Effect Of Al And Mn On Microstructure And Mechanical Properties Of CoCrFeNiTi High-entropy Alloy System

Multi-component high-entropy alloys (HEAs), obtaining a breakthrough of traditional single principal element alloy design patterns, is a new alloy system composed of five or more elements in equimolar or almost equimolar ratio.According to regular solution approach of a solid alloy solution, the system entropy of mixing becomes higher with increasing number of principal elements, which favors the formation of stable multi-element solid solution rather than multiple intermetallic compounds.Thus, it has received worldwide extensive attention in the field of metal material, since the HEAs concept was put forward.Currently, HEAs are still in the research stage, not widely used in the production yet. Previous studies indicated that most HEAs possess high hardness and strength, wear resistance, corrosion resistance and high temperature oxidation resistance and other excellent mechanical and physical and chemical properties by optimum design alloy composition.In this paper,the preparation, microstructure and mechanical properties of new alloys based on Co, Cr, Fe, Ni-based HEAs were studied by the methods of Al, Ti, Mn and C four elements addition.The HEAs ingots of AlxCo1.5CrFeNi1.5Tiy (x=0,0.2; y=0.5,1.0) multi-component HEAs were prepared by vacuum arc furnace under a Zr-gettered high-purity argon atmosphere,while those of AlxC0.1CoCrFeNiTi0.5Mny (x=0,0.2;y=0.1,0.2) multi-component HEAs were prepared by intermediate frequency induction melting furnace with EMS under a high-purity argon atmosphere. X-ray diffractometer (XRD, Shimadzu XRD-6000) with Cu Ka radiation, scanning electron microscope with energy dispersive spectrometry (SEM, Zeiss supra55), compressive properties tester, tensile properties tester and Vickers microhardness tester model (MH-60) were used characterize the microstructure and mechanical properties of the alloys.The experimental results are as follows:Four AlxCo1.5CrFeN1.5Tiy (x=0,0.2; y=0.5,1.0) high entropy alloys maintained the crystal structure of FCC. Both A00Ti05 and A02Ti05 alloys showed a distinct two-phase structure, While AOOTi10 and A02Ti10 alloys have a dendrite-interdendrite structure. Intergranular precipitate of four alloys was the hard η,-(Co+Ni)3Ti phase having a HCP crystal structure. It was found that the hardness of AlxCo1.5CrFeNi1.5Tiy (x=0,0.2;y=0.5,1.0) alloys enhanced significantly with a increasing element content of both Al and Ti. Wherein, the A02Ti10 alloy obtained the highest microhardness value of 671.6HV. A00Ti05 and A02Ti05 alloys exhibited more excellent ductility than AOOTi10 and A02Ti10 alloys. Among all the four HEAs, A02Ti05 alloy reflected the best compression performance, possessing the maximum deformation of 43.5% and a breaking strength reaching 2740MPA.Four AlxC0.1CoCrFeNiTi0.5Mny (x=0,0.2;y=0,0.2) high entropy alloys maintained FCC FCC crystal structure. Polygonal CTi nanoscale particles evenly dispersed in the grain boundaries of four kinds of alloys. Dendrite gaps of AOMO and A0M2 alloys constituted Ni, Ti-rich phase and Ti,Co-rich phase,while that of A2M0 alloy was composed fo Fe, Cr-rich phase and needle η-(Co+Ni)3Ti phase. Differently, the dendrite gap of A2M2 alloy showed two forms, one constituted of Fe, Cr-rich phase and needle η-(Co+Ni)3Ti phase and the other composed fo Fe, Cr-rich phase and spinodal decomposition organized system. Besides, as the result of the addition of Mn,the elastic modulus of AOMO and A0M2 alloys demonstrated a substantial increase by reaching 6.9 GPA and 6.4 GPA, respectively.With the addition of Al, the hardness of A2M0 and A2M2 alloys alloys increased slightly by reaching 543.2HV and 532.3HV. However, the decreases of the fracture strength and elastic modulus caused by the addition of Al also led to a poor deformation capacity of the alloys.Among the four HEAs, had relatively fine deformation capacity, the breaking strength of AOMO and A0M2 alloys reached 638.3MPA and 608.3 MPA and the maximum tensile strain of those attained 11.8% and 12.9%, respectively,which displayed relatively fine deformation capacity.