Microstructure Evolution And Mechanical Properties Of CoFeNiV(Mo,Nb) High Entropy Alloys

High entropy alloys (HEAs) are mainly consisted of at least five principal elements with 5-35at.% concentrations for each element. This novel alloy design concept breaks through the conventional alloy design methods which are based on one or two principal elements, and also brodens the research field of metallic materials. Because of the unique multiprincipal element composition, HEAs can possess special properties. These include high strength, high hardness, outstanding wear resistance and exceptional thermal stability, making HEAs promising in academic research and industrial application.According to the characteristics of solid solution and Hume-Rothery rules, the parameters of atomic size difference δ, mixing enthalpy ΔHmix and valence electron concentration VEC were proposed to predicate the formation of pure single solid solution structured HEAs. Through systematically summarizing the relationship between the crystal structures and the parameters of δ ΔHmix、VEC of representative HEAs, the model was developed. Based on this, the CoFeNi0V0.5 alloy with single solid solution was designed. Further, considering the d-orbit energy level model, Mo and Nb elements with high Md value were chosen to strengthen the alloy through forming intermetallics. In addition, the deformation mechanisms of CoFeNi2V0.5Mo0.2 and CoFeNiV0.5Nb0.2 alloy with excellent integrated mechanical properties were investigated by synchrotron radiation in-situ tensile tests、strain field distribution tests and TEM analyses. Heat treatment experiments were conducted to explore the thermal stability of CoFeNi2V0.5Nb0.75 alloy with eutectic microstructure; Laser cladding method was adopted to prepare HEA coatings from CoFeNi2V0.5Nb0.75 and CoFeNi2V0.5Nb compositions with excellent wear resistance. The properties of the substrate and coatings were also investigated.Main conclusions can be drawn and listed as follows:(1) According to the characteristics of solid solution and Hume-Rothery rules, the parameters of atomic size difference 8, mixing enthalpy ΔHmix and valence electron concentration VEC were proposed to predicate the formation model of single solid solution structured HEAs. The single-phase FCC solid solution tends to form on condition of 8< 3.75%,-7.27kJ·mol-1<ΔHmix<4kJ·mol-1 and VEC> 8, while the single-phase BCC solid solution prefers to form in the case of δ< 3.75%,-7.27kJ·mol-1<ΔHmix<4kJ·mol-1 and VEC <6.87.(2) Based on the model of pure single solid solution, a CoFeNi2V0.5 HEA with single FCC solid solution was designed. And considering the d-orbit energy level model, Mo and Nb elements with high Md value were chosen to strengthen the alloy through forming intermetallics. Further, the effects of Mo and Nb contents on microstructure and properties were also investigated. Results showed that with increasing Mo or Nb contents, the microstructure variation tendency was identical. Both transformed from single-phase FCC solid solution, to hypoeutectic microstructure, then to fully eutectic microstructure, and finally to hypereutectic microstructure.(3) The optimized CoFeNi2V0.5Mo0.2 alloy showed single FCC solid solution, and high tensile strength of 528.1MPa, excellent elongation of 72.8%. During solidification, electromagnetic stirring (EMS) was applied to investigate its effects on microstructure and properties. It was found that the crystal grains refined after EMS, and the tensile strength increased to 547.0MPa, the elongation increased to 82.1%. In addition, the CoFeNi2V0.5Mo0.2 alloy exhibited excellent combination of strength and ductility in the temperature range of-196-800 ℃, accompanying with homogeneous plastic deformation without necking.(4) The strain field distribution tests were conducted to analyze the deformation mechanism of CoFeNi2V0.5Mo0.2HEA. It was found that the alloy exhibited alternate strain field during tensile deformation. Analysis showed that the lattice distortion in the HEAs contributed to the alternate deformation mode, and effectively dispersed the strain localization, which relieved the stress of the areas with lattice defects. As a result, the onset of necking was avoided and the ductility was improved.(5) The optimized CoFeNi2V0.5Nb0.2 alloy showed excellent integrated mechanical properties in the temperature range of -196-700℃. TEM analysis of the interrupted tensile samples indicated that the deformation mechanism of the present CoFeNi2V0.5Nb0.2 HEA relied on dislocation planar slip. Complex dislocation configurations, e.g., Taylor lattices, HDDWs and Lomer-Cottrell locks, increased the resistance to dislocation movement and improved the strength of the CoFeNi2V0.5Nb0.2 HEA. While the high work hardening capability postponed the onset of necking instability and increased the ductility.(6) The optimized CoFeNi2Vo.5Nbo.75 alloy showed fully eutectic microstructure, high hardness of 640.1HV and excellent thermal stability. It was found that the alloy had best integrated properties after quenched at 800 ℃, with fracture strength of 2586.8MPa, yield strength of 2075.2MPa, plastic strain of 16.7%, and Vickers hardness of 640.7HV.(7) The HEA coatings based on the compositions of CoFeNi2Vo.5Nbo.75 and CoFeNi2V0.5Nb with excellent wear resistance were prepared by laser cladding method. Results showed that the wear resistance of the HEA coatings was twice higher than that of the SUS304 steel.