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.