The hardness of Fe–Cu alloys is known to increase by the precipitation of Cu clusters during thermal aging. We reported previously that the precipitation of Cu clusters accelerate due to the introduction of lattice defects by pre-strain obtained through tensile test or cold rolling, thus reducing the starting temperature of the hardness increase. However, the mechanism has not yet been clarified. In this study, the positron annihilation lifetime and coincidence Doppler broadening techniques have been used to investigate the recovery behavior of lattice defects such as vacancies, vacancy clusters and dislocations as well as the diffusion behavior of Cu atoms of cold rolled and thermally-aged Fe–Cu alloys.
Lattice defects were densely introduced when Fe–1.5%Cu alloys were cold rolled at a reduction rate of more than 30%, and vacancies were approximately 20% of the defects. Both vacancies and dislocations were annealed out through the aging process. On the other hand, some dislocations remained even after aging at 550°C, although the vacancies almost disappeared after aging at 300°C. Cu clusters started to precipitate mainly on the dislocations above 200°C in the cold rolled Fe–1.5%Cu alloys. The hardening due to the Cu clusters occurs with the recovery of vacancies. Thus, both vacancies and dislocations accelerate the diffusion of Cu atoms and the precipitation of Cu clusters.
