Role of defectivity on the crystallography of martensitic transformations in Ti50Ni40Cu10: an XRD investigation

Martensitic transformations in Ni50Ti40Cu10 are well known to proceed with a two-step process, from B2 austenite to monoclinic B19? with intermediate orthorhombic B19. These transformations can be readily followed by X-ray diffraction especially in solution heat-treated materials through split and distribution of the main diffraction lines, while peaks broadening and overlap make the transformations more difficult to be described in highly defective materials. The present study addresses the effect of defects and chemical inhomogeneities on the martensitic transformation, placing particular emphasis on the crystallography of the low temperature B19 to B19? phase transition. Lattice strains proved to be a powerful tool to monitor the martensitic transformations: whereas a clear discontinuity is observed for the solution heat-treated sample, defects promote a continuous progressive distortion from B19 to B19?. Calorimetry and internal friction investigations were added as a reference to verify the occurrence of the transformations and define the corresponding temperatures.