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David Armstrong

Professor David Armstrong
Associate Professor or Materials
Corpus Christi College
Royal Academy of Engineering Research Fellow

Department of Materials
University of Oxford
16 Parks Road
Oxford OX1 3PH
UK

Tel: +44 1865 273708 (Room 110.20.08)
Tel: +44 1865 273777 (reception)
Fax: +44 1865 273789 (general fax)

Materials for Fusion and Fission Power
Oxford Micromechanics

Summary of Interests

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

 

Current Research Projects

Micromechanical testing
Dr David E.J. Armstrong, Dr Jicheng Gong, Prof Angus J. Wilkinson
The project develops new methods of testing mechanical properties at the micron scale, using a combination of focussed ion beam machining (to produce specimens) and atomic force microscopy / nanoindentation (to test them). The methods are applied to testing thin films, ion-irradiated layers, interfaces and properties of individual grains and grain boundaries in alloys. A newly-commissioned machine will enable tests to be perfomed in the temperature range -50 to +750C. Supported by EPSRC and CCFE Culham (Junior Research Fellowship at St Edmund Hall: D. Armstrong)

Tungsten for fusion power applications
R. Abernethy, C. Beck, Dr. D.E.J. Armstrong, Professor S.G. Roberts, Dr, M. Reith*
Tunsten and tungsten alloys (especailly W-Ta and W-Re) are candidate materials for the "divertor" (helium exhaust) of a fusion power plant. During operation, neutron irradiation gradually transmutes tungsten into W-Re then W-Re-Os alloys. This, and the radiation danage itself, will affect its mechanical properties.  In this project, we fabricate W-Re and W-Ta alloys and subject them to W-ion irradiation, mimicking the neutron irradiation. Micromechanical test methods are used to study effects of radiation on strength. The migration of Re & Ta to grain boundaries, and how the this changes grain boundary strength, will also be studied. Funded by EPSRC and CCFE. (* Karlsruhe Institute of Technology)

Understanding Mechanical Size Effects in Oxide Dispersion Strengthened Steels
C. Jones, Dr D.E.J. Armstrong, Professor S.G. Roberts
For high temperature applications in both future fusion and fission reactors oxide dispersion strengthened steels (ODS Steels) are seen promising materials. However there is a lack of data available on how the mechanical properties (both yield strength and fracture toughness) change after neutron irradiation. Due to the time and cost constraints of neutron irradiation ion implantation can be used to mimic the neutron damage, however the damaged layer is typically only a few microns deep. Micro-cantilever bending techniques can be used to study the mechanical properties of the damaged layer, but to extract useful engineering properties the influence of size effects, due to the reduced specimen size, and how these vary with key microstructural length scales, must be understood. This project is using both Eurofer97 and ODS alloys developed in Oxford to study the size effects present in micro-cantilever tests and developing methods to allow comparisons of results from micro and macro mechanical tests.

Brittle-ductile transitions in BCC metals for fusion power applications
Dr D.E.J. Armstrong, Dr. E. Tarleton, Professor S.G. Roberts, Dr. A.J. Wilkinson, Professor S.L. Dudarev*
The project investigates the brittle-to-ductile transition in tungsten and iron-chromium alloys up to 12%Cr (all these metals are the basis for proposed fusion power plant alloys). Pre-cracked miniature bend specimens of single crystals and polycrystalline materials are fracture tested in the temperature range 77 - 450K. The effect of dislocation motion around the crack tips on fracture stress is examined, and modelled using dynamic-dislocation simulations. Funded by EPSRC and CCFE. (*EURATOM/CCFE)

