Are nanotwinned structures in fcc metals optimal for strength, ductility and grain stability?

The optimality of nanotwinned structures in imparting maximum strength and stability has been investigated by performing atomistic simulations of dislocation interactions with various types of grain boundaries. By developing an understanding of the mechanism of the instability, we predict that nanocrystalline metals with grain sizes below 50 ± 10 nm or 70 ± 10 nm (in Cu) are inherently unstable at 0 or 300 K, respectively, in contrast to nanotwinned face-centered cubic structures which are stable and optimal.