Oxygen nanoclustering evades inverse Hall-Petch softening

Grain refinement can drastically increase the strength of metals. However, this approach tends to become less effective or even inverses once grain sizes are reduced to very small scales, generally below 10 ~ 20 nanometers. This softening effect emerges from grain boundary instability and the limited ability of dislocations to form and move within such nanosized grains. However, grain boundary stability can be tuned by solute decoration or grain boundary relaxation. In this context, we present a strategy to achieve superior strength and plasticity in nanograined metals simultaneously. The formation of oxygen (O)-rich clusters at grain boundaries can significantly improve grain boundary stability, even at the 3 nm grain size model (CoCrNi)₈₇O₁₃ (at.%) alloy investigated in this study. Furthermore, the presence of O-rich clusters in grain interiors promotes the accumulation and multiplication of dislocations, which facilitates strain hardening during deformation. Consequently, despite being situated in the inverse Hall-Petch regime, this (CoCrNi)₈₇O₁₃ alloy exhibits a remarkable yield strength of ~3.6 GPa and retains a uniform plastic strain of over 50% under micropillar compression. These findings therefore provide a universal design strategy for nanograined metals aimed at utilizing O clusters to achieve the highly desired combination of high strength and large deformability.