Processing and properties of tungsten coatings for fusion reactors
E. Rowe, Dr D. Armstrong, Professor P.S. Grant
Tungsten is the key plasma facing material for use in any future nuclear fusion device due to its high melting point, good sputter resistance and low activity. However its refractory nature leads to inherent difficulties in its processing and many traditional production routes are not available. Vacuum plasma spraying is one of the most attractive methods of producing tungsten coatings for this application, but thermal mis-match between the tungsten and substrates such as steel or copper lead to the development of complex residual stresses which degrade the performance of the coating. Other challenges include microstructural control, and characterising the properties of tungsten in the coating arrangement. This project will use recently upgraded vacuum plasma spraying equipment to produce both pure and alloyed tungsten coatings on novel substrates. In-situ process data will be recorded including temperatures, deflections, etc. Coatings will be characterised using state of the art microscopy and micro-mechanical testing facilities, as well as thermal loading, and finite element analysis used to understand the evolution of the stress state. By consideration of the process conditions in manufacture, the microstructure and the properties of the coatings, optimisation will be performed to produce practical tungsten coatings for further testing and application. Funded by EPSRC and CCFE

Silicon Carbide for Nuclear Applications
A. Leide, H. Pratt. Dr E. Zayachuck, Professor R.I. Todd Professor S.G. Roberts, Dr D.E.J. Armstrong
We are undertaking research into a range of silicon carbide based materials for nuclear applications. These include direct sintered materials for fundamental studied of radiation damage, SiC/SiC composites for use as fuel cladding in next generation fission reactors and reaction bonded silicon for use in future nuclear fusion systems. We are using micromechanical testing and high resolution microscopy to understand the complex deformation processes that occur in these materials and to understand how they will behave under extreme radiation conditions. In addition we are developing our own processing capabilities for RB-SiC. This work is in collaboration with UC-Berkeley, General Atomics and UKAEA. 

Micro-mechanical Testing of the hex-BN Interlayer in SiC/SiC Composites for Aerospace Application
Robin De Meyere, Prof. David Armstrong, Prof. James Marrow
This project - funded by Rolls-Royce plc - investigates the degradation of the hex-BN interlayer in SiC/SiC composites used in engine components. Silicon carbide fibre/silicon carbide matrix composites are finding renewed interest in the aerospace community for use as high temperature components in engines due to the potential for significant weight savings over metallic parts. Due to the low fracture toughness of monolithic silicon carbide, the most promising concepts use silicon carbide fibres embedded in a silicon carbide matrix. The toughness of the composite is achieved by the application of interphase coating to the fibre before the matrix is applied, which allows for cracks to deviate from the matrix, increasing toughness from fibre pullout and crack bridging. It is critical for the interphase coating to maintain the appropriate properties (modulus, interfacial shear strength, friction, etc.) to maintain the desired composite behaviour. Over time, the interphase coating can degrade due to cyclic loading and environmental exposures as a result of frictional heating, oxidation, and other environmental mechanisms. These can change the properties of the fibre interphase resulting in regions that are embrittled or have poor load sharing. This project aims to use novel micro-mechanical testing methods to measure the strength and toughness of the individual composite components as well as the interfaces in virgin material, and in materials which have been environmentally aged for different lengths of time. These tests will be carried out both at room temperature and elevated temperatures up to 1000ºC in the hot-temperature nano-indenter. Advanced electron microscopy techniques, including EDX, EBSD and TKD will be used to image both environmental and mechanical damage. Macro mechanical-testing in conjunction with X-ray tomography (both lab and synchrotron based) will be performed and contrasted to finite element simulations, in order to relate the micro-scale properties as a function of temperature and degradation to bulk behaviour. 

7 public active projects

Research Publications

For an upto date publication list please follow the link below 

D.E.J. Armstrong Publication List

Projects Available

Mechanical Behaviour of solid state lithium ion batteries
Professor David Armstrong, Professor Peter Bruce

Developing a solid state lithium ion battery would have a transformative impact on energy storage, particularly in the area of personal transport, especially if it can be combined with a metallic lithium anode. This would facilitate increased distances through increased energy densities, and improved safety through the removal of the reactive liquid electrolytes. Whilst there are a large range of materials systems being studied and many different cell and battery architectures under development deployment of these is hampered by a fundamental understanding of the mechanical properties of the materials. This is of upmost importance as the anode, cathode and electrolyte are by necessity in contact in a SSB and as the system charges and discharges stresses are developed and the microstructure evolves and eventually  these limit the system lifetimes.

