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)HfxNb20Mo10 (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.