Microstructure Evolution And Mechanical Properties Of (Fe₆₅Ni₁₅Co₁₀Cr₁₀)₉₂Ti₅Al₃ With Heterogeneous Structures

In recent years,non-isoatomic ratio high entropy or intermediate entropy alloys have broken the traditional design idea of isoatomic ratio high entropy alloys.Fe-based medium entropy alloys（FeMEAs）,with their extremely high proportion of Fe atoms,on the one hand reduce the cost of high entropy alloys while maintaining excellent mechanical properties at low temperature.The excellent mechanical properties at low temperature are due to TRIP effect.However,the effect of stress-induced phase change does not occur at room temperature,and the low yield and tensile strength of FeMEAs at room temperature and its weak work-hardening capability limit its wide application in engineering.There is an urgent need to develop a FeMEA that can be strong and synergistic at room temperature.As a new strengthening method,the heterogeneous structure can realize the coordination of strengthening and toughening of alloys to a certain extent by using the interaction between soft zone and hard zone.However,the strengthening ability of a single heterogeneous structure is limited.The heterogeneous structure of(Fe₆₅Ni₁₅Co₁₀Cr₁₀)₉₂Ti₅Al₃ alloy was achieved by rolling and annealing.The phase structure and microstructure of as-cast and annealed(Fe₆₅Ni₁₅Co₁₀Cr₁₀)₉₂Ti₅Al₃ samples were characterized,and the mechanical properties of the samples were studied at room temperature.The microstructure and mechanical properties of the annealed alloy were regulated by aging treatment,and the effects of aging temperature and time on the microstructure were studied.The results are as follows:（1）After cold rolling and low temperature annealing,the optimal structure of samples annealed at 750℃was obtained.The heterogeneous structure was composed of grain size,FCC-BCC biphase and nanometer precipitated phases.After annealing at 750℃,the samples showed excellent strength-plastic synergy,with yield strength of 960 MPa,tensile strength of 1513 MPa,and total elongation of about 26%.The good strength plastic synergistic effect is mainly caused by heterogeneous structure,nanometer precipitated phase,fine crystal strengthening and stress-induced phase transformation effect（DIMT）.The DIMT effect at room temperature is mainly attributed to the serious lattice distortion caused by the addition of Ti and Al and the Ni segregation caused by the precipitated phase.（2）All the aged samples still showed stratified heterostructure,in which the ratio of deformed microstructure to recrystallized grains changed.After aging,all the samples showed an increase in yield strength.With the increase of aging temperature and time,the samples gradually showed plastic instability,and the total elongation decreased,leading to a decrease in tensile strength.After aging,the contribution of heterogeneous structure to yield strength was still at a high level,but the proportion of contribution was different.When the aging temperature increased from 400℃ to 450℃,the contribution of back stress was the highest,reaching 76%.The main source of aging strengthening is precipitated phase in the sample.The average size of precipitated phase decreases from 56.124 nm at 450℃ to 46.649 nm at 550℃,which is closer to the critical size of precipitated phase.In addition,as the density of precipitated phase increases,the spacing of precipitated phase decreases,which leads to the further reduction of SFE,the difficulty of dislocation deposition increases,and the DIMT effect is easier to occur,which promotes the plastic instability of the alloy.