A precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties and a preparation method thereof

Abstract

The present invention relates to the field of high-temperature structural materials, and particularly to a precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties and a preparation method thereof. The chemical composition of the alloy is as follows by weight percentage: Ni (43% - 52%); Co (25% - 31%); Cr (13% - 18%); Al (2.5% - 3.5%); Ti (5.5% - 6.5%); C (0.01% - 0.05%); Zr (0.01% - 0.05%); B (0.003% - 0.05%). The method melts each metal element into an alloy ingot by an arc melting method and naturally cools it in a copper crucible. The melted alloy ingot is sealed in a quartz tube for heat treatment. Through heat treatment, a heterogeneous structure with small L12 phases and large L12 phases distributed along dendrites and interdendrites is regulated, and primary carbides tend to precipitate in the large L12 phase region. This structure endows it with excellent high-temperature mechanical properties. And the density of the alloy is 7.8 - 7.9 g / cm 3 , greatly improving the application potential of high-entropy alloys in the field of high-temperature structural materials.

Description

Technical Field

[0001] The present invention relates to the field of high-temperature structural materials, and particularly to a precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties and a preparation method thereof. Background Art

[0002] Superalloys are the most complex material systems that humans can currently manufacture and use, and have significant application values in the fields of aviation, aerospace, petroleum, chemical industry, ships, etc. With the development of the country and society, the demand for superalloys in civilian fields such as automobiles is increasing. However, so far, the density of most superalloys is above 8.2 g / cm 3 and some superalloys with more refractory elements added can even reach above 10 g / cm 3 This far exceeds the density of ordinary steel. Whether in the aerospace, navigation, or automotive fields, high density means high cost and low energy efficiency.

[0003] In 2004, Ye Junwei et al. first proposed the concepts of multi-component alloys and high-entropy alloys, and found that high-entropy alloys have unique physical characteristics and performance advantages. High-entropy alloys are not limited to the use of a single main element, and their composition has extended to the central region of the phase diagram, which provides more possibilities for the composition design of high-temperature structural materials and more solutions to the high-density problem of traditional superalloys. However, currently, BCC-based high-entropy alloys generally face problems such as high density and low oxidation resistance in high-temperature applications, while the application research of FCC-based high-entropy alloys in high-temperature fields is still less. Therefore, how to design and prepare a low-density high-entropy alloy with good comprehensive mechanical properties under high-temperature conditions is a technical problem that urgently needs to be solved. Summary of the Invention

[0004] The main purpose of the present invention is to provide a precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties and a preparation method thereof. This alloy does not use expensive refractory metal elements such as Nb, Mo, Ta, and W, has a relatively low density compared to traditional superalloys, and achieves good mechanical properties at both room temperature and 750 °C.

[0005] The purpose of the present invention and the solution to its technical problems are achieved by adopting the following technical solutions:

[0006] A precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties, by weight percentage, the chemical composition of this high-entropy alloy is as follows:

[0007] Ni: 43% to 52%; Co: 25% to 31%; Cr: 13% to 18%; Al: 2.5% to 3.5%; Ti: 5.5% to 6.5%; C: 0.01% to 0.05%; Zr: 0.01% to 0.05%; B: 0.003% to 0.05%.

[0008] The precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties, the density of the high-entropy alloy is 7.8 g / cm 3 to 7.9 g / cm 3 .

[0009] The precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties, the microstructure of the high-entropy alloy consists of FCC phase, L12 phase and carbides.

[0010] The precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties, the FCC phase in the high-entropy alloy is the matrix phase, with atoms disorderly arranged, and its volume content is 40% to 49.5%; the L12 phase is the precipitation-strengthening phase, with atoms orderly arranged, and its volume content is 50% to 59.5%; there are two types of carbides MC and M 23 C6 in the alloy, the volume content of MC is 0.1% to 0.5%, and the volume content of M 23 C6 is 0.01% to 0.05%.

[0011] The precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties, there are two scales of L12 phases in the high-entropy alloy, the size range of the small L12 phase is 20 nm to 250 nm, the size range of the large L12 phase is 300 nm to 2 μm, and the small L12 phase and the large L12 phase are distributed along dendrites and interdendrites; MC carbides precipitate in the large L12 phase region, and M 23 C6 carbides precipitate discontinuously at grain boundaries.

[0012] The preparation method of the precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties includes the following steps:

[0013] (1) Weigh raw materials according to the preset composition ratio, and prepare alloy ingots by arc melting;

[0014] (2) Let the alloy ingots cool naturally in the copper crucible of the arc melting furnace;

[0015] (3) Seal the cooled alloy ingots into quartz tubes, and obtain the precipitation-strengthened NiCoCrAlTi high-entropy alloy through solution treatment and aging treatment.

