Neutron diffraction study of NiTi during compressive deformation and after shape-memory recovery

Neutron diffraction measurements of internal elastic strains and texture were performed during uniaxial compressive deformation of martensitic NiTi. Rietveld refinement of the diffraction spectra was performed in order to obtain lattice parameter variations and preferred orientation of martensitic variants during deformation. The elastic internal strain is proportional to the externally applied stress but strongly dependent on crystallographic orientation. Plastic deformation by matrix twinning is consistent with Type I (1-1-1) twinning, whereby (100) and (011) planes tend to align perpendicular and parallel to the load axis, respectively. The preferred orientation ratio according to the model by March and Dollase is found to be proportional to the macroscopic plastic strain for (100) and (011) planes for loading, unloading and shape-memory recovery. To the best of our knowledge, this is the first in situ bulk measurement of reversible twinning in NiTi. Finally, shape-memory recovery results in a marked change of NiTi cell parameters.