DYNAMIC PRECIPITATION PROCESSES OF AL2CU IN AL-CU THIN-FILMS BY IN-SITU TRANSMISSION ELECTRON-MICROSCOPY

Dynamic precipitation processes of the Al2Cu (theta) phase in Al-1.5 w t % Cu thin films were studied by in situ heat treatments with transmi ssion electron microscopy for films deposited at high temperatures in both the single-phase and the two-phase regions. The film deposited at 325 degrees C, in the two-phase Al(alpha)-Al2Cu region, has large irr egular precipitates. Upon heating to 380 degrees C, the precipitates s plit and grow, and then at 450 degrees C, they dissolve. Upon cooling , theta precipitates form at new locations, namely, at grain boundarie s (GBs) and at triple points (TPs). Reheating and cooling does not sig nificantly change their location and morphology. The film deposited at 465 degrees C, in the single-phase Al(alpha) region, has fine precipi tates at the GBs and TPs. Heating causes growth and then dissolution o f precipitates, but upon cooling, precipitates reform at the original locations. The coarse theta precipitates, in the as-deposited film at 325 degrees C, can increase corrosion susceptibility compared to the f iner platelike morphology present after annealing or from deposition i n the single-phase region.