Poster Presentation 24th Australian Conference on Microscopy and Microanalysis 2016

Mechanisms of void shrinkage in aluminium (#303)

Zezhong Zhang 1 , Tianyu Liu 1 , Andrew E Smith 2 , Nikhil Medhekar 1 , Philip Nakashima 1 , Laure Bourgeois 1 3
  1. Department of Materials Science and Engineering, Monash University , Melbourne, Victoria, Australia
  2. School of Physics and Astronomy, Monash University , Melbourne, Victoria, Australia
  3. Monash Centre for Electron Microscopy, Monash University , Melbourne, Victoria, Australia

Voids in aluminium can significantly affect materials performance and a key question is their evolution.12 We characterised the shrinkage of voids in quenched pure aluminium by performing in-situ annealing in a JEOL JEM 2100F transmission electron microscope (TEM) operated at 200 kV and 160 kV. The in-situ annealing was performed with a Gatan double-tilt heating holder. 

We found that void shrinkage is a two-stage process. Voids first shrink anisotropically from a non-equilibrium to an equilibrium shape and then shrink isotropically, keeping their equilibrium shape until their full dissolution into the aluminium matrix. This phenomenon can be explained through a surface energy analysis: the void shrinks so to maximise the total surface energy reduction per vacancy emitted. By imaging the shrinkage of voids  at near atomic resolution, we showed that it is quantised, taking place one atomic layer and one facet at a time throughout the whole shrinkage process, regardless of the irradiation. We reproduced the measured void shrinkage rates using bulk diffusion kinetics modified to take into account electron beam irradiation, the void geometry and the quantised shrinkage.  Electron beam irradiation remarkably accelerates the diffusion of vacancies from the void into the matrix. This is due to the sputtering of vacancies on the void surface of void caused by the electron beam. Our calculated sputtering rates are consistent with the vacancies emission rates measured from the TEM images.3 

  1. R. E. Smallman and R. J. BiShop, Modern Physical Metallurgy and Materials Engineering. Elsevier, 1999, pp. 84–124.
  2. T. E. Volin and R. W. Balluffi, Phys. status solidi, 25, 163 (1968).
  3. Z. Zhang, T. Liu, A. E. Smith, N. V. Medhekar, P. N. H. Nakashima and L. Bourgeois, Mechanisms of Void Shrinkage in Aluminium. (under review)