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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 Tc2; a B2 superstructure between Tc2 and Tc1, and a disordered BCC structure above Tc1. 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.

The present work deals with the analysis of the atomic-order dependence of the elastic constants of a BCC binary alloy near the A3B composition. The authors assume that each atom interacts with nearest and next-nearest neighbours through a pairwise interatomic potential. The configurational phase diagram of the system has been obtained from Monte Carlo simulation of a lattice model for such an alloy system. The DO3, B2 and A2 phases are found to be stable for appropriate values of the interaction parameters. The elastic constants of partially ordered configurations are calculated at T=0 K, after quenches from different temperatures Tq ranging over the stability regions of the three phases. The results are compared with experimental data corresponding to the Cu-Zn-Al shape memory alloy and their relevance in connection with the martensitic transformation is discussed.