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Mechanical properties of ceramics and metals. Most research revolves around oxide ceramics, microstress measurements and superplastic metals. Current interests include the processing and properties of ceramic nanocomposites, with either particulate or carbon nanotube reinforcements, high spatial resolution (~100nm) measurements of stresses in metals and ceramics, and high spatial resolution surface studies of superplastic flow.

• IOM3 Verulum Medal and Prize, 2012.
• Pfeil Award, Institute of Materials, 2001.

Surface effects in superplastic deformation
M.A. Rust, Dr. R.I. Todd
The superplastic deformation of aluminium and Sn-Pb alloys is being studied with particular reference to surface observations. FIB is being used to etch submicron reference grids on the surface and EBSD is being used to characterise grain boundaries of interest. Aspects being studied include the origin of surface ridges and other inhomogeneous aspects of flow, co-operative grain boundary sliding, intragranular deformation and the nature of grain boundaries. (In collaboration with Superform Metals)

Carbon nanotube reinforced ceramics
Dr. F. Dillon, G. Otieno, Professor R.I. Todd, Professor N. Grobert
There have been several attempts recently to make ceramic nanocomposites in which the reinforcing phase consists of carbon nanotubes. None has resulted in a viable composite, either because the nanotubes have been destroyed by the high firing temperatures used, or because the nanotubes have not been properly dispersed in the ceramic matrix. We are trying to solve these problems using a variety of techniques and using both single- and multi-walled nanotubes. (Supported by The Royal Society, and ERC Starting Grant)

Oxide Nanocomposites
A. Mukhopadhyay, Dr. R.I. Todd
Work in Oxford has shown that alumina/SiC nanocomposites offer enormous improvements in resistance to severe wear compared with pure alumina. Commercial takeup of these materials has been limited, however, owing to the requirement to sinter these materials in inert gas in order to prevent oxidation of the SiC. Routes for producing 100% oxide nanocomposites are being explored that will avoid this problem.

Plasticity in oxide nanocomposites
D. Shi, Dr. R.I. Todd
Alumina/SiC nanocomposites exhibit much more surface plasticity and much less brittle fracture than pure alumina when subjected to severe grinding. This project is investigating the extent to which this also applies to bulk deformation of MgO based nanocomposites.

Understanding and Improving Ceramic Armour
Dr. C.E.J. Dancer, H. Curtis, S. Bennett, S. Falco*, S. Huang**, Dr. S. Ghosh**, Dr. H. Wu**, Dr. B. Vaidhyanathan**, Professor J. Binner**, Dr. N. Petrinic*, Dr. R.I. Todd
Ceramic materials are used for both personnel and vehicle armour since they can be very effective at stopping ballistic projectiles by breaking and eroding them. However, such armour is generically fairly heavy and does not have multihit capability due to its fragmentation during impact. The development of new ceramics for armour is further hindered by the limited understanding of the mechanisms involved in their success and therefore what the characteristics of the ideal ceramic should be. We are taking a holistic approach to this problem, by combining modelling and experimental studies on traditional monolithic and novel nanocomposite ceramics. Testing a range of ceramics with systematically differing characteristics will give comparative information about which microstructural features and mechanical properties successful ceramics possess, as well as enabling the development of a fundamental understanding of the high strain rate performance. This will then be used to produce armour with improved properties. (*Department of Engineering, University of Oxford; **Department of Materials, Loughborough University)

5 public active projects

Russell-Stevens, M., Todd, R.I. and Papakyriacou, M. (2006). 'Thermal expansion behaviour of ultra-high modulus carbon fibre reinforced magnesium composite during thermal cycling' Journal of Materials Science 41(19) 6228-6236.

Todd, R.I. (2006). 'Particulate Composites'. "Ceramic Matrix Composites - Microstructure Properties and Applications". Low, I.M., Woodhead Publishing Ltd., Cambridge, UK and CRC Press LLC, Boca Raton, USA.

Todd, R.I. and Armstrong, D. (2006). 'Gum Metal and Related Alloys'. "Encylopedia of Materials: Science and Technology, 2006 on-line update". Buschow, J., Flemings, M., Cahn, R., Veyssiere, P.,Kramer, E., Elsevier, Kidlington, UK. on-line at www.sciencedirect.com.

Ortiz, J.M., Cock, A., Roberts, S.G. and Todd, R.I. (2005). 'Quantitative surface fractography of alumina and alumina-SiC composites' Key Engineering Materials 290 149-159.

