Numerical Simulation And Experimental Verification Of NbMoTaW Type Refractory High-Entropy Alloys

Refractory high-entropy alloys（RHEAs）represented by NbMoTaW shows excellent properties such as high hardness,wear and corrosion resistance,and structural stability.RHEAs retain high strength at ultra-high temperatures and have great development potential.Considering the rapid development of the aerospace industry,the service temperature requirements for high-temperature materials in the future are likely to reach over 2000℃.Therefore,it is necessary to further improve the strength of RHEAs.The room temperature plasticity of NbMoTaW alloy is poor,which is not conducive to its practical engineering application.As a new type of high-temperature alloy,the underlying logic of the phase structure evolution,strengthening mechanism,microstructure,and atomic bonding characteristics of RHEAs is still unclear,which hinders its rapid updating and replacement.In order to further improve the comprehensive mechanical properties of NbMoTaW RHEAs,this thesis applies material calculation methods,such as CALPHAD and first-principle calculations,to design a series of new alloys based on NbMoTaW.The microstructure characteristics,phase evolution rules,and mechanical properties of these alloys were thoroughly studied at the atomic/electronic scale,and the physical mechanisms of related phenomena were analyzed and discussed in detail.The research content of this thesis mainly includes three parts.In order to further improve the strength of the alloy,the effect of Re element alloying on the structure and mechanical properties of NbMoTaW was studied.The calculation results indicate that the NbMoTaWRe_x alloy has a single-phase BCC structure,but the phase stability gradually decreases with the increase of Re content.The first principles calculation results indicate that with the increase of Re content,the lattice constant of the alloy decreases,but the elastic modulus and hardness of the alloys are significantly enhanced.The electronic structure analysis shows that the low electronic density of states at the Fermi level is an important reason for the NbMoTaWRe_x alloy to maintain the single-phase BCC structure.The experimental results confirm the BCC phase of the as-cast NbMoTaWRe_x alloys,and the errors between the calculated lattice constants and the experimental values are less than 1%.In addition,the experimental modulus and hardness of the alloys increase with the increase of Re content,which is consistent with the calculated results.However,the addition of Re element leads to a deterioration of the plasticity of NbMoTaW alloy.Therefore,a small amount of Re element should be added in NbMoTaW,or Re element should be co alloying with metal elements such as Ti/Zr/Hf that can improve the plasticity of NbMoTaW.This thesis also investigated the structure and mechanical properties of a new alloy formed by completely replacing one of NbMoTaW elements with Re element.Except for Nb Mo WRe alloy,the substitution of Re element resulted in a certain degree of plasticity reduction,indicating that the content of Re element should be carefully controlled in the NbMoTaW system to avoid a decrease in the comprehensive mechanical properties due to the excessive brittleness.To improve the plasticity of NbMoTaW based RHEAs,the effects of Hf element alloying on the structure and mechanical properties of NbMoTaW were studied.The first-principle calculations results indicated that the BCC phase is more advantageous in energy than the FCC/HCP phase in NbMoTaWHf_x alloy.The lattice distortions of NbMoTaWHf_x alloy significantly increased with the increasing Hf content.The XRD analysis results indicated that the alloy is single BCC phases when x=0.92,while a two BCC phase structure is formed when x>0.92.The energy spectrum analysis results indicated that there is a large amount of Hf element enrichment in the precipitated BCC2 phase.With the increase of Hf content,the strength and plasticity of NbMoTaWHf_xalloy were improved.When x=0.92,the comprehensive mechanical properties of the alloy reached the best,with yield strength and plastic strain of 1550 MPa and 4.21%,respectively.There is a linear positive correlation between the reduced yield strength and lattice distortion of NbMoTaWHf_x alloy,indicating that lattice distortion plays a dominant role in the strengthening of this alloy system.The new alloy formed by completely replacing one element of NbMoTaW with Hf element exhibits excellent comprehensive mechanical properties.The maximum compressive strength（2.31 GPa）of NbHfTaW is increased by～80%compared to NbMoTaW（1.28 GPa）,and the plastic strain（～18.2%）is six times that of NbMoTaW（～3.0%）.In addition to its excellent mechanical properties,NbHfTaW is also an ideal elastic isotropic material（A_z=1.008）with great application potential.The effects of non-metallic element C on the structure and properties of NbMoTaW and NbHfTaW alloys were studied.We first used the CALPHAD method to study the effect of C element content on the microstructure of（NbMoTaW）_1-xC_x and（NbHfTaW）_1-xC_x（0=x=0.5）materials,and analyzed the phase fractions of x=0.1,0.2,0.3,0.4,and 0.5.As the content of C increases from 0 to 0.5,a complex phase transition occurs in sequence of BCC→BCC+FCC/HCP→BCC+FCC+HCP→FCC+HCP→FCC.We selected（Nb0.25Mo0.25Ta0.25W0.25）C and（Nb0.25Hf0.25Ta0.25W0.25）C（i.e.x=0.5）for in-depth analysis.The calculation results showed that both materials have very high elastic modulus,hardness,and yield strength,with Young’s modulus of 434 GPa and445 GPa,hardness of 16.8 GPa and 19 GPa,and yield strength of 5.6 GPa and 6.3 GPa,respectively.The covalent bond formed by orbital hybridization between C and refractory metal elements is the main reason for the high hardness and strength of materials.Through in-depth research on NbMoTaW based RHEAs,we can develop and apply refractory metal minerals in China more scientifically and reasonably,achieve efficient resource utilization,and more conducive to ensuring the autonomy of core technologies in China’s future high-temperature material field.The research results of this thesis can not only promote our understanding of the structural characteristics and strengthening mechanisms of RHEAs,but also provide some reference for the development of new RHEAs in the future.