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 | Dr Richard I ToddReader in Materials |
[ Quicklinks: Research Summary Current Research Projects Recent Publications D.Phil. Projects Available ]
Summary of Research Interests
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 (including functional nanocomposites based on BaTiO3), 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.
- Pfeil Award, Institute of Materials, 2001.
Current Research Projects
Processing of nanograined alumina
M. Brooke-Hitching, Dr. R.I. Todd
There are several papers in the literature about various aspects of powder processing of submicron alumina. These have shown some success but few if any of the papers have used all the ideas in the literature at the same time. This project aims to produce an optimised processing route that considers both the production of green bodies by wet shaping methods and the details of the sintering cycle to ascertain how far this combined approach can reduce the sintered grain size.
PLZT microstructures for high strain piezoelectric applications
M. Waring, Dr. R.I. Todd, Dr. L.P. Walker*, Dr. K.S. Knight*
PLZT compositions close to the tetragonal/rhombohedral phase boundary are known to produce an exceptionally large strain for a given applied electric field. There are three contributions to the strain, namely electrostriction, the converse piezoelectric effect, and a field induced phase change. We are using electro-mechanical testing, Raman and electron microscopy and XRD to develop an improved understanding of these effects through a thorough study of the relationship between microstructure, and the grain size in particular, and properties. (*QinetiQ)(Supported by EPSRC)
Perovskite-based ceramic nanocomposites
A. Ross, H. Wang, Dr. H. Zheng, Dr. R.I. Todd
Functional ceramics based on perovskite structures have many interesting and useful properties (e.g. they can be piezo- and pyro-electric). Much research has gone into tailoring their properties to particular applications by changing their composition, but relatively little work has been done on changing their properties by the addition of second ceramic phases. Recent work in Oxford has shown that very small volume fractions (e.g. 1-2%) of nanophase additions can have dramatic effects on the properties of structural ceramics, and research elsewhere gives reason to believe that this might also be the case with functional ceramics. Furthermore, some of these effects might be synergistic in that they could improve both the mechanical and the functional properties of the material. The aim of the project is to explore the interaction between internal stresses, ferroelectric domain structure and functional and mechanical properties of such nanocomposites, starting with the barium titanate/SiC system. (In collaboration with Morgan Electro Ceramics)
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)
Wear and indentation of alumina/SiC nanocomposites
A. Limpichaipanit, Dr. S.G. Roberts, Dr. R.I. Todd
The wear and indentation of alumina/SiC nanocomposites are being investigated with a view to explaining the reduction in size of surface pullouts and increased resistance to microcrack nucleation observed in extensive previous work in this Department. (With support from the Thai Government)
Internal stresses and stored energy in steels
E. Clarke, Dr. A.J. Wilkinson, Dr. R.I. Todd, Dr. D. Crowther*, Dr. A. Howe*
Several methods of measuring microstresses in steels are being investigated, with a view to developing a robust method of measurement. The main methods to be tried are (i) XRD line profile analysis, (ii) EBSD, and (iii) nanoindentation. The microstresses are thought to be important in many steels, and link processing to properties in much the same way as microstructural features such as grain size. (In collaboration with *CORUS and with support from EPSRC and CORUS)
Measurement and mapping of stresses in alumina with submicron resolution using cathodoluminescence in the SEM.
Dr. R.I. Todd, Dr. P.R. Wilshaw, Dr. S. Galloway*
Stress measurement in alumina and other oxides using the shift of Cr3+ fluorescence peaks is well established, and is most often accomplished by stimulating the fluorescence using laser light focused on the specimen through an optical microscope. The stress can be measured with a spatial resolution down to several micrometers by this means. The same fluorescence lines can be stimulated by incident electrons, and we are investigating the possibility of making stress measurements with submicron resolution using the Cr3+ cathodoluminescence given off by Cr-doped alumina in the electron beam of an SEM. (* Gatan Ltd.)
Carbon nanotube reinforced ceramics
T.T. Chu, Dr. N. Grobert, Professor M.L.H. Green*, Dr. R.I. Todd
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.
Surface mechanical properties of alumina-based materials
S. Guo, Dr. R.I. Todd
The project aims to develop a better understanding of the nature and origin of surface damage in ceramics based on alumina. The residual stresses caused by indentations, single scratches and standard grinding and polishing operations are being mapped using ruby R-line piezospectroscopy. High spatial resolution is being achieved by stimulating this fluorescence using light in conjunction with an optical microscope and electrons in the SEM (cathodoluminescence). The stress measurements are being correlated with direct SEM observations of cracking in sectioned specimens.
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
A. Mukhopadhyay, 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 alumina and MgO based nanocomposites.
Synthesis and microstructure of MgB2 powders and wires
C. Dancer, Professor C.R.M. Grovenor, Dr. R.I. Todd, Dr. P. Kovac*, Dr. T. Prikhna**
MgB2 is a most promising new superconducting material for high current applications at temperatures below 30K. We are studying new methods for the chemical synthesis of high quality MgB2 powder with controlled particle size and impurity content. At the same time we are collaborating with the Kovac research group in Slovakia on the analysis of the microstructure of high current wires fabricated with commercial starting powder. XRD, SEM and EPMA analysis are all key aspects of the work, with high resolution SIMS analysis of oxygen and H contents. (*Institute of Electrical Engineering, Slovak Academy of Sciences, **Institute for Superhard Materials, Ukraine). (Funding provided by EPSRC DTA studentship for CD.)
12 public active projects
Research Publications
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.
Projects Available
Nanoceramics by Precipitation
R.I. Todd
The last decade has seen sustained interest in nanostructured ceramics (including both nanograined monolithics and nanocomposites) because of the improved properties they offer. To our knowledge, however, there are no commercial applications of such materials. The reason is not hard to find: they are difficult and expensive to make. Expensive nanopowders, carefully controlled conditions within a small processing window, high temperatures and exotic densification techniques such as spark plasma sintering or microwave heating may be required. Recently, however, we have developed a cheap and elegant method of making nanostructured ceramics by precipitating a nanoscale second phase from a dense, alumina-based solid solution. The ingredients are cheap, the whole process takes place at moderate temperatures, the processing window is wide and the resulting nanoceramics show the significant property improvements compared with pure alumina (the most widely used structural ceramic) that have been found with more expensive and less versatile processing schemes. This project aims to continue this work by understanding the mechanisms involved in the mechanical property improvements, the microstructural development during precipitation and by exploring new systems. The research will involve ceramic processing, mechanical testing and microstructural characterisation.
Also see homepages:Richard Todd
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:
1. Factors controlling the strong size-effects on yield and work-hardening in micro-cantilever, micro-tension and micro-compression specimens;
2. Technique and equipment development, especially to low and high test temperatures and use of controlled test environments
3. Characterising stress-corrosion cracking rates as a function of stress and boundary character for individual grain boundaries in steels;
4. Mechanical behaviour of microporous materials;
5. Grain boundary strength and sliding in superplasticity;
6. 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.
