Ordering and quasi-martensitic transformations in Au-Cu-Al alloys

A gold alloy with 18 wt% Cu and 6 wt% Al can undergo a reversible displacive transformation. However, the effect is weakly developed until the B2 parent phase produced by rapid solidification is subjected to an aging heat treatment. The surface upheaval generated during the transformations is fully reversible in the first cycle, but not after repeated transformation. This is evidently due to the pinning of parent phase interfaces by the crossing of laths, which produces a checkerboard microstructure. The laths of both parent and product phase display other curious properties, such as self-accommodation, split tips, bent interfaces, and the ability to cross grain boundaries. An especially puzzling property is the tendency of parent and product phase laths to change morphology during aging between the A(f) and M-s temperatures.