LOW-FREQUENCY INTERNAL-FRICTION PEAKS ASSOCIATED WITH THE MARTENSITIC PHASE-TRANSFORMATION OF NITI

Anelastic measurements in the low frequency range have been made on a martensitic equiatomic NiTi alloy. On cooling, an internal friction peak was observed with a corresponding sharp minimum of frequency at the same temperature. In some specimens, a second internal friction peak was found with no frequency minimum. In comparing the anelastic curves with resistivity measurements, these peaks were associated on the one hand with premartensitic effects and on the other hand with the martensitic transformation itself. These peaks have also been studied for various heating and cooling rates, frequencies and amplitudes of measurements and have been compared to the predictions of different theories of the internal friction observed during the phase transformation. One part of the damping, which depends on cooling and heating rate has been interpreted as the result of a plastic strain due to the preferential growth of the martensite plates in the direction of the applied stress and the other part, independent of the heating and cooling rate, has been attributed to the viscous motion of the twin dislocations. © 1979.