Lattice stability and martensitic transition in β-phase Cu-based shape memory alloys: Long-wavelength acoustic mode anharmonicity

Abstract

We report measurements of the complete set of third-order elastic constants of β-phase Cu—Al—Ni and Cu—Al—Be alloys as a function of temperature. These data provide the experimental information needed for the study of the vibrational anharmonicity of long-wavelength acoustic modes in Cu-based alloys. The long-wavelength acoustic mode Grüneisen parameters have been computed for different families of Cu-based alloys. The modes associated with the two shears necessary to accomplish the lattice distortion from the β phase to the close-packed martensite structure have been found to exhibit the largest anharmonicity. It has also been found that the Grüneisen parameters of these two relevant modes take fixed values (independent of alloy composition and structure of the martensitic phase) at the transition temperature. Based on the results, the mechanical stability of the β phase and the nucleation mechanism is discussed in terms of the vibrational anharmonicity.