MARTENSITIC PHASE IN NITI AND CUALNI STUDIED BY ISOTHERMAL MECHANICALSPECTROSCOPY

Internal friction has been measured in isothermal conditions over a la rge frequency range (10(-3) Hz-10 Hz) in NiTi (49.6% Ni) and CuAlNi (1 5% Al, 3% Ni) alloys. After convenient thermal treatments, the samples present a stable martensitic phase between room temperature and simil ar to 350 K for NiTi and similar to 873 K 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% stra ined NiTi sample, have evidenced a decrease Ea. From these results we have assumed that the damping increase could be associated to the inte rface motion of the martensite variants. In other words, the high dens ity of the variants in a non strained sample could inhibit their displ acements.