NITI AND CUALNI MARTENSITE STUDIED BY ISOTHERMAL MECHANICAL SPECTROSCOPY

Internal friction has been measured in isothermal conditions over a la rge frequency range (5x10(-4) Hz-40 Hz) in NiTi (49.6 at.% Ni) and CuA lNi (12 wt.% Al, 3 wt.% Ni) alloys. A low frequency increase which dep ends on temperature has been found on NiTi sample in the martensitic p hase. This damping effect has been ascribed to interface motions betwe en martensite variants. As to confirm that the process is linked to th e martensitic structure, CuAlNi alloy was also studied. It has been ob served that the low frequency damping behaviour of two samples looks t otally 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 quenche d sample where the effect is reduced and delayed as it appears only at much higher temperature. Several annealings have been performed on th e quenched sample and the ageing effects have also been observed by mo difications on the internal friction level. In addition, the structura l evolution of this last sample has been observed by X-ray diffraction experiments.