Electron diffraction contrast from ledges at the interfaces of faceted θ′ precipitates

Ledges have been observed at the habit-plane interface of faceted ø’ precipitates in an aluminium-copper alloy. The ledges are between 15 and 65 Å in height and are visible both by strain field and thickness contrast effects. The strain field behaves qualitatively as a prismatic dislocation loop lying at the precipitate-matrix interface. When a precipitate reflection only is excited, a characteristic black-white contrast is observed in dark field and this enables the sense of the loop to be determined, i.e. growth or dissolution ledges can be distinguished.