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

• Published in: Thin solid films Vol. 545; pp. 71 - 80
• Main Authors: Koker, M.K.A., Schaab, J., Zotov, N., Mittemeijer, E.J.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 31.10.2013; Elsevier
• Subjects: Austenite; Composition and phase identification; Condensed matter: structure, mechanical and thermal properties; Diffraction; Exact sciences and technology; Film curvature; Intermetallics; Lattice strain; Macroscopic stress; Martensitic transformations; Nickel titanides; Nickel titanium; Phase transformation kinetics; Physics; Rietveld analysis; Shape memory alloy; Shape memory alloys; Stresses; Structure and morphology; thickness; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Thin film structure and morphology; Thin films; Thin films; Film curvature; Shape memory alloy; Phase transformation kinetics; Lattice strain; Rietveld analysis; Nickel titanium; Macroscopic stress; Rietveld method; Cubic lattices; Memory; XRD; Elastic deformation; Reverse martensitic transformations; Rocking curve; Titanium; Stress effects; Martensite; Synchrotron radiation; Crystal structure; Temperature dependence; Phase transformations; Martensitic transformations; Monoclinic lattices; Heat treatments; Nickel; Microstructure; Curvature; Thermal cycling
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/j.tsf.2013.07.033

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. •The martensite crystallite size remains constant as a function of temperature.•The austenite crystallite size suggests heterogeneous nucleation and growth.•Strongly anisotropic microstrain in the martensite phase.•The state of stress in the austenite is biaxially rotationally symmetric.•Thermal lag between macroscopic stress and austenite phase fraction