The Effect of Hydrostatic Pressure on the Shrinkage of Cavities in Metals

Abstract

Specimens of magnesium, tough-pitch copper, and oxygen-free, high conductivity copper, in which cavities situated on grain boundaries had been introduced by creep, were subsequently annealed in an inert atmosphere over a range of temperatures and at atmospheric and higher pressures. The rate of density decrease on annealing, when cavities remained attached to grain boundaries, was associated with activation energies for the three materials of (1.0 ± 0.1) × 10⁵, (1.34 ± 0.12) × 10⁵, and (1.26 ± 0.12) × 10⁵ J/mol, respectively, and these values were not significantly altered by environmental pressure up to 30 MN/m² The decrease in sintering rate with time and the influence of hydrostatic pressure were consistent with a model of cavity closure by grain-boundary self-diffusion, which could also incorporate an effect of residual gas. The model of sintering was developed quantitatively, taking into account the effect of varying cavity size, and can be used to predict rates of sintering or, alternatively, to permit estimates of grain-boundary self-diffusion coefficients where data on these are not available.