Theoretical Studies On Thermal And Mechanical Properties Of NbTiZr-based Refractory High Rntropy Materials

High entropy alloy(HEA)is a new material containing multiple principal elements,it breaks the constraint in conventional alloys with one or two primary elements,and provides a new way for exploring new materials and new properties.Due to the unique composition of HEAs,many properties of HEAs are superior to those of conventional alloys,such as high strength and hardness,outstanding wear resistance and corrosion resistance,good thermal stability,etc.Therefore,HEAs have become a hot topic in material science and engineering filed in recent years.Rrefractory high entropy alloys(RHEAs)based on refractory elemets have shown very promising high?temperature mechanical properties and being considered as novel high?temperature structural materials beyond conventional superalloys.At present,the studies on HEAs are mostly focused on the experements,mainly on the studies of structure,hardness,compressive strength,plasticity,corrosion resistance and so on,but the studies on thermodynamic and elastic mechanical properties are relatively few.Moreover,due to the limitations of the experimental environment,there are few studies on the properties of RHEAs under high temperature.In this work,the thermal and mechanical properties of Nb Ti Zr-based RHEAs are theoretically explored in detail by using the exact muffin-tin orbitals(EMTO)method based on density functional theory combined with the coherent potential approximation(CPA)method.The main research contents are as follows:Firstly,temperature dependence of mechanical and thermodynamic properties of refractory high entropy alloys Ti_(25+x)Zr₂₅Nb₂₅Ta_(25-x)(x?20)are studied from EMTO-CPA method and quasi-harmonic Debye-Grüneisen model,and the influences of partial substitution of Ti for Ta are emphasized.The partial substitution of Ti for Ta results in the significantly reduced densities and the apparently enhanced specific elastic properties of Ti_(25+x)Zr₂₅Nb₂₅Ta_(25-x)alloys.Moreover,the increases of Ti content obviously improve ductility.At high temperature,the alloys with higher Ti content demonstrate the stronger thermal expansion behavior and thermal conductivity.Importantly,the alloys with higher Ti content possess the greater specific elastic properties at high temperature.In the studied temperature range,the mechanical properties of the alloys show a weak temperature dependence,which indicates that the present alloys exhibit good anti-softening ability.Therefore,the partial replacement of Ta by Ti in refractory Ti_(25+x)Zr₂₅Nb₂₅Ta_(25-x)alloys is beneficial for formation of novel high-temperature structural materials with lower density,better ductility and stronger specific mechanical properties.Secondly,the mechanical properties of body-centered cubic Nb Ti Zr M(M=Hf,Ta,W)refractory high entropy alloys are systematically studied by EMTO-CPA method,and the correlation between mechanical properties and valence electron concentration(VEC)is appreciably revealed.The present investigation demonstrates that both the single-crystal elastic parameters and the polycrystalline elastic moduli of Nb Ti Zr M alloys presents an upward tendency with increase of atomic number of M in sequence of Nb Ti Zr Hf<Nb Ti Zr Ta<Nb Ti Zr W,similar to the variation trend of VEC.Hence,the strength and hardness of Nb Ti Zr M alloys exhibit apparent correlation with the VEC.Because the Poisson's ratio and the Pugh's ratio suggest the alloys are more ductile with decrease of atomic number of M,the Nb Ti Zr M alloys with lower VEC show better ductility.Moreover,with increasing VEC,the elastic anisotropies of Nb Ti Zr M alloys are predicted to decrease,whereas the Debye temperatures increase obviously.Noticeably,the VEC is an effective adjusting parameter to obtain excellent mechanical properties of alloys.