Professor Angus Wilkinson
Department of Materials
Tel: +44 1865 273700 (switchboard)
Fax: +44 1865 273764
Micromechanics Group Website
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.
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)
6 public active projects
?a? Prismatic, ?a? basal, and ?c+a? slip strengths of commercially pure Zr by micro-cantilever tests
J Gong, TB Britton, MA Cuddihy, FPE Dunne, AJ Wilkinson
Acta Materialia (2015) vol. 96, 249-257
Measurements of stress fields near a grain boundary: Exploring blocked arrays of dislocations in 3D
Y Guo, DM Collins, E Tarleton, F Hofmann, J Tischler, W Liu, R Xu, AJ Wilkinson, TB Britton
Acta Materialia (2015) vol. 96, 229-236
On the mechanistic basis of deformation at the microscale in hexagonal close-packed metals
TB Britton, FPE Dunne, AJ Wilkinson
Proceedings of the Royal Society A (2015) vol. 471, 20140881
Evolution of intragranular stresses and dislocation densities during cyclic deformation of polycrystalline copper
J Jiang, TB Britton, AJ Wilkinson
Acta Materialia (2015) vol. 94, 193-204
The effect of pattern overlap on the accuracy of high resolution electron backscatter diffraction measurements
V Tong, , J Jiang, AJ Wilkinson, TB Britton
Ultramicroscopy (2015) vol. 155, 62-73
Using transmission Kikuchi diffraction to study intergranular stress corrosion cracking in type 316 stainless steels
M Meisnar, A Vilalta-Clemente, A Gholinia, Michael Moody, AJ Wilkinson, N Huin, S Lozano-Perez
Micron (2015) vol. 75, 1-10
The orientation and strain dependence of dislocation structure evolution in monotonically deformed polycrystalline copper
J Jiang, TB Britton and AJ Wilkinson
International Journal of Plasticity (2015) vol. 69, 102-117
A synchrotron X-ray diffraction study of in situ biaxial deformation
DM Collins, M Mostafavi, RI Todd, T Connolley and AJ Wilkinson
Acta Materialia (2015) vol. 90, 46-58
A discrete dislocation plasticity study of the micro-cantilever size effect
E Tarleton, DS Balint, J Gong, and AJ Wilkinson
Acta Materialia (2015) vol. 88, 271-282
Measurement of probability distributions for internal stresses in dislocated crystals
Evolution of dislocation density distributions in copper during tensile deformation
Slip Band - Grain Boundary Interactions in Ti alloys
Prof A J Wilkinson
Over the last few years we have had good success using EBSD to map local stress distributions at the head of slip bands intersecting with grain boundaries and the work has received considerable interest. This project will extend the measurements by looking at how material alloying and mechanical parameters alter the response. In Ti oxygen content and Al alloying have profound effects on strength and alter slip planarity. This can alter the amount of plastic strain localised into a particular slip band and hence influence the magnitude of stress ‘hot spots’ that develop at grain boundaries. Similarly altering the mechanical loading conditions from quasi-static tension, to creep or cyclic ratcheting may also alter the strength of grain boundaries as barriers to slip transfer.
Also see homepages: Angus Wilkinson
Fatigue Crack Initiation in Ti alloy Linear Friction Welds
Prof Angus J Wilkinson and Dr Jicheng Gong
We have developed novel methods for very rapid (~106 cycles in 1 min) fatigue testing of very small (~0.5 to 200 µm across) material volumes. This has opened up new possibilities for studying fatigue crack initiation in (very) high cycle fatigue. In this project the method will be used to examine fatigue crack initiation in Ti-6Al-4V linear friction welds - the key technology in manufacture of bladed disks 'blisks' for areoengines. During linear friction welding very intense but localised heating is generated and followed by rapid cooling as the process stops which leads to very dramatic changes in microstructure over a narrow weld region. The new testing method will allow individual parts of this microstructure to be isolated and its fatigue response established. We aim to provide a very complete analysis of the weld zone's fatigue behaviour and link it to microstructural variation and processing conditions.
This project will be carried out in close conjunction with Rolls Royce and will link to activities under the flagship EPSRC HexMat programme grant.
Also see homepages: Angus Wilkinson
Fundamentals of Fatigue Crack Initiation
Prof Angus J Wilkinson
The development within the group of the high resolution EBSD method provides a route to map the intra-granular distributions of local stress and dislocation density in a routine way. This project will exploit this development in trying to gain insight into cyclic deformation leading to fatigue crack initiation. Polycrystals processed to give different characteristic length scales will be examined at various points through the early stages of fatigue. The formation of 'hot spots' of high stresses and/or high dislocation density will be characterised. We will also attempt to identify local microstructural features that tend to encourage hot spot formation and crack initiation.
Also see homepages: Angus Wilkinson
Strains Induced by Hydride Formation in Zirconium
Prof Angus J Wilkinson
In service temperature cycling of nuclear fuel cladding can lead to repeated sequences of precipitation and dissolution of hydrides in zirconium based alloys. During the transformation from hydrogen in solid solution to the hydride phase there is a considerable volume expansion. This project will explore the links between nucleation sites, hysteresis between temperatures for precipitation and dissolution, the stress field and local plasticity induced by the transformation strain and the precipitation morphology. The following techniques will be used: high resolution EBSD, digital image correlation of SEM images, in situ thermal cycling, finite element based-crystal plasticity simulations. This project will be carried out in close conjunction with Rolls Royce and other partner Universities within the HexMat flagship EPSRC programme (http://www3.imperial.ac.uk/hexmat).
Also see homepages: Angus Wilkinson
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; strength and toughness of individual grain boundaries in engineering ceramics. Funding may be available (for UK/EU applicants), depending on the outcome of some currently pending research grant applications.
Also see a full listing of New projects available within the Department of Materials.