Ridge-twin boundaries as prolific dislocation sources in low stacking-fault energy metals and alloys

Dislocation activities are crucial in facilitating plastic deformation, even in low stacking-fault energy (SFE) materials that are prone to deformation twinning. The high initial strain-hardening rate commonly observed in low-SFE materials is believed to originate from dislocation slip, as twinning typically occurs at large plastic strains. However, twin boundaries account for a significant proportion of the total boundaries in these materials, and it remains unclear whether twin boundaries can effectively nucleate dislocations. Combining multi-scale and in situ electron microscope characterizations, here we report the discovery of a novel type of prolific dislocation sources, which are nano-sized ridges residing along the borders between different twin variants in low-SFE materials. These sources act as dislocation generators that promote dislocation interaction and accumulation, spreading plastic strain and leading to robust strain hardening at the early stage of plastic deformation. Molecular dynamic simulations indicate that the formation of nano-sized ridge-twin structures is energetically favorable at the junctions between multiple twins, explaining why such structures are ubiquitous in low-SFE materials. Decreasing the SFE can significantly increase the population of ridge-twin boundaries, facilitating dislocation emission and hence strain hardening to sustain the stability of plastic flow. These findings provide new insights into the origin of dislocation plasticity and the high early-stage strain hardening rate in low-SFE materials.