Aging behavior of ultrafine grained Al–2 wt%Cu alloy severely deformed by accumulative roll bonding

Abstract

Aging behavior of an ultrafine grained Al–Cu alloy was studied. Al–2 wt%Cu alloy sheets were solution-treated at 550 °C for 1.8 ks and then severely deformed up to an equivalent strain of 4.8 by the accumulative roll bonding (ARB) process at ambient temperature. The ARB processed material showed the ultrafine lamellar boundary structure with mean boundary interval of 67 nm. The ARB processed sheets and the starting sheets having conventionally coarse grains were aged at 190 °C. The coarse grained specimen for comparison showed typical aging behavior showing in-grain precipitation of the thin-plate shaped θ’ phase coherent to {001} crystallographic plane of the matrix aluminum. Hardness of the coarse grained specimen increased as precipitation proceeds, reached the maximum around HV60, and decreased owing to over-aging. On the other hand, the ARB processed sample with the ultrafine-grained structure showed very high hardness of HV115 in the as-deformed state, and the hardness decreased monotonously with increasing the aging time. The detailed microstructural observations clarified that recovery and grain growth of the matrix proceeds during 190 °C aging and the precipitation only at grain boundaries occurs in the ultrafine grained material. Instead of the θ′ phase, the equilibrium θ phase preferentially appeared in the ultrafine grained specimen. The coarsening of the matrix was mainly responsible for monotonous decrease of hardness. The kinetics of precipitation in the ARB processed sample was much faster than that of the coarse grained materials. It was concluded that the aging behaviors in severely deformed material having the ultrafine grained structure are completely different from the conventional ones in the coarse-grained materials.