Anharmonicity of Cu-based shape-memory alloys in the vicinity of their martensitic transition

We present measurements of the complete set of third-order elastic constant s (TOEC) of Cu-Al-Ni and Cu-Al-Be shape-memory alloys, as functions of temp erature. These data quantify the elastic anharmonic behavior of these alloy s on approaching their martensitic transformation. We provide experimental evidence that the martensitic transition in Cu-based shape-memory alloys oc curs at a fixed value (similar or equal to 60 GPa) of the TOEC combination C-3 = (1/8 root 3)(3C(112)-2C(123)-C-111), which is the relevant combinatio n for the shear mechanisms leading the bcc structure to the close-packed ma rtensite. By means of a Landau model for elastic phase transitions, the val ue of C-3 has been estimated from the crystal structure of the martensitic phase. The value obtained conforms reasonably with that found experimentall y. [S0163-1829(99)04001-1].