A study of precipitate formation in aluminium-copper alloys by positron annihilation and transmission electron microscopy

Abstract Polycrystalline Al─Cu samples containing 0.5, 2.0 and 4.0 wt.% Cu were quenched, after homogenization, from a temperature of 823 to 200 K. Positron annihilation Doppler broadening measurements were then made after isochronal annealing steps with 30 min holding times. Transmission electron microscopy was used to study a set of similarly prepared specimens after several annealing steps. The results indicate that there is a high density of vacancies at or near Guinier-Preston (GP) zones. Positron annihilation is shown to be insensitive to the growth of GP zones and to the transition from GP1 to GP2, but is extremely sensitive to the formation of the partially incoherent θ' phase. We propose that the interface between the incoherent θ' precipitate and the Al matrix provides efficient trapping sites for positrons.