Grain boundary precipitation in aluminum alloy 7075

The stress corrosion resistance of 7XXX-series aluminum alloys is related to the distribution of precipitates on grain boundaries. In particular, an increase in the spacing of grain boundary precipitates has been correlated with an increase in the resistance to stress corrosion cracking [1]. In this study, three approaches to heat treating aluminum alloy 7075 were explored in an attempt to increase the grain boundary precipitate spacing relative to that in commercially processed 7075-T651 plate. The first approach consisted of quenching from the solution treatment temperature directly to the aging temperature, the second approach included a step quench to an intermediate annealing temperature after solution treating and before quenching to room temperature and aging, and the third approach consisted of step quenching to an intermediate annealing temperature followed by a relatively high-temperature aging treatment. Grain boundary precipitate spacings significantly larger than those in commercially processed 7075-T651 were obtained without sacrificing hardness using the third approach. The increase in precipitate spacing is attributed to lower grain boundary nucleation kinetics at the intermediate annealing temperature and subsequent coarsening of grain boundary precipitates at the high aging temperature.