A novel ZrNbMoTaW refractory high-entropy alloy with in-situ forming heterogeneous structure

NbMoTaW-based refractory high-entropy alloys (RHEAs) are now at the research frontier of advanced metallic materials due to outstanding mechanical properties at elevated temperatures. However, the poor plasticity of NbMoTaW-based RHEAs at room temperature hinders further development. The present work proposes a new strategy to tailor in-situ forming heterogeneous structures and improve the room-temperature mechanical properties of NbMoTaW-based RHEAs. A novel series of as-cast ZrxNbMoTaW (x = 0.1, 0.5, and 1, denoted by Zr0.1, Zr0.5, and Zr1, respectively) RHEAs was designed and prepared. The effect of Zr on the mechanical properties and microstructure evolution was investigated. The Zr1 alloy had a compressive yield strength of 1558 MPa and a strain of 15.8% at room temperature, superior to most reported NbMoTaW-based RHEAs. Surprisingly, the Zr1 alloy also showed an ultra-high strength at 1000 °C, which compressive yield strength was 555 MPa without fracture at the strain of 25%. Moreover, all ZrxNbMoTaW RHEAs were composed of two disordered body-centered cubic (BCC) phases. The nanoindentation results showed that the hardness of the dendrite exceeded that of the interdendritic region. Such heterogeneous structure of the Zr1 alloy had a hetero-deformation-induced enhancing effect on strength and plasticity. The heterogeneous structure was mainly triggered by increased mixing enthalpy, severe lattice distortion, and non-equilibrium solidification. The double cross slipping was the dominant deformation mechanism of the Zr1 alloy, which served as a new dislocation source besides the Frank-Read one. Accordingly, the dislocation multiplication was promoted, enhancing the strength and plasticity. This study successfully introduced a high-strength and high-plasticity NbMoTaW-based RHEA, gaining more insight into the alloy design strategies of RHEAs.