Microstructure Control And Mechanical Behavior Research Of Dual-Phase High Entropy Alloy

In recent years,high entropy alloys?HEAs?have become one of the research hotspots in the field of metal materials because of novel design concept,broad design space and unique performance characteristics.To date,400 kinds of high entropy alloys with virious properties have been successfully designed,however,most of them are difficult to achieve an optimal combination of strength and plasticity.Designing the dual-phase HEAs combining soft and hard phases is one of the typical methods to obtain the HEAs with well match of strength and plasticity,and one of the key points of this method is how to effectively control the microstructure of two phase taking fully into account the characteristics of each phase in alloy.In this paper,the Al_0.6CoCrFeNi dual-phase HEA?FCC+BCC?was prepared,the solidification control and thermo-mechanical treatment were used to improve the microstructure of alloy.The microstructure evolution regulation of alloy was analyzed,the process conditions to improve microstructure were obtained,and the mechanical properties of alloy were improved.The main conclusions are summarized as follows:The different diameters and casting methods were adjusted to regulate the cooling rate for the microstructure refinement of alloys.With an increase in the cooling rate,the x range of the Al_xCoCrFeNi?x is the molar ratio,0.4?x?1.0?HEAs with dual-phase structure was reduced,the grain size of alloys was more refined,and the segregation degree of alloys was reduced,thereby the hardness and yield strength of alloys were improved.In addition,with an increase in the cooling rate,the structure of the Al_0.6CoCrFeNi HEA changed from the dual-phase to single BCC phase structure.The microstructure of alloy transformed from columnar dendritic to lamellar,and then to equiaxed crystal microstructure.Both the hardness and yield strength of alloy increased correspondingly.The thermo-mechanical treatment was further carried out on the Al_0.6CoCrFeNi dual-phase HEA for improving the microstructure of the Al_0.6CoCrFeNi HEA.The in situ thermal field study of Al_0.6CoCrFeNi dual-phase HEA shows that,with an increase in temperature,the BCC phase gradually transformed to the FCC phase?880K?,then the B2phase precipitated form the FCC phase?950K?,after that the B2 phase completely dissolved in the FCC phase?1473K?.Meantime,the volume fractions of the FCC and BCC phases reached to a stable value,80%and 20%,respectively.Based on the above results,the heat treatment temperature was determine by the temperature at which the B2 phase?which is harmful to the plasticity of alloy?dissolved in the FCC phase.The homogenization treatment?1473K,24h?could reducs the lattice strain caused by the compoment segregation by an order of magnitude.Then the cold/hot deformation with a50%deformation degree followed by an annealing treatment were carried out.It was found that after annealing at 1473 K?0.9T_m?for 20 h,both the FCC and BCC phases in alloy completely recrystallized,and the mechanical properties performace of alloy was improved.Compared with the cast alloy,the tensile fracture strain of the annealed alloy increased by67%.Therefore,it is considered that the high temperature and the long annealing time is necessary for the formation of recrystallized equiaxed microstructure and the improvement of plasticity of the Al_0.6CoCrFeNi dual-phase HEA.A reversible deformation-induced martensitic transformation was found occurring in Al_0.6CoCrFeNi dual-phase HEA for the first time.During deformation,the BCC phase transformed to the martensitic phase with nano-scale and orthorhombic structure.The transformation preferentially occurred in the grains with an orientation of B-[001]//loading direction and B-[110]//transverse direction.During deformation,the FCC phase yielded prior to the BCC phase,and the BCC phase bore more stress in the plastic deformation stage despite having less volume fraction.Designing the HEAs with deformation-induced recoverable martensitic transformation characteristics could be used as a helpful way to further enhance the mechanical performance of HEAs.