Microstructure And Mechanical Properties Of L1₂/B2-type High-entropy Intermetallics

High-entropy intermetallic compounds are a new type of intermetallic compounds which formed by applying the multi-principal component design idea of high-entropy alloys to intermetallic compounds.In this paper,the related parameters of L1₂/B2 high-entropy intermetallic compound model alloys have been calculated theoretically,and the interrelationship between the parameters has been analyzed.The microstructures and properties of five as-cast and annealed high-entropy intermetallic compound alloys have been studied.At the same time,the effect of heat treatment on the microstructures of alloys has been studied.The atomic radius difference???,enthalpy of _mixing,valence electron concentration and configuration entropy of 27 kinds of high-entropy intermetallic compound model alloys have been calculated and analyzed,and the correlation among the HEI related characterization parameters has been obtained.When the valence electron concentration?VEC?>7.6,the crystal structure of the alloy is body-centered cubic FCC/L1₂ structure,and when VEC<7.6,the crystal structure of the alloy is face-centered cubic BCC/B2structure.The VEC,electronegativity and configuration entropy of different HEI alloys are different.The VEC value and electronegativity of HEI-II alloys are higher than that of HEI-III alloys,but the configuration entropy is lower than that of HEI-III alloys.The distribution of?-?and?G-?is affected by the content of Si in the alloy.It is also noted that for high-entropy intermetallic compounds containing Si,there may be a negative linear correlation between?G and?.According to the theoretical calculation results,five typical components were selected and named HEI1,HEI2,HEI3,HEI4 and HEI5 alloys.The alloy ingots were prepared by induction melting.In this paper,structure and properties of as-cast high-entropy intermetallics have been studied.HEI1 and HEI3 belong to type B2 high-entropy intermetallic compounds.HEI2,HEI4 and HEI5 belong to L1₂ type high entropy intermetallic compounds.HEI1alloy is composed of BCC/B2 matrix and precipitated phase of FCC/L1₂ structure.HEI3alloy is composed of B2 matrix phase and dense hexagonal structure of?-Co solid solution.HEI2,HEI4 and HEI5 alloys are composed of matrix phase and precipitated phase?G phase?of FCC/L1₂.The compressive yield strength of the five alloys shows the rule of B2 structure>L1₂ structure.Tensile tests show that the plasticity of L1₂ structural alloys is slightly better than that of B2 structural alloys?HEI1 and HEI3?at room temperature.However,the plasticity of B2 structural alloys increased significantly at850?,while that of L1₂ structural alloys did not change significantly.The ductile-brittle transition temperatures of HEI1 and HEI3 alloys are 750?-850?.Among the five alloys,HEI5 alloy has higher strength and shows medium temperature brittleness at 750?.After homogenization annealing,rod-like white precipitates were uniformly precipitated in the grain of HEI1 alloy.Fine needle-like white phases in HEI3 alloy gradually dissolve and coarsen during annealing,forming rod-like or strip-like white precipitates.In addition,a small amount of G phase with FCC structure is produced.The phase composition of HEI2 and HEI4 alloys has no obvious change compared with that before annealing.G phase and Al-rich phase appeared in HEI5 alloy after annealing.The compressive yield strength of annealed alloy shows the rule of B2 structure>L1₂structure.Tensile tests show that HEI1 alloy exhibits obvious cleavage fracture characteristics at room temperature,but excellent plasticity at high temperature.HEI3alloy exhibits quasi-cleavage brittle fracture at room temperature and 750?.The plasticity increases remarkably at 850?.HEI5 alloy has good strength-plasticity combination at room temperature.Yield strength is 434 MPa,and tensile strength is 705MPa and elongation is about 1.5%.The elongation decreases to 0 at 750?.The ductility of the alloy increases slightly at 850?,and the elongation is about 3%.After homogenization annealing,the precipitation phase changes in HEI1,HEI3 and HEI5 alloys are obvious.The compressive yield strength of annealed alloy is B2 structural alloy>L1₂ structural alloy.Tensile tests show that HEI1 alloy exhibits obvious cleavage fracture characteristics at room temperature.After annealing,the room temperature tensile strength of HEI1 alloy increases greatly to 657 MPa,and the ductile-brittle transition temperature decreases to below 750?.HEI3 alloy exhibits brittleness at room temperature and 750?,and its plasticity increases significantly at 850?.The yield strength of HEI5 alloy at room temperature is 434 MPa,the tensile strength is 705 MPa and the elongation is about 1.5%.The alloy exhibits medium temperature brittleness at750?,and the elongation decreases to 0.At 850?,the elongation increases to 3%.Finally,the effect of heat treatment on the microstructure of HEI5 alloy was studied.The initial dissolution temperature of L1₂ ordered phase in HEI5 alloy was measured by DSC at 1034?.The initial melting temperature of the alloy is 1188.7?.Water quenching shows that HEI5 alloy is composed of Al-rich phase,G-rich phase,matrix phase and reticulated chain precipitate phase at 1150?-1175?.After solid solution treatment at1150,1175 and 1200?,it was found that with the increase of solid solution temperature or time,Al-rich eutectic precipitates gradually spheroidized and their size gradually increased.The initial melting of low melting point phase occurs at grain boundary at1200?,and the dispersed network-like precipitates disappear in the matrix.After solution treatment at 1175?/2h/WC,HEI5 alloy aged at 900?for 4h,8h,16h and 24h,respectively.It was found that there was an approximate linear relationship between the size of L1₂ ordered phase and aging time.At the same time,both of them accorded with Ostwald's law,and the volume fraction of L1₂ ordered phase increased.There are not obvious change about the properties of HEI5 alloy after aging treatment.