Dual-gradient structure induced spreadable phase transformation for simultaneously improving strength and ductility

Gradient structure can achieve excellent mechanical properties in metallic materials through synergistic effects from heterogeneity. Conventional surface heterostructuring typically develops one single coupling gradient (i.e., hard surface and soft core). In this study, we present a novel approach to attain a dual-gradient structure, characterized by a series of hard surface-soft middle-hard core, through simple laser-scanning on cold-rolled metastable metallic materials. The resultant dual-gradient structure with significant synergistic strengthening improves the yield strength by more than two-fold compared to its annealed counterparts. Notably, the significant improvement in yield strength does not sacrifice ductility due to the unique deformation behavior of the dynamic-spreading phase transformation. The designed dual-gradient structure induces spatial-distinct phase stability along the depth, enabling to spreading phase transformation from the surface to the center during deformation. The inward-propagating phase transformation contributes to sustaining the heterostructure and the transformation-induced plasticity effect over a wide stress range while detouring stress concentration. Our study suggests a promising process design for fabricating a dual-gradient structure that possesses inward-spreading phase transformation to enhance the mechanical properties of heterostructured materials without compromising their ductility.