Hf-induced transition from twinning to complete kink bands in NbTaTiZr alloy during cryogenic compression: An integrated computational and experimental study

Developing refractory high-entropy alloys (RHEAs) with integrated high strength and ductility remained a core challenge in structural materials for extreme environments, as conventional NbTaTiZr-based alloys suffered from low-temperature brittleness. Herein, NbTaTiZr and its Hf-doped derivative HfNbTaTiZr were investigated via EBSD, TEM, EDS and first-principles calculations to clarify their macroscopic properties, microstructural evolution and deformation mechanisms under quasi-static compression at 223 K. Macroscopic tests showed NbTaTiZr exhibited high strength but low ductility with distinct shear cracks, while HfNbTaTiZr achieved strength-ductility synergy with uniform deformation and no obvious cracks. EBSD revealed deformation heterogeneity: NbTaTiZr featured “dislocation slip + Σ3 twinning” in the surface layer and “incomplete kink bands + high-density low-angle grain boundaries” in the core (GND density: 1.04 ×10¹⁴ m⁻², three times that of the surface), with Zr segregation at grain boundaries; HfNbTaTiZr had complete kink bands in the core without twins, lower GND density (0.32 ×10¹⁴ m⁻²) and no segregation. Schmidt factor analysis indicated NbTaTiZr relied on {110}< 1–11 > single slip system in the edge and multiple slip systems in the core, whereas HfNbTaTiZr was dominated by {112}< 11–1 > slip system due to modified critical resolved shear stress. First-principles calculations demonstrated Hf increased bulk modulus (99.66→123.18 GPa) and Poisson's ratio (0.36→0.39), slightly reduced shear modulus (29.89→29.65 GPa), suppressing twinning and promoting complete kink bands by regulating lattice rigidity and stacking fault energy. This work clarified Hf's regulatory mechanism, providing theoretical guidance for RHEA composition design and their extreme-environment applications.