Browsing by Author "Koker, Margaret Kolbe Annellen"

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Diffraction analysis of materials in a state of stress: elastic loading and phase transformations
(2013) Koker, Margaret Kolbe Annellen; Mittemeijer, Eric J. (Prof. Dr. Ir.)
Externally or internally applied stresses and strains can pronouncedly influence material properties. Hence, the role of stress on material behavior is an important and developing field of research. Variations in stress throughout a material can lead to either strengthening or weakening of a specimen or engineering component. A clear understanding of stress and strain, and the ability to predict the magnitude of its variation, in a material (as a function of material processing), in the absence and presence of external loading, is of utmost importance to optimize material properties. Lattice-strain variation in massive, polycrystalline aggregates provides a wealth of information about the grain interaction in an externally loaded specimen. Each grain within the body is confined by its neighbors, and the compliance of these neighboring bodies provides the extent to which a grain in a massive polycrystalline body may deform under loading. As single crystals (with tungsten being an exception) are intrinsically elastically anisotropic, the direction of the applied loading with respect to the grain's crystallographic orientation must also be considered. Various elastic grain-interaction models can be used to approximate the average lattice strain within a crystallite based on its orientation with respect to the aggregate and the external loading. Each of these grain-interaction models is based on its own set of assumptions for the grain interaction. (See Table 2.1 in Ch. 2 for details on each of the discussed elastic grain-interaction models.) Two main categories of grain interaction can be defined: (i) isotropic grain interaction, where the interactions of the grains in all directions adhere to the same assumptions, and (ii) anisotropic grain interaction, where, conversely, the interactions of the grains do not adhere to the same assumptions in all directions. Three categories of strain variation may be present in an elastically loaded polycrystalline aggregate: (i) macro-, (ii) meso-, and (iii) microvariation in strain. The applicability of each set of grain-interaction assumptions (e.g. the individual grain-interaction model) is highly dependent on the specimen and the loading conditions. One shortcoming of the elastic grain-interaction models is that all grains of the same crystallographic orientation are considered to experience identical (average) lattice strains. Also, the lattice-strain variation within an individual grain cannot be calculated according to the models. Hence, these models only calculate approximate solutions for the macrovariation of strain and a portion of the mesovariation of strain, not taking into account any strain variation induced by local heterogeneities in the neighborhood around the individual crystallites. Therefore, the elastic grain-interaction models provide only an underestimate of the total strain variation in a loaded, polycrystalline body. Near equiatomic compositions of NiTi are shape memory alloys, which are characterized by two unique behaviors: pseudoelasticity (also called super elasticity) and shape memory effect. These material properties make NiTi thin films a prime candidate for application in microelectromechanical systems (MEMS). Due to the abrupt structural transition associated with the phase transformation, the material lends itself well to investigation via x-ray diffraction (XRD) techniques. Diffraction line-profile analysis of XRD patterns during such in situ experiments is a powerful tool. The study in Ch. 4 focuses on synchrotron XRD experiments of substrate-bound NiTi thin films (49.2(5) at.%Ni) during in situ heating. Through in situ high temperature XRD measurements, experiments have been performed to track the phase fraction, stress, and crystallite size of the phases during the transformation. A combination of XRD techniques were applied for the investigation to measure the phase fraction (Rietveld analysis) and stresses (curvature and sin2psi methods) as a function of temperature. Measurements of similar samples demonstrated good reproducibility. The macroscopic film stress was observed to increase with temperature. The stresses pertaining to the individual phases were separated, demonstrating that the magnitude of stress is highly dependent on the phase fraction of austenite in the substrate-bound NiTi thin film. Despite increasing in magnitude, the stress in the austenite phase remains biaxially, rotationally symmetric throughout the entire transformation.