[0016] In the preparation method of the precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties, in step (3), the solution treatment temperature is 1150°C ± 20°C, the holding time is 2 - 5 h, and it is air-cooled to room temperature; the aging treatment temperature is 750°C ± 20°C, the holding time is 4 - 24 h, and it is air-cooled to room temperature.

[0017] In the preparation method of the precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties, the performance indexes of the high-entropy alloy are as follows: When tensile tested at room temperature, the yield strength is 950 MPa to 1050 MPa, the tensile strength is 1100 MPa to 1200 MPa, and the elongation is 7% to 15%; when tensile tested at 750°C, the yield strength is 850 MPa to 900 MPa, the tensile strength is 950 MPa to 1050 MPa, and the elongation is 12% to 15%.

[0018] The design idea of the present invention is as follows:

[0019] (NiCoFe) 86 The (NiCoFe)Al7Ti7 high-entropy alloy exhibits excellent room-temperature mechanical properties after a series of deformations and heat treatments, but the high-entropy alloy has not been tested and studied for high-temperature mechanical properties. Based on this alloy, the present invention has carried out an improved design according to the requirements of the high-temperature properties of the alloy. In order to enable the designed alloy to have good oxidation resistance at high temperatures, we added 13% - 18% mass fraction of Cr element. Since the Cr element and the Fe element are very likely to form brittle TCP phases such as σ phase and Laves phase that have an adverse effect on the alloy properties when together, in order to avoid excessive precipitation of TCP phases in the alloy, we choose not to add Fe element in the alloy. For the requirement that the density of the alloy is lower than 8 g / cm 3 ³, no high-density refractory elements are added in the alloy, but this may lead to insufficient strength of the alloy under high-temperature conditions. In order to have a high room-temperature and high-temperature yield strength, we choose to maintain relatively high mass fractions of Al (2.5% - 3.5%) and Ti (5.5% - 6.5%) elements to ensure the volume fraction of the high L12 precipitation strengthening phase. In order to maintain a low stacking fault energy and make it easy to activate stacking faults and increase ductility during the deformation of the alloy, the mass fraction of Co is set to 25% - 31%. Since the high-entropy alloy of the present invention is a polycrystalline structure and is extremely prone to grain boundary weakening and intergranular sliding at high temperatures, in order to strengthen the grain boundary bonding force at high temperatures, we added trace elements such as C (0.01 - 0.05 wt.%), Zr (0.01 - 0.05 wt.%), and B (0.003 - 0.05 wt.%).

[0020] In addition, in the present invention, each metal element is melted into an alloy ingot by an arc melting method and naturally cooled in a copper crucible. The cooled alloy ingot is sealed in a quartz tube for heat treatment. The specific heat treatment regime is as follows: solution treatment at 1150°C for 2 to 5 hours, aging treatment at 750°C for 4 to 24 hours, and all cooling methods are air cooling. Through the above heat treatment, a heterogeneous structure with small L12 phases and large L12 phases distributed along dendrites and interdendrites is prepared, and primary carbides tend to precipitate in the large L12 phase region.

[0021] By means of the above technical solution, the precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties of the present invention has at least the following advantages:

[0022] 1. The precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties of the present invention has a density of 7.8 g / cm 3 to 7.9 g / cm 3 , which is much lower than the density range of traditional superalloys greater than 8.2 g / cm 3 . Its preparation process flow is simple, and it has excellent high-temperature mechanical properties, greatly improving the application value and application potential of high-entropy alloys in the field of high-temperature structural materials.

[0023] 2. The precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties of the present invention has good room-temperature tensile properties. When tensile tested at room temperature, the yield strength is 950 MPa to 1050 MPa, the tensile strength is 1100 MPa to 1200 MPa, and the elongation is 7% to 15%.

[0024] 3. The precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties of the present invention has excellent high-temperature tensile properties. When tensile tested at 750°C, the yield strength is 850 MPa to 900 MPa, the tensile strength is 950 MPa to 1050 MPa, and the elongation is 12% to 15%. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is the XRD pattern of the NiCoCrAlTi low-density high-entropy alloy.

[0026] Figure 2 is the microstructure of the as-cast NiCoCrAlTi low-density high-entropy alloy. Among them, (a) is the microstructure under low-magnification scanning electron microscopy, (b) is the large-sized MC carbides precipitated within the grains, and (c) is the large-sized carbides precipitated along the grain boundaries.

