Adiabatic Shear Band versus Twinning: Hf-driven cryogenic deformation mechanism transition in NbTaTiZr at high strain rate

This study systematically investigated the dynamic deformation mechanisms and microstructural evolution of NbTaTiZr and HfNbTaTiZr refractory high-entropy alloys (RHEAs) under cryogenic conditions (223 K) and high strain rates (3000-6000 s⁻¹) through integrated experimental characterizations and molecular dynamics simulations. For NbTaTiZr, dislocation slip dominated at 3000 s⁻¹, with twinning becoming the primary deformation mechanism at 5000 s⁻¹, leading to shear fracture at 6000 s⁻¹. In contrast, HfNbTaTiZr exhibited progressive transition from kink band nucleation at grain boundaries to fully developed adiabatic shear bands (ASBs) at 4000-6000 s⁻¹, with suppressed twinning activity. Molecular dynamics simulations revealed that Hf addition elevated the generalized stacking fault energy (GSFE) from 607 to 700 mJ/m², thermodynamically inhibiting twin formation, while reducing thermal conductivity from 13.00 to 8.45 W/m·K, which enhanced adiabatic heating and facilitated ASB nucleation. These results demonstrated that Hf triggered a deformation mechanism transition from twinning to ASB-dominated behavior, providing critical insights for designing high-performance RHEAs in cryogenic and dynamic loading applications.