The formed semi-finished product can be, for example, a strip (hot-rolled strip), a sheet, a block, a bar, etc. The hot forming process 4 can include, for example, rough rolling 4.1 in a rough rolling mill and (optionally) further forming steps, such as multi-line rolling (at 4.2) in a multi-line rolling mill, or optionally, a forging treatment.

[0057] In the next step of the process, the formed semi-finished product is annealed at reference numeral 5. The annealing temperature can range between 1000°C and 1200°C. The annealing time can, for example, be from 0.5 hours to 10 hours.

[0058] Solution annealing (5) is performed to remove residual precipitates and pronounced textures from production. For this purpose, the formed pre-product is immersed in solution at a high temperature (e.g., equal to or greater than 1050°C) for a sufficiently long period of time (e.g., 3 hours or more). Solution annealing serves to homogenize concentration differences of the alloying elements through diffusion and to reduce microstructural inhomogeneities.

[0059] Solution annealing (5) allows for very precise control of the phase fractions, particularly the austenite-ferrite fractions. It has been shown that it is possible to achieve a (high) ferrite content, which can reach up to 45 vol.% of the microstructure (also referred to as microstructure). The additional ferritic phase increases the strength of the shape memory alloy.

[0060] In particular, the solution annealing 5 can be carried out such that the shape memory alloy has an austenite phase (in vol. %) between 35 and 85%, in particular 45 to 80% or 52 to 75%, and / or a ferrite phase (in vol. %) between 15 and 45%, in particular 15 to 40% or 20 to 40%. An e-martensite phase may optionally be present (in which case the microstructure of the shape memory alloy lies in the three-phase region) and amounts, for example, to between 1 and 30%, in particular 1 and 20% or 1 and 15% or 1 and 10% (in vol. %).

[0061] The grain size of the austenite grains is also controlled by the process parameters (annealing temperature, annealing time) of the solution annealing. The microstructure of the shape memory alloy after solution annealing 5 can exhibit a mean grain size (diameter) of the austenite grains between 10 pm and 200 pm, particularly between 50 pm and 200 pm. Specifically, the mean grain size of the austenite grains can be approximately 100 pm. The grain sizes are determined according to the standard ASTM E112. After solution annealing 5, the microstructure no longer changes with respect to the phase fractions; that is, the values ​​mentioned above also apply to the final microstructure after the complete heat treatment process.

[0062] Ferrite does not contribute to the SME. Surprisingly, however, it has been shown that even with a ferrite content of up to 45 vol%, a very good SME can still be achieved (due to the remaining austenite phase).

[0063] The phase composition in the microstructure depends on the annealing temperature and duration. The higher the annealing temperature and / or the longer the annealing duration, the more ferrite forms. Therefore, according to some embodiments, solution annealing 5 is carried out at a higher temperature and / or for a longer duration to create the desired austenite-ferrite composition of the shape memory alloy.

[0064] In summary, the manufacturing process involves the transformation of the alloy between the ferrite and austenite phases, possibly with the involvement of the e-martensite phase, in order to achieve the desired microstructure and, in particular, the desired mean grain size of the austenite grains and the desired properties.

[0065] Since chromium (Cr) is a ferrite former, ferrite can contain more Cr than austenite. Therefore, with prolonged solution annealing (5), the Cr content in ferrite should increase, and the Cr content in austenite should decrease. However, it has been shown that despite solution annealing (5), Cr remains in both ferrite and austenite at a Cr content > 11 wt.%, and particularly > 12 wt.%, to ensure the corrosion resistance of the shape memory alloy. Solution annealing (5) can be terminated by cooling, e.g., by quenching (rapid temperature reduction). Quenching can be achieved, for example, by adding water. It is also known as quench hardening. Quenching prevents the formation of precipitates during the cooling process.Furthermore, quenching freezes the phase mixture present during solution annealing 5 (ferritic phase (bcc), austenitic phase (fcc) and to a lesser extent martensitic phase (hcp) and carbides).

