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 displ acive phase transformation between an incompletely ordered L2(1) parent pha se and a tetragonal product. The characteristics of these transformations w ere studied using acoustic emission, dilatometry, X-ray diffraction, and me tallography. The morphology of the transformation products, the structure o f the parent phase, and the generation of significant acoustic emission dur ing the transformations indicate that they are at least quasi-martensitic, if not martensitic, and that this system is an example of a beta-phase shap e-memory alloy (SMA). The onset temperatures of the transformations depend on the prior thermal history of the sample. The martensite start (M-s) temp erature is between 30 degrees C and 20 degrees C, The system exhibits hyste resis 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, fr eshly cast or solution-annealed and quenched samples of the alloy do not tr ansform to the tetragonal phase. Aging of such material at temperatures bet ween 30 degrees C and 200 degrees C is required before they will manifest t he displacive transformation. The "martensite" phase is considerably more r esistant to aging-induced stabilization than that of most other SMAs.