Ridged twin boundaries as prolific dislocation sources in high-entropy alloys and other low stacking-fault energy metals

Dislocation activities play an important role in mediating plastic deformation, even in metals that are prone to deformation twinning. Combining multi-scale and in situ electron microscope characterizations, here we report a discovery of a unique type of dislocation sources that are particularly fertile in low stacking-fault energy materials, including CrCoNi-based high-entropy alloys and twinning-induced plasticity steel. These sources reside on nano-sized ridges, which form strings along the borders between different twin variants to accommodate the multiple twinning relationships. Upon plastic deformation, such ridge-twin structures act as an effective dislocation generator, from which dislocations are emitted into the coherent twin boundaries or cross slip into the interior of grains. At larger strain, the incoherent boundaries of the nano-sized ridge-twins emit partial dislocations to mediate deformation twinning as well. 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 ubiquitously populous in the twinned architectures that are commonplace in the several low stacking-fault energy materials that we investigated. These results shed light on understanding the origin of dislocation plasticity in low stacking-fault energy metals and alloys including those known for twinning-induced plasticity.