[0066] Solution annealing 5 followed by cooling (e.g., quenching) can optionally be followed by an aging treatment 6 of the hot-rolled strip (or products, components, or samples manufactured from it). During this aging treatment, also known in engineering as "aging," V(C,N) can be selectively cultivated. V(C,N) precipitates have a positive effect on the SME (which only occurs in the austenitic phase). At the same time, V(C,N) has a strength-enhancing effect.

[0067] Another beneficial effect of V(C,N) is that it binds C and thus suppresses chromium carbide precipitates. Chromium carbide precipitates are undesirable because they bind Cr, which would then no longer be available for corrosion protection.

[0068] The aging treatment 6 of the cooled, formed pre-product can be carried out at a temperature between 550°C and 800°C, particularly between 600°C and 700°C. In particular, the aging treatment can be performed at temperatures below 750°C, 700°C, 650°C, 640°C, or 630°C. The duration of the aging treatment can be between 1 hour and 300 hours, particularly between 3 hours, 4 hours, 5 hours, 8 hours, 10 hours, or 15 hours and 50 hours. The optional aging treatment ("aging") no longer changes the phase fractions of the shape memory alloy or the grain sizes.

[0069] It has been shown that during aging treatment at lower temperatures, the nucleation rate is higher, resulting in the formation of more finely dispersed secretions (e.g., V(C,N) secretions). However, the nucleation rate is lower, meaning more time is required to reach the ideal secretion size. Therefore, favorable conditions for aging treatment can be created, for example, by using comparatively low temperatures and longer treatment times.

[0070] After solution annealing and cooling of the formed preform and (optional) aging treatment, the formed preform can be delivered to a customer (at the dashed line in Figure 1). The following process step 7 comprises a pre-stretching ("stretching") of the formed preform or a product manufactured from it. This process step 7 can be carried out at the customer's site or on-site during the installation of the shape memory alloy, for example, in a component (e.g., a concrete part). The pre-stretching 7 causes the martensitic transformation.

[0071] The term shape memory alloy includes, in the following, the alloy itself as well as products made from this alloy, e.g. strips, sheets (plates), rods, beams, samples, etc.

[0072] One advantage of the manufacturing process illustrated in Figure 1 is that the shape memory alloy can be produced via a single manufacturing route under atmospheric conditions. Table 1 summarizes ranges of chemical composition for shape memory alloys according to the disclosure. In addition to the limit values, preferred and especially preferred range limits are given (oG: upper limit; Bev.oG: preferred upper limit; Sp bev.oG: specially preferred upper limit; uG: lower limit; Bev.uG: preferred lower limit; Sp_bev.uG: specially preferred lower limit). All values ​​are given in wt.%.

[0073] Table 1 (Alloy composition [wt.%])

[0074] Table 1 (continued - alloy composition [wt.%)) The shape memory alloy (SNA) disclosed here is also referred to as duplex SMA, since it consists (almost entirely, see Table 2) of the oc phase (ferrite) and the y phase (austenite). Conventional shape memory alloys often consist practically only of the austenite phase. Surprisingly, it has been found that with the shape memory alloys according to the invention, despite high ferrite content, a very good SME (due to the remaining austenite phase) is still achievable.

[0075] Table 2 summarizes the ranges of phase fractions in the microstructure of shape memory alloys according to the disclosure. In addition to the limit values, preferred and especially preferred range limits are listed with the abbreviations explained in Table 1. The data refer to a shape memory alloy after heat treatment (solution annealing 5 and cooling (optional quenching)) or after the optional aging treatment 6 (which leaves the phase fractions unchanged). The measurements were performed using the EBSD analysis method. Electron backscatter diffraction (EBSD) is a measurement technique used to analyze the crystalline structure of a material. EBSD systems are used in scanning electron microscopes.

[0076] Table 2 (Microstructure [Vol.-%] )

[0077] A shape memory alloy according to the disclosure can further be characterized by the fact that the Cr content in both the austenitic and ferritic phases can be, for example, more than 11 wt.%, and in particular more than 12 wt.%. This results in increased corrosion resistance of the shape memory alloy. The measurement is carried out, for example, by EDS point analysis in a scanning electron microscope (EDS: Energy Dispersive X-ray Spectroscopy). In this method, several points (e.g., 30 points; 15 points in the ferrite, 15 points in the austenite) are selectively measured and evaluated in the ferrite and austenite phases.