[0027] Figure 3The microstructure after solution treatment of the NiCoCrAlTi low-density high-entropy alloy. Among them, (a) is the microstructure under low-magnification scanning electron microscopy, (b) is the heterogeneous structure formed by large L12 phase and small L12 phase, (c) is the dendrite and interdendritic structure under optical microscopy, and the large-sized MC carbide precipitated along the dendrite and large L12 phase region is indicated by the white arrow.

[0028] Figure 4 The microstructure at the grain boundary of the NiCoCrAlTi low-density high-entropy alloy. Among them, (a) is after solution treatment, and (b) is after aging treatment.

[0029] Figure 5 The morphology of L12 of the NiCoCrAlTi low-density high-entropy alloy. Among them, (a) is as-cast, (b) is after solution treatment, and (c) is after aging treatment.

[0030] Figure 6 The tensile stress-strain curves of the examples and the control examples. Among them, (a) is at room temperature, and (b) is at 750 °C.

[0031] Figure 7 The comparison results of the specific yield strength and ductility of the examples and the traditional cast superalloys at 750 °C. In the figure, the abscissa Fracture strain represents the fracture strain (%), and the ordinate σ YS / ρ represents the specific yield strength (MPa·g -1 ·cm 3 ). Specific Embodiments

[0032] The following combines the accompanying drawings and examples to describe the specific embodiments of the present invention in more detail, and understand the solutions of the present invention and the advantages of its various aspects. However, the specific embodiments and examples described below are only for illustrative purposes and are not limitations on the present invention.

[0033] In the specific implementation process, the present invention provides a precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties. The chemical composition, microstructure and specific preparation technology of the alloy are as follows:

[0034] 1. By weight percentage, the chemical composition of the high-entropy alloy is as follows: Ni: 43% - 52%; Co: 25% - 31%; Cr: 13% - 18%; Al: 2.5% - 3.5%; Ti: 5.5% - 6.5%; C: 0.01% - 0.05%; Zr: 0.01% - 0.05%; B: 0.003% - 0.05%.

[0035] 2. The microstructure characteristics of the high-entropy alloy are described as follows:

[0036] (1) The microstructure of the alloy consists of FCC phase, L12 phase and carbides.

[0037] (2) The FCC phase is the matrix phase with disordered atomic arrangement; the L12 phase is the precipitation strengthening phase with ordered atomic arrangement, and its main component is Ni3(Al,Ti). The carbides are large-sized MC carbides and small-sized M 23 C6 carbides. The main component of the MC carbide is TiC, and the main component of the M 23 C6 carbide is Cr 23 C6.

[0038] (3) Small L12 phases and large L12 phases are distributed along the dendrites and between the dendrites; MC carbides precipitate in the large L12 phase region, and M 23 C6 carbides precipitate discontinuously at the grain boundaries.

[0039] 3. The preparation method of the high-entropy alloy comprises the following steps:

[0040] (1) Pure metals such as Ni, Co, Cr, Al, Ti with a purity greater than 99.95 wt.% are polished, pickled and ultrasonically cleaned with alcohol. Subsequently, the cleaned metal raw materials are dried. Weigh the above-treated metal raw materials and raw materials such as C, Zr, B according to the preset component ratio. The chemical composition of the alloy is shown in Table 1. First, the vacuum chamber is pre-evacuated to a pressure of 3.5×10 -3 Pa, and then argon with a volume purity of 99.999% is filled to make the pressure in the vacuum chamber 4×10 4 Pa. The melting current of the alloy starts from 0 A and is gradually increased to 400 A, and it is melted for 2 minutes. After each melting, the alloy is flipped and melted again, and at least 5 times are repeated to make the alloy fully homogenized. Finally, the current is slowly turned off, and the alloy ingot is naturally cooled in the copper crucible.

[0041] (2) The cooled alloy ingot is sealed in a vacuum quartz tube for solution treatment. The pressure in the vacuum quartz tube is 2×10 -3 Pa, the solution treatment temperature is 1150 °C, the holding time is 2 h, and it is air-cooled to room temperature.

[0042] (3) The solution-treated alloy ingot is put into a heat treatment furnace again for aging treatment. The temperature is 750 °C, the holding time is 16 h, and it is air-cooled to room temperature, and finally a precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties is obtained.

[0043] In the present invention, different treatment stages of the high-entropy alloy are used as control examples. The specific components and densities are shown in Table 1, the heat treatment processes are shown in Table 2, the room-temperature mechanical properties are shown in Table 3, and the high-temperature mechanical properties at 750 °C are shown in Table 4.

