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Department of Materials

University of Oxford

  2. David Armstrong

David Armstrong

Professor David Armstrong
Associate Professor of Materials
+44 1865 273708
david.armstrong@materials.ox.ac.uk

My research group work on understanding the behaviour of materials under extreme environments, such as radiation damage, high temperatures or high stresses. By developing an understanding of the mechanical behaviour and defects which control materials behaviour we then try and develop materials better able to operate under extreme conditions. We are now working to take technqiues we have developed for traditional engineering materials and apply them to questions in other areas such as solid state batteries for energy storage and geological materials.

Much of our work is centred on developing mechanical testing techniques at the nano and micro scale. We have a state of the art high temperature nanoindentation system which allows us to perform tests up to temperature above 1000K. These techniques are being used to study a range of important materials for both nuclear power and aerospace applications. We also work with leading groups in Oxford and elsewhere to use understanding gained from our experiments to process new materials better suited to working under extreme conditions.

Materials systems being studied include; ceramic composites, high entropy alloys, refractory alloys, high strength steels and zirconium alloys. This is carried out with a range of partners including, UKAEA, General Atomics, Rolls Royce, Karlsruhe Institute of Technology, Germany and  UC Berkeley, and University of Wisconsin-Maddison, USA, as well as many collaborators within Oxford. Particular areas of current research include:

  • Materials for nuclear fission and fusion
  • Development of micromechanical testing techniques
  • Fundamentals of fracture
  • High temperature mechanical properties
  • Time dependent deformation
  • Ceramic composite materials
  • Aerospace materials
  • Deformation in geological materials

Selected Publications

  • Linking microstructure and local mechanical properties in SiC-SiC fiber composite using micromechanical testing

    2019
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    Journal article
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    Acta Materialia
    © 2019 Acta Materialia Inc. Local mechanical properties of SiC-SiC fiber-reinforced composite – matrix, fiber and interphases – were evaluated using nanoindentation and microcantilever fracture testing. The fracture toughness was found to be ∼4.25 MPa*m 1/2 in the matrix, ∼2 MPa*m 1/2 in the fibers and ∼0.8 MPa*m 1/2 at the interphases. Nanoindentation hardness was found to vary from ∼17 GPa in the center of the fibers to ∼40 GPa in the matrix. Values obtained with micromechanical testing were found to be in good agreement with the available data on bulk mechanical properties. The mechanical property variations in the different components of the composite can be explained by the variations in the microstructure. The matrix has complex hierarchical microstructure with elongated grains, often featuring twinning, growing radially from the fibers in predominantly <111> direction and forming sets of concentric rings around them. The fibers contain equiaxed grains with carbon precipitates at the grain boundaries. It was found that in the matrix fracture is transgranular, while in the fibers it can be both trans- and intergranular; at the interphases the fracture occurs at the carbon-fiber boundary. The differences in mechanical properties between the matrix and the fibers are attributed to the presence of carbon inclusions in the fibers, which reduce both hardness and fracture toughness.

  • Temperature dependence of strain rate sensitivity, indentation size effects and pile-up in polycrystalline tungsten from 25 to 950 °C

    2018
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    Journal article
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    Materials and Design
    © 2018 Elsevier Ltd Elevated temperature nanoindentation measurements were performed on polycrystalline tungsten to 950 °C under high vacuum conditions with very low thermal drift. The temperature dependence of the hardness, elastic modulus, strain rate sensitivity, activation volume and the indentation size effect in hardness were studied. More significant time-dependent deformation was observed from 850 °C. Strain rate sensitivity determined by analysis of indentation creep data increased with temperature. Activation volume reached a peak of ~50 b3 at 750–800 °C. Decreasing activation volume >800 °C was a consequence of the increased strain rate sensitivity. For a bcc metal lattice resistance depends on T/Tc (where Tc, the critical temperature, at which flow stress becomes insensitive to temperature, is 527 °C for W); size effects would be expected scale with this relative temperature. Stronger indentation size effects in hardness were found at elevated temperatures. The influence of the time-dependent deformation on the unloading data was accounted for by a viscoelastic compliance correction. After correction the elastic moduli were to within ~1% of literature values at 750–800 °C and to within 6% at 950 °C. These small remaining differences are consistent with AFM measurements which show that pile-up is significant in these high temperature indentations.

