Effect Of Rare Earth On As-cast And Aging Microstructure Of FeMnCrNiCo High-entropy Alloy

High-entropy alloys(HEAs)take up five or more than five principal elements in equiatomic or close-to-equiatomic ratios.Simple FCC,BCC or HCP solid solution phase tend to be formed in HEAs due to high entropy effect.In addition,nanoprecipitates and amorphous phase might form in the matrix under some heat treatment conditions.Therefore,it usually has higher strength and better thermal stability and corrosion resistance than traditional alloys.FeMnCrNiCo high-entropy alloy is FCC solid solution phase,with outstanding plasticity and toughness,but its strength is not high.In this thesis,we mainly study the effect of La on the as-cast and aging microstructure of FeMnCrNiCo high-entropy alloy,which carry out basic exploration for the application of rare earth elements in high-entropy alloys.First,FeMnCrNiCo and FeMnCrNiCoLa high-entropy alloys(in which the mass percentage of La is 9.3%)was prepared by medium frequency induction furnace.The influence of La on the microstructure of FeMnCrNiCo high-entropy alloy was analyzed by SEM,XDR and EDS.The results showed that with the addition of La,the structure of the FeMnCrNiCo HEA changed from single FCC phase to FCC phase and LaNi phase,and the brittle LaNi phase distributed continuously in the interdendrite.In addition,the grain was refined,the hardness also increased from 112 HV to178HV.In addition,in order to eliminate the adverse effect of the large brittle LaNi phase on the properties of the alloy,this study performed remelting and rapid solidification of the alloy.The results indicated that the rapid solidification quickly avoided the crystallization temperature range of the LaNi phase,so that the La was retained to room temperature in the form of supersaturation.Lastly,in order to make supersaturated La in the alloy after rapid solidification treatment be reprecipitated,we performed aging treatments on the alloy with different parameters.The results showed that the fine LaNi phase was precipitated after aging treatment at 400?.With the increase of aging temperature,the LaNi phaseincreased and dispersed in the matrix.After aging at 700? and higher temperature,the number and volume of LaNi phase had no abvious changes.It is found that the LaNi phase is the finest and the hardness reaches a peak value after aging at 600? 60 min.After analysis,we conclude that the precipitation of LaNi phase is due to the fact that the aging treatment intensifies the diffusion of atoms,thereby promoting the dissolution of supersaturated solid solution elements.In addition,since the mixing enthalpy of La and Ni is the lowest,they are easily combined to form compounds.