The role of copper twin boundaries in cryogenic indentation-induced grain growth

Nanostructured Cu films with and without a high density of twins were indented at approximately 77 K with a load of 0.5 N. Utilizing precession-enhanced electron diffraction in the transmission electron microscope, the crystallographic texture, grain size, and grain-to-grain misorientation were quantified. In both orientations, the nanotwinned Cu underwent grain growth in the pile-up region of the indent, with the 〈100〉 oriented nanotwinned Cu having marked increases in the Σ3 and Σ5 boundary fractions as compared to the 〈111〉 orientated nanotwinned Cu. The Cu film without twin boundaries predominately experienced grain distortion and refinement. The twinned grain structure seems to facilitate the observed grain growth, either as a result of the increased mobility of the twin boundaries through a complex mechanically-induced detwinning mechanism and/or microstructure dependent dynamic recrystallization.