Anna Kareer

Anna Kareer
David Clarke Research Fellow
anna.kareer@materials.ox.ac.uk

My research is focused on developing novel micromechanical testing techniques, in order to quantify the mechanical properties of materials where only small volumes are accessible; the main application being structural materials used in nuclear power plants. The methods I develop will enable the extraction of bulk mechanical properties of very small volumes of materials exposed to irradiation, in order to predict their long term performance in operating conditions.

I primarily use nano-indentation with varying tip geometries along with EBSD for the microstructural analysis and strain mapping and FE modelling to extrapolate the bulk mechanical properties, up to and beyond the yield point. I am interested in advanced steels, particularly ferritic/martensitic steels and ODS varieties, exposed to reactor environments and irradiated using self-ions to emulate the damaged microstructure.

My work is in collaboration with several US research institutions including the University of Michigan and Oak Ridge National Laboratories, the UK Atomic Energy Authority and Rolls Royce Plc. I am currently funded via a Davis Clarke Fellowship award from the Energy Technologies Institute (ETI)  and the EPSRC.

Oxford MicroMechanics Group

Selected Publications

  • Scratching the surface: Elastic rotations beneath nanoscratch and nanoindentation tests

    November 2020
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    Journal article
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    Acta Materialia
    In this paper, we investigate the residual deformation field in the vicinity of nano-scratch tests using two orientations of a Berkovich tip on an (001) Cu single crystal. We compare the deformation with that from indentation, in an attempt to understand the mechanisms of deformation in tangential sliding. The lattice rotation fields are mapped experimentally using high-resolution electron backscatter diffraction (HR-EBSD) on cross-sections prepared using focused ion beam (FIB). A physically-based crystal plasticity finite element model (CPFEM) is used to simulate the lattice rotation fields, and provide insight into the 3D rotation field surrounding nano-scratch experiments, as it transitions from an initial static indentation to a steady-state scratch. The CPFEM simulations capture the experimental rotation fields with good fidelity, and show how the rotations about the scratch direction are reversed as the indenter moves away from the initial indentation.
    cond-mat.mtrl-sci

  • A more holistic characterisation of internal interfaces in a variety of materials via complementary use of transmission Kikuchi diffraction and Atom probe tomography

    October 2020
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    Journal article
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    Applied Surface Science
    © 2020 Elsevier B.V. Changes in the chemistry of internal interfaces, particularly grain boundaries, are known to affect the macroscopic properties of a wide range of material systems. Solute segregation to grain boundaries is dependent on, amongst other factors, the physical structure of the grain boundary. We demonstrate how complementary use of transmission Kikuchi diffraction (TKD) and atom probe tomography (APT) can provide a more holistic characterisation of grain boundaries in a variety of materials. Structural information is reported from TKD data for a model steel, a titanium alloy, and a multicrystalline silicon sample. Complementary APT analyses are used to determine the segregation behaviour to these interfaces. A novel specimen preparation protocol allows for the grain boundary to be positioned more reliably within the apex of an APT specimen. Meanwhile, a method that allows a grain boundary's five macroscopic degrees of freedom to be determined from TKD data alone is also proposed.

  • Short communication: ‘Low activation, refractory, high entropy alloys for nuclear applications’

    December 2019
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    Journal article
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    Journal of Nuclear Materials

  • Micro mechanical testing of candidate structural alloys for Gen-IV nuclear reactors

    August 2018
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    Journal article
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    NUCLEAR MATERIALS AND ENERGY

  • An analytical method to extract irradiation hardening from nanoindentation hardness-depth curves

    January 2018
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    Journal article
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    JOURNAL OF NUCLEAR MATERIALS
    Fe alloys, Ion-irradiation, Nano-indentation, Irradiation hardening

  • The existence of a lateral size effect and the relationship between indentation and scratch hardness in copper

    December 2016
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    Journal article
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    Philosophical Magazine
    © 2016 Informa UK Limited, trading as Taylor & Francis Group. Indentation size effects (ISEs) are well known in static indentation of materials that deform by dislocation-based mechanisms. However, whilst instrumented indentation techniques have become rapidly established as a means of determining the near-surface mechanical properties of materials, scratch testing has been much less widely used. Hardness is used in wear models as a proxy for the yield stress, and the design of materials and hard coatings has often sought to exploit size-derived performance enhancements through length-scale engineering. Yet, it is not known directly whether (or not) length-scale effects also apply to scratch (and thus wear) performance at small scales, or what the functional form of this effect is. This work directly demonstrates that there is a lateral size effect (LSE) and shows that there are questions to be answered if the use of hardness as an indicator of wear performance is to remain valid. We report on constant load scratch experiments using a Berkovich indenter on single-crystal, annealed copper, using a range of applied normal forces and compare results from three scratch hardness calculation methods to indentation hardness (ISO 14577:2002) measured on the same sample at the same loads. Scratch tests were performed with the Berkovich indenter aligned either edge forward or face forward to the scratch direction. In all cases, we demonstrate that there is a very significant (approximate factor of two) effect of scratch size (an LSE) on scratch hardness. The results also show that the deformation mechanisms occurring in scratch tests are different to those occurring beneath a static indentation and that different mechanisms dominated for different stylus orientations (face-forward vs. edge-forward orientation). This is, to our knowledge, the first direct demonstration of an LSE akin to the ISE in metallic materials. The results have significant implications for using static indentation as a predictor of deformation during wear processes.

  • The interaction between Lateral size effect and grain size when scratching polycrystalline copper using a Berkovich indenter

    December 2016
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    Journal article
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    Philosophical Magazine
    © 2016 Informa UK Limited, trading as Taylor & Francis Group. It has been reported previously that, for single and polycrystalline copper (fcc), the indentation size effect and the grain size effect (GSE) can be combined in a single length-scale-dependent deformation mechanism linked to a characteristic length-scale calculable by a dislocation-slip-distance approach (X. D. Hou and N. M. Jennett, ‘Application of a modified slip-distance theory to the indentation of single-crystal and polycrystalline copper to model the interactions between indentation size and structure size effects,’ Acta Mater., Vol. 60, pp. 4128–4135, 2012). Recently, we identified a ‘lateral size effect (LSE)’ in scratch hardness measurements in single crystal copper, where the scratch hardness increases when the scratch size is reduced (A. Kareer, X. D. Hou, N. M. Jennett and S. V. Hainsworth ‘The existence of a lateral size effect and the relationship between indentation and scratch hardness’ Philos. Mag. published online 24 March 2016). This paper investigates the effect of grain size on the scratch hardness of polycrystalline copper with average grain sizes between 1.2 and 44.4 μm, when using a Berkovich indenter. Exactly the same samples are used as in the indentation investigation by Hou et al. (‘Application of a modified slip-distance theory to the indentation of single-crystal and polycrystalline copper to model the interactions between indentation size and structure size effects,’ Acta Mater., Vol. 60, pp. 4128–4135, 2012). It is shown that, not only does the scratch hardness increase with decreasing grain size, but that the GSE and LSE combine in reciprocal length (as found previously for indentation) rather than as a superposition of individual stresses. Applying the same (as indentation) dislocation-slip-distance-based size effect model to scratch hardness yielded a good fit to the experimental data, strongly indicating that it is the slip-distance-like combined length-scale that determines scratch hardness. A comparison of the fit parameters obtained by indentation and scratch on the same samples is made and some distinct differences are identified. The most striking difference is that scratch hardness is over four times more sensitive to grain size than is indentation hardness.
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