Mechanisms of void shrinkage in aluminium

Voids can significantly affect the performance of materials and a key question is how voids form and evolve. Voids also provide a rare opportunity to study the fundamental interplay between surface crystallography and atomic diffusion at the nanoscale. In the present work, the shrinkage of voids in aluminium from 20 to 1 nm in diameter through in situ annealing is imaged in a transmission electron microscope. It is found that voids first shrink anisotropically from a non-equilibrium to an equilibrium shape and then shrink while maintaining their equilibrium shape until they collapse. It is revealed that this process maximizes the reduction in total surface energy per vacancy emitted. It is also observed that shrinkage is quantized, taking place one atomic layer and one void facet at a time. By taking the quantization and electron irradiation into account, the measured void shrinkage rates can be modelled satisfactorily for voids down to 5 nm using bulk diffusion kinetics. Continuous electron irradiation accelerates the shrinkage kinetics significantly; however, it does not affect the energetics, which control void shape.