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Oxford Micromechanics Group

The Oxford Micromechanics Group are interested in how materials (engineered and naturally occurring) respond, at the microstructural level, to externally applied loading - mechanical, thermal, and/or environmental (chemical, irradiation).  The complex patterning of local stress and strain distributions and how they evolve, and are linked to particular aspects of the microstructure, provides many fascinating intellectual challenges.  Technical impact comes from building sound understanding and models of how materials fail.  This is central to setting safe performance windows, and developing new alloys and microstructures with greater capability.
Our centre of mass is in the Department of Materials, but we also span across to the Department of Engineering Science and Department of Earth Science.  We work on a range of materials systems including those for nuclear, aerospace, and automotive sectors, as well minerals.  We also have made significant contributions to development of new testing and characterisation methods allowing us to gain new insights. 

Recent Papers & Pre-Prints

  • 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

  • On the Brittle-to-Ductile Transition of the As-cast TiVNbTa Refractory High-entropy Alloy

    October 2020
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    Journal article
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    Materialia

  • Cold creep of titanium: Analysis of stress relaxation using synchrotron diffraction and crystal plasticity simulations

    October 2020
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    Journal article
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    Acta Materialia
    © 2020 It is well known that titanium and some titanium alloys creep at ambient temperature, resulting in a significant fatigue life reduction when a stress dwell is included in the fatigue cycle. It is thought that localised time dependent plasticity in ‘soft’ grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring ‘hard’ grain that is poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. In this work, synchrotron X-ray diffraction during stress relaxation experiments was performed to characterise the time dependent plastic behaviour of commercially pure titanium (grade 4). Lattice strains were measured by tracking the diffraction peak shifts from multiple plane families (21 diffraction rings) as a function of their orientation with respect to the loading direction. The critical resolved shear stress, activation energy and activation volume were established for both prismatic and basal slip modes by fitting a crystal plasticity finite element model to the lattice strain relaxation responses measured along the loading axis for three strong reflections. Prismatic slip was the easier mode having both a lower critical resolved shear stress (τcbasal = 252 MPa and τcprism = 154 MPa) and activation energy (ΔFbasal= 10.5×10−20J = 0.65 eV andΔFprism = 9.0×10−20J = 0.56 eV). The prism slip parameters correspond to a stronger strain rate sensitivity compared to basal slip. This slip system dependence on strain rate has a significant effect on stress redistribution to ‘hard’ grain orientations during cold dwell fatigue.

  • Dislocation interactions during low-temperature plasticity of olivine and their impact on the evolution of lithospheric strength

    August 2020
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    Journal article
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    Earth and Planetary Science Letters
    © 2020 The Author(s) The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, strain hardening may cause significant evolution of strength in the colder load-bearing portion of the lithosphere. Recent rheological data from low-temperature deformation experiments on olivine suggest that strain hardening occurs due to the presence of temperature-independent back stresses generated by long-range elastic interactions among dislocations. These interpretations provided the basis for a flow law that incorporates hardening by the development of back stress. Here, we test this dislocation-interaction hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of ≤4.3GPa or in a nanoindenter at applied contact stresses of ≥10.2GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 1014m−2 and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned in slip bands and interacting with dislocations of other types that act as obstacles. The resulting accumulations of dislocations in their slip planes, and associated stress heterogeneities, are consistent with strain hardening resulting from long-range back-stresses acting among dislocations and thereby support the form of the flow law for low-temperature plasticity. Based on these observations, we predict that back stresses among dislocations will impart significant mechanical anisotropy to deformed lithosphere by enhancing or reducing the effective stress. Therefore, strain history, with associated microstructural and micromechanical evolution, is an important consideration for models of lithospheric strength. The microstructural observations also provide new criteria for identifying the operation of back-stress induced strain hardening in natural samples and therefore provide a means to test the applicability of the flow law for low-temperature plasticity.
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