Deformation induced martensite stabilization of NiTi in constrained composite systems

This study investigated the effect of tensile deformation on the transformation behaviour of NiTi within the constrained environment of NiTi-Nb composites. Particular attention is given to the deformation-induced stabilisation of the stress-induced martensite manifested as the increase of the reverse transformation temperature. Two different forms of the NiTi-Nb composites were used, including nanowires and microparticles, in comparison with the single-component binary NiTi alloy. This stabilisation effect has been reported for NiTi in the literature. The effect is attributed to the changes in the internal elastic and plastic states of the martensite variant structure caused by the deformation. The NiTi-Nb nanocomposites present different internal mechanics environments. It was found that the presence of a second phase within the NiTi matrix hinders the lattice distortion martensitic transformation, enlarging its hysteresis in the order of the binary NiTi, the NiTi-Nb microparticle composite and the NiTi-Nb nanowire composite. After deformation, the NiTi-Nb microparticle composite exhibited the highest stabilisation effect, followed by the NiTi-Nb nanowire composite and then the binary NiTi alloy. These contrasting observations are attributed to dual effects of the Nb inclusions within the NiTi matrix, to obstruct the lattice distortions of the martensitic transformation, and to retain elastic strains and thus stresses that may assist and resist a transformation depending on their relative directions.