Coarsening kinetics of silica in copper

Coarsening experiments have been conducted in the copper-silica system to study the aging behavior of compound precipitates in metallic systems under various conditions of temperature and atmosphere. Most of the experiments were conducted in a two-zone heating furnace, wherein a copper-cuprous oxide mixture was heated in one zone and the copper-silica sample was heated in the other zone, all enclosed in a quartz capsule. The temperature of the former zone determined the oxygen partial pressure in the system and the temperature of the latter zone determined the coarsening temperature. The silica particles from the coarsened samples were extraction-replicated and photographed in an electron microscope. The average radii obtained from these photographs were used to determine coarsening rate constants and activation energies. Isothermal coarsening kinetics followed the γ^-3 vs t law, indicating volume diffusion control. Activation energies were obtained for four different sets of experimental conditions. In each case the activation plot was linear. For coarsening under variable oxygen pressure the experimental activation energy of 530 kJ (127 Kcal) agrees fairly well with the predicted value of 514 kJ (123 Kcal) which takes into consideration the heats of solution and dissolution, as well as the activation energy for diffusion. For coarsening under constant oxygen pressures the experimental activation energies do not compare well with the predicted values, calculated on the basis of complete equilibration of oxygen between the gaseous phase and the sample. However, the experimental activation energies of 524 kJ (125 Kcal) and 447 kJ (107 Kcal) lie in the predicted range calculated on the basis of fixed amounts of oxygen in solution in copper. In no case is the activation energy for coarsening equal to that for diffusion of either species in the matrix as is sometimes assumed.