Intrinsic toughening by motivating multiple slip and fracture systems in NbTaTiHf RHEAs

As a class of single-phase body-centered cubic (BCC) refractory high-entropy alloys (RHEAs), NbTaTiHf, especially non-equi-atomic ones, exhibit excellent synergy of strength and toughness, which are promising structural materials. However, there still lacks a fundamental understanding of the physical origin of the intrinsic toughening. In this work, the orientation-dependent plastic and fracture deformation mechanisms of the equi-atomic NbTaTiHf RHEA under tensile and shear loads are firstly investigated, respectively, using ab initio density-functional theory (DFT) calculations. Subsequently, the effects of relative compositions of group IV (Ti and Hf) compared to group V (Nb and Ta) elements on these deformation mechanisms of the non-equi-atomic NbTaTiHf RHEAs are discussed. The results show that the RHEAs are intrinsically ductile under shear, with stacking fault nucleation dominating the plastic deformation mechanism, while exhibit brittle facture behavior during tension. The critical shear stress for stacking fault nucleation, the critical tensile stress for crack initiation, and their anisotropies generally decrease with the increasing of the relative concentration, indicating that multiple slip and fracture systems can be activated in the RHEAs, which are attributed to the spatially heterogeneous bond strengths dominated by the local lattice distortion (LLD) and the charge density contribution (CDD). These findings can provide valuable insights into the intrinsic toughness of BCC RHEAs.