Dynamic precipitation processes of Al₂Cu in Al-Cu thin films by in situ transmission electron microscopy

Dynamic precipitation processes of the Al₂Cu (θ) phase in Al-1.5 wt % Cu thin films were studied by in situ heat treatments with transmission electron microscopy for films deposited at high temperatures in both the single-phase and the two-phase regions. The film deposited at 325°C, in the two-phase Al(α)-Al₂Cu region, has large irregular precipitates. Upon heating to 380°C, the precipitates split and grow, and then, at 450°C, they dissolve. Upon cooling, θ precipitates form at new locations, namely, at grain boundaries (GBs) and at triple points (TPs). Reheating and cooling does not significantly change their location and morphology. The film deposited at 465°C, in the single-phase Al(α) region, has fine precipitates at the GBs and TPs. Heating causes growth and then dissolution of precipitates, but upon cooling, precipitates reform at the original locations. The coarse θ precipitates, in the as-deposited film at 325°C, can increase corrosion susceptibility compared to the finer platelike morphology present after annealing or from deposition in the single-phase region.