Observations of the Microstructure of Pre-precipitates in an Al–3%Cu Alloy by a Lattice Imaging Technique

Microstructures of pre-precipitates in an Al–3 mass%Cu single crystal alloy have been investigated by the lattice imaging technique using a high resolution electron microscope. In the early stage of aging, images of G.P. [1] zones with a typical structure of a single Cu-rich layer (single layer zone) and in the subsequent stages images of G.P. [2] zones, in which two Cu-rich layers are separated by three layers of the Al(200) plane, were clearly observed. Moreover it was also observed that there coexisted pre-precipitates such as those having a ‘many Cu-rich layer’ structure, or various periodic structures composed of Cu-rich layers and Al(200) planes, which were different and more complex than the typical G.P. [1] and G.P. [2] zones structure.Through precise measurements of the spacing of the lattice fringes adjacent to the G.P. [1] and G.P. [2] zones, it was shown that the maximum lattice displacement in the normal direction is 9–12% and that the displacement profile is asymmetrical on both sides of the G.P. zones. A small displacement in the radial direction of G.P. zones was also observed.