Displacive transformations in Au-18 wt pct Cu-6 wt pct Al

A gold alloy with 18 wt pct Cu and 6 wt pet Al undergoes a reversible displacive phase transformation between an incompletely ordered L2(1) parent phase and a tetragonal product. The characteristics of these transformations were studied using acoustic emission, dilatometry, X-ray diffraction, and metallography. The morphology of the transformation products, the structure of the parent phase, and the generation of significant acoustic emission during the transformations indicate that they are at least quasi-martensitic, if not martensitic, and that this system is an example of a beta-phase shape-memory alloy (SMA). The onset temperatures of the transformations depend on the prior thermal history of the sample. The martensite start (M-s) temperature is between 30 degrees C and 20 degrees C, The system exhibits hysteresis and will revert to the parent phase when reheated, with an austenite start (A(s)) temperature between 55 degrees C and 80 degrees C. However, freshly cast or solution-annealed and quenched samples of the alloy do not transform to the tetragonal phase. Aging of such material at temperatures between 30 degrees C and 200 degrees C is required before they will manifest the displacive transformation. The "martensite" phase is considerably more resistant to aging-induced stabilization than that of most other SMAs.