Effects Of Rolling At Medium Temperature And Microalloying On Microstructure And Properties Of CoCrFeNi High Entropy Alloy

The effects of room temperature and medium temperature rolling and addition of Al-10Er（10wt.%Er）master alloy on the microstructure and mechanical properties of CoCrFeNi high entropy alloy were studied.CoCrFeNi（Al-10Er）x high entropy alloys（x=0,0.01,0.05 and 0.1）were prepared by vacuum arc melting.CoCrFeNi high entropy alloy（x=0）were rolled at room temperature,500℃,650℃and 800℃with the reduction deformation of 10%,40%and 70%.The rolling of CoCrFeNi（Al-10Er）_0.05 high entropy alloy was at 650℃and 800℃with 50%deformation.The microstructure and mechanical properties of the prepared alloys before and after rolling were analyzed by X-ray diffraction（XRD）,electronic universal testing machine,metallographic microscope,scanning electron microscope（SEM）,electron backscatter diffraction（EBSD）and transmission electron microscope（TEM）.The strengthening mechanism of CoCrFeNi（Al-10Er）x high entropy alloys under different rolling conditions and the optimal rolling process parameters were obtained.The main research contents are as follows:The effects of room temperature and medium temperature rolling on Microstructure and mechanical properties of CoCrFeNi high entropy alloy were studied.The results show that:The rolling temperature and deformation degree has no influence on the structure of CoCrFeNi high entropy alloy,keeping face centered cubic structure.The prepared CoCrFeNi high entropy alloy has good phase-structure stability.Compared with room temperature rolling,the density of slip bands decreases and the degree of grain deformation increases.For room temperature rolling of the prepared CoCrFeNi high entropy alloy,the deformation mechanism is thought to be violent cross slipping of dislocations,and hence the working hardening effect is great.When rolling at500℃and 650℃,the softening mechanism of the prepared alloy is mainly dynamic recovery.However,when rolling at 800℃,it is mainly dynamic recovery-recrystallization.When rolling is at 800℃with 70%of deformation,a large number of fine equiaxed grains are produced and a small amount of twins are observed,suggesting that obvious recovery-recrystallization has occurred during rolling.At this state,the mechanical properties of the alloy after rolling are excellent:yield strength ultimate tensile strength and the elongation after fracture are 700MPa,734MPa and 22.3%,respectively.The effect of Microalloying on Microstructure and mechanical properties of CoCrFeNi high entropy alloy was studied.Trace addition of Al-10Er master alloy in CoCrFeNi alloy does not change the structure of the base alloy.When x=0.05,the mechanical properties of this alloy are better than that of the other,with yield strength ultimate tensile strength and the elongation after fracture of 192MPa,434Mpa and 54.2%respectively.Fracto-graphs observation on CoCrFeNi（Al-10Er）x high entropy alloys after tensile test indicate that the fracture mode is ductile when x=0,0.01 and 0.05.However,when x=0.1,the fracture becomes ductile-brittle mode.Compression test reveals that the homogenized CoCrFeNi（Al-10Er）x high entropy alloys have much excellent compression deformation performances:the compression ratio limit is about 83%,and the true compressive strength above 2.4GPa with more than 1.7 true strain.For CoCrFeNi（Al-10Er）_0.05 high entropy alloy after rolling at 800℃ with 50%deformation,besides the cross slipping of dislocations,there are dynamic recovery-recrystallization（incomplete recrystallization）occurring and annealing twins forming in nano scale,which jointly depend on the strength and plasticity:yield strength ultimate tensile strength and the elongation after fracture are 652MPa,678MPa and 21.8%,respectively.The significant increase in the volume fraction of recrystallization grains strongly indicates that the trace addition of Al-10Er can greatly promote the recovery-recrystallization during medium temperature rolling.