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A gold alloy with 18 wt pct Cu and 6 wt pct 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 ␤-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 ЊC and 20 ЊC. The system exhibits hysteresis and will revert to the parent phase when reheated, with an austenite start (A s ) temperature between 55 ЊC and 80 ЊC. 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 ЊC and 200 ЊC 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.

The Al-Au-Cu system and its associated ternary alloys and intermetallic compounds is surprisingly poorly known, and the authors could find no phase diagram for it in the literature. This article addresses this omission by presenting an isothermal section at 500 ЊC, derived with the aid of X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), metallography, and hardness measurements. The samples studied had generally received an anneal of 2 hours at 500 ЊC, primarily in order to complete any transformations that occurred during solidification and cooling of the castings. The possibility of further changes on protracted annealing at 500 ЊC is not ruled out, and the diagram presented is, therefore, applicable only to material prepared by thermal processing of an industrial nature. The presence of a ternary ␤ phase with a nominal stoichiometry of AlAu 2Ϫx Cu 1+x (0 Յ x Յ 1) was confirmed, and its phase field at 500 ЊC was determined. A number of the binary intermetallic phases were found to exhibit some solid solubility of the ternary element. In particular, the ␥ -Al 4 Cu 9 phase extends deep into the ternary and, in the vicinity of the commercially interesting 18-carat line, appears to exist in a ternary ordered form, designated here as ␥ 2 .

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The Au 7 Cu 5 Al 4  electron phase transforms displacively from an L2 1 parent to a nominally bodycentred tetragonal martensite with c/a<1. The compound is of interest because it has the potential to serve as an 18 carat shape memory alloy in jewellery. Analysis of its X-ray diffraction spectra indicates that the martensite is modulated by a [110]/[ 10 1 ] transverse shear wave, showing that it belongs, strictly speaking, to the generic B19 structure type. The martensite is also twinned, and the probable twinning structure is explored. A ) 6 , 9 ( 15R stacking sequence is deduced, which for reasons of the L2 1 ordering inherited from the parent phase, must be doubled to produce a notional ) 6 , 9 , 6 , 9 ( 30R martensite that properly repeats. However, although the measured X-ray diffraction spectra can be substantially explained by the structures derived, the martensite probably also has additional, higher-order lattice modulations.