Angus Wilkinson
Head of Department (Interim), Professor of Materials
+44-1865-273737
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. |
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New Postgraduate Research Projects Available
Selected Publications
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On the brittle-to-ductile transition of the as-cast TiVNbTa refractory high-entropy alloy
December 2020|Journal article|Materialia -
Scratching the surface: Elastic rotations beneath nanoscratch and nanoindentation tests
November 2020|Journal article|Acta MaterialiaIn this paper, we investigate the residual deformation field in the vicinity of nano-scratch tests using two orientations of a Berkovich tip on an (001) Cu single crystal. We compare the deformation with that from indentation, in an attempt to understand the mechanisms of deformation in tangential sliding. The lattice rotation fields are mapped experimentally using high-resolution electron backscatter diffraction (HR-EBSD) on cross-sections prepared using focused ion beam (FIB). A physically-based crystal plasticity finite element model (CPFEM) is used to simulate the lattice rotation fields, and provide insight into the 3D rotation field surrounding nano-scratch experiments, as it transitions from an initial static indentation to a steady-state scratch. The CPFEM simulations capture the experimental rotation fields with good fidelity, and show how the rotations about the scratch direction are reversed as the indenter moves away from the initial indentation.cond-mat.mtrl-sci, cond-mat.mtrl-sci -
Cold Creep of Titanium: Analysis of stress relaxation using synchrotron diffraction and crystal plasticity simulations
October 2020|Journal article|Acta MaterialiaThere is a long standing technological problem in which a stress dwell during cyclic loading at room temperature in Ti causes a significant fatigue life reduction. It is thought that localised time dependent plasticity in soft grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring hard grain poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. This work uses a novel approach of in situ synchrotron X-ray diffraction during stress relaxation tests, to quantify the time dependent plasticity. Measured lattice strains from multiple lattice families (21 diffraction rings) were compared with simulated lattice strains from crystal plasticity finite element (CPFE) simulations. The prism slip parameters were found to show stronger strain rate sensitivity compared to basal slip, and this has a significant effect on stress redistribution to hard grain orientations during cold creep.cond-mat.mtrl-sci, cond-mat.mtrl-sci -
Dislocation interactions during low-temperature plasticity of olivine and their impact on the evolution of lithospheric strength
August 2020|Journal article|Earth and Planetary Science Letters© 2020 The Author(s) The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, strain hardening may cause significant evolution of strength in the colder load-bearing portion of the lithosphere. Recent rheological data from low-temperature deformation experiments on olivine suggest that strain hardening occurs due to the presence of temperature-independent back stresses generated by long-range elastic interactions among dislocations. These interpretations provided the basis for a flow law that incorporates hardening by the development of back stress. Here, we test this dislocation-interaction hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of ≤4.3GPa or in a nanoindenter at applied contact stresses of ≥10.2GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 1014m−2 and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned in slip bands and interacting with dislocations of other types that act as obstacles. The resulting accumulations of dislocations in their slip planes, and associated stress heterogeneities, are consistent with strain hardening resulting from long-range back-stresses acting among dislocations and thereby support the form of the flow law for low-temperature plasticity. Based on these observations, we predict that back stresses among dislocations will impart significant mechanical anisotropy to deformed lithosphere by enhancing or reducing the effective stress. Therefore, strain history, with associated microstructural and micromechanical evolution, is an important consideration for models of lithospheric strength. The microstructural observations also provide new criteria for identifying the operation of back-stress induced strain hardening in natural samples and therefore provide a means to test the applicability of the flow law for low-temperature plasticity. -
Microstructural understanding of the oxidation of an austenitic stainless steel in high-temperature steam through advanced characterization
August 2020|Journal article|ACTA MATERIALIAStainless steel, Internal oxidation, Nanoporous, Transmission electron microscopy, Atom probe tomography -
Tetragonality of Fe-C martensite -- a pattern matching electron backscatter diffraction analysis compared to X-ray diffraction
June 2020|Journal article|Acta MaterialiaMeasurements of the local tetragonality in Fe-C martensite at microstructural length-scale through pattern matching of electron backscatter diffraction patterns (EBSPs) and careful calibration of detector geometry are presented. It is found that the local tetragonality varies within the complex microstructure by several per cent at largest and that the scatter in the axial ratio is increased at higher nominal carbon content. At some analysis points the local crystal structure can be regarded as lower symmetry than simple body centred tetragonal. A linear relation between the nominal carbon content and averaged local tetragonality measured by EBSD is also obtained, although the averaged axial ratio is slightly below that obtained from more classical X-ray diffraction measurements.cond-mat.mtrl-sci, cond-mat.mtrl-sci -
Tetragonality of Fe-C martensite – a pattern matching electron backscatter diffraction analysis compared to X-ray diffraction
June 2020|Journal article|Acta Materialia -
Surface Evolution of Lithium Titanate upon Electrochemical Cycling using a Combination of Surface Specific Characterization Techniques
April 2020|Journal article|Advanced Materials Interfaces -
Tension–compression asymmetry of 〈c+a〉 slip in Ti–6Al
March 2020|Journal article|Scripta Materialia -
On the assessment of creep damage evolution in nickel-based superalloys through correlative HR-EBSD and cECCI studies
February 2020|Journal article|Acta Materialia© 2019 The evolution of dislocation density with creep strain in single-crystal superalloys is studied quantitatively using high-resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging under controlled diffraction conditions (cECCI). Data regarding dislocation density/structure is measured for deformation at 900 °C and 450 MPa up to ≈ 1% plastic strain. Effects of chemical composition are elucidated via three purpose-designed superalloys of differing rhenium and ruthenium contents. The evidence indicates that dislocation avalanching is already prevalent at plastic strains of ≈ 0.1%; thereafter, an exponential decay in the dislocation multiplication rate is indicative of self-hardening due to dislocation constriction within the matrix channels, as confirmed by the imaging. The results are rationalised using discrete dislocation dynamics modelling: a universal dislocation evolution law emerges, which will be useful for alloy design efforts.