[0078] Furthermore, it is known that the SME is strongly dependent on the stacking fault energy. A comparatively low stacking fault energy (SFE < 20 mJ / m) is required. 2) , in order to preferentially obtain s-martensite during deformation (process 7) of the shape memory alloy and thus achieve SME. If the stacking fault energy (SFE) is too high, SME does not occur, as only twins then form in the austenite (see Figure 3).

[0079] The stacking fault energy (SFE) was calculated using Qi-Xun's formula. The calculation of the stacking fault energy (SFE) is given in the unit mJ / m². 2 refers to the entire shape memory alloy, i.e., to all phases of this alloy (where the elements denote their corresponding wt.%).

[0080] SFE = 39 + 1.59*Ni - 1.34*Mn + 0.06*Mn 2 - 1.75*Cr + 0.01*Cr 2 + 15.21*Mo - 5.59*Si + 26.27* (C + 1.2*N) * (Cr + Mn + Mo) 0 ' 5 + 0.61* [Ni* (Cr + Mn) ] 0 ' 5 - 60, 69* (C + 1,2*N) °' 5 m J / m 2 .

[0081] The formula shows that austenite-forming elements, such as Mn, C, and N, increase the stack fault energy SFE. Ferrite-forming elements, such as Cr and Si, decrease the stack fault energy SFE empirically determined according to the equation above.

[0082] A shape memory alloy according to the disclosure can be characterized by having a stacking fault energy (SFE) (based on the entire alloy) e.g. in the range of -40 mJ / m 2 < SFE < 40 mJ / m 2 , in particular -35 mJ / m 2 < SEE < 30 mJ / m 2 or -25 mJ / m 2 < SFE < 20 mJ / m 2 exhibits.

[0083] For the shape memory alloys according to the disclosure, activation temperatures T akt < 300°C and especially e.g.

[0084] T akt < 200°C, T akt < 180°C or T akt< 170°C is reached. Particularly in applications in concrete construction, activation temperatures of 200°C or less are required, and lower activation temperatures are advantageous.

[0085] Examples

[0086] Table 3 shows examples of the alloy compositions of shape memory alloys SMA_1 to SMA_3 according to the disclosure and a reference alloy SMA R. The reference alloy SNA R is also an Fe-Mn-Si-based shape memory alloy, but the Cr content is too low for effective corrosion protection. In all cases, the residual content consists of iron and the unavoidable impurities, and optionally also of the aforementioned optional elements (whereby neither Al, Mo, Co, Nb nor rare earth metals were added in the examples).

[0087] Table 3 (Alloy compositions - examples)

[0088] All values ​​are given in wt.%.

[0089] Figure 2 shows an EBSD analysis of the microstructure of the shape memory alloy SMA_2. It can be seen that, in addition to the austenitic phase (fcc), a relatively high proportion of the ferritic phase (bcc) is present. The martensitic phase (hcp) is present only in very small amounts in the initial state of the shape memory alloy.

[0090] The microstructure shown in Figure 2 is present after solution annealing and quenching (process step 5) of the shape memory alloy SMA_2.

[0091] The essentially two-phase structure (“duplex SMA”) of the microstructure leads to high strength (due to the phase boundaries, which increase strength) of the alloy. Furthermore, the yield strength is increased, resulting in an increase in the retransformation stress Rr. It should be noted that the term “duplex SMA” used here, according to the present disclosure, does not preclude the possibility that the microstructure—as already mentioned—may also exist in a three-phase structure (austenite + ferrite + e-martensite), where the martensite component is reversible and thus responsible for the SMA effect. The mean grain size of the austenite grains in the shape memory alloys SMA 1, SMA 2, and SMA 3 was measured after aging treatment 6 (which no longer alters the grain size) according to ASTM E112. The mean grain size of the austenite grains was approximately 100 pm in each case.

[0092] Table 4 shows the manufacturing parameters of alloys SMA_1, SMA_2, SMA_3, and SMA_R. The annealing treatment (solution annealing 5) was carried out for all alloys at 1070°C for a duration of 4 hours. The subsequent aging treatment following cooling (e.g., quenching) was performed at lower temperatures (equal to or below 660°C) for SMA_1, SMA_2, and SMA_3 than for SMA_R (700°C).

