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 D-SF below which a stacking fault is stable are found to scale inversely with the normalized intrinsic stacking fault energy, gamma(I)/mu b, where mu is the shear modulus and b is the value of the Burgers vector. More significantly, we reveal that a grain size smaller than D-SF is a necessary but not sufficient condition for twinning. Rather, it is shown that deformation twinning additionally scales with Delta(SFE) = (gamma(U) - gamma(I))/ mu b, where gamma(U) is the unstable stacking fault energy. The combined effects of the material gamma-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. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.