In-situ synchrotron X-ray diffraction texture analysis of tensile deformation of nanocrystalline NiTi wire in martensite state

Tensile deformation of nanocrystalline superelastic NiTi wire in the martensite state at -90 °C until fracture was investigated by in-situ synchrotron X-ray diffraction texture analysis. The results were interpreted in terms of the activity of various transformation and deformation twinning modes. The strain components into the load axis as well as discontinuous rotations of the martensite crystal lattice due to activation of various twinning modes were calculated using an abstraction of “ideal fibre textured polycrystal” and used to estimate the evolution of the martensite texture during the tensile test. It is found that the four fibre texture of selfaccommodated martensite transforms during the martensite reorientation stage in the tensile test into the sharp two fibre texture of the reoriented martensite reflecting its (001) compound twinned microstructure. Plastic deformation of martensite is claimed to proceed via peculiar deformation mechanism involving deformation twinning and kinking assisted by [100]/(011) dislocation slip. This deformation mechanism enables the nanocrystalline NiTi wire to deform at ∼1 GPa engineering stress up to very large plastic strains ∼50% with small strain hardening and brings about refinement of the austenitic microstructure of the NiTi wire down to nanoscale.