On the origin of acoustic emission in the stress-induced martensite regime of shape memory alloys

The superelastic (SE) deformation behavior of Heusler-type Co-Ni-Ga shape memory alloy (SMA) single crystals is investigated employing complementary in situ techniques. Findings obtained by optical microscopy, neutron diffraction and acoustic emission (AE) provide deep insights into the microstructural events taking place during compressive loading. In addition to the martensitic forward and reverse transformation, unloading of the stress-induced martensite is found to be accompanied by the emission of acoustic signals. In the unloading martensite regime, neutron diffraction gives clear evidence for a change in the volume fractions of martensite domain variants upon unloading, i.e. reorientation of martensite by the growth of one martensite domain variant at the expense of the other one. Hence, it is shown that the origin of the AE signals occurring in that unloading martensite regime can be ascribed to twin boundary motion.