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 a lloys close to the eutectic composition. X-ray diffraction measurement s have been performed to characterize the ordered structure and to fol low the evolution of the degree of long-range order with temperature. Results show that the transition is first order and takes place, on co oling, from the disordered to a D0(3) structure. Taking into account t hese results, a mean held model has been developed in order to study t he superlattice formation in these alloys. Present X-ray measurements together with previous calorimetric measurements of the martensitic tr ansition undergone by Cu-Al-Be alloys, have enabled a determination of their metastable phase diagram. It is remarkable that although the ad dition of a small quantity of Be has a strong effect on the martensiti c transformation, it does not modify the symmetry properties of the bi nary Cu3Al.