David Armstrong
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 acces to in-situ and ex-situ high temperature nanoindentation systems which allows us to perform tests up to temperatures above 1200K. 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, Tokamak Energy, 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
- Mechanical behviour of materials for energy storage
- Development of micromechanical testing techniques
- High entropy and nanostructured alloy development alloys
- High temperature mechanical properties
- Time dependent deformation
- Ceramic composite materials for energy and aerospace
- Deformation in geological materials
New Postgraduate Research Projects Available
Selected Publications
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2020 roadmap on solid-state batteries
August 2020|Journal article|Journal of Physics: Energy -
Measuring the brittle-to-ductile transition temperature of tungsten–tantalum alloy using chevron-notched micro-cantilevers
April 2020|Journal article|Scripta Materialia -
Statistically sound application of fiber push-out method for the study of locally non-uniform interfacial properties of SiC-SiC fiber composites
April 2020|Journal article|Journal of the European Ceramic Society© 2019 Elsevier Ltd Push-out tests were performed on SiC-SiC fiber composites with single- and multi-layered pyrolytic carbon fiber-matrix interphases. It is shown that experimental scatter is significant and a large number of tests is necessary in order to obtain statistically relevant values of interfacial shear strength. A difference between the regions of an individual fiber tow is observed, linked to local porosity. Interfacial debonding occurs along the boundary between the fiber and the first carbon layer, regardless of the structure of the interphase, and therefore interfacial shear strength is not directly linked to the structure of the interphase. -
Microstructural and micromechanical assessment of aged ultra-fast sintered functionally graded iron/tungsten composites
March 2020|Journal article|Materials and DesignFunctionally graded (FG) iron/tungsten (Fe/W) composites are considered for stress-relieving interlayers in tungsten-steel joints, required in future fusion reactors. The macroscopic gradation of the two materials allows relaxation of thermally-induced stresses and hence extend the lifetime of the cyclic-loaded dissimilar materials joints. While many properties, e.g. thermal expansion and strength, of the as-manufactured Fe/W composites are promising with respect to the anticipated application, the temperature-induced microstructural changes and their effect on the material properties remain largely unexplored. Given that the thermodynamic system of FeW contains two types of intermetallic phases, understanding the microstructural changes in the FG Fe/W composites are crucial for long-term operation of fusion reactors. In the present work, the microstructure of ultra-fast sintered Fe/W composites containing 50 and 75 vol% tungsten is studied via electron microscopy (SEM) and X-ray diffraction (XRD) in as-manufactured and thermal aged conditions (300, 500, and 800 °C for up to 72 h). The hardness and modulus of selected composites are measured via nanoindentation, and the fracture toughness of the FeW interfaces is tested via notched micro-cantilever bending tests. The results from microstructural and micromechanical analyses are discussed, and the materials are evaluated for their application in fusion reactors based on the microstructure-to-property relationship.FFR, nuclear fusion, first wall, micro-cantilever bending, DEMO, tungsten-steel composites, multi-scale characterization, nanoindentation, functionally graded materials (FGM) -
Sodium/Na β″ Alumina Interface: Effect of Pressure on Voids.
January 2020|Journal article|ACS applied materials & interfacesThree-electrode studies coupled with tomographic imaging of the Na/Na-β″-alumina interface reveal that voids form in the Na metal at the interface on stripping and they accumulate on cycling, leading to increasing interfacial current density, dendrite formation on plating, short circuit, and cell failure. The process occurs above a critical current for stripping (CCS) for a given stack pressure, which sets the upper limit on current density that avoids cell failure, in line with results for the Li/solid-electrolyte interface. The pressure required to avoid cell failure varies linearly with current density, indicating that Na creep rather than diffusion per se dominates Na transport to the interface and that significant pressures are required to prevent cell death, >9 MPa at 2.5 mA·cm-2. -
Effects of neutron irradiation on the brittle to ductile transition in single crystal tungsten
December 2019|Journal article|JOURNAL OF NUCLEAR MATERIALS -
Decoration of voids with rhenium and osmium transmutation products in neutron irradiated single crystal tungsten
December 2019|Journal article|Scripta Materialia© 2019 High temperature, neutron irradiated single crystal tungsten, with a post irradiation composition of W - 1.20 ± 0.11 at.%Re - 0.11 ± 0.05 at.%Os - 0.03 ± 0.01 at.%Ta was characterised using a combination of Atom Probe Tomography (APT) and Scanning Transmission Electron Microscopy (STEM). APT showed that within nanoscale clusters of Re/Os, the atomic density was above the theoretical limit. Complimentary High Angle Annular Dark Field (HAADF) imaging shows that some clusters contain voids at their centre which are leading to APT aberrations and enhancing the atomic density. High resolution Energy Dispersive X-ray (EDX) spectroscopy shows that voids are decorated with a shell of rhenium with a small osmium cluster to one side. -
Short communication: ‘Low activation, refractory, high entropy alloys for nuclear applications’
December 2019|Journal article|Journal of Nuclear Materials -
Evaluation of Fracture Toughness Measurements Using Chevron-Notched Silicon and Tungsten Microcantilevers
October 2019|Journal article|JOM -
Radiation-induced segregation in W-Re: from kinetic Monte Carlo simulations to atom probe tomography experiments
October 2019|Journal article|EUROPEAN PHYSICAL JOURNAL B