Periodic antiphase domain structure in the off-stoichiometric CuAu II phase

The ordered structures of Cu-Au alloys containing 56 to 65 at.% Au have been investigated by electron diffraction and electron microscopy, using thin foils prepared from bulk specimens. Single-crystal diffraction patterns taken from specimens in the CuAu II phase region are interpreted basically in terms of the CuAu II-type structure. In the composition range of 59-63 at.% Au, however, the splitting direction of superlattice reflections at 110 and equivalent positions is not parallel to the [100] direction, but deviates slightly. To explain the deviation of the splitting direction, regular steps have to be introduced into the periodic antiphase domain boundaries of the CuAu II-type. The high-resolution electron microscope study has confirmed the existence of the boundary steps.