High-resolution electron backscatter diffraction: an emerging tool for studying local deformation
AJ Wilkinson, EE Clarke, TB Britton, P Littlewood, PS Karamched
Journal of Strain Analysis for Engineering Design, (2010), vol. 45, 365-376
Electron backscatter diffraction (EBSD) is a widely available and relatively easy-to-use... more
Electron backscatter diffraction (EBSD) is a widely available and relatively easy-to-use scanning-electron-microscopy-based diffraction technique. Recently, Wilkinson, Meaden, and Dingley presented two papers on a new cross-correlation-based analysis of EBSD patterns which allow variations in the elastic strain and lattice rotation tensors to be measured at a sensitivity of about 10^−4 at high spatial resolution. This paper briefly describes the basis of the technique and how the resulting lattice curvatures can be used to estimate the geometrically necessary dislocation (GND) content in a sample. To illustrate the utility of the method for microscale deformation studies the following examples are described: first, nanoindentation near a grain boundary in α-Ti; second, transformation-induced GNDs in a dual-phase steel; third, thermally-induced and mechanically-induced deformation near carbides in a superalloy; fourth, GND accumulation during fatigue of a polycrystalline Ti–6Al–4V alloy.
Electron backscatter diffraction study of dislocation content of a macrozone in hot-rolled Ti–6Al–4V alloy
TB Britton, S Birosca, M Preuss and AJ Wilkinson
Scripta Materialia, (2010), vol. 62, 639-642
We compare the dislocation substructure within macrozone and non-macrozone regions of hot-rolled Ti–6Al–4 V.... more
We compare the dislocation substructure within macrozone and non-macrozone regions of hot-rolled Ti–6Al–4 V. Hough-based and cross-correlation-based analysis of electron backscatter diffraction (EBSD) patterns are used to establish the grain orientations and intra-granular misorientations, respectively. The set of geometrically necessary dislocations (GNDs) that support measured lattice curvatures and minimize the total GND line energy are calculated. The GND content in the macrozone is approximately twice that in the non-macrozone region, and <a> GNDs are present at densities 10 times higher than <c + a> GNDs.
The effect of crystal orientation on the indentation response of commercially pure titanium: experiments and simulations
TB Britton, H Liang, FPE Dunne and AJ Wilkinson
Proceedings of the Royal Society A, (2010), vol. 466, 695-719
This study combines nanoindentation, electron backscatter diffraction (EBSD) and crystal plasticity finite element... more
This study combines nanoindentation, electron backscatter diffraction (EBSD) and crystal plasticity finite element analysis to examine the anisotropy in the indentation behaviour of individual grains within an α-Ti polycrystal. Nanoindentation is utilized to mechanically probe small volumes of material within grains for which orientations are known from prior EBSD mapping. Both indentation modulus and hardness decrease significantly as the indentation axis is inclined further from the c-axis; the plastic response showing the more marked anisotropy. Recently developed high angular resolution EBSD has been utilized to examine selected indents, providing maps of elastic strain variations and lattice rotations. From such maps lower bound solutions for the density of geometrically necessary dislocations (GNDs) have been established. Crystal plasticity modelling showed promise in capturing correctly the orientation dependence of load–displacement response and in lattice rotations local to the indenter, particularly for indentation into a basal plane which generated threefold rotational symmetry about an axis parallel with the indentation direction which was also observed in experiments.
High resolution electron back-scatter diffraction analysis of thermally and mechanically induced strains near carbide inclusions in a superalloy
PS Karamched and, AJ Wilkinson
Acta Materialia, (2011), vol. 59, 263-272
Cross-correlation-based analysis of electron back-scatter diffraction (EBSD) patterns has been used to obtain high... more
Cross-correlation-based analysis of electron back-scatter diffraction (EBSD) patterns has been used to obtain high angular resolution maps of lattice rotations and elastic strains near carbides in a directionally solidified superalloy MAR-M-002. Lattice curvatures were determined from the EBSD measurements and used to estimate the distribution of geometrically necessary dislocations (GNDs) induced by the deformation. Significant strains were induced by thermal treatment due to the lower thermal expansion coefficient of the carbide inclusions compared to that of the matrix. In addition to elastic strains the mismatch was sufficient to have induced localized plastic deformation in the matrix leading to a GND density of 3 × 10^13 m–2 in regions around the carbide. Three-point bending was then used to impose strain levels within the range ±12% across the height of the bend bar. EBSD lattice curvature measurements were then made at both carbide-containing and carbide-free regions at different heights across the bar. The average GND density increases with the magnitude of the imposed strain (both in tension and compression), and is markedly higher near the carbides particles. The higher GND densities near the carbides (order of 10^14 m–2) are generated by the large strain gradients produced around the plastically rigid inclusion during mechanical deformation with some minor contribution from the pre-existing residual deformation caused by the thermal mismatch between carbide and nickel matrix.
Measurement of residual elastic strain and lattice rotations with high resolution electron backscatter diffraction
TB Britton and AJ Wilkinson
Ultramicroscopy, (2011), available online from 26 May 2011
A set of dynamically simulated electron backscatter patterns (EBSPs) for α-Ti crystals progressively rotated by 1°... more
A set of dynamically simulated electron backscatter patterns (EBSPs) for α-Ti crystals progressively rotated by 1° steps were analysed using cross-correlation to determine image shifts from which strains and rotations were calculated. At larger rotations the cross-correlation fails in certain regions of the EBSP where large shifts are generated. These incorrect shifts prevent standard least square error procedures from obtaining a valid solution for the strain and rotation, where the applied rotation exceeds 8°. Using a robust iterative fitting routine reliable strains and rotations can be obtained for applied rotations of up to and including 11° even though some image shifts are measured incorrectly. Finally, high resolution electron backscatter diffraction has been used to analyse the residual elastic strain, lattice rotations and density of stored geometrically necessary dislocations in a sample of copper deformed to 10% total strain. The robust iterative fitting analysis allows reliable analysis of a larger proportion of the map, the difference being most obviously beneficial in regions where significant lattice rotations have been generated.
Download (.pdf) Stress Potentiates Early and Attenuates Late Stages of Visual Processing
Shackman, A. J., Maxwell, J. S., McMenamin, B. W., Greischar, L. L. & Davidson, R. J. (2011). Stress potentiates early and attenuates late stages of visual processing. Journal of Neuroscience,31, 1156-1161.
Stress is a universal experience that can fundamentally alter neural responses to incoming information. Recent... more Stress is a universal experience that can fundamentally alter neural responses to incoming information. Recent research has begun to clarify the substrates of stress-induced modulations of neural processing. Based on this work, it has been hypothesized that stress and anxiety shift the balance of attention away from a task-directed mode, governed by prefrontal cortex (PFC), to a sensory-vigilance mode, governed by the amygdala and other threat-sensitive regions. A key untested prediction of this framework is that stress exerts dissociable effects on different stages of information processing. Here, we exploited the temporal resolution afforded by event-related potentials (ERPs) to disentangle the impact of stress on sensory-vigilance, indexed by early perceptual activity, from its impact on task-directed cognition, indexed by later postperceptual activity. Results indicated that threat-of-shock produced sustained increases in both the experience and expression of stress, measured using retrospective ratings and concurrent facial electromyography (EMG). During this same period, stress double dissociated the early sensory-specific and later task-directed (postperceptual) processing of emotionally-neutral stimuli. In particular, stress amplified N1 (184-236 ms) and attenuated P3 (316-488 ms) visually-evoked activity. These observations provide compelling new evidence that stress can exert fundamentally different consequences across successive stages of information processing. Consistent with recent suggestions, stress amplified earlier extrastriate activity in a manner consistent with vigilance for threat, but disrupted later activity associated with the evaluation of task-relevant information. These results provide a novel basis for understanding how stress can modulate information processing in everyday life and disorders characterized by hypersensitivity to threat.
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