While a Hall-Petch-type dependence was known for deformation twinning in Cu and other metals of conventional grain sizes (D>1 μm), with D decreasing into the nanocrystalline regime, the propensity for deformation twinning turns around to increase, exhibiting an inverse grain size dependence. This trend was inversed again at still smaller grain sizes, returning to the behavior of increased difficulty in deformation twinning with D going further down. This double-inverse behavior with respect to the normal Hall-Petch D dependence was demonstrated here for nanocrystalline Cu films, deformed in tension at room temperature and slow strain rates. The non-monotonic D dependence of deformation twinning was explained by modelling the competing grain size effects upon the emission of the first partial dislocation and the plane-to-plane promotion of partial dislocation slip afterwards.

Double-Inverse Grain Size Dependence of Deformation Twinning in Nanocrystalline Cu. J.Y.Zhang, G.Liu, R.H.Wang, J.Li, J.Sun, E.Ma: Physical Review B, 2010, 81[17], 172104