Formation mechanism of bulk nanocrystalline aluminium with multiply twinned grains by liquid quenching: A molecular dynamics simulation study

The formation mechanism of bulk nanocrystalline aluminium with multiply twinned grains has been investigated by a large-scale molecular dynamics simulation. The results show that bulk nanocrystalline aluminium can be obtained directly by quenching liquid at an appropriate cooling rate window. Most nanograins do not merge with each other in the coarsening stage due to the fast cooling rate, but are separated by high-angle grain boundaries. The nanograins exhibit a narrow grain-size range with an average diameter of 6.4 nm and random crystallographic orientations. These microstructure features are consistent with some of the bulk nanocrystalline alloys prepared by melt casting in experiments. The nanograins display various twinned morphologies, and they can be described by three twin elements of parallel, cross and fivefold twinning in varying amounts. The multiply twinned nanograins come from the successive formation of twin HCP planes during the layer-by-layer growth of FCC stacking blocks, and the growth kinetics plays a critical role in the grain morphology. This formation mechanism is different from that of fivefold deformation and annealing twins in the nanocrystalline metals.