Holme, P.H., Rust, M.A., Huang, Y. and Todd, R.I.: 'Surface studies in superplastic materials using focused ion beams (FIB)' Proceedings of the 4th European Conference on Superplastic Forming, London, IoMMM (2005) 29.

Merino, J.L.O., Cock, A., Roberts, S.G. and Todd, R.I.: (2005). 'Quantitative surface fractography of alumina and alumina-SiC composites during diamond grinding'. Fractography Of Advanced Ceramics Ii. 290: 149-159.

Ortiz-Merino, J.L. and Todd, R.I.: 'Relationship between wear rate, surface pullout and microstructure during abrasive wear of alumina and alumina/SiC nanocomposites' Acta Materialia 53 (12) (2005) 3345-3357.

Robinson, J. and Todd, R.I.: 'Mechanisms of microsuperplasticity' Proceedings of the 4th European Conference on Superplastic Forming, London, IoMMM (2005) 31-36.

Russell-Stevens, M., Todd, R. and Papakyriacou, M.: 'Microstructural analysis of a carbon fibre reinforced AZ91D magnesium alloy composite' Surface And Interface Analysis 37 (3) (2005) 336-342.

Russell-Stevens, M., Todd, R. and Papakyriacou, M.: 'The effect of thermal cycling on the properties of a carbon fibre reinforced magnesium composite' Materials Science And Engineering A-Structural Materials Properties Microstructure And Processing 397 (1-2) (2005) 249-256.

Todd, R.I. (2005). Alumina-SiC Nanocomposites. Nanomaterials. Cantor, B. Bristol, Institute of Physics: 220-232.

Todd, R.I. and Derby, B.: 'Thermal stress induced microcracking in alumina-20% SiCp composites.' Acta Materialia 52 (6) (2004) 1621-1629.

Cock, A.M., Shapiro, I.P., Todd, R.I. and Roberts, S.G.: 'Effects of yttrium on the sintering and microstructure of alumina-silicon carbide "nanocomposites"' Journal Of The American Ceramic Society 88 (9) (2005) 2354-2361.

Hill P.S., Todd R.I. and Ridley N.: 'Mechanism of the HIP bonding of Zircaloy-4 in the alpha-phase field' Journal of Materials Processing Technology 135, 131-136 (2003).

Merino J.L.O. and Todd R.I.: 'Thermal microstress measurements in Al2O3/SiC nanocomposites by Cr3+ fluorescence microscopy' Journal of the European Ceramic Society 23, 1779-1783 (2003).

Chen, K.H., Liu, H.W., Zhang, Z., Li, S. and Todd, R.I.: 'The improvement of constituent dissolution and mechanical properties of 7055 aluminum alloy by stepped heat treatments.' Journal Of Materials Processing Technology 142 (1) (2003) 190-196.

Griffiths, S., Whittle, D., Ridley, N. and Todd, R.I. (2003). Superplasticity in commercial Al 7475. Superplasticity In Advanced Materials. 447-4: 283-288.

Merino, J.L.O. and Todd, R.I. (2003). Alumina/SiC nanocomposites for improved surface finish and wear resistance. Proceedings of the American Society for Composites 17th Technical Conference. Kim, C.T.S.a.H., CRC Press / American Society for Composites: 172.

Todd, R.I. (2003). Residual stresses in ceramic materials. Analysis of residual stress by diffraction using neutron and synchrotron radiation. Lodini, M.E.F.a.A. London, Taylot and Francis: 334-348.

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

Mechanism and Modelling of Superplastic Deformation
R.I. Todd

Superplasticity is a phenomenon in which metals and ceramics can exhibit spetacularly large tensile elongations to failure under certain conditions (the world record approaches 10 000!). By using submicron surface marker grids we have recently shown conclusively that superplastic deformation takes place by stress-directed diffusion and does not involve significant lattice dislocation activity under optimum conditions. This has made a clear step forward in the understanding of the phenomenon and has settled the 75 year old question of how grain boundary sliding is “accommodated” at grain boundary triple lines. At the same time, this advance raises a new set of questions, and in particular why the kinetics of superplasticity do not correspond to those of classical diffusion creep. This project aims to answer these questions by expanding our surface studies to different materials and different deformation regimes. A further aim is to incorporate this new understanding of superplasticity into improved modelling of superplastic forming. The research will involve Focused Ion Beam milling, SEM, mechanical testing over a range of temperatures and nanoindentation.

Also see homepages: Richard Todd

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