Effect Of Cold Rolling And Annealing On Microstructure And Mechanical Properties Of CoCrFeNiMnC_X High Entropy Alloy

High entropy alloys(HEAs)with single face centered cubic(FCC)structure possessmany advantages,such as single phase composition,thermodynamic stability and excellent mechanical properties,which have great promise for engineering applications.However,the strength and hardness of the HEAs with FCC structure are relatively low,which greatly limits its wide application in engineering field.The common treatment methods-cold working treatment-can effectively improve the strength of the alloy,but at the same time,it will deteriorate its plasticity,and the reversion and recrystallisation will be occur at high temperature,resulting in changes in the mechanical properties of the alloy.Therefore,it is of great significance to seek effective strengthening means to improving their recrystallization resistance while strengthening.In this study,the strategy of interstitial carbon doping combined with cold rolling and heat treatment is employed to modulate the size and distribution of grains and second phases in the alloy,increase the strength without significantly reducing the plasticity of the alloy,improve the overall mechanical properties and recrystallization resistance of the alloy,and reveal its mechanisms of strengthening and reversion and recrystallization,CoCrFeNiMnCx(x=0,0.5,1.0,1.5at.%)HEA was prepared by non consumable vacuum arc furnace.X-ray Diffraction(XRD),Scanning Electron Microscope(SEM),Transmission Electron Microscope(TEM),Energy Dispersive Spectroscopy(EDS),Vickers hardness tester and universal electroni tester were used to characterize the materials.The effects of cold rolling and annealing on the microstructure and mechanical properties of CoCrFeNiMnCxHEA were investigated.The main conclusions of the study are as follows:(1)CoCrFeNiMnCxhigh entropy alloy shows typical dendritic structure and single FCC structure;After the alloy was homogenized at 1100℃/1h,the dendrites disappeared and equiaxed crystals were formed,and the phase did not change.(2)Cold rolling will not cause phase transformation of CoCrFeNiMnCxalloy,but cellular substructure will be formed in the alloy crystal.With the increase of cold rolling reductions,the size and quantity of substructure will decrease and increase,respectively.Especially after 90%cold rolling,nanocrystalline structure is formed in the alloy.(3)CoCrFeNiMn-1at.%The yield strength of C alloy increases from 210 MPa in homogenized state to 1670 MPa in 90%cold rolled state,which is about 7 times higher.The relationship between the increment of alloy hardness and yield strength and the true strain of cold rolling meets the Hollomon relationship.(4)90%of the cold-rolled state CoCrFeNiMnCxdid not produce phase transformation after annealing treatment at(575℃,615℃and 700℃),but different degrees of reversion and recrystallisation occurred.Among them,the percentage of recrystallised grains was 31.3%,74.6%and 90%for the alloy without C doping after annealing at 575℃,615℃and 700℃for 1h,respectively,while the percentage of recrystallised grains was 5.6%,21.7%and 78.1%for the high entropy alloy with1at.%C doping,respectively.The doping of 1at.%C increased the recrystallisation temperature of the alloy by approximately 85℃.(5)The interstitial carbon atoms are dissolved in the alloy.The initial recovery stage of carbon free alloy is mainly related to vacancy migration and the recovery activation energy is about 122.5k J/mol.Due to the migration of vacancy and interstitial C atoms the main recovery activation energy is 177k J/mol with doping of1at%C.The recovery activation energy of the two alloys in the middle and late recovery stage is equivalent to the self diffusion activation energy of the replacement atoms in the alloys,and the recovery mechanism may be mainly dislocation climbing.