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.

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The Al-Ag system is thought to be a well-understood model system used to study diffusional phase transformations in alloys. Here we report the existence of a new precipitate phase, ζ, in this classical system using scanning transmission electron microscopy (STEM). The ζ phase has a modulated structure composed of alternative bilayers enriched in Al or Ag.Our in situ annealing experiments reveal that the ζ phase is an intermediate precipitate phase between GP zones and γ . First-principles calculations show that ζ is a local energy minimum state formed during Ag clustering in Al. The layered structure of ζ is analogous to the well-known Ag segregation at the precipitate-matrix interfaces when Ag is microalloyed in various aluminium alloys.

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