On the mechanistic understanding of annealing-induced strength enhancement of ultrafine-grained high-Mn steel

Abstract

Severe plastic deformation (SPD) introduces both high dislocation density and profound grain refinement in metals, triggering metastable microstructures associated with non-equilibrium boundaries and vacancy agglomerates. Subsequent solute migration induced by low-temperature annealing has the potential to release complex microstructures, and thereby increase strength. However, a mechanistic understanding of the annealing-induced strength enhancement in the SPD-processed high-Mn steel with ultrafine grains has remained elusive, particularly at the nanoscale. Here, the impact of subsequent annealing on the tensile properties of the SPD-processed high Mn steel with ultrafine grains was investigated. C detection and dislocation density measurements proved that both severe lattice distortion and high density of dislocations (resulting from the SPD route) are the precursors of C-clusters during the subsequent annealing. Formation of the nano-sized C-clusters in the SPD-annealing-treated samples interrupts dislocation gliding upon loading, increasing the yield strength by 15% at comparable ductility compared to those of the SPD-processed samples with same grain sizes. We attribute the increased strength to dislocation forest hardening via annealing-driven C-clusters, qualifying a low-temperature annealing step as a replicable strategy for further improving the strength of the SPD-processed nanostructured materials.