Effect Of Al+Ni And Al+Ti On Microstructure Of The FeCoNiCrAl Based High Entropy Alloys

In this paper,the effects of（Al+Ni）and（Al+Ti）mole fraction on the microstructure of FeCoNiCrAl high entropy alloys in as cast and heat treated states were investigated.The mixing enthalpy,the mixing entropy,the atomic radius difference,the electron negative and the electron vacancy concentration were used to calculated the phase stability of FeCoNiCrAl based high entropy alloys.Then the experiment was used to validate the effect of（Al+Ni）and（Al+Ti）mole fraction on the as cast and heat treatment of FeCoNiCrAl based high entropy alloy.The mixing enthalpy,entropy of mixing,atomic radius difference,electronegativity,the electron vacancy concentration are employed to calculated the phase stability of FeCoNiCrAl based high entropy alloy The results show that FeCoNiCrAl high entropy alloys exist in solid solution state if the Al/Ni is the ratio of 1:3 and 1:4 as well as the mole fraction is 30% of Al+Ni,while the solid solution and compound coexist in others.Among the FeCoNiCrMo_0.1Al_xTi_y（x+y=0.5,y=0.1,0.2,0.3,0.4）alloys,the only FeCoNiCrMo_0.1Al_0.4Ti_0.1alloy present the solid solution structure,other alloys include intermetallic compounds（I）and solid solution（s）.It is found that when Al /Ni is the ratio of 1:4,dendrites form in the FeCoNiCrAl high entropy alloys with（Al+Ni）mole fraction from 30-60%.Dendrites and interdendritic regions have no obvious segregation,forming only FCC crystal structure.With（Al+Ni）increasing,Ni and Al is more rich in the interdendritic.Eutectic type microstructure form in the FeCoNiCrAl alloy when Al/Ni is the ratio of 1:2 and（Al+Ni）mole fraction is 50%,phase transform from FCC structure to FCC+BCC structure.The（Al+Ni）mole fraction has a greater impact on the microstructure of FeCoNiCrAl alloy in the heat treatment state.（Al+Ni）content also has important on the microstructure of FeCoNiCrAl alloy in the heat treatment state,when（Al+Ni）mole fraction is 30%-40%,the morphology of dendrite form andand the precipitated phases enriched（Ni,Al）appear at different aging temperatures in the dendritic at different aging temperature and no enrichment（Fe,Cr）particles precipitate in the interdendritic.When（Al+Ni）the mole fraction is 50%,side plate structure form and no spinodal decomposition and particle precipitation appear in different aging temperatures.When（Al+Ni）the mole fraction is 60%,NiAl is the matrix where（Fe,Cr）enriched particles precipitate at different aging temperatures.The effect of Mo on the microstructure of FeCoNiCrAl_0.3 high entropy alloy was investigated.The results showed that the FeCoNiCrMo_0.1Al_0.3 alloy Mo mainly in solid solution formation exist in the alloy,and along with the increase of Mo content,（Cr,Mo）（Ni,Co）type sigma phase precipitate in the interdendritic of the FCC matrix.with the increase of Mo content,a lot of sigma phase at the grain boundaries exhibit stripe.The dendrite structure of FeCoNiCrMo_0.1Al_xTi_y（x+y=0.5,x=0.1,0.2,0.3,0.4）alloy with different Al+Ti content in as cast condition was studied.No segregation appears between the dendrite and the dendrite with less Ti content in the FeCoNiCrMo_0.1Al_0.4Ti_0.1 alloy.However,with the increase of Ti content,the content of Ti in the interdendrite is more and more concentrated than dendrite.γ phase precipitate from the FeCoNiCrMo_0.1Al_xTi_y alloy with the different content of Al+Ti at 1050 ℃ solid solution for 8 hours and 750°C aged 24 hours.When the aging temperature is 850 ℃,the precipitates gradually grow up,and with the increase of Ti content,the coarsening effect becomes more and more obvious.When the aging temperature is 950 ℃,completely dissolved temperature of γ phase reached,gamma phase remelt into the matrix.