  • On the Influence of Nb/Ti Ratio on Environmentally-Assisted Crack Growth in High-Strength Nickel-Based Superalloys

    2018
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    Journal article
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    METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE

  • The effect of helium implantation on the deformation behaviour of tungsten: X-ray micro-diffraction and nanoindentation

    2018
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    Journal article
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    SCRIPTA MATERIALIA
    Nanoindentation, Laue micro-diffraction, Ion implantation damage, Fusion armour materials

  • The effect of helium implantation on the deformation behaviour of tungsten: X-ray micro-diffraction and nanoindentation

    2018
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    Journal article
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    Scripta Materialia
    © 2017 Elsevier Ltd. The effect of helium-implantation-induced defects on deformation behaviour is examined by comparing spherical nano-indents in unimplanted and helium-implanted regions of a tungsten single crystal. Helium-implantation increases hardness and causes large pileups. 3D-resolved X-ray micro-diffraction uniquely allows examination of the complex lattice distortions beneath specific indents. In the ion-implanted material we find reduced lattice rotations and residual strains due to indentation, indicating a more confined plastic zone. Together, our observations suggest that helium-induced defects initially act as efficient obstacles to dislocation motion, but are weakened by the subsequent passage of dislocations, causing a reduction in work hardening capacity.

  • Ceramic composites: A review of toughening mechanisms and demonstration of micropillar compression for interface property extraction

    2018
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    Journal article
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    JOURNAL OF MATERIALS RESEARCH
    ceramic composite, internal friction, neutron irradiation, chemical vapor deposition, coating, toughness, fracture, nuclear materials, nano-indentation

  • Characterisation of ODS Fe-14Cr-2W-0.3Ti before and after high temperature triple and low temperature single ion irradiations

    2018
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    Journal article
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    MATERIALS CHARACTERIZATION
    Oxide dispersion strengthened steels, Ion irradiation, Transmission Electron Microscopy, Nanoindentation, Atom Probe Tomography, Nanoparticles

  • Hybrid electrolytes with 3D bicontinuous ordered ceramic and polymer microchannels for all-solid-state batteries

    2018
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    Journal article
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    ENERGY & ENVIRONMENTAL SCIENCE

  • Size effects resolve discrepancies in 40 years of work on low-temperature plasticity in olivine.

    2017
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    Journal article
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    Science Advances
    The strength of olivine at low temperatures and high stresses in Earth's lithospheric mantle exerts a critical control on many geodynamic processes, including lithospheric flexure and the formation of plate boundaries. Unfortunately, laboratory-derived values of the strength of olivine at lithospheric conditions are highly variable and significantly disagree with those inferred from geophysical observations. We demonstrate via nanoindentation that the strength of olivine depends on the length scale of deformation, with experiments on smaller volumes of material exhibiting larger yield stresses. This "size effect" resolves discrepancies among previous measurements of olivine strength using other techniques. It also corroborates the most recent flow law for olivine, which proposes a much weaker lithospheric mantle than previously estimated, thus bringing experimental measurements into closer alignment with geophysical constraints. Further implications include an increased difficulty of activating plasticity in cold, fine-grained shear zones and an impact on the evolution of fault surface roughness due to the size-dependent deformation of nanometer- to micrometer-sized asperities.

  • Development of High Temperature Nanoindentation Methodology and its Application in the Nanoindentation of Polycrystalline Tungsten in Vacuum to 950 A degrees C

    2017
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    Journal article
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    EXPERIMENTAL MECHANICS
    Nanoindentation, High temperature, Nanomechanics, Tungsten, Vacuum
    • More
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