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

We report measurements of the complete set of third-order elastic constants of beta-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 al loys. The long-wavelength acoustic mode Gruneisen parameters have been comp uted for different families of Cu-based alloys. The modes associated with t he two shears necessary to accomplish the lattice distortion from the beta phase to the close-packed martensite structure have been found to exhibit t he largest anharmonicity. It has also been found that the Gruneisen paramet ers of these two relevant modes take fixed values (independent of alloy com position and structure of the martensitic phase) at the transition temperat ure. Based on the results, the mechanical stability of the beta phase and t he nucleation mechanism is discussed in terms of the vibrational anharmonic ity.