Advanced Nanoindentation Techniques for the Extraction of Material Flow Curves

Alexander Leitner

    Research output: ThesisDoctoral Thesis

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    Abstract

    For decades material scientists long for an efficient testing tool to characterize materials with respect to mechanical, but also rate-dependent properties and the possibility to conduct experiments in a broad temperature range. Nanoindentation turns out to meet these demands, but there remain uncertainties in the analysis, especially in the comparability to uniaxial data. On the example of nanoporous Au, it is demonstrated that modern nanoindentation approaches allow to comprehensively characterize the thermo-mechanical behavior of bench scale materials. The examined foam exhibits a distinct hardening-by-annealing behavior and dislocation/interface interactions as dominating deformation mechanism in the tested temperature range from ambient temperatures to 300 °C. This is followed by investigations of the impact of dynamic measurement methods on the obtained mechanical properties. Thereby it is verified that differences in hardness between static and dynamic testing protocols mainly originate from strain-rate discrepancies owed to the hold segment used in static tests. Identification of this effect allowed to introduce a predictive model which reduces the error from up to 17% down to an average of 2.5%. Accounting for this effect allows to subsequently exploit sharp tip nanoindentation by using four tips with varying apex angle equivalent to different representative strains. Testing various Ni and W samples with microstructures ranging from single crystalline to nanocrystalline enables to compare nanoindentation to Vickers hardness tests and to identify Hall-Petch parameters in dependence of the strain. Finally, flow curves are constructed using the obtained modulus and by converting hardness to equivalent stresses. Uniaxial literature data are in good accordance with obtained numbers if the indentation size effect is adequately considered. To further increase the local resolution and to maximize the output, finally spherical tip geometries are thoroughly investigated as this geometry combined with dynamic measurement techniques enables the extraction of flow curves from individual impressions. Improved calibrations of tips, based on spherical geometries, and the introduction of strain-rate controlled tests enable a reliable determination of the conversion factor between hardness and stress. The accuracy of the material dependent constraint factor is verified on body-centered cubic (W, Cr) and face-centered cubic materials (Cu, Ni) with refined microstructures. The present thesis closely investigates modern nanoindentation approaches to identify restrictions and required parameters necessary for the comparison with other testing techniques. Occurrent challenges in terms of flow curve extraction are faced by novel advancements which moves the technique one step further towards its feasibility to extract material flow curves.
    Translated title of the contributionInnovative nanomechanische Methoden zur Bestimmung der Fließkurven von Hochleistungswerkstoffen
    Original languageEnglish
    QualificationDr.mont.
    Awarding Institution
    • Montanuniversität
    Supervisors/Advisors
    • Durst, Karsten, Assessor B (external), External person
    • Kiener, Daniel, Assessor A (internal)
    Publication statusPublished - 2018

    Bibliographical note

    no embargoed

    Keywords

    • nanoindentation
    • mechanical properties
    • strain-rate effects
    • spherical indentation

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