Microstructure,Strengthening Mechanisms And Mechanical Properties Of CrMnFeCoNi-based High Entropy Alloys

The design concept of high-entropy alloys,bright a new trend and development prospect of structural materials,especially the alloys with high strength,high hardness and excellent properties at both room and elevated temperatures,that they have not only high academic value in the field of scientific research,but also broad prospect in industrial production and engineering application.High-entropy alloys,as a new alloy system,although receiving wide attention and research achievements in recent years by scholars both at home and abroad,the study of high-entropy alloys just began,compared with traditional alloys.Both theoretical research and experimental results are relatively few.The acknowledgement about the alloying mechanism,deformation behavior as well as the basic scientific problems are limited.In this paper,CrMnFeCoNi alloy with outstanding ductility but low strength was selected as baseline alloy to study the formation of hard phase to strengthen this ductile alloy by adding Al or Cu element into it.The results indicate that there is a phase structure transition from FCC to BCC when adding enough Al into CrMnFeCoNi.The yield strength and hardness increase with increasing the content of Al,due to the formation of hard BCC phase in the ductile CrMnFeCoNi alloy.An example of tailoring the strength and ductility of CrMnFeCoNiAl_0.75.75 high-entropy alloy is demonstrated in this paper.In addition,the CrMnFeCoNiAl_x alloy exhibit excellent corrosion resistance and oxidation resistance,due to the formation of NiAl phase and Al₂O₃ oxide film.The effects of Cu addition on the phase change and mechanical properties of CrMnFeCoNiCu_x high-entropy alloy were investigated.The results indicated that there was an evolution of phase structure from FCC1 to FCC2 with the increase of Cu content.The FCC2 was identified to be（Cu,Mn）-rich phase.In the alloys with high amount of Cu,Cu-depleted dendrites and（Cu,Mn）-rich inter-dendrites was observed,due to the positive mixing enthalpy between Cu and other composition elements.The yield strength and hardness increased with increasing Cu content.The yield strength increased from 188.04 MPa for Cu0 to 350.63 MPa for Cu1 alloy.The high strength of CrMnFeCoNiCu alloy was attributed to the uniformly dispersed nano-size Cu-rich precipitates in the matrix,which impeded effectively the dislocation motion during alloy deformation.The Cu-added CrMnFeCoNiCu_x alloys also showed excellent strain hardening ability during compression test.The as-cast high-entropy alloys were not always in stable state.Phase structure,phase composition and microstructure would change after aging treatment.the aging alloys CrMnFeCoNiAl_x and CrMnFeCoNiCu_x,precipitated Cr-rich phase,NiAl phase and CrMn phase,respectively.The mechanical properties were improved largely,on account of the second phase strengthening.In this paper,the relationships between phase structure and high-temperature properties were also studied.The results shown that the yield strength of high-entropy alloys rose to a critical value as the improving temperature,and then it reduced due to the pronounced thermal softening.For example,the yield strength of CrMnFeCoNiAl alloy at room temperature,400 ^oC and 600 ^oC were 852.09 MPa,989.76 MPa and723.27 MPa,respectively.However,the dual-phase CrMnFeCoNiAl_0.75.75 alloy shown an excellent combination of strength and ductility at both room and elevated temperature.It showed a 1.070 GPa yield strength and 15.30%ductility at 500 ^o C,mainly attributed to the superalloy-like microstructure,which was characterized by cuboidal B2 precipitates coherently embedded in the FCC matrix.