The effect of isochronal annealing on the deformation-induced defects in pure Cu and Cu-Ni-Si alloys is studied by positron annihilation spectroscopy.For the cold-rolled Cu,annealing up to 900℃ causes a gradual recovery of the deformation-induced defects and monotonous decrease of the hardness.This indicates that its hardening is mainly related with defects such as dislocations.However,for the hot-rolled and quenched Cu-Ni-Si alloy,although there is a partial recovery of defects after annealing below 500℃,formation of additional defects is observed after annealing above 500℃.The hardness of Cu-Ni-Si alloy has a maximum value after annealing at 500℃,which suggests that the hardening of Cu-Ni-Si alloy is not due to defects,but primarily due to the precipitation formed during annealing.Further annealing of the Cu-Ni-Si alloy above 500 C results in over-aging effect and the precipitates lose coherence with the host matrix,which leads to positron trapping by vacancy clusters in the incoherent interface region.