Structure and ordering of the 18-carat Al–Au–Cu β-phase

Gold alloys with around 30 at% copper and 25 at% aluminium undergo a reversible displacive transformation with Ms and As temperatures of about 30 and 75°C respectively. However, the nature of the parent and product phases in this system has not previously been clear. It is confirmed here that the parent phase is a β electron compound. If it is annealed and quenched, it has ordering intermediate between the A2 and B2 states, and in this case the displacive transformation between the parent phase and the low temperature (martensite) phase is poorly developed, and has some second order characteristics. Ageing of the parent phase causes it to take up L21-type ordering after which the transformation becomes classically displacive and reversible, with many of the characteristics associated with the known shape memory systems. The site occupancies of the parent phase have been estimated.