Relationship between monolayer stacking faults and twins in nanocrystals

A density functional theory-phase field dislocation dynamics model is used to study stress-induced emission of defects from grain boundaries in nanoscale face-centered cubic (fcc) crystals under ambient conditions. The propensity for stable stacking fault formation and the maximum grain size DSF below which a stacking fault is stable are found to scale inversely with the normalized intrinsic stacking fault energy, γI/μb, where μ is the shear modulus and b is the value of the Burgers vector. More significantly, we reveal that a grain size smaller than DSF is a necessary but not sufficient condition for twinning. Rather, it is shown that deformation twinning additionally scales with ΔSFE=(γU-γI)/μb, where γU is the unstable stacking fault energy. The combined effects of the material γ-surface and nanograin size for several pure fcc metals are presented in the form of a twinnability map. The findings may provide useful information in controlling nanostructures for improved mechanical performance.