Texture, Strain, and Phase-Fraction Measurements during Mechanical Cycling in Superelastic NiTi

parameters in-situ during the transformation and to obtain SUPERELASTICITY is a deformation mechanism phase-specific information. However, neutrons penetrate observed in NiTi and other allotropic, ordered alloys where deeper than X-rays (several millimeters, compared to a few the high-temperature cubic phase (austenite), when subjected micrometers for X-rays from a conventional source). Thus, to an external applied stress, transforms to a monoclinic neutrons are uniquely suited to study the bulk behavior of phase (martensite), thus producing a macroscopic strain that polycrystalline samples without free-surface effects. This augments the elastic strain. On removing the stress, the has been demonstrated in previous publications, where the martensite is no longer stable and transforms back to austenphase fraction, texture, and strain evolution of mechanically ite, resulting in a complete recovery of the macroscopic loaded superelastic NiTi were characterized by in-situ neustrain at the end of the mechanical cycle.[1,2] In various tron diffraction during a single mechanical cycle.[12,13] engineering applications,[3] superelastic alloys are subjected In the present study, we report on the use of in-situ neutron to repetitive loading and unloading accompanied by forward diffraction during multiple compressive load-unload cycles and reverse martensitic transformations of the parent austento monitor the evolution of the stress-induced transformation ite phase. in polycrystalline NiTi. As any initial texture is expected to Previous studies[4–11] have documented changes in the significantly affect the properties of the transformation,[14] stress-strain response of cyclically loaded superelastic NiTi. we used a polycrystalline sample with randomly oriented Some of the changes previously observed with an increasing grains, unlike most of the previous studies cited earlier. number of cycles include an increase in the residual plastic Furthermore, we used compressive cycles, which have rarely strain in the unloaded state; a decrease in the stresses at been investigated but are known to differ from tensile which the martensitic transformation starts and finishes; a cycles.[15] reduction in the load-unload stress hysteresis; and a decrease