Microstructure of AuCu II in the disordered f.c.c. phase of AuCu-14at.%Ag alloy

Abstract Microstructural changes related to AuCu ordering and two-phase decomposition were studied by means of conventional transmission electron microscopy and high resolution electron microscopy in an AuCu-14at.%Ag pseudobinary alloy. Two different types of precipitates, spindle type and chain type, are formed by a nucleation and growth process upon annealing at 643 K. These precipitates are termed “composite precipitates” because they show a sandwich structure consisting of blocks of a silver-rich f.c.c. phase (α 2 ) and AuCu II ordered thin platelets along two of their three faces. Only their lateral faces were not decorated and connected directly with the copper-rich α 1 matrix. By aging just below the spinodal line, the blocks of α 2 f.c.c. were completely sandwiched by the AuCu II ordered thin platelets. A model explaining these configurations is proposed. In the present alloy a modulated structure is also found by aging for short periods at low temperature.