THE LONG-PERIOD SUPERSTRUCTURES IN BINARY AU3+XZN1-X ALLOYS - STABILITY AND OFF-STOICHIOMETRY EFFECTS

A detailed analysis by means of high-resolution electron microscopy (HREM) of the long-period anti-phase boundary structures in Au3+xZn 1-x alloys is presented. The initial complex one-dimensional superstructures, consisting of a uniform mixture of anti-phase domains of two and three L12 unit cells width, are found to be metastable. Comparison between observed and calculated HREM images along two distinct zone axes indicates that the anomalous contrast in the imaging of the minority (3) domains in the off-stoichiometric alloys can be attributed to a combined decrease of the order parameter and a localised accommodation of excess Au. Monte Carlo simulations in the context of the three-dimensional face-centred cubic Ising model are presented, which show a similar localised decrease of the order parameter and the localisation of excess Au at an isolated (3) wall in a (2) structure, the first effect being dominant at high temperatures whereas the latter effect is more important at low temperatures. The localisation of excess Au at the (3) walls is shown to play an important role in the decomposition process, which causes a compositional segregation after prolonged annealing, with the commensurate (2) structure as one of the end-products. The electron microscope observations are interpreted in terms of a possible microscopic model for the segregation mechanism.