A surface stacking fault energy approach to predicting defect nucleation in surface-dominated nanostructures

We present a surface stacking fault (SSF) energy approach to predicting defect nucleation from the surfaces of surface-dominated nanostructure such as FCC metal nanowires. The approach leads to a criterion that predicts the initial yield mechanism via either slip or twinning depending on whether the unstable twinning energy or unstable slip energy is smaller as determined from the resulting SSF energy curve. The approach is validated through a comparison between the SSF energy calculation and low-temperature classical molecular dynamics simulations of copper nanowires with different axial and transverse surface orientations, and cross sectional geometries. We focus on the effects of the geometric cross section by studying the transition from slip to twinning previously predicted in moving from a square to rectangular cross section for 〈 100 〉 / { 100 } nanowires, and also for moving from a rhombic to truncated rhombic cross sectional geometry for 〈 110 〉 nanowires. We also provide the important demonstration that the criterion is able to predict the correct deformation mechanism when full dislocation slip is considered concurrently with partial dislocation slip and twinning. This is done in the context of rhombic 〈110〉 aluminum nanowires which do not show a tensile reorientation due to full dislocation slip. We show that the SSF energy criterion successfully predicts the initial mode of surface-nucleated plasticity at low temperature, while also discussing the effects of strain and temperature on the applicability of the criterion.