EFFECT OF REINFORCEMENT ON THE AGING RESPONSE OF CAST 6061 AL-AL2O3 PARTICULATE COMPOSITES

The effect of 10 and 15 vol pct alumina particulate addition on the age hardening behavior of cast 6061 Al-matrix composites was studied using microhardness, electrical resistivity, differential scanning calorimetry, and transmission electron microscopy (TEM). It was found that the kinetics of precipitation in the matrix alloy are significantly accelerated due to the presence of reinforcements. This acceleration is attributable to the decrease in incubation time required for nucleation and the increase in solute diffusivity and hence precipitate growth rate resulting from the increase in the matrix dislocation density due to coefficient of thermal expansion (CTE) mismatch between the matrix and the reinforcements. The relative amounts of the various phases were also observed to be affected by reinforcement addition. Increasing reinforcement content decreased the volume fractions of the beta' and beta-precipitates while increasing the volume fraction of the GP-I zones. The volume fraction of silicon clusters (which are the precursors to GP zones in 6061 Al) formed during postsolution treatment aging was found to decrease with increasing reinforcement addition. The above effects have been discussed with respect to the associated mechanisms, and plausible explanations have been offered.