ENHANCED DIFFUSION OF LIQUID-COATED GAS-BUBBLES IN SOLIDS

Gas bubble diffusion in solids is typically due to surface diffusion of matrix atoms along the gas-solid interface. Insoluble impurities that segregate to the interface tend to suppress surface diffusion by chemically combining with the surface atoms, as in the creation of a metal oxide layer, or by simply presenting a physical barrier to the kinetic jump process. We propose a mechanism by which surface self-diffusion (and hence gas bubble diffusion) may in fact be enhanced during high-temperature anneals, for a special class of matrix-impurity couple. A liquid layer at the gas-solid interface, comprised of the impurity species and an equilibrium concentration of matrix atoms, acts as a conduit for rapid mass transport, as equilibrium is maintained by continuous dissolution and precipitation of matrix atoms at the solid surface. This liquid dissolution diffusion process thus requires that the impurity species melt at a temperature lower than the annealing temperature, that the liquid lower the gas solid interface energy and so wet the solid surface, and that the matrix species be soluble in the liquid impurity. Recent in situ observations by transmission electron microscopy of dramatically enhanced diffusive motion of helium bubbles in aluminium alloyed with lead and with indium may be evidence for this diffusion mechanism.