[0093] Furthermore, the aging treatment for SMA_1, SMA_2, SMA_3 was performed over a longer period (duration equal to or greater than 19 hours).

[0094] Table 4 (Manufacturing parameters)

[0095] The specified manufacturing parameters for SNA 1, SNA 2, and SNA 3 are applicable to all shape memory alloys according to the disclosure. This means that the temperature of the aging treatment can generally be, for example, equal to or less than or greater than 660°C, 632°C, or 600°C, and / or its duration can generally be, for example, equal to or less than or greater than 19 hours or 24 hours. That is, all values ​​specified in Table 4 are also disclosed as upper or lower limits of ranges. In general, as already mentioned, the temperature of the aging treatment can, in particular, be equal to or less than, for example, 750°C, 700°C, 650°C, 640°C, or 630°C, and / or the duration of the aging treatment can, in particular, be equal to or greater than, for example, 3 hours.

[0096] 4 hours,

[0097] It could be 5 hours, 8 hours, 10 hours or 15 hours.

[0098] Table 5 summarizes the phase fractions and stacking fault energy of the alloy examples SMA_1, SMA_2, SMA_3 and the reference alloy SMA_R (after solution annealing and quenching).

[0099] Table 5 (Microstructure)

[0100] SMA_R exists almost exclusively in the austenitic phase. SMA 1, SMA2, and SNA 3 exhibit ferritic content between 24 and 37 vol.%. Furthermore, SMA_1, SMA_2, and SMA_3 contain a smaller martensitic phase component. The stacking fault energy (SFE) (relative to the total alloy) is significantly lower for SMA 1, SMA 2, and SMA 3 than for SMA_R. Considering only the austenitic phase of SMA_1, SMA_2, and SMA_3, the stacking fault energy is similar to that of SMA_R. In both the austenitic and ferritic phases of SMA 1, SMA 2, and SMA 3, more than 12 wt.% Cr (measured by EDS point analysis) was found.

[0101] The values ​​for SMA_1, SMA_2, SMA_3 given in Table 5 are applicable to all shape memory alloys according to the disclosure, i.e. the values ​​given for the phase fractions and / or stacking fault energies are also disclosed as upper or lower limits of ranges.

[0102] Figure 6 characterizes the microstructure of the alloy examples SMA_1, SMA_2, and SMA_3 in a phase diagram (so-called Schaeffler diagram) of Ni and Cr equivalents (according to Speidel, M. O.; Uggowitzer, P.J.: High Manganese, High Nitrogen Austenitic Stainless Steels: Their Strength and Toughness. In: Lula, R.A. (ed.): Proceedings of two Conferences, ASM International, 1993, pp. 135–142). The equations for the Ni and Cr equivalents are:

[0103] Ni äq = %Ni + %Co + 30*%C + 0, l*%Mn - 0.01* (%Mn) 2 + 18% N and Cr äq = %Cr + l.5* (%Mo+%W) + 0, 48*%Si + 2.3*%V + l.75*%Nb.

[0104] It is evident that examples SMA 1, SMA 2, and SMA 3 lie in the three-phase region of austenite + ferrite + e-martensite. Other chemical compositions according to the disclosure lie in the two-phase region of austenite + ferrite. In particular, the chemical compositions according to the disclosure do not lie, for example, in the single-phase regions of e-martensite or ferrite or austenite, and also not, for example, in the two-phase regions of austenite + e-martensite and / or ferrite + e-martensite.

[0105] Figure 6 also shows the microstructures of examples from documents CN 111 235 491 A,

[0106] CN 108 588 368 A, CN 109 457 091 A and CN 109 182 662 A describe the well-known shape memory alloy based on Fe-Mn-Si. The microstructures of these alloys are neither in the two-phase region of austenite + ferrite nor in the three-phase region of austenite + ferrite + e-martensite.

[0107] Due to the significantly higher ferritic phase content in the microstructure, shape memory alloys according to the disclosure (e.g., SMA_1, SMA_2, SMA_3) exhibit magnetic properties. Magnetic shape memory alloys can be advantageous in various applications.

