Surface severe plastic deformation-enabled deformation behavior control and mechanical property enhancement in metastable ferrous medium-entropy alloys

Abstract

Stacking fault energy (SFE) is an intrinsic property that governs the deformation behavior of metallic materials, including dislocation slip, deformation twinning, and phase transformation. In this study, we present a mechanistic perspective demonstrating that the ‘apparent’ SFE and the associated deformation behavior can be tailored by modifying only the localized microstructure (∼100 μm from the surface) through the application of surface severe plastic deformation. This process generates a well-defined gradient microstructure in the near-surface region through grain refinement and an increase in dislocation density. The reduction in apparent SFE induced by localized gradient structure enhances the driving force for martensitic transformation compared to its homogeneous counterpart. This effect originates from the preferential martensite nucleation sites provided by the localized gradient region, as well as from dynamic stress partitioning facilitated by phase interfaces and gradient heterostructure, which synergistically accelerate the growth of martensitic phase. As a result, the deformation behavior was effectively modulated, leading to significantly enhanced mechanical properties. In particular, partial microstructural modification enabled strength enhancement while minimizing the loss of ductility, in clear contrast to conventional approaches based solely on grain refinement or dislocation density enhancement. This work therefore provides phenomenological insight into how localized microstructural engineering can regulate deformation mechanisms and mechanical performance, representing advancements beyond the conventional understanding of mechanical behavior of heterostructured materials.