Study On Microstructure And Properties Of Carbide Reinforced NiCoCrAl High Entropy Alloy Matrix Composites

High entropy alloys（HEAs）have great potential for structural materials,functional materials,high temperature coatings and the nuclear industry due to their excellent strength,hardness,corrosion resistance,thermal stability and irradiation resistance.At present,introducing suitable reinforcing phase into the HEAs can form high entropy alloy matrix composites（HEAMCs）,which has become an effective method to further improve the mechanical properties of HEAs.In this paper,the Ni Co Cr Al eutectic high entropy alloy（EHEA）has been designed by combining machine learning and CALPHAD methods,an newly designed EHEA was used as the matrix,WC reinforced Ni Co Cr Al HEA composites were prepared by arc melting and mechanical alloying（MA）+fast hot press sintering（FHPS）,the phase,microstructure and mechanical properties of the composites under different preparation processes were studied,the formation and strengthening mechanism of carbides in the composites were clarified,and the influence of heat treatment on the microstructure and properties of as-cast composites was investigated.Basically,the following conclusion can be drowned:（1）Based on literature data and thermodynamic calculations results,combining machine learning and CALPHAD method was a reliable means of designing the composition of EHEA.Through this method,new Ni Co Cr Al EHEA consisting of FCC（L1₂）and BCC（B2）phases has been successfully designed.FCC（L1₂）phases was enriched in Co and Cr,and BCC（B2）phases was enriched in Ni and Al.（2）WC/Ni₄₉Co₁₆Cr₁₆Al₁₉ composites prepared by arc melting and MA+FHPS consisted of four phases:FCC phase,BCC phase,W-rich carbide,and Cr-rich carbide.The main sources of carbides in composites include:（1）added WC particles;（2）diffusion and in-situ formation of W,C,and other elements;（3）direct reaction of WC particles with matrix elements.When the WC content was up to 15 wt.%,the carbides of as-cast composites appeared clustering phenomenon,while the carbides of as-MA+FHPSed composites were diffusely distributed.（3）The addition of WC effectively increased the hardness and strength of the Ni₄₉Co₁₆Cr₁₆Al₁₉ HEA,which was mainly attributed to the interaction of strengthening mechanisms,such as second-phase strengthening,solid solution strengthening,dislocation strengthening and thermal mismatch strengthening.Compared with as-cast composites,the MA+FHPS composites with the same WC content have higher hardness and compressive yield strength.The 15WC/Ni₄₉Co₁₆Cr₁₆Al₁₉ composite prepared by MA+FHPS had the highest hardness and compressive yield strength of 904.6HV₃ and 3049MPa respectively,but the worst plasticity.（4）After heat treatment,a transition from the BCC phase to FCC phase was observed in the as-cast WC/Ni₄₉Co₁₆Cr₁₆Al₁₉ composites,which was mainly the result of the diffusion of W and other elements.In addition,the hardness and compressive yield strength of as-cast composites were further increased due to the precipitation of a large number of secondary carbides,but the plasticity was reduced.This paper contains 58 figures,10 tables and 123 references.