Structural aspects of AuCu I or AuCu II and a cuboidal block configuration of f.c.c. disordered phase in AuCuPt and AuCuAg pseudobinary alloys

A crystallographic analysis of the relative orientation of the microstructure of the two-phase (AuCu I or AuCu II) alloy coexisting with cuboidal block configurations of disordered f.c.c. phase was performed by means of transmission electron microscopy (TEM) and high-resolution electron microscopy complemented with selected area electron diffraction (SAED) examinations in (AuCu)1−X−PtX (X = 0.0135, 0.0248, 0.0469 and 0.0692 in atomic ratio), AuCu9 at.% Ag and AuCu14 at.% Ag alloys. In (AuCu)1−X−PtX alloys, a characteristics checkerboard-like microstructure which was composed of cuboidal blocks of disordered f.c.c. enclosed by two parallelogram-shaped orientation variants of AuCu IX and AuCu IY ordered regions without twin relation was observed, although the SAED pattern showed twinning. Evidence for a twin relation was found in small ordered regions observed in the cuboidal blocks unavoidably during quenching from an ageing temperature to ice brine. The cuboidal block was enclosed by four parallelogram-shaped AuCu I regions. The orientation relationship was deduced as (001)AuCu I//{100}α2 and the interface was defined as the {102}AuCu I. The microstructure consisted of the cuboidal block of a silver-rich f.c.c. α2 phase surrounded by three different orientation variants of the AuCu II ordered regions in which two of three variants had twin relation in AuCu9 at.% Ag and AuCu14 at.% Ag alloys. The orientation relationship was deduced as (001)AuCu II//{100}α2 and the interface was defined as the (001)AuCu II. The formation sequence of the checkerboard-like microstructure in (AuCu)1−X−PtX alloys was also analyzed using dark-field images in conventional TEM and SAED observations.