Study On The Regulation And Mechanism Of Trace Ti Element On The Properties Of FeCoNi Mid-entropy Alloy

As one of the theoretical breakthroughs in the field of metal materials in recent years,high-entropy alloys have important theoretical research significance and application potential,which have attracted extensive scholars' attention.High-entropy alloys are designed by alloying multiple alloying elements with equal or near equiatomic ratios,thus obtaining unique composition,microstructure,and physicochemical properties.Normally,single face-centered cubic(fcc)structured medium-entropy alloys and high-entropy alloys exbit high plasticity but relatively low strength which should be applied effective strengthenings to improve their strength and ductility,making its mechanical properties meet the needs of practical applications.Therefore,FeCoNi medium-entropy alloy with a single-phase fcc structure was choosed as base alloy in this paper.After adding different contents of Ti element and different heat treatments,we have systematically studied the influence of Ti elements on the microstructure,mechanical properties controlling and physical mechanisms of the FeCoNi medium-entropy alloys:(1).The contents of Ti element control the microstructures and properties of(FeCoNi)1-xTix alloys:Firstly,FeCoNi base alloys with equal atomic ratio were prepared by vacuum arc melting and homogenizing.The FeCoNi alloy exbits a high plasticity of 49%,but the yield strength(YS)and ultimate tensile strength(UTS)are only 103 MPa and 320 MPa,respectively.After forging(? 50%reduction),the YS and UTS only increase to 454 MPa and 551 MPa respectively,but the plasticity drops to 26%.Different strengthening mechanisms are introduced through mechanical process and heat treatment with further adding different contents of Ti to the FeCoNi matrix,finding that the addition of Ti element have significantly improved the overall mechanical properties of the alloys.Through optimizing the component,we found that the YS and UTS of(FeCoNi)94Ti6(Ti6 hereafter)alloy reach to 697 MPa and 968 MPa,respectively.Compared with the FeCoNi matrix alloy after forging,they are increased by 54%and 76%respectively,while maintaining an elongation of 26%.The higher yield strength of FeCoNi alloy after adding Ti comes from the effects of solid solution strengthening,dislocation strengthening.And the contribution of solid solution and dislocation strengthening is about 534 MPa.(2).The effects of heat treatments on the structures and mechanical properties of(FeCoNi)1-xTix alloys and the corresponding mechanisms:Different from the"softening" of FeCoNi alloys after post-deformation annealing at high temperatures,the strength of(FeCoNi)97Ti3(Ti3 hereafter)and Ti6 alloys are abmormal increased,which is attributed to the Ni3Ti precipitations formed during the high temperature annealing process.Under the strengthening effect of Ni3Ti precipitations,the UTS of Ti6 alloy increases from 968MPa to 1263MPa while the elongation decreases from 26%to 24%.At post-deformation annealing at 1000?,the the L12 precipitates in Ti3 are L12-type nanoclusters and the size of the precipitates in the matrix are tiny while the precipitates in Ti6 alloy have a larger size.A high density of L12-type Ni3Ti nanoparticles with an average size of 47 nm were observed by TEM in the Ti6 alloy.The precipitates in Ti6 exhibit coherent with the matrix and the coherent interface could effectively enhance the strength of matrix while maintain a good ductility.Theoretical calculations show that the precipitation strengthening contributes up to 600 MPa to the yield strength.