Electron microscopic observations of coherent boundaries in γ- (Cu-Al)

Electron microscopic evidence is obtained for the presence of a possible two-phase structure inside the γ-phase field of the Cu-Al system. Alloys with composition 34.54 at.% Al show a banded microstructure. When the bands are observed edge on adjacent plates differ in brightness. They are visible as fringe patterns when lying in planes inclined to the foil plane. Electron diffraction experiments show that the only crystallographic difference between two adjacent plates is the size of the unit cell. The probable composition of the coexisting phases is suggested to be Cu9Al4 and Cu17Al9.