Factors that Influence Overaging
Temperature
Increasing temperature can increase the rate of diffusion which may accelerate the dissolution and growth rates and therefore enhance particle coarsening. With increasing temperature, the maximum solute solubility in the matrix also increases and precipitates may dissolve to release solute in solid solution. Both these mechanisms increase the interparticle spacing and this in turn can soften the alloy. (example of ageing in Al-Cu alloy given on next page)
Diffusion Coefficient
During coarsening, solute has to be transported from the dissolving phases to the growing phases. Faster diffusion species accelerate coarsening and the reverse is true for slower diffusing species.
Size of Precipitates
Smaller precipitates dissolve faster compared to larger ones. Therefore, the contribution of dislocation-particle interaction to strengthening is lost faster with smaller particles compared to larger ones.