AGE-HARDENING IN 6061AL ALLOY AND AL2O3 6061AL COMPOSITE/

6061 aluminium alloy and the alloy containing 15% volume fraction of A l2O3 particles were aged at 398 and 443 K. It was found that the hardn ess in reinforced alloy reached its peak value at much shorter ageing time compared to the monolithic alloy when aged at 443 K. However, the reinforced material did not show any acceleration in precipitation ha rdening when aged at 398 K. Transmission electron microscopic investig ation showed that hardness increased as fine needle-shaped zones (GP-I I or called beta'') appeared, and the material attained peak hardness when the structure contained a high density of needle-shaped zones. As the ageing continued, the zones became coarser and fewer in number wh ich led to decrease in the hardness. The presence of large Al2O3 parti cles accelerated coarsening of the beta'' zones resulting in peak hard ness which occurred at shorter ageing time in reinforced material comp ared to that in the monolithic alloy at 443 K. However, at a lower age ing temperature of 398 K, the coarsening of the zones was very sluggis h in both reinforced and monolithic materials. It was found that defor mation prior to ageing of the material could modify the formation and coarsening of the precipitates. The microscopic observations revealed that formation of fine needle-shaped beta'' zones was depressed by for mation of beta'' phase in deformed material. Since the beta'' zone is the only hardening agent in 6061Al alloy, reduction or absence of beta '' can considerably or totally diminish the hardening ability in the d eformed alloy.