High-density metastable annealing twin boundaries-induced extraordinary strain hardening in ultrafine-grained high-entropy alloy

Twin boundaries (TBs) are well known to play a critical role in affecting the mechanical properties of face-centered cubic (FCC) metals and alloys. However, most existing strategies that leverage TB-dislocation interaction for strengthening while maintaining ductility rely primarily on stable TBs, following the widely held “stable-is-better” belief. Here, we report, for the first time, the fabrication of highly dense, metastable annealing TBs (ATBs) with the highest density of 1.93 μm⁻¹ ever achieved, within an ultrafine-grained FCC Fe₄₉Mn₃₀Co₁₀Cr₁₀C high-entropy alloy. This structure delivers a superior strength–ductility synergy while preserving a remarkable strain hardening ability typically observed in coarse-grained materials, even surpassing that of base material with stable ATBs upon deformation and, thus, demonstrating the paradigm “metastable is also better”. On this basis, we reveal a previously unidentified strain hardening mechanism driven by densely metastable ATBs, which interact with dislocations to dynamically transform into high-angle grain boundaries. This transformation enables the production of high-density dislocations, generates a new type of dynamic Hall–Petch effect, and promotes cross slip. These effects play a major role in enhancing strain hardening, resulting in sluggish dislocation slip controlled by the high driving force and high generalized stability. Our findings provide new insights into the role of ATBs upon deformation that is different from that conventionally reported in the literature, and highlight the potential of manipulating high-density metastable ATBs to significantly improve the strain hardening capability of FCC metals and alloys.