Diffusion of Gold in Lead under Hydrostatic Pressure

Diffusion coefficients of gold in lead single crystals have been determined between 60 and 353°C under hydrostatic pressures ranging from 1 to 10 000 kg ${\mathrm{cm}}^{-2\mathrm{}}$. For each pressure, a unique activation energy for diffusion has been found at all temperatures. Both the pre-exponential factor and the activation energy increase with pressure from values ($D_0=2.5\mathrm{}\times {10}^{-3\mathrm{}}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$, $Q=8700\mathrm{}$ cal/mole) in good agreement with those obtained in previous experiments without pressure, reaching saturation values ($D_0=2.0\mathrm{}\times {10}^{-2\mathrm{}}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$, $Q=12\mathrm{}000$ cal/mole) at higher pressures. Correspondingly, the activation volume of the processes involved appears to vary between about 0 and about 24 ${\mathrm{\AA }}^3$/atom in the explored ranges of temperature and pressure. The results seem to indicate that at least two competitive mechanisms are responsible for diffusion in this system and that the relative importance of the two mechanisms depends on the pressure. A simple combination of two mechanisms, which could explain the experimental results, is discussed.