precipitates which in turn change to rhombohedral a9 (R phase). The R phase grows to overcome the coherent strains of GP zones. The a9 (aa950.3992 nm) or R (aR5 0.4005 nm) phase particles have stronger resistance to the movement of dislocations. Therefore as the R phase grows, the micro hardness value and the electrical resistivity of the matrix alloy show a steady increase. In case of composites, the effect is more pronounced due to an increase in the dislocation density arising out of thermal mismatch between the matrix and reinforcement. The increase in dislocation density results in a decrease in incubation time required for R phase nucleation in the GP zone. This is because the dislocations act as sites for nucleation to take place so that nucleation rate becomes more. Further, dislocations act as short circuit path for solute diffusivity enhancing precipitate growth. The reduced incubation time leads to the formation of a significant amount of R phase particles at an early stage of aging.
This explains the result that the time to reach peak hardness in case of composites is less than the time taken
by the unreinforced alloy.