Ultrafine κ-Phase and De-faulting mechanisms enables 2GPa strength with ductility in a VCoNi alloy via ultra-fast heat treatment

Medium-entropy alloys have become promising structural materials owing to their exceptional mechanical performance. Strategies to reduce enthalpy in these systems have proven effective in enhancing strength, while they often cause a dramatic loss of both strength and ductility, due to overgrowth of precipitates or intermetallic compounds. Here, we employ ultra-short-time high-temperature heat treatment—enabled by a Gleeble system—to process cold-rolled VCoNi medium-entropy alloy samples within the single-phase solid-solution regime. The alloy achieves a remarkable tensile strength of 2 GPa while retaining 4 % uniform elongation. We reveal that the formation of ultrafine κ phases is inherently coupled with a high density of nanoscale stacking faults. These faults participate cooperatively in κ-phase deformation and give rise to a previously unreported “de-faulting” mechanism, whereby a substantial fraction of stacking faults is recovered post-deformation. We propose that partial dislocations emitted from grain boundaries is critical in mediating this process.