Microstructure And Properties Of NbTaW_0.5M Refractory High Entropy Alloys By Carbon Element

With the continuous development of modern science and technology,aerospace and nuclear industry have put forward more stringent requirements for high temperature materials.The traditional alloy based on refractory metals is difficult to meet the high temperature performance requirements above 1300°C.Therefore,it is urgent to develop more excellent superalloys.The unique multi-component design concept of high-entropy alloys（HEAs）not only subverts the traditional alloy design concept,but also shows great potential in the field of superalloy.Therefore,the paper mainly develops refractory high entropy alloys（RHEAs）with high strength and excellent plasticity.In the paper,based on the phase parameters and solid solution strengthening model,the NbTaW_0.5,NbTaW_0.5Ti and NbTaW_0.5Hf_0.25 HEAs with excellent plasticity were developed taking Nb-Mo-Ta-W high entropy alloys as the research object.Subsequently,second phase reinforced NbTaW_0.5（Mo₂C）_x,NbTaW_0.5TiC_xand NbTaW_0.5Hf_0.25C_x RHEAs were developed by adding carbon through the addition of ceramic particles,and the thermal stability of the RHEAs was researched by isothermal annealing at 1 400°C for 12 h.The crystal structure,mechanical properties and microstructure of the as-cast and annealed samples were analyzed.The main findings are as follows:（1）With the increase of ceramic particle content,the NbTaW_0.5（Mo₂C）_x as-cast RHEAs transform from single-phase BCC structure to BCC and HCP dual-phase structure,while the NbTaW_0.5TiC_x,NbTaW_0.5Hf_0.25C_x as-cast RHEAs changes from single phase BCC structure to BCC and FCC structure.After 1400℃/12 h isothermal annealing treatment,the phase structure of NbTaW_0.5TiC_x and NbTaW_0.5Hf_0.25C_x RHEAs did not change,while the orthogonal structure carbides were newly precipitated in NbTaW_0.5（Mo₂C）_x series alloys.（2）All RHEAs show obvious dendritic morphology due to the preparation of arc smelting technology.With the increase of ceramic addition,carbides are formed in the interdendrites and the volume fraction gradually increases.Among them,the NbTaW_0.5Hf_0.25C_x RHEAs forms obvious eutectic-like structure in the interdendrites.After isothermal annealing treatment,the microstructures of three RHEAs changed significantly.The NbTaW_0.5（Mo₂C）_x RHEAs precipitates lamellar orthogonal phase sub-carbides in the matrix,and the NbTaW_0.5TiC_xRHEAs precipitates secondary FCC carbides（mainly TiC）in the matrix and secondary BCC phase（Nb,Ta-rich）in the matrix,and the NbTaW_0.5Hf_0.25C_x RHEAs precipitates fine FCC carbides（mainly Hf C）dispersed on the matrix.（3）As the volume fraction of the second phase increases,the yield strength and Vickers hardness of the three series as-cast and annealed RHEAs at room temperature gradually increase,while the room temperature plasticity of the RHEAs except for NbTaW_0.5Hf_0.25C_x RHEAs decreases significantly.The yield strength of as-cast and annealed NbTaW_0.5（Mo₂C）_0.2 are 1615MPa and 1304 MPa,respectively,and the fracture strain is 13.6%and 22.9%,respectively.The peak yield strength of the NbTaW_0.5Hf_0.25C_x RHEAs can reach more than 1300 MPa,and the fracture strain exceeds 30%.Although the crystal structure of the carbides in the NbTaW_0.5TiC_xRHEAs is the same as that of NbTaW_0.5Hf_0.25C_x RHEAs,but carbides in Ti-containing RHEAs will eventually completely wrap the grains,which is not conducive to the propagation of dislocations between grains.In addition,the formation of secondary precipitates after annealing will reduce the plasticity of the RHEAs.（4）The NbTaW_0.5（Mo₂C）_x and NbTaW_0.5Hf_0.25C_x RHEAs have excellent high temperature performance.Among them,the yield strength of NbTaW_0.5（Mo₂C）_0.2 alloy at 1473 K and 1673K is 1026 and 697 MPa,respectively,and the yield strength of NbTaW_0.5Hf_0.25C_0.25 at 1273 K,1473 K and 1673K is 868 MPa,792 MPa and 749 MPa,respectively,far exceeding the currently reported RHEAs.The two series of RHEAs with such excellent high temperature strength strongly depend on the contribution of the second phase strengthening.Although the structure and morphology of these two series of RHEAs change after annealing,the RHEAs still have excellent performance stability and have strong advantages in the field of ultra-high temperature alloys.In addition,although the NbTaW_0.5（Mo₂C）_0.1 and NbTaW_0.5Hf_0.25C_0.15 RHEAs sacrifice part of the high temperature strength,they possess better room temperature plasticity（37%and39.7%,respectively）,so they also have huge application potential.