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My group is concerned with the mechanical behaviour of materials, mostly metals and alloys, at the microstructural level. We are at the forefront of developing the SEM based EBSD technique for mapping stress distributions at high spatial resolution. Other techniques used extensively within the group are nano-indentation, micro-cantilever bend testing and micro-pillar compression testing, digital image correlation and dislocation-based modelling. We aim to help improve mechanistic understanding of deformation, fracture and fatigue processes in metals and alloys. Currently we are working on Ti, Cu, Ni, Fe-Cr and W based alloys.

High Resolution EBSD - Mapping Strain, Lattice Rotation and Dislocation Density
Dr. A.J. Wilkinson, Dr T. B. Britton, Jun Jiang
We are continuing to develop the cross-correlation based analysis of electron back scatter diffraction (EBSD) patterns that was pioneered by the group for mapping elastic strain tensors at high spatial resolution. Current interests are (i) in using the measured lattice curvatures to map the geometrically necessary dislocation density, (ii) in moving from our small deformation formulation to a finite strain one, (iii) overcoming the ‘reference pattern problem’. The method is being applied to a wide range of materials systems including semiconductors (SiGe/Si, and GaN) and metals (Ti, Zr, Ni, Cu, Fe-Cr, and W alloys deformed in various ways).    

Micromechanical Testing of Ti Alloys
Dr J Gong, Dr A. J. Wilkinson
We are using FIB to cut micron scale cantilevers in polycrystalline Ti alloy samples. The cantilevers are then tested in bending using a nanoindenter. The load-displacement data is analysed using crystal plasticity finite element simulations to extract elastic constants (Young’s modulus) and plastic flow properties (critical resolved shear stress). We are examining the effects of alloying additions and the presence of beta phase on the CRSS of the various slip systems in near alpha alloys. We are also investigating the effects of individual grain boundaries on plastic deformation. (Funded by EPSRC with support from Rolls Royce)

Fundamentals of cyclic deformation and fatigue crack initiation
Jun Jiang, Dr. A.J. Wilkinson, Professor S.G. Roberts, Professor F. P.E. Dunne*
Plastic deformation of metals leads to an increase in the dislocation content, the development of residual stresses and changes in the flow stress. In a polycrystal grain-grain interactions lead to inter- and intra-granular inhomogeneity in the dislocation density and residual stress distributions. Characterising the evolution of this patterning is key to understanding strain localisation and subsequent crack initiation. We are using the high resolution EBSD method to map geometrically necessary dislocation density in model FCC polycrystals (Cu, brass) deformed in tension to varying strain levels, and in fatigue to varying numbers of cycles. Results will be used to guide development of crystal plasticity models by our collaborators. (*Engineering Science, Oxford)

Local stress fields and deformation generated by twinning and martensitic phase transformation
Dr A. J. Wilkinson
Deformation twinning makes a sizeable contribution to the plastic deformation of many bcc and hcp metals and alloys.  The twinning shear strain is often very large, and consequently leads to very high, but localised, stresses within the surrounding matrix.  Further plastic flow through dislocation motion can occur in the matrix so as to relax the high stresses and accommodate some of the twinning strain. Furthermore deformation twinning is often cited as a mechanicsm leading to fracture initiation.  Martensitic phase transformations have some similarity with deformation twinning though hydrostatic strains can be caused in addition to shearing.  In this project the new cross-correlation based EBSD method will be used to map elastic strains (ie stresses) and lattice rotations around deformation twins and martensite laths.  The following interactions will be studied: twin-grain boundary, twin-dislocation slip, twin-crack and twin-twin.  The project will concentrate on fundamental aspect of these processes using various model material systems.

Micromechanical testing
Dr. D.E.J. Armstrong, Dr T.B. Britton, Dr J. Gong, Professor S.G. Roberts, Dr. A.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)

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)

Mechanical properties of multicrystalline silicon for solar cell applications
B.R. Mansfield, Dr. T.B. Britton, Dr. D.E.J. Armstrong, Dr. S. Senkader*, Dr. A.J. Wilkinson, Dr. J.D. Murphy
The thickness of multi-crystalline silicon (mc-Si) wafers used for photovoltaic applications is predicted to halve in the next five to ten years to just 100microns. The mechanical properties of such wafers are therefore of increasing importance and this project aims to further the understanding of these properties. The research will focus on effects pertaining to silicon carbide and silicon nitride precipitates - defects which are abundant in cast mc-Si. The project exploits novel methods of testing materials at the micron scale which have been developed in Oxford. These use a combination of focussed ion beam machining (to produce specimens) and atomic force microscopy / nanoindentation (to test them). The fracture toughness of individual grain boundaries decorated with nitride and carbide precipitates will be measured. Further experiments will be performed using high-resolution electron backscatter diffraction strain mapping to understand strains around these particles. The research is being performed in collaboration with leading suppliers of silicon to the photovoltaic industry. (* REC Wafer, Norway).

7 public active projects

Micromechanical testing of individual grain boundaries
A J Wilkinson / S G Roberts / R I Todd

Marked differences exist in the mechanical behaviour of different types of grain boundaries in metals and alloys and this has a large impact on their overall mechanical properties. Within this project we will further develop our micro-mechanical testing methodologies so as to study a range of grain boundary behaviours, including dislocation generation, slip transfer/transmission, grain boundary sliding as a function of grain boundary geometry. A focus ion beam (FIB) will be used to fabricate micro-cantilever beams which contain a single isolated grain boundary. Load-displacement data for the deformation of such micro-cantilevers will be generated using a nano-indenter. AFM, EBSD and TEM methods will be used to characterize the grain boundaries and micro-cantilevers before and after deformation. Studies will centre on simple model systems (ie elemental metals) and be aimed at generating fundamental understanding of grain boundary related phenomena.

Also see homepages: Steve Roberts Richard Todd Angus Wilkinson

Micromechanical testing of Zirconium and it alloys
A J Wilkinson

Zr and its alloys are of great importance within the nuclear industry and are used in safety critical components.  Their elastic and plastic properties are known to be highly anisotropic so that deformation processing leads to strong crystallographic textures and significant directionality in the performance of the final product.  There are still significant gaps in the fundamental understanding of deformation in these systems.  For example, the few data available for critical resolved shear stress (CRSS) in the literature show huge scatter, and do not even agree on which slip systems are the most easily activated.  In this project a FIB will be used to machine micron-scale cantilever beams within suitably oriented grains of polycrystalline samples, which will then be taken to a nano-indenter for mechanical testing.  The elastic and plastic behaviour will be analysed by comparing experimental load-displacement data with the response of finite element crystal plasticity simulations, so as to determine Young’s modulus and CRSS values.  In the latter stages of the programme methods may be adapted to explore the effects of irradiation damage, hydride percipitates and/or temperature on the mechanical deformation.

Also see homepages: Angus Wilkinson

Fundamentals of cyclic deformation and fatigue crack initiation
A J Wilkinson / S G Roberts / F P E Dunne (Eng. Sci.)

Understanding of the basic nature of fatigue crack initiation and growth in metals has long been a problem. The SEM based techniques of ECCI and EBSD allow detailed study of the interaction of dislocation patterning and crack initiation and growth; data can be gathered easily from a large specimen volume throughout the whole fatigue life. The work will be on pure Cu and Cu-Al, or Cu-Zn alloys. The aim is to study how variations in, grain size, solid solution hardening and in stacking-fault energy, both of which are dependent on alloy chemistry, affect the cyclic stress-strain curve, the dislocation patterning behaviour and ultimately the initiation and growth of fatigue cracks.  Knowledge gained from experimental studies will be used to guide development and validate crystal plasticity simulations undertaken in collaboration with Prof Dunne in Engineering Science.

Also see homepages: Steve Roberts Angus Wilkinson

Nanoscale mapping of local strains in semiconductors
A J Wilkinson

Our development of cross-correlation based analysis of EBSD has produced a step change in performance allowing elastic strain and lattice rotations to be measured in the SEM at high spatial resolution and with sensitivity that is competitive with large synchrotron facilities. The project will continue pushing this development by establishing routes to determine the detector geometry with the accuracy required to allow the use of dynamical diffraction simulations as reference patterns. This will dramatically increase the scope of the method. The method will be use it to map strains distributions in a variety of semiconductor devices and structures, including (i) GaN epilayers which have been grown using lateral overgrowth from misfitting substrates in an attempt to minimise defect density, (ii) SiGe epilayers grown on Si, and GaAs/AlGaAs vertical cavity surface emitting laser structures. The EBSD analysis will be complemented by finite element modeling of strains distributions and cathodoluminescence, AFM and TEM observations as appropriate.

Also see homepages: Angus Wilkinson

Materials for fission and fusion power
S G Roberts / A J Wilkinson / P Bagot / P S Grant

Do you want to help to solve the future energy crisis? Are you interested in doing novel and exciting experimental work? Would you like opportunities for international collaborations and travel?

Fusion reactors potentially offer a complete solution to the problem of future energy supply, and are environmentally friendly: they emit no greenhouse gases, and so would not contribute to global warming. The recent success of the JET project at Culham in the south of Oxfordshire, which proved that plasma could be heated and controlled to produce fusion, has demonstrated the feasibility of the concept. The next step is to construct a prototype reactor (ITER) which is now the focus of a major international project. This will be followed by a prototype commercial reactor (DEMO).

So far relatively little demand has been made on the properties of the materials used for JET and other prototype reactors, since they had only to contain an operating plasma for very short times. For actual fusion power plants, materials issues will be crucial to success.

In the closer future, advanced fission reactors will be needed to meet some at least of the world energy demand, as fossil stocks dwindle and as their use become less environmentally acceptable. Materials degradation by radiation damage is a serious issue in current-generation reactors, and the new “generation IV” reactors currently being proposed will place even heavier demands on materials.

The materials needed will operate at temperatures of 600ºC or more, will need to withstand stresses up to 300MPa, and will accumulate over their lifetime radiation damage from fast neutrons amounting to ~100 displacements per atom. In fusion reactors, an additional problem will arise due to the high levels of helium and hydrogen produced in transmutation reactions. It is essential that any material used maintains adequate strength and toughness, while suffering minimal dimensional change through swelling and creep. These are very demanding requirements, which cannot be met by conventional structural materials. It will be necessary to develop and evaluate new materials.

Ion irradiation is currently the only non-activating method of mimicking the fast neutron damage produced in nuclear fission and fusion reactors (with or without co-implantation with He or H, to mimic the effects of fusion by-products). Implanted layers are ~2-3 microns deep or less.  We have developed micromechanical test methods, using ion beam machining to make specimens only a few microns in size, for determination of the elastic, plastic and fracture properties of candidate materials. This is linked with parallel electron microscopy and atom-probe microscopy studies of the development and nature of the radiation damage, and of the interactions between radiation damage and mobile dislocations which give rise to hardening and embrittlement.The experimental work is closely linked to the development and verification of computer models of radiation defects and their interactions.

A new large research project in this area, an EPSRC programme grant centred at Oxford University, started in early 2010. It involves UK and European partners (especially Liverpool University, CCFE Culham laboratory, the Commissariat à l'Énergie Atomique (CEA), and Rolls Royce). It aims at providing a thorough understanding of the mechanical properties and irradiation response of materials with potential for fusion and advanced fission reactor applications, including low-activation ferritic-martensitic steels, oxide-dispersion strengthened steels, model Fe-Cr alloys and tungsten alloys.

Individual research student projects started in 2009, 2010 and 2011 in the areas of:

• Tungsten-based alloys,
• Radiation hardening and embrittlement, 
• Grain boundary irradation embrittlement,
• Grain boundary stress-corrosion cracking,
• Processing & properties of Oxide Dispersion Strengthened alloys

Further projects will be available, to start October 2012. Applications for studentships in these areas from well-qualified applicants of all nationalities are welcome, but the note that available funding covers fees and stipend of UK and EU nationals only.

Also see homepages: Paul Bagot Patrick Grant Steve Roberts Angus Wilkinson

Dislocation-based modelling of Crack-Microstructure Interactions
Dr. A.J. Wilkinson

This project will continue to develop and apply computer simulation methods based on using dislocation dipoles to represent cracks and the associated localised plastic flow fields. An array of edge dislocation can represent the mode I opening (Burgers vector normal to the crack) and mode II shearing (Burgers vector along the crack). Previous work within the group began simulating multiply deflected and branched crack paths that are typical in IGSCC and lead to extensive crack-crack interactions and complicated variations in the local driving force for crack advancement (K). This project will aim to incorporate the effects of pre-existing residual stress variations on the crack propagation so as to simulate the technologically important cold work effects on micro-structurally short cracks.

Also see homepages: Angus Wilkinson

Micromechanics
S G Roberts / A J Wilkinson / R I Todd

We have demonstrated over the last two years that focussed ion-beam (FIB) machining can be used to produce specimens for mechanical testing on a length scale of microns to tens of microns. These can then be imaged using a nanoindentation system in AFM mode and loaded to produce a load-displacement curve from which stress-strain data can be derived. This type of testing only became possible with the advent of precision FIB equipment, and is greatly facilitated by the use of the recent dual-beam (electron & gallium) instruments that allow imaging without simultaneous cutting & damage. The Oxford group is one of thee groups worldwide (the other two being in the USA and in Austria) currently leading in this new and very rapidly-developing area. For the first time, we can make quantitative studies of mechanical behaviour at the scale of materials’ microstructures, the scale that control their behaviour. These techniques will form an integrated part of many of the “fusion reactor materials” projects. We have devised specimen geometries that contain only the thin ion-irradiated layer in the deforming region of the specimen, and have shown that we can obtain full stress strain curves of irradiated materials from such specimens. In other aspects of micromechanical testing, we are now looking to recruit researchers to develop these new techniques and to apply them to the understanding of the microstructural basis of the mechanical behaviour of materials.In particular, we aim to initiate projects focussing on: Factors controlling the strong size-effects on yield and work-hardening in micro-cantilever, micro-tension and micro-compression specimens; Technique and equipment development, especially to low  and high test temperatures and use of controlled test environments Characterising stress-corrosion cracking rates as a function of stress and boundary character for individual grain boundaries in steels; Mechanical behaviour of microporous materials; Grain boundary strength and sliding in superplasticity; Grain boundary embrittlement in ferritic steels. Funding may be available (for UK/EU applicants), depending on the outcome of some currently pending research grant applications.

Also see homepages: Steve Roberts Richard Todd Angus Wilkinson

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