In situ Transmission Electron Microscopy Investigation of Nucleation of GP Zones Under Natural Aging in Al-Zn-Mg Alloy

This investigation reveals very early-stage (within an hour) precipitation during natural-aging of Al-Zn-Mg alloy can have very complex processes with strong fluctuations in precipitate types and formation mechanisms. In situ heating followed by quenching and subsequent natural-aging at room temperature was carried inside transmission electron microscope to study the formation and evolution of early-stage precipitation. Investigation reveals formation of solute clusters at ~2 mins and subsequent nucleation of GP-II zone adjacent to clusters at ~10 mins during very early-stage natural-aging. Moreover, nucleation of GP-II is found to be related to GP-I but not in one unique mechanism. GP-I precipitates were observed to act as sites for the formation of GP-II either by ‘separated’ nucleation (i.e. from the interface between GP-I and Al matrix) or via in situ nucleation (i.e. within a GP-I precipitate). Occasionally, some GP-II precipitates were observed to dissolve into Al matrix without further transforming to ƞ՛ precipitates.