Size-dependent mechanical responses of twinned Nanocrystalline HfNbZrTi refractory high-entropy alloy

Atomistic simulations are performed to study the size-dependent mechanical responses of HfNbZrTi refractory high-entropy alloy (RHEA) containing ultrafine grains and highly oriented twin boundaries (TBs). The strength and flow stress of nanocrystalline RHEA (NC-RHEA) under tensile loadings are explored versus decreasing grain size d. The transition from classical Hall-Petch (HP) strengthening to inverse HP softening at a critical grain size d_c = 5.91 nm is attributed to the change of plastic deformation mechanisms from dislocation emission and phase transformation to grain boundary (GB) activities. Besides, the intragranular TBs considerably enhance the strength of nanotwinned RHEA (NT-RHEA); the enhancing effect reduces with decreasing twin thickness λ. As the volume fraction of GB increases with decreasing d, GB activities dominate the plasticity of NT-RHEA and cause comparable mechanical properties with NC-RHEA. Moreover, the influences of dislocation glide, phase transformation and twinning on the mechanical properties of RHEA are quantified and separately analyzed to further verify our simulation results. Findings of this study not only promote insights into the nanostructure-property relation of HfNbZrTi, but also shed the light on performance enhancement through nanostructural design.