[0108] The thermomechanical properties of the shape memory alloys SMA_1, SMA_2, SMA_3, and SMA_R were measured by pre-stretching and activation tests. The pre-stretching corresponds to process step 7 in Figure 1.

[0109] Figure 4 shows the stress-strain curves measured during pre-stressing for the aforementioned shape memory alloys. All shape memory alloys were subjected to a 2% strain and then fully unloaded. The non-linear stress-strain behavior of the shape memory alloys is attributable to martensitic transformation, plastic flow, or a combination of these two effects.

[0110] Figure 4 illustrates that the shape memory alloys SMA_1, SMA_2, and SMA_3, as disclosed, achieved a higher yield strength RpO,l than the reference alloy SMA_R. The yield strength RpO,l was determined at 0.1% plastic strain (intersection of the stress-strain curves with the straight line intersecting the x-axis at 0.1% strain). The higher yield strengths RpO,l of the alloys SMA_1, SMA_2, and SMA_3 (see also Table 6) result in an increase in the retransformation stress compared to the reference alloy SMA_R.

[0111] After pre-stressing (martensitic transformation), the alloys SMA_1, SMA_2, SMA_3, and SMA_R were activated, as shown in Figure 5. Activation involves heating and cooling the alloys. During this process, the alloys are mechanically clamped (fixed), meaning they can neither expand nor contract.

[0112] In the example shown here, activation begins at approximately 23°C with a tensile stress of slightly over 100 MPa. As the temperature increases, the alloy initially expands, thereby reducing the tensile stress in the clamped state. An alloy without SME would continue to expand with further temperature increases, eventually transitioning into the compressive stress range (see dotted line). The SME, with the austenite transformation that begins at K (kink), causes contraction and thus prevents a pressure build-up as the alloy heats up further.

[0113] During the subsequent cooling of the shape memory alloy, it attempts to contract, but this is prevented by the constrained state of the alloy (in practice, for example, it is embedded in a concrete component). This results in an increase in tensile stress. At the end of the cooling process (i.e., top left in Figure 5), the stresses specified in Table 6 (so-called reversion stress Rr) are generated. These stresses are used to reinforce components (e.g., concrete components) in which the shape memory alloy is incorporated. In practice, concrete slabs, beams, bridges, silos, etc., can be strengthened against loads in this way, thereby reducing the amount of concrete required for the respective structure or component. Table 6 (Mechanical parameters, measured at pre-stress and activation)

[0114] The pre-strain tests (Figure 4) and the activation tests (Figure 5) were performed using a Zwick Z020 tensile testing machine equipped with a climate chamber. A mini-MFA 2 strain gauge, with an accuracy class of 0.2 according to EN ISO 8513, was used to measure the strain. The linearity error of the strain gauge, including hysteresis, was within 0.05%. During activation (with an activation temperature Ti), the following measurements were taken: akt At a temperature of 160°C, the strain was directly monitored via the strain gauge, whose thermal expansion was compensated for by the testing machine after calibration. The measurement was performed on SMA samples cut into a dog bone shape, measuring 32 mm in length and 1.6 mm in width.

[0115] The reconversion stress Rr (see Figure 5) of the shape memory alloys according to the disclosure can generally be greater than 300 MPa, in particular 320 MPa. Further tests showed that reconversion stresses Rr > 400 MPa can be achieved. The yield strength Rp0,l of the shape memory alloys according to the disclosure can generally be greater than 300 MPa, in particular 400 MPa or 430 MPa.

Claims

Patent claims 1. Shape memory alloy based on Fe-Mn-Si, containing (in wt.%): Mn: 15 - 20%, Si: 4 - 9%, Cr: 11 - 18%, Ni: 2.5 - 7%, V: 0.3 - 0.9% C: 0.03 - 0.3% N: 0.005 - 0.5%, where Cr + Si: 15 - 22%, and the optional elements Al: < 0.1%, P: < 0.1% S: < 0.05% Mo: < 3%, Co: < 1%, Nb: < 1%, rare earth metals in total: < 0.3%, the remainder iron and unavoidable impurities.

