Abstract
Vacancies highly in excess of equilibrium are thought to be responsible for the high rates of formation of Guinier-Preston zones (G.P. zones) after the quench from high temperatures. A kinetic model is developed for the growth of spherical G.P. zones based on the diffusion of solute-vacancy complexes to the zones. Under certain favorable physical conditions, when the complex arrives at the zone-matrix interface, the solute and vacancy are no longer bound; and the vacancy can now contribute to enhancing a vacancy concentration gradient directed radially outward from the zone. The vacancies are thus driven back into the matrix to again collide with solute atoms, once more becoming complexes. In this way the zone acts as a vacancy pump, maintaining a high vacancy concentration for long periods of time. Mathematical equations are derived based on this model, and kinetic data for Al-rich Zn is considered. A rather good comparison with experiment is found for early in the zone-forming process.
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