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Title
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Influence of nano and microstructural features and defects in finegrained NiTi on the thermal and mechanical reversibility of the martensitic transformation
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Author
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Abstract
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| The main properties of Ni–Ti alloys, i.e., shape memory and superelasticity, are used in engineering applications usually through a cyclic shape recovery during thermal or strain cycling, respectively. As these properties originate from the martensitic transformation, the functional stability of the material depends on the reversibility of this martensitic transformation. Therefore, the reversibility of martensitic transformation for Ni–Ti material under thermal cycling was investigated for bulk and micro–wire Ni–Ti. In the first part of this work the effect of low temperature thermal cycling combined with room and elevated temperature aging on the martensitic transformation of some bulk and micro–wire Ni–Ti samples was studied. The cluster model was used to interpret strong structured diffuse intensities condensed in specific periodic loci in selected area electron diffraction patterns which revealed the formation of micro–domains in the shape of needle clusters of pure Ni atoms. Quantitative comparison between samples with and without a differential scanning calorimetry cycle revealed that the more DSC cycles a sample has received, the more condensed the diffuse intensity becomes which is expected to be caused by longer Ni clusters and enhancement of short–range ordering. A novel method to use a conventional twin–jet electropolishing apparatus for thin wires was also introduced. In the second part of this work in–situ transmission electron microscopy tensile tests on nano–scale single crystals and polycrystalline Ni–Ti specimens was performed. The formation of stress–induced martensite was observed and stress–strain curves were plotted based on the obtained mechanical data. The stress plateau height shows an increase by decreasing specimen thickness but remains independent of the grain size since the latter is, on average, larger than the specimen thickness. Martensitic transformation starts at edges of the specimen for the single crystal and on the edges and grain boundaries for the polycrystalline specimen. When a martensite plate approaches a grain boundary in the polycrystalline specimen, it provokes the transformation in the neighboring grain at the other side of the grain boundary. After releasing the load, depending on the totally induced strain, some residual martensite remains in the specimen indicating the existence of induced plasticity in the martensite at large strains. |
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Language
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English
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Publication
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Antwerpen
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Universiteit Antwerpen, Faculteit Wetenschappen, Departement Fysica
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2020
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Volume/pages
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166 p.
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Note
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Verlinden, B. [Supervisor]
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Schryvers, D. [Supervisor]
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Full text (open access)
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