Ultrasonic attenuation by dislocation formation in NiTi shape memory alloys

Dislocations are induced in polycrystalline NiTi shape memory alloy by thermal and mechanical cycling. The variation of the ultrasonic velocity and the attenuation with frequency in the range 2–60 MHz was measured at room temperature. The frequency dependent experimental data are well described in the frame of a vibrating string-model. The fits yield length and density of the dislocations in cycled and non-cycled samples. Depositing the sample at 300 K for a couple of days leads to a reduction of the dislocation density accompanied with an increase of the transition temperature.