Twin thickness-dependent tensile deformation mechanism on strengthening-softening of Si nanowires.

Twin thickness-dependent deformation and the transition from strengthening to softening in twinned silicon nanowires are investigated using molecular dynamics simulations with cylindrical and hexagonal cross sections. The results show that the transition from strengthening to softening occurs at critical twin thicknesses of 8.1 nm (11.0 TB s) with cylindrical cross section and 11.0 nm (8 TBs) with hexagonal cross section with decreasing twin thickness, and that the strongest twin thickness originates from a transition in the initial plasticity mechanism from full dislocation nucleation and interaction with the TBs to partial dislocation nucleation and gliding parallel to the TBs. Moreover, it is found that the relationship between peak stress and twin thickness can be divided into two regions. Several full and partial dislocations are formed in the regions with strengthening twin thickness range. The accumulation and pile-up of these dislocations and their interaction with the TBs at high density cause the Hall-Petch strengthening behavior. In contrast, few full and partial dislocations are formed with softening twin thickness range. These dislocations are nucleated and propagate parallel to the TBs, resulting in TB migration that causes inverse Hall-Petch softening behavior. Our simulation results provide sufficient insight into the mechanical behavior of twinned silicon nanowires with cylindrical and hexagonal cross sections. The study will be helpful to the further understanding of CTB-related mechanical behaviors of non-metallic materials and non-metallic system.