Tuning deformation mechanisms in refractory high-entropy alloys: slip plane preference and dislocation behavior

Refractory high-entropy alloys (RHEAs) exhibit exceptional high-temperature strength but typically suffer from limited tensile ductility at room temperature. In this study, we investigate the mechanical properties and underlying deformation mechanisms of single-phase body-centered cubic (BCC) Ti₃₅Zr_(35-x)Hf_xNb₂₀Mo₁₀ (x = 0, 2.5, 5, 7.5, and 10) alloys. Increasing Hf content significantly enhances tensile ductility while maintaining a high yield strength above 1 GPa. Notably, the fracture elongation of Ti₃₅Zr₂₅Hf₁₀Nb₂₀Mo₁₀ alloy is 27.7%, nearly double that of the Hf-free Ti₃₅Zr₃₅Nb₂₀Mo₁₀ alloy (14.4%). In-situ electron backscatter diffraction EBSD analysis shows that Hf additions promote the activation of the {112} slip plane, whereas the {123} slip plane is consistently active across all compositions. Transmission electron microscopy (TEM) analysis further reveals distinct dislocation behavior depending on the slip plane: screw dislocations dominate on the {110} plane, while edge and mixed dislocations preferentially glide on high-order planes. These wavy mixed dislocations facilitate cross-slip and the development of secondary planar-slip bands, thereby improving strain uniformity and mitigating local stress concentrations. Moreover, kink bands are observed exclusively in Hf-containing alloys. Their formation is associated with the relaxation of localized strain and stress, contributing to improved fracture resistance. Collectively, these findings offer a detailed understanding of the deformation mechanisms in RHEAs and suggest a promising alloy design strategy to simultaneously enhance strength and ductility—critical for structural applications under extreme thermal and mechanical loading conditions.