Temperature dependence of elastic properties of cubic and orthorhombic phases in Cu–Al–Ni shape memory alloy near their stability limits

Elastic constants reflect the fundamental thermodynamic properties of the crystal lattice in solids. Essential information on the phase stability of transforming materials can be deduced from their temperature and stress dependencies. In this work, temperature dependence of all elastic constants of the cubic austenite and orthorhombic 2H martensite phases of the Cu–Al–Ni alloy in the transformation temperature range determined by resonance ultrasonic spectroscopy are reported. All measurements were performed on the same sample, existing first as a martensite single crystal (measurement during heating) and, after the reverse transformation took place, as the austenite single crystal (measurement during cooling). It was found that some elastic constants of the austenite phase (C′) and martensite phase (C55, CMS) significantly soften when approaching transformation temperatures. The martensite softening is by far more pronounced. While the austenite softening appears solely in the directions 〈1 1 0〉A (C′ modes), the 2H martensite softens in the lattice corresponding directions [1 0 0]M and [0 0 1]M (C55 modes) as well as along directions from a zone crossing the basal plane characterized by a elastic constant CMS newly introduced in our earlier related work. The elastic anisotropy of the both phases significantly increases when approaching transformation temperatures, the anisotropy factor of martensite increases about ten times more strongly than in the case of austenite.