Martensitic Phase in NiTi and CuAlNi Studied by Isothermal Mechanical Spectroscopy

Abstract
Internal friction has been measured in isothermal conditions over a large frequency range (10^-3 Hz-10 Hz) in NiTi (49.6%Ni) and CuAlNi (15% Al, 3%Ni) alloys. After convenient thermal treatments, the samples present a stable martensitic phase between room temperature and ≈350K for NiTi and ≈873K for CuAlNi. We have found an increase of Q^-1 at very low frequencies which depend on temperature. The activation energy deduced from different spectra is 0.85 eV for NiTi and 0.45 eV for CuAlNi. Other experiments, performed on a 6% strained NiTi sample, have evidenced a decrease Ea. From these results we have assumed that the damping increase could be associated to the interface motion of the martemite variants. In other words, the high density of the variants in a non strained sample could inhibit their displacements.