Study On The Evolution Of Phase Composition And Mechanical Properties Of Ti-Zr-Nb-V High-entropy Alloys

High-entropy alloys is a new type of alloys which is composed of multi-components with equiatomic or near-equiatomic proportions.It has shown good potential in extreme applications such as those under ultra-low temperature,ultra-high temperature or ultrahigh loading speed.At present,the research on high-entropy alloys mainly focuses on the structure and properties of a specific component,but less attention is paid to the basic problems such as how the entropy increase caused by the increase of components in alloys affects the phase composition and what affects the intrinsic strength of single-phase highentropy alloys.In view of the above problems,this paper takes Ti-Zr-Nb-V high entropy alloy as the research subject,and studies the influence of mixing entropy on the phase composition of high-entropy alloys,the influence of valence electron concentration on the intrinsic strength of them,the dynamic mechanical behavior of them,directional solidification process and heat treatment process by using the methods of theoretical design,experimental verification and model extraction.Firstly,the evolution of phase composition duiring binary to ternary and then to quaternary alloys was studied by using the mixing entropy as the main variable,and the influence and onset threshold of mixing entropy on the phase composition were determined.Ternary and quaternary alloys with different ideal mixing entropy values were obtained by adding different contents of Ti or/and Nb in VZr binary alloy.The actual content of Laves phase in different alloys was obtained from the experimental results by image analysis technology and compared with the prediction values of V(Nb)-Zr(Ti)pseudo binary phase diagram.According to the mixing entropy range when the prediction values lose their accuracy,it was found that the threshold of ideal mixing entropy inhibiting Laves phase was 1.1-1.21R.Secondly,the variation law of the intrinsic strength of single-phase solid solution Ti-Zr-Nb-V high-entropy alloys was studied with valence electron concentration as a variable.The classical theory of relationship between strength and valence electron concentration in pure metals was successfully extended to the fields of multi-component single-phase alloys.A series of(TiNb)1-x-yZryVx alloys with the same atomic size mismatch and different valence electron concentrations were designed and prepared.Based on the theory of relationship between metal bond binding energy and valence electron concentration,a universal relationship model between the intrinsic yield strength(σy)and valence electron concentration(VEC)of single-phase solid solution high-entropy alloys was constructed as σy=-437.4+100.3 ·(VEC)2·Rn-1。Thirdly,the dynamic mechanical properties and impact energy release characteristics of TiZrNbV high-entropy alloy were studied.The crack "self-healing"phenomenon of the alloy during dynamic compression was found,and the factors affecting the energy release rate during impact were excavated.Through the quasi in-situ characterization of the microstructure evolution and resistivity change of TiZrNbV highentropy alloy during dynamic loading at 7400 s-1 strain rate,it is found that there is a crack "self-healing" phenomenon in TiZrNbV high entropy alloy during dynamic compression,and it is judged that the formation reason is explosive welding between the two sides of the crack.By analyzing the impact reaction products of TiZrNbV highentropy alloy,the chemical model of deflagration reaction of the alloy during high-speed impact was constructed,and the decisive effect on the energy release rate was revealed to be the proportion of fragments whose sizes are less than 100 μm.Finally,the induction remelting-directional solidification process and heat treatment process were explored for TiZrNbV high-entropy alloy.The induction remeltingdirectional solidification platform was independently built.By studying the microstructure of the as cast alloys under different crucible-dropping rates,the optimal crucible-dropping rate that can simultaneously realize defect concentration and avoid component separation was found to be between 160～400 μm/s.Multi-phase TiZrNbV high-entropy alloy prepared by directional solidification was heat treated at 1200℃ to transform to a single phase.By analyzing the phase evolution during the heat treatment,it is found that the phase transformation process of the alloy can be divided into three stages:structure transformation of the Zr-rich phase,solid solution of the Zr-rich phase and the fusion of the remained two phases.Then,based on the JMA model of isothermal diffusion type phase transformation,the relationship model between the monophasic degree and heat treatment time is fitted.The above research results partially solved some problems about the basic theory and preparation process of high-entropy alloys,and provided some references for the research and development of high-entropy alloys.