This project will develop a fundamental understanding of the mechanical properties of the ceramic lithium ion conductors such as  Li1.4Al0.4Ge1.6(PO4)3 (LAGP) and  Li7La3Zr2O12 (LLZO) which have been shown to be promising electrolyte materials for solid state lithium ion batteries. While their electrochemical properties have been well studied there is comparatively little information on the mechanical properties of these materials. This will then be followed by measuring at interfacial mechanical behaviour of these materials in contact with both anode and cathode materials. The data produced in this way will not only be useful for seeding mechanical models of cells and batteries but also allow optimisation of processing routes for producing electrolytes with improved lifetimes.

This project will use a range of nano and mico-mechanical indentation methods to study, the hardness, elastic modulus, yield stress and fracture toughness of both materials. These properties will be related to local microstructural features through the use of scanning electron microscopy (SEM), Electron back scattered diffraction (EBSD) and Raman Spectroscopy. In particular a newly commissioned pico-indenter in an SEM intergrated glove box will allow testing of reactive materials susch as LLZO and lithium without exposure to the air. This system is globally unique and has been funded by both the Faraday and Royce Institutes with which this project will interact.

This project would suit a graduate with a background in materials science or engineering with a strong and demonstrable interest in working on the integration of mechanical testing, materials processing and electrochemistry.  

 

Also see homepages: David Armstrong

Understanding degradation of nuclear steels using micro-mechanical methods
Professor David Armstrong,

Nuclear grade steels are widely used across the nuclear industry in a wide range of structural applications. Typically in these environments they are exposed for long time periods (upto 80 years in future civil reactors) to high temperatures and radiation damage. This can result in degradation of mechanical properties through formation of new precipitates and phases which are not seen at shorter time periods at lower temperatures or without radiation damage. Simulation of this is difficult and so called thermal aging treatments are used to accelerate the formation of phases. Whilst much is understood about the chemical and microstructural changes how this effects the mechanical properties is less well understood. This project will develop in-situ high temperature micro-mechanical methods to study the degredation induced in these samples and compare it to conventional macroscopic data, produced by industrial collaborators.

The proposed project involves the application of a range of micromechanical techniques to specific nuclear steels; the mechanical properties of most interest are yield strength, work hardening behaviour, hardness and fracture toughness.  A range of materials of key interest will be studied in the datum condition and after irradiated by ion implantation or test reactor exposure as appropriate. Testing will be carried out from room temperature to reactor specific temperatures in vac (likely 350oC). Thermal ageing post exposure will be explored for selected samples. For the samples which show the most change or interesting results additional in-situ tests will be performed in the SEM, using a newly purchased Pico Indenter. This will allow direct observation and local strain measurements to be by developing robust methods for performing DIC on these tests.

Typical specimen length scales are sub-micron to tens of microns and test geometries include tensile tests, compression and micro-bending. This small scale offers radically reduced difficulty and costs associated with testing irradiated materials. The small size of the tests, however, creates a size effect in the results that must be understood in detail to maximise the usefulness of the micromechanical testing results. These methods are widely used in other projects involving industrial collaborators studying both fundamentals of deformation as well as in service materials failures in a range of materials including titanium alloys, nickel super alloys and ceramic composites. This project is expected to have additional industrial funding confirmed in January 2019.

 

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Micro-Mechanical Properties of Dissimilar Metals Diffusion Bonds
Professor David Armstrong

Diffusion bonding is an attractive method of making high quality joints between materials (similar or dissimilar) based on the solid state diffusion of atomic species between the two parts being joined. As no filler material is required the quality of the joint can be very high and little residual stress is generated compared to traditional welding methods. However in the case of dissimilar metal joints interdiffusion will result in a compositional gradient. Depending on the phase diagrams and reaction kinetics this can result in the generation of a range of second phases, often intermetallic compounds which are usually brittle and deleterious to bulk mechanical behaviour.