2. Shape memory alloy according to claim 1, wherein Cr: 12 - 16%, in particular 13 - 15%.

3. Shape memory alloy according to claim 1 or 2, wherein Cr + Si: 16 - 21%, in particular 17 - 20%.

4. Shape memory alloy according to any one of the preceding claims, wherein V: 0.35 - 0.7%, especially 0.4 - 0.5%.

5. Shape memory alloy according to one of the preceding Claims, with an austenite phase (in vol.-%) between 35 and 85%, in particular between 45 and 80% or 52 and 75% and / or a ferrite phase (in vol.-%) between 15 and 45%, in particular between 15 and 40% or 20 and 40%.

6. Shape memory alloy according to claim 5, comprising an e-martensite phase (in vol.%) between 1 and 30%, in particular between 1 and 20% or 1 and 15%.

7. Shape memory alloy according to one of the preceding claims, wherein the microstructure of the shape memory alloy is in the two-phase region austenite + ferrite or in the three-phase region austenite + ferrite + e-martensite.

8. Shape memory alloy according to one of the preceding claims, wherein (in wt.%) more than 11% Cr, in particular more than 12% Cr, is present in an austenite phase of the shape memory alloy and / or more than 11% Cr, in particular more than 12% Cr, is present in a ferrite phase of the shape memory alloy.

9. Shape memory alloy according to one of the preceding claims, wherein in an austenite phase of the shape memory alloy austenite grains have a mean grain size between 10 pm and 200 pm.

10. Shape memory alloy according to one of the preceding claims, which has a stacking fault energy (SFE) in the range -40 mJ / m 2 < SFE < 40 mJ / m 2 , in particular -35 mJ / m 2 < SFE < 30 mJ / m 2 , or -25 mJ / m 2 < SFE < 20 mJ / m 2 exhibits.

11. Shape memory alloy according to one of the preceding Claims that specify a yield strength Rp0, l > 300 MPa, in particular Rp0, l > 400 MPa.

12. Shape memory alloy according to one of the preceding claims, having a reverse conversion voltage Rr > 300 MPa, in particular Rr > 320 MPa.

13. Shape memory alloy according to one of the preceding Claims, wherein an activation temperature Takt the shape memory alloy T akt < 200°C, especially T akt < 170°C.

14. Method for producing a shape memory alloy based on Fe-Mn-Si, comprising the following steps: Melting a metal melt containing (in wt.%): Mn: 15 - 20%, Si: 4 - 9%, Cr: 11 - 18%, Ni: 2.5 - 7%, V: 0.3 - 0.9% C: 0.03 - 0.3% N: 0.005 - 0.5%, where Cr + Si: 15 - 22%, as well as the optional elements Al: < 0.1% P: < 0.1% S: < 0.05% Mo: < 3%, Co: < 1%, Nb: < 1%, rare earth metals in total: < 0.3%, the remainder iron and unavoidable impurities; Pouring the molten metal into a semi-finished product; Hot forming of the semi-finished product into a formed semi-finished product; and Solution annealing of the formed pre-product at an annealing temperature between 1000°C and 1200°C for an annealing duration of 0.5 to 10 hours.

15. Method according to claim 14, wherein the annealing temperature is above 1050°C, in particular above 1070°C, for an annealing duration of over 3.0 hours, in particular over 3.5 hours or 4.0 hours.

16. Method according to claim 14 or 15, wherein the solution annealing is carried out such that the shape memory alloy has an austenite phase (in vol.%) between 35 and 85%, in particular 45 to 80% or 52 to 75% and / or a ferrite phase (in vol.%) between 15 and 45%, in particular 15 to 40% or 20 to 40%.

17. Method according to any one of claims 14 to 16, further comprising: Cooling, in particular quenching, of the reshaped pre-product after solution annealing; and Aging treatment of the cooled, reshaped pre-product at a temperature between 550°C and 800°C, especially 600°C and 700°C.

18. Method according to claim 17, wherein the aging treatment is carried out for a period of time between 1 hour and 300 hours, in particular between 3 hours and 50 hours.

19. Method according to one of claims 17 or 18, wherein C > 0.1% and the aging treatment is carried out such that V (C, N) -excretions are produced.

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