Novel high-density refractory high-entropy alloys with excellent mechanical properties at high temperatures and high strain rates

Based on traditional refractory high-entropy alloy (RHEA) NbMoTaW alloy, two novel RHEAs, Nb_0.2Mo_0.8Ta₂WHf and Nb_0.5Mo_0.5TaWHf with densities of respectively 14.7 and 15.57 g/cm³ were designed and fabricated by vacuum arc melting method. Phase components, microstructures, mechanical properties and fracture morphologies of the two RHEAs were investigated with comparison to the traditional NbMoTaW RHEA. Experimental results show that the two novel RHEAs present dual-BCC phase structure, which is different to that of the traditional NbMoTaW RHEA. Grain size of NbMoTaW is the largest, followed by Nb_0.2Mo_0.8Ta₂WHf and Nb_0.5Mo_0.5TaWHf. The phenomenon of severe components segregation is observed in the two novel RHEAs. Nb_0.5Mo_0.5TaWHf has the best mechanical properties with microhardness of 653 ± 30 Hv, quasi-static yield stress of 1880 MPa and ductility of 2.6 %. Especially, the material still exhibits excellent strength at high temperatures of 600 – 1000 ℃. NbMoTaW and Nb_0.2Mo_0.8Ta₂WHf show strain rate sensitivity within the strain rate range of 0.001 – 3000 s⁻¹. Compared to NbMoTaW and Nb_0.5Mo_0.5TaWHf, Nb_0.2Mo_0.8Ta₂WHf has the best performance under dynamic loadings. Fracture morphologies of the three RHEAs agree with mechanical properties well. Chemical composition optimization is proven to be an effective method in improving both the density and the mechanical properties of the traditional NbMoTaW RHEA.