NEUTRON-DIFFRACTION STUDY OF LONG-RANGE ATOMIC ORDER IN CU-ZN-AL SHAPE MEMORY ALLOYS

Neutron diffraction experiments have been carried out in order to investigate the ordered structures of the high temperature phase of two different Cu-Zn-Al alloys. It has been confirmed that the alloys exhibit an L21 superstructure below a critical temperature T(c2); a B2 superstructure between T(c2) and T(c1), and a disordered BCC Structure above T(c1). The evolution of the degree of order has been investigated by measuring the change with temperature of the intensity of the 111 and 200 reflections. Results indicate that both transitions are second order. The critical temperatures have been determined as well as the critical exponent-beta. Within the experimental accuracy, it has been found that both transitions belong to the universality class of the three dimensional Ising model.