Non-generic concentrations for shape-memory alloys; the case of CuZnAl

As is well known, the remarkable properties of many active crystalline materials, such as shape-memory alloys, originate from a symmetry-breaking martensitic phase transformation. A recent analysis (see Pitteri and Zanzotto, Acta mater., 1998, 46, 225) has shown that if a special condition on the strain parameters is satisfied, the twinning ability of a crystal undergoing a cubic-to-monoclinic transformation is considerably increased. As this feature may improve the memory behavior of cubic–monoclinic alloys, it is suggested that these predictions be tested experimentally. A procedure is given to determine explicitly which “non-generic” concentrations allow suitable alloys to produce the extra twins; which concentrations give a volume-preserving transformation are also determined (this condition guarantees self-accommodation of the martensite). As an example, the ternary CuZnAl system is considered, for which, based on the available experimental data, the non-generic concentrations are established. The work indicates that there is a scarcity of the necessary data on the mapping of lattice parameters vs concentrations in alloys with a potential for shape memory. A specific experimental program aiming at the systematic gathering of this information is proposed: building such a database may prove of considerable help in the search and synthesis of new materials with improved performance.