Changes in a 3.5nm-diameter precipitate, which contained up to 2.0at% of vacancies, were studied by using molecular dynamics techniques. The diffusion coefficients were calculated for a single vacancy and for vacancy concentrations, within the precipitate, of 0.5, 1 or 2at%. It was found that the diffusion coefficients for each set of vacancies changed with time. During the initial stages, the diffusion of vacancy populations was very close to that of single vacancies. This initial stage was explained in terms of vacancy arrangements which contained some clusters and which were temperature-dependent. At about 600K, these clusters contained few vacancies whereas, at temperatures of about 1100K, they consisted mainly of di-vacancies. These clusters were unstable, and changed quite rapidly with time. Changes in larger vacancy-containing precipitates were monitored during aging at a constant temperature; followed by cooling at various rates. In the case of a 7nm precipitate which contained 6.0at% of vacancies and which had been cooled from 1000K at a speed of 5K/ps, small nuclei of a face-centered cubic phase were formed.

J.N.Osetsky, A.Serra: Philosophical Magazine A, 1997, 75[4], 1097-115