Martensitic transformation of Cu-based shape-memory alloys: Elastic anisotropy and entropy change

We have studied the martensitic transformation in a family of Cu-Al-Be crystals: We have measured the entropy change of the transition using a high-sensitivity calorimeter, and the elastic anisotropy of the high-temperature phase as a function of temperature by an ultrasonic method. Both entropy change and elastic anisotropy at the transition temperature are shown to be independent of the alloy composition. On the other hand, the vibrational entropy change and the elastic anisotropy have been theoreticallay related by an expression containing the fraction of low-energy modes. This relation has been specified for two different situations: (i) The region of soft modes is limited to wave vectors very close to the [ξξ0] direction in the reciprocal space and (ii) the region of soft modes is allowed to spread up to wave vectors with directions differing substantially from [ξξ0]. Our experimental results and other results for other families of Cu-based alloys are consistent with the two relations, suggesting a small (or at least composition independent) electronic contribution to the total entropy change.