A high-resolution transmission electron microscopy investigation of the δ'–θ' precipitate structure in an Al-2 wt% Li-1 wt% Cu alloy

Abstract

High-resolution transmission electron microscopy was used to investigate the atomic structure of the two-phase Δ'–θ' precipitate which forms in Al–Li–Cu alloys. The results show that θ' plates often thicken in multiples of two unit cells and that the interphase boundary at the plate faces is atomically flat and coherent with the surrounding δ' phase. The positions of Cu atoms in the top half and bottom half of the θ' unit cell causes the ordered δ' phase to form in an antiphase relationship on opposite faces of the θ' precipitate. This antiphase relation is continued into the δ' beyond the plate edge, which is largely coherent, forming an antiphase boundary. These observations are explained in terms of conservative mechanisms of precipitate growth involving (a/2)[100] and (a/2)[010] transformation dislocations and bonding tendencies among Al, Li and Cu atoms.