Moderating martensitic transformation rate enables simultaneous enhancements of ductility and strength

The rate of martensitic transformation plays a pivotal role in determining mechanical properties of TRIP materials. However, optimizing this rate to simultaneously achieve high strength and ductility in single-phase materials remains challenging. Here we report a moderate martensitic transformation rate can enable a maximum uniform elongation and improved yield strength in a gradient-dislocation structured 321 stainless steel, significantly outperforming its coarse-grained counterpart. The gradient dislocation structure was produced by cyclic twisting processing, which introduced dislocation entanglements and Lomer-Cottrell (L-C) locks. During the tensile testing, dislocation slip, stacking faults, nanotwinning and martensitic transformation were activated. This synergistic interplay effectively moderated the martensitic transformation kinetics. Notably, the sustained emission of Shockley partials from L-C locks and γ/α′ interfaces facilitated persistent nanotwinning at comparatively low stress levels, contributing to continuous work hardening. This study presents a promising strategy for regulating the martensitic transformation kinetics to enhance the mechanical properties of TRIP materials.