Twinning in nanocrystalline metals

Abstract

Twinning is a commonly observed deformation mode in lowsymmetry crystals such as hexagonal-close packed metals, but not in high-symmetry metals that possess many possible slip systems with lower critical resolved shear stress (CRSS) than the twinning stress. However, if the CRSS for slip can be increased to a level greater than the twinning stress, then twinning may be favored as a deformation mechanism even in cubic materials. Nanostructured metals present such a scenario. Following the observation of deformation twinning in nano-crystalline metals, even in high stacking fault energy metals such as aluminum, recently there has been signifi cant interest in studying twinning as a deformation mode in nanocrystalline face-centered cubic (fcc) metals. Another important breakthrough is the fabrication of fcc metals with nanotwinned structures using methods such as electro-deposition or sputter deposition. Nanoscale growth twins have also been observed in nanowires. Nanotwinned fcc metals made via sputtering can have twin boundary spacing as small as 2–4 nm and preferential alignment of {111} twin boundaries normal to the growth direction. These as-grown nanotwinned structures exhibit unusual properties as compared to nanocrystalline metals. Typically, in the equiaxed grain nanocrystalline metals with high-angle grain boundaries the increased strength is accompanied by a loss in ductility, thermal stability, and electrical conductivity. Twinning in Nanocrystalline Metals