[0044] Table 1 Chemical Compositions (wt.%) and Densities of the High-Entropy Alloys in the Embodiments and Comparative Examples of the Present Invention

[0045] Ni Co Cr Al Ti C Zr B <![CDATA[Density (g / cm 3 )]]> 47.145 28 15.21 3.45 6.13 0.02 0.03 0.015 7.83

[0046] Table 2 Heat Treatment Processes of the High-Entropy Alloys in the Embodiments and Comparative Examples of the Present Invention

[0047] Heat treatment process Control Example 1 Without heat treatment Control Example 2 Solution treatment at 1150°C for 2 h → Air cooling Example 1 Solution treatment at 1150°C for 2 h → Air cooling → Aging treatment at 750°C for 16 h → Air cooling

[0048] Table 3 Room-Temperature Mechanical Properties of the High-Entropy Alloys in the Embodiments and Comparative Examples of the Present Invention

[0049]

[0050]

[0051] Table 4 High-Temperature Mechanical Properties of the High-Entropy Alloys in the Embodiments and Comparative Examples of the Present Invention

[0052] Yield strength (MPa) Tensile strength (MPa) Elongation rate (%) Control Example 1 816 866 1.1 Control Example 2 693 796 4.8 Example 1 864 1006 12.9

[0053] As Figure 1 shown, the phase compositions of the comparative examples and embodiments of the present invention are presented, which are the FCC matrix phase and the L12 strengthening phase. Due to the low carbide content, they were not identified by XRD. However, in Figure 2 the microstructure under low magnification of the scanning electron microscope, large-sized MC carbides can be observed to precipitate, with most of them distributed at the grain boundaries and a small part distributed within the grains.

[0054] As Figure 3 shown, the microstructure of the NiCoCrAlTi low-density high-entropy alloy after solution treatment is presented. It can be seen that small-sized L12 phases and large-sized L12 phases are distributed along the dendrites and between the dendrites, forming a heterogeneous structure. MC carbides precipitate between the dendrites and in the regions of the large-sized L12 phases. This is because the inter-dendritic regions are rich in Ti element, and Ti element is an important component element of MC carbides.

[0055] As Figure 4 shown, the microstructure at the grain boundaries of the NiCoCrAlTi low-density high-entropy alloy is presented. It can be seen that no precipitate phases precipitate at the grain boundaries after solution treatment, while fine and dispersed M 23 C6 carbides appear at the grain boundaries after 16 h of aging treatment. After aging treatment, the FCC phase in this high-entropy alloy is the matrix phase with disordered atomic arrangement, and its volume fraction is 44.47%; the L12 phase is the precipitation strengthening phase with ordered atomic arrangement, and its volume fraction is 55.3%; there are two types of carbides in the alloy, namely MC and M 23 C6 carbides. The volume fraction of MC is 0.21%, and the volume fraction of M 23 C6 is 0.02%.

[0056] As Figure 5 shown, the small-sized L12 morphology of the NiCoCrAlTi low-density high-entropy alloy at different processing stages is presented. The L12 phase in the as-cast state is the primary L12 phase with a small size and a small gap between L12 and L12. The L12 phase after solution treatment is the secondary L12 phase, which is larger in size than the primary L12 phase, and has a larger gap between L12 and L12, and a longer dislocation movement channel. After aging treatment at 750 °C for 16 h, the secondary L12 phase further coarsens, but a large number of fine and dispersed tertiary L12 phases appear in the FCC channels. After aging treatment, the high-entropy alloy has small-sized L12 phases and large-sized L12 phases. The size of the small L12 phases is 40 nm to 210 nm, and the size of the large L12 phases is 310 nm to 1.8 μm.

[0057] As Figure 6 shown, the mechanical properties of the NiCoCrAlTi low-density high-entropy alloy at different processing stages are presented. Comparative Example 1 is the high-entropy alloy in the as-cast state. Benefiting from the finely dispersed L12 phases, it achieves a yield strength of 917 MPa and a room-temperature ductility of 9.2% at room temperature; it achieves a high yield strength of 816 MPa and a high tensile strength of 866 MPa at 750 °C, but due to the problem of grain boundary weakening at high temperature, the ductility is only 1.1%. Comparative Example 2 is the high-entropy alloy after solution treatment. Due to the larger-sized secondary L12 phases, the interaction mechanism with dislocations changes from a cutting mechanism to a bypassing mechanism, and the larger spacing between L12 and L12 phases also provides more space for dislocation movement, enabling the continuous slip of dislocations, thereby increasing the ductility of the alloy at room temperature by 27%. The increase in ductility at high temperature benefits from the pinning effect of MC carbides on the grain boundaries at high temperature. Example 1 is the high-entropy alloy after aging treatment. M 23 C6 can pin the grain boundaries and dislocations, and the precipitation of fine and dispersed tertiary L12 phases at the FCC channels will also produce a cutting mechanism with dislocations, which will greatly increase the strength of the alloy at room temperature and high temperature. M 23 C6's pinning of the grain boundaries also greatly improves the ductility of the high-entropy alloy at high temperature, but reduces the ductility of the alloy at room temperature.

