Origins of strengthening and toughening effects in twinned nanocrystalline alloys of low stacking fault energy with heterogeneous grain structure

Low stacking-fault energy (SFE) nanocrystalline alloys with pre-existing nanotwins (PNTs) and heterogeneous grain structure (HGS) exhibit exceptional strength-ductility combinations. This has been mainly ascribed to the multiple concurrent deformation mechanisms promoted by the low SFE, and back stress hardening from the strain gradient imposed by PNTs and HGS via activities of geometric necessary dislocations (GNDs). However, the influence of PNTs on dislocation configurations, and the role of intragranular statistically stored dislocations (SSDs) in the mechanical response are still unclear. Here, we uncover the origins of strengthening and toughening effects in a prototype low SFE multicomponent CoCrNi alloy with PNTs and HGS via atomistic simulations. The results show that HGS leads to significant strain partitioning between the smaller- and larger-grain domains and promotes the strain localization, while PNTs alleviate the strain localization and facilitate homogeneous deformation for better toughness. Moreover, PNTs efficiently block intragranular mobile Shockley partials and enhance the dislocation reaction probability, increasing the densities of sessile SSDs. The strengthening effects from both, dynamically increased sessile structures and plastic resistance induced by localized dislocations, are revealed by quantitative analysis on dislocation distributions. This indicates that the dislocation configurations correlated with strain localization are fortified owing to the presence of PNTs and HGS, which further enhance the flow stress on deformation. The present study demonstrates that, PNTs can enable efficient controlling of dislocation configurations upon deformation, while the strengthening and toughening effects of alloys with heterostructures are achieved by both GNDs correlated with localized strains and SSDs generated via dislocation reactions.