Ultra-high-strain-rate shearing and deformation twinning in nanocrystalline aluminum

Nanocrystalline aluminum films with grain sizes of 50–100 nm were made through E-beam evaporation. The films were then subjected to high-rate shearing deformations with strain rates of 105–106 s−1, with local shear stresses of the order of 10 GPa. The experimental configuration is that of a compression–torsion Kolsky bar, where the specimen is a thin film (thickness 200 nm) mounted on a silicon wafer ring. Strain rates during the shearing are determined from the measured torsional waves in the bars. Site-specific TEM samples are prepared using focused ion beam micromachining to investigate the regions of large plastic deformation. Deformation twins and stacking faults are found to develop under the high-strain-rate shearing. The formation of twinning in pure aluminum with comparatively large grain sizes and the high-strain-rate promotion of twinning are discussed, and possible mechanisms are considered.