NiTi and CuAlNi Martensite Studied by Isothermal Mechanical Spectroscopy

Abstract
Intemal friction has been measured in isothermal conditions over a large frequency range (5x10^-4 Hz-40 Hz) in NiTi (49.6 at.% Ni) and CuAlNi (12 wt.% Al, 3 wt.% Ni) alloys. A low frequency increase which depends on temperature has been found on NiTi sample in the martensitic phase. This damping effect has been ascribed to interface motions between martensite variants. As to confirm that the process is linked to the martensitic structure, CuAlNi al!oy was also studied. It has been observed that the low frequency damping behaviour of two samples looks totally different whether the sample has been slowly cooled from 1073 K or if it has been water quenched. Indeed, the low frequency increase is clearly evidenced in the first case contrarily to the water quenched sample where the effect is reduced and delayed as it appears only at much higher temperature. Several annealings have been performed on the quenched sample and the ageing effects have also been observed by modifications on the internal friction level. In addition, the structural evolution of this last sample has been observed by X-ray diffraction experiments.