X-ray diffraction study of the reverse martensitic transformation in NiTi shape memory thin films

The development of stresses, phase fractions and the microstructure of thin equiatomic NiTi substrate-bound films was investigated during the reverse transformation from martensite to austenite. Synchrotron X-ray diffraction (XRD) experiments were performed during the heating portion of thermal cycling applied to the thin films to capture, in particular, the reverse martensitic phase transformation (monoclinic martensite → cubic austenite). The phase fractions and microstructure, as a function of temperature and thermal cycling, were analyzed through the application of Rietveld refinement to the diffraction data. Further, using the XRD data, the overall macroscopic stress in the film (derived from the curvature of the film/substrate system determined by XRD rocking curve measurements) and the stress in the austenite phase (derived from the lattice strain) during the transformation were tracked as a function of the degree of the transformation. The state of the stress in the austenite was found to remain biaxially, rotationally symmetric, even in the two-phase (martensite and austenite) film. The developments of the total stress in the film and the stresses in each of the two phases are discussed in terms of the transformation-induced volume misfit and its accommodation by elastic deformation.