Enabling high strength yet ductility in a refractory high-entropy alloy through oxygen-nitrogen synergistic effect

Most refractory high-entropy alloys (RHEAs) are susceptible to brittle fracture when subjected to tensile loading. The TiZrHfNb RHEA and its derivative possess good tensile ductility, but still encounter issues with low yield strength. In this study, we propose an Oxygen-Nitrogen Synergistic Effect (ONSE) for reinforcing the TiZrHfNb RHEA. Oxygen atoms create local chemical order (LCO), seeding more dislocation nucleation sites, promoting dislocation multiplication and ensuring uniform deformation. Nitrogen atoms induce lattice distortion, hinder dislocation motion, and generate a significant solid solution strengthening effect. The introduction of ONSE results in high-density mobile dislocations distributed throughout the alloy, as opposed to the conventional belief that dislocations are limited to narrow slip bands by LCOs. The model alloy (TiZrHfNb)₉₈N_1.5O_0.5 (at. %) designed by ONSE showed a significantly improved yield strength of 1412.9 ± 13.5 MPa, representing a 92 % enhancement compared to the TiZrHfNb RHEA, while maintaining a fracture elongation of 10 %. Our strengthening approach provides a new option for improving the strength of other body-centered-cubic (BCC) RHEAs.