This project will use a combination of high resolution SEM based EDX and EBSD and TEM based diffraction and EDX microscopy to study the production of such phases in a range of diffusion couples based on the copper-titanium, copper-iron and iron-titanium systems. These systems have been chosen as they represent the three common metallic crystal structures, and are all of interested in the nuclear community. The mechanical properties of the phases produced will be studied using ultra high resolution nanoindentation using newly installed equipment, funded through the Royce Institute.

There are three major outputs envisaged for this project. One is the development of the testing methodologies and protocols for studying chemo-mechanical relationships in diffusion bonds. This is of interest to diffusion bonds in the nuclear industry which are not easily studied, due to cost and safety, and as such not suitable for development work. Secondly the systems chosen are of technological interest to the wider nuclear industry and understanding the evolution of joints in these will allow better design of complex structures such as the divertor in fusion rectors. Thirdly diffusion bonds can be used to generate a wide compositional gradient which can be used for combinatorial alloy design and gaining a better understanding of

The project will be in collaboration with industrial contacts in the nuclear industry and may also be funded through the nuclear fusion CDT. It would suit a student with strong background in materials science or engineering with an interest in developing high resolution micromechanical testing methods and applying these.

Brief summary of the new project available

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Exploring metal plasticity through atomic imaging of core structure
Prof P D Nellist, Prof D E J Armstrong

Almost all materials we use in our civilisation are crystals, and the things that make crystals interesting are their defects. One of the most important crystals defects are dislocations, and they are key to understanding how materials deform plastically. In some materials, for examples the tungsten used in fusion reactors, certain types of dislocations can behave in unusual ways, by having low mobility making the materials much more brittle. The explanation of this unusual behaviour probably lies in the detailed atomic arrangement at the core of the dislocation, but a full 3D characterisation of such defects has not before been possible. Here we make use of a novel “optical sectioning” procedure we have developed in our laboratory to determine the structure of dislocations at atomic resolution in 3D using electron microscopy. Using this approach to relate atomic structure to materials properties allows the rational design of alloys to improve the ductility of important structural materials.

This project would suit someone who enjoys challenging experiments but also wants to experience the excitement of seeing atoms in materials. In addition to hands on experiments, the project will involve data processing using scripting in software packages such as Matlab.

Also see homepages: David Armstrong Peter Nellist

Understanding High Temperature Small Scale Mechanical Performance of Materials for Nuclear Fusion
Dr D.E.J. Armstrong, Dr E. Tarleton, Professor A.J. Wilkinson,

Future nuclear power systems, both fission and fusion, rely on the development of materials which can withstand some of the most extreme engineering environments. These include temperatures up to 1500oC, high fluxes of high energy neutrons and effects of gaseous elements produced by transmutation and implantation from the plasmas. Due to efforts to minimise the production of nuclear waste by such reactors the elements which may be used in structural components is limited and in many cases there is a lack of understanding of the basic deformation processes occur in ether pure materials or alloys and importantly how these are affected by temperature, radiation damage and gas content. This project will build upon the expertise in the MFFP and Micromechanics groups on high temperature mechanical testing at the micro and nano-scale. Facilities include two high temperature nanoindenters (-50oC to 950oC), high temperature microhardness (RT to 1500oC) and dedicated FIB-SEM and FEG-SEM with EBSD as well as state of the art computer codes for strain gradient crystal plasticity finite element modelling and discrete dislocation plasticity modelling. Both nanoindentation, micro-compression and micro-bend experiments will be used to study plastic deformation, fracture and creep in a range of novel high temperature materials (likely Fe, SiC or Zr based) with potential for use in future fusion reactors. HR-EBSD and AFM will be used to study deformation structures produced during testing and to inform strain gradient crystal plasticity finite element and discrete dislocation models. This will allow for a fuller understanding of the underlying physics of deformation in these materials both before and after irradiation or gas implantation. Strong links will be made to activities within the Science and Technology of Fusion Energy (EPSRC Centre for Doctoral Training) and the Culham Centre for Fusion Energy.

Also see homepages: David Armstrong Edmund Tarleton Angus Wilkinson

Also see a full listing of New projects available within the Department of Materials.