Revealing the grain size-dependent twinning variants and the associated strengthening mechanisms in a carbon-free austenitic steel

Twinning-induced plasticity (TWIP)-assisted steel shows great promises in various engineering applications due to their exceptional strength and ductility. However, the dominant strengthening mechanism of TWIP-assisted steels is still ambiguous, with much debate over the respective contributions of dislocations and deformation twinning. Here, samples with various grain sizes were prepared to tailor the deformation mechanisms of TWIPassisted steel, demonstrating that the twinning-induced strengthening is grain size-dependent. Specifically, dislocation slip dominates plasticity when grain sizes are small. With increasing grain sizes, single-oriented twinning appears, followed by multi-oriented twinning at the largest grain sizes. Strengthening contributions from each mechanism were analyzed, and it was found that the single-oriented twin variants are not as effective as their multi-oriented counterparts in terms of strengthening. Such an enhanced strengthening effect associated with multi-oriented twin variants is due to the strong interactions between deformation twins and dislocations. Moreover, a fine-grained sample exhibits higher values for both strength and uniform elongation compared to the medium-grained sample, indicating that grain refinement is responsible for better strain hardening capability, in contrast to the kinematic hardening induced by single-oriented twinning.