Influence of martensite variant orientation on their mobility in a CuAlNi alloy

The study has been performed on a Cu-13.6Al-3.1Ni (wt. %) single crystal. Two specimens have been submitted to a treatment at 1133 K during 10 min. followed by a water quench at room temperature. One sample was longitudinally strained, with a residual deformation of 4.5 %. Then, the internal friction has been measured in isothermal conditions over a large frequency range from 40 Hz to 10-4 Hz for measurement temperatures ranged between 294 K and 497 K. The damping measured at high frequency (>10-2 Hz) on the two samples is quite similar and is not changed significantly during the transitions (direct and reverse). On the contrary, at low frequency two effects are evidenced. First of all, on cooling, a sharp damping increase is observed from the beginning of the direct transformation on both samples. The damping is then, continuously decreased down to the martensite state. These evolutions have been ascribed to the fractal growth of the martensite. Second, during the first reverse transition, i.e. the first heating, the damping measured in the strained sample exhibits a maximum. This effect is diminished for the second reverse transition and is not present on the unstrained sample. The damping increase is discussed taking into account the incidence of the variant orientation on their mobility and the likely motion of structural defects introduced by the straining