[0058] Figure 7 The comparison results of the specific yield strength and ductility of the examples and traditional cast superalloys at 750 °C are shown. It can be seen that the examples of the present invention have better specific yield strength and better high-temperature ductility compared to traditional cast superalloys.

[0059] The implementation results show that the NiCoCrAlTi low-density high-entropy alloy prepared by the present invention through arc melting and appropriate heat treatment processes mainly consists of FCC phase and L12 phase. The synergistic effect of the MC carbide and M 23 C6 carbide precipitated during the heat treatment process and the heterogeneous structure of different sizes of L12 phase has greatly improved the high-temperature mechanical properties of the alloy: a yield strength of 864 MPa, a tensile strength of over 1 GPa, and a ductility of 12.9% are achieved at 750 °C; meanwhile, it has a yield strength of 990 MPa, a tensile strength of 1166 MPa, and a ductility of 8.2% at room temperature, and the specific yield strength and ductility at high temperature are excellent. The excellent properties of the NiCoCrAlTi low-density high-entropy alloy at room temperature and high temperature have greatly improved the application value and application potential of high-entropy alloys in the field of high-temperature structural materials.

[0060] The described embodiments are the preferred embodiments of the present invention, but the present invention is not limited to the above embodiments. Without departing from the essential content of the present invention, any obvious improvements, substitutions or modifications that those skilled in the art can make all fall within the protection scope of the present invention.

Claims

1. A precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties, characterized in that, By weight percentage, the chemical composition of the high-entropy alloy is as follows: Ni: 43% - 52%; Co: 25% - 31%; Cr: 13% - 18%; Al: 2.5% - 3.5%; Ti: 5.5% - 6.5%; C: 0.01% - 0.05%; Zr: 0.01% - 0.05%; B：0.003%~0.05%； The microstructure of the high-entropy alloy consists of FCC phase, L12 phase and carbides; In this high-entropy alloy, the FCC phase is the matrix phase with disordered atomic arrangement and its volume fraction is 40% - 49.5%; the L12 phase is the precipitation strengthening phase with ordered atomic arrangement and its volume fraction is 50% - 59.5%; there are two types of carbides, MC and M 23 C6 in the alloy. The volume fraction of MC is 0.1% - 0.5%, and the volume fraction of M 23 C6 is 0.01% - 0.05%. There are two sizes of L12 phases in this high-entropy alloy. The size range of the small L12 phase is 20 nm to 250 nm, and the size range of the large L12 phase is 300 nm to 2 μm. The small L12 phase and the large L12 phase are distributed along the dendrites and interdendrites; MC carbides precipitate in the large L12 phase region, and M 23 C6 carbides precipitate discontinuously at the grain boundaries.

2. The precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties according to claim 1, wherein, The density of the high-entropy alloy is 7.8 g / cm 3 to 7.9 g / cm 3 .

3. A method for preparing a precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties according to any one of claims 1 to 2, characterized in that, It includes the following steps: (1) Weigh raw materials according to a preset composition ratio and prepare alloy ingots by arc melting; (2) Let the alloy ingots cool naturally in the copper crucible of the arc melting furnace; (3) Seal the cooled alloy ingots into quartz tubes, and obtain a precipitation-strengthened NiCoCrAlTi high-entropy alloy through solution treatment and aging treatment.

4. The preparation method of the precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties according to claim 3, characterized in that, In step (3), the solution treatment temperature is 1150°C ± 20°C, the holding time is 2 - 5 h, and it is air-cooled to room temperature; the aging treatment temperature is 750°C ± 20°C, the holding time is 4 - 24 h, and it is air-cooled to room temperature.

5. The preparation method of the precipitation-strengthened NiCoCrAlTi low-density high-entropy alloy with excellent high-temperature mechanical properties according to claim 4, characterized in that, The performance indexes of the high-entropy alloy are as follows: When tensile tested at room temperature, the yield strength is 950 MPa to 1050 MPa, the tensile strength is 1100 MPa to 1200 MPa, and the elongation is 7% to 15%; when tensile tested at 750°C, the yield strength is 850 MPa to 900 MPa, the tensile strength is 950 MPa to 1050 MPa, and the elongation is 12% to 15%.

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