Double-inverse grain size dependence of deformation twinning in nanocrystalline Cu

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