Papers by Author: John Wheeler

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Authors: John Wheeler, Zhenting Jiang, David J. Prior, Jan Tullis
Abstract: It is generally agreed that the driving force (plastic strain energy) is much too small to allow "classical" nucleation during static and dynamic recrystallisation, and that rotation/growth of subgrains is an alternative. The latter explanation predicts that new grains should begin at low angles to old grains. We have used electron backscatter diffraction on an experimentally deformed quartz polycrystal that has deformed by dislocation creep and partially recrystallised. In a region shortened by about 30% new grains are at high angles (much greater than 15º) to adjacent parent grains. A histogram of misorientation versus number of boundaries shows a gap at 15-20º. In its simple form we expect the subgrain rotation model to predict a spectrum of misorientations but with most of them being low angle. Instead, the histogram suggests that new boundaries began life as high-angle structures, so current models for deformation-induced nucleation require refinement.
Authors: John Wheeler, J.M. Ford
Abstract: Numerical and analytic models for diffusion creep have commercial and geological uses. For single phase polycrystals, numerical models of interface diffusion creep illustrate how grains rotate and what the relative contributions of grain shape change and grain boundary sliding are to the overall strain. In particular they shows that an equi-axed starting material will initially show large grain angular velocities but that these slow down as grain become slightly elongate. A steady state microstructure with some grain elongation and little or no grain rotation is reached. Consequently the equi-axed grain shapes seen in superplastic deformation require additional processes for a full explanation. For two phase aggregates, the mathematical framework cannot be simply extended it breaks down as the system becomes mathematically overdetermined. Further work is required to solve this problem. If the second phase is insoluble, the mathematics can, though, be extended successfully, paving the way for models of diffusion creep with insoluble second phase particles.
Authors: John Wheeler, E. Mariani, S. Piazolo, D.J. Prior, P.J. Trimby, M.R. Drury, D. McNamara, M.A. Pearce
Abstract: Misorientation can be calculated over large datasets and a theme of this paper is the usefulness of examining the results statistically. Comparing the statistics of misorientations calculated from neighbouring pixels (or grains) with those calculated from pairs of pixels (or grains) selected at random helps to indicate deformation and recrystallisation mechanisms. Taking boundary length into account provides a link to grain boundary energy, and boundary length versus misorientation data should be used to examine how boundaries with different misorientations evolve through time. Time lapse misorientation maps indicate how orientation changes through time at particular points in a microstructure during in situ experiments. The size of areas which have changed orientation by particular amounts can be linked to boundary length and boundary migration velocities. When dealing with different phases, the statistics of angular relationships, akin to intraphase misorientation analysis, can indicate orientation relationships in the absence of prior knowledge, which is advantageous in investigating the plethora of minerals that make up the Earth.
Authors: David J. Prior, Michel Bestmann, Angela Halfpenny, Elisabetta Mariani, Sandra Piazolo, Jan Tullis, John Wheeler
Abstract: Misorientation analysis, using EBSD data sets, has enabled us to constrain better recrystallization mechanisms in rocks and minerals. Observed microstructures are not explicable in terms of recovery, boundary bulging and migration alone. We have to invoke either a nucleation process (physics unknown) or grain rotations that are not related to grain