Deformation kinetics and hetero-deformation-induced strengthening in additively manufactured 304L stainless steel

Additive manufacturing (AM), particularly laser power bed fusion (LPBF) technique, enables the formation of hierarchical heterogeneous microstructures that markedly enhance the mechanical performance of materials. However, a quantitative, kinetics-based understanding of how AM-induced heterostructures govern the evolution of deformation mechanisms and hetero-deformation-induced (HDI) strengthening during plastic flow remains lacking. Here, we employ metastable 304 L stainless steel, with its intrinsically low stacking fault energy that activates slip, twinning, and martensitic transformation, as a model system to elucidate the interplay between deformation kinetics and hetero-deformation in AM alloys. By integrating strain-rate jump and stress-relaxation experiments with loading–unloading hysteresis measurements and multiscale microstructural characterisation, we quantitatively resolve the evolution of dislocation mobility, dislocation storage, and HDI back stress across different deformation stages. The results reveal a sequential yet overlapping transition of dominant plasticity carriers: from dislocation slip in sub-micron cellular structures and planar slip band, to deformation twinning, and finally to deformation-induced martensitic transformation. This mechanistic evolution is manifested by a continuous reduction of activation volume from over 100 b³ to tens of b³, coupled with a non-monotonic evolution of mobile dislocation density. Throughout deformation, HDI back stress contributes up to ~75% of the flow stress, primarily associated with the massive accumulation of geometrically necessary dislocations at cellular walls, twin boundaries, and martensitic interfaces. These findings establish a quantitative linkage between AM-induced heterogeneous microstructures, deformation kinetics, and HDI strengthening, providing a kinetics-based framework for the mechanistic design of AM-enabled heterostructured materials. • Kinetics-based framework links AM microstructures to plastic deformation. • Activation volume drops from ~120 b³ to ~30 b³ across deformation stages. • Multi-modal deformation mechanisms govern rate control and sustain work hardening. • HDI back stress contributes ~75% of flow stress in LPBF 304 L stainless steel. • Transient tests quantify dislocation mobility, exhaustion, and back-stress evolution.