Personal Homepages

Mechanical behaviour of materials. Materials for nuclear (fusion & fission) power plants. Micromechanical testing. Brittle-ductile transitions. Studies aim at linking modelling at the defect and dislocation level with experimental studies of well-characterised materials.

Atomic scale studies of oxide dispersion strengthened (ODS) steels
C.A. Williams, Dr P. Edmundson, Dr P. Bagot, Professor S.G. Roberts
Oxide dispersion strengthened steels have great potential for application in future nuclear fusion reactors, because of their high thermal stability and resistance to degradation under intense irradiation. This project involves atomic-scale studies of the microstructure, phase composition and solute segregation behaviour in a range of such steels, with the aim of optimising their overall design and performance.

Processing of oxide dispersion strengthened alloys for fission and fusion power
Z. Hong, M. Gorley, Dr. H. Zhang, Professor S.G. Roberts, Professor P.S. Grant
Oxide dispersion strengthened alloys comprise a metallic alloy with a dispersion of sub-micron oxide particles. The fine scale dispersion of the ceramic particles gives rise to strain fields around the particles, which can confer strength and other properties by interaction with dislocations in a manner similar to that of fine scale precipitates produced by ageing heat treatments in conventional metallurgical alloys. The particles also have the potential to stabilize microstructural features such as grain size at intermediate temperature. A further potential benefit of these particles in steels for nuclear applications is that they or the interface between the particles and the matrix may act as a 'sink' for vacancies and He induced by a neutron flux environment, partially mitigating otherwise severely damaging effects such as embrittlement. It is known that the type (size, volume fraction, chemistry, etc) of particles and the homogeneity of their dispersion in the matrix is influential on the final ODS alloy properties and the extent to which potential benefits are realised in practice. However, there are few systematic studies that allow the detail of the oxide particle mixing/dissolution and re-precipitation behaviour to be reconciled in terms of the processing parameters of practical interest. In part, this derives from the long times associated with the design-make-characterise-irradiate-test cycle. In this project we combine in-house processing of high quality ODS steel powders by mechanical means, the subsequent manufacture of consolidated ODS alloys. The study focusses on the dynamics of the critical metallic-ceramic mixing process and aims to develop ideas for identifying and assuring the "quality" of milled powders so that downstream properties are evolved optimally. Alternative processes to mechanical mixing are also being explored. Funded by EPSRC.

Tungsten for fusion power applications
J. Gibson, 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)

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)

TEM studies of radiation damage in materials for fusion reactors
S. Xu, X. Yi, Dr M. Briceno, Professor S.G. Roberts, Dr. M.A. Kirk*
Materials issues will be crucial to the success of future fusion reactors. Structural materials will operate at temperatures up to 600C, 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, combined with 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 changes through swelling and creep. Candidate materials include ferritic/martensitic steels based on iron ~9% chromium, ODS steels, vanadium alloys and tungsten. This project uses advanced techniques in electron microscopy to characterise the development of radiation damage in these materials and in model FeCr alloys. We perform both high-energy (MeV) ion irradiations of bulk materials and in-situ heavy-ion irradiations. We work closely with the materials modelling group who are applying multiscale modelling techniques to the same materials.(Funded and in collaboration with EURATOM/UKAEA, *Argonne National Laboratory)

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)

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)

7 public active projects

Dislocation dynamic modelling of the brittle-ductile transition in tungsten, E Tarleton and S.G. Roberts, Phil. Mag. 89, 2009, 2759-2769; DOI: 10.1080/14786430902992619

Characterisation of plastic zones around crack-tips in pure single-crystal tungsten using electron backscatter diffraction, J D Murphy, A J Wilkinson and S G Roberts, Mat. Sci. & Eng. 3 (2009) 012015; DOI:10.1088/1757-899X/3/1/012015

Measuring anisotropy in Young’s modulus using micromechanical testing in single crystal Cu, D.E.J. Armstrong, A.J. Wilkinson and S.G. Roberts, J Mater Res, 24, 2009, 3268-76 DOI: 10.1557/JMR.2009.0396

3D DD modelling of the prismatic loops and dislocations interaction in pure iron, R.Novokshanov and  S.G. Roberts: J. Nucl. Materials, 386,, 2009, 64-66.

Micromechanical testing of stress corrosion cracking of individual grain boundaries, D.E.J. Armstrong, M. Rogers and S.G. Roberts, Scripta Mater., 61, 2009, 741-743; http://dx.doi.org/10.1016/j.scriptamat.2009.06.017

Measuring local mechanical properties using FIB machined cantilevers, D.E.J. Armstrong, A.J. Wilkinson, S.G. Roberts, in Probing Mechanics at Nanoscale Dimensions, edited by N. Tamura, A. Minor, C. Murray, L. Friedman (Mater. Res. Soc. Symp. Proc. 1185, Warrendale, PA, 2009), paper II-02-08.

 Effects of Y₂O₃ additives and powder purity on the densification and grain boundary composition of Al₂O₃/SiC nanocomposites I.P. Shapiro, R.I. Todd, J.M. Titchmarsh, S.G. Roberts, J. Euro Ceram. Soc,29, 2009, 1613-1624; http://dx.doi.org/10.1016/j.jeurceramsoc.2008.09.021  

The mechanical properties of tungsten grown by chemical vapour deposition, J.D. Murphy, A. Giannattasio, Z. Yao, C.J.D. Hetherington, P.D. Nellist, S.G. Roberts, J. Nucl. Materials, 386, 2009, 583-586; doi:10.1016/j.jnucmat.2008.12.182  

Nanoindentation and micromechanical testing of iron-chromium alloys implanted with iron ions, F.M. Halliday, D.E.J. Armstrong, J.D. Murphy and S.G. Roberts, Extremat meeting, San Sebastian, Advanced Materials Research, 59, 2009, 304-307.

Brittle-to-ductile transition of poly-crystalline iron-chromium alloys, M. Tanaka, A.J. Wilkinson, S.G. Roberts, J. Nucl. Mater., 378, 2008, 305-311; http://dx.doi.org/10.1016/j.jnucmat.2008.06.039  

Brittle-ductile transition in single-crystal iron, M. Tanaka, E. Tarleton, S. G. Roberts, Acta Mater.56, 2008, 5123-29; http://dx.doi.org/10.1016/j.actamat.2008.06.025

Residual stress distributions around indentations and scratches in polycrystalline Al₂O₃ and Al₂O₃/SiC nanocomposites measured using fluorescence probes, H.Z. Wu, S. G. Roberts and B. Derby, Acta Mater., 56, 2008, 140-149.

Low-temperature fracture mechanisms in a spheroidised reactor pressure vessel steel, A. Kumar, S.G. Roberts and A. J. Wilkinson, Intl. J. Fracture, 144, 2007, 121-129.

Quasi-cleavage fracture planes in spheroidised A533B steel, A. Kumar, A.J. Wilkinson and S.G. Roberts, J. Microsc, 227,  2007, 248–253

Brittle-ductile transitions in vanadium and iron-chromium,  T.D. Joseph, M. Tanaka, A.J. Wilkinson and S.G. Roberts, J. Nucl. Mater., 367–370, 2007,  637–43.

Micro-fracture testing of Ni-W microbeams produced by electrodeposition and FIB machining, D. Armstrong, A. Haseeb, A.J. Wilkinson, S.G. Roberts, in Focused Ion Beams for Analysis and Processing, edited by W. MoberlyChan, H. Colijn, R. Langford, A. Marshall (Mater. Res. Soc. Symp. Proc. 983E, Warrendale, PA, 2007), 0983-LL08-07.

Strain-rate dependence of the brittle-to-ductile transition temperature in tungsten, A. Giannattasio and S. G. Roberts, Phil. Mag. 87, 2007, 2589-98.

An empirical correlation between temperature and activation energy for brittle-to-ductile transition single-phase materials, A. Giannattasio, M. Tanaka, T. D. Joseph and S. G. Roberts, Physica Scripta, T128, 2007, 87-90

Sub-surface damage in ground and annealed alumina and alumina-silicon carbide nanocomposites, B.K. Tanner, H.Z. Wu and S.G. Roberts, J. Am. Ceram. Soc., 89, 2006, 3745–50

High-resolution parallel-beam powder diffraction measurement of sub-surface damage in alumina-silicon carbide nanocomposite, B K Tanner, H Z Wu and S G Roberts, Advances in X-ray Analysis, 49, 2006 169-174

Characterising dislocation structure evolution during cyclic deformation using electron channelling contrast imaging, J.Ahmed, S. G. Roberts, and A. J Wilkinson, Phil. Mag. 86, 2006, 4965–81

Crack initiation in the fracture of ferritic steels, M. Coates, A. Kumar and S.G. Roberts, Fatigue Fract Engng Mater Struct 29, 2006, 661–671.

Modelling the upper yield point and the brittle-ductile transition of silicon wafers in three-point bend tests, S.G. Roberts and P.B. Hirsch, Phil. Mag. 86, 2006, 4099-4116.

The use of closely spaced Vickers indentations to model erosion of polycrystalline α-Al2O3, A. Franco and S.G. Roberts,  MRS Proceedings 843: Surface engineering 2004 - fundamentals and applications, eds: S.N. Basu, J.E. Krzanowski, J. Patscheider & Y. Gogotsi (Materials Research Society, Warrendale, PA, USA, 2005), pp143-148.

Effects of yttrium on the sintering and microstructure of alumina - silicon carbide “nanocomposites”, A.M. Cock, I.P. Shapiro, R.I. Todd and S.G. Roberts, J. Amer. Ceram. Soc. 88, 2005, 2354-61.

Quantitative surface fractography of alumina and alumina-SiC composites during diamond grinding, J.M. Ortiz, A.M. Cock, S.G. Roberts and R.I. Todd, Key Eng. Mats 290, 2005, 149-159

Direct evidence for compressive elastic strain at ground surfaces of nanocomposite ceramics, B.K. Tanner, H.Z. Wu and S.G. Roberts, Appl. Phys. Letts, 86, 2005, art. 061909

Measuring fracture toughness of coatings using FIB-machined microbeams, D. Di Maio and S.G. Roberts. J. Mater. Res, 20 (2), 2005, 299-302.

Substrate and elastic recovery effects in hardness measurement of CVD WC-based coatings, D. Di Maio and S.G. Roberts, Phil. Mag,, 85, 2005, 33-44.

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

Applications for studentships in these areas from well-qualified applicants of all nationalities are welcome, but note that all funded projects for 2012 start have now been allocated

Also see homepages: Paul Bagot Patrick Grant Steve Roberts 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

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

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

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