Study of the order-disorder phase transitions in Cu-Al-Be shape memory alloys

We have investigated the order-disorder phase transition in Cu-Al-Be alloys close to the eutectic composition. X-ray diffraction measurements have been performed to characterize the ordered structure and to follow the evolution of the degree of long-range order with temperature. Results show that the transition is first order and takes place, on cooling, from the disordered to a D0(3) structure. Taking into account these results, a mean held model has been developed in order to study the superlattice formation in these alloys. Present X-ray measurements together with previous calorimetric measurements of the martensitic transition undergone by Cu-Al-Be alloys, have enabled a determination of their metastable phase diagram. It is remarkable that although the addition of a small quantity of Be has a strong effect on the martensitic transformation, it does not modify the symmetry properties of the binary Cu3Al.