Transmission electron microscopy observations on the microstructure of naturally aged Al–Mg–Si alloy AA6022 processed with an electric field

The influence of an electric field applied during the solution heat treatment and during a short subsequent initial natural aging of AA6022 on the nature of the precipitates which occurred after natural aging for a long time (2–3 years) was determined employing transmission electron microscopy and selected area electron diffraction. The precipitates were spherical in shape ranging from 2 nm to 40 nm in diameter d _p with an average of 7.9 nm. Their average size was larger than that (4.9 nm) in a specimen processed without field and their size distribution had a greater number of precipitates with d _p>∼3 nm. Moreover, their crystal structure differed from that in specimens processed without a field. The increase in yield stress which occurred with the electric field treatments is attributed to an increase in the interaction force between the precipitates and dislocations, which resulted from the larger size and different crystal structure of the precipitates by the field.