Study of copper precipitates in alpha-iron by computer simulation .3. The evolution of vacancies within precipitates

The evolution of a copper precipitate with a diameter of 3.5 nm containing up to 2.0 at.% of vacancies at different temperatures has been studied by molecular dynamics. The diffusion coefficients were calculated for a single vacancy and vacancy concentrations inside the precipitate of 0.5, 1.0 and 2.0 at.%. The diffusion coefficients for each set of vacancies change with lime. During the initial stage, the diffusion of populations of vacancies is very close to that of single vacancy diffusion. The initial stage is interpreted in terms of arrangements of vacancies which contain-some clusters and are temperature dependent. For low temperatures (about 600-K) these clusters contain few vacancies whereas for high temperatures (about 1100 K) they are mainly formed by divacancies. These clusters are not stable and change in time quite rapidly. The evolution of larger precipitates with vacancies was followed under ageing at constant temperature followed by cooling at one of several constant speeds. For a precipiate of 7 nm diameter containing 6.0 at.% of vacancies and cooled from 1000 K with a speed of 5.0 K ps(-1), small nuclei of fee phase were formed. However, the complete bcc --> fcc transformation of the precipitate was not obtained, and possible mechanisms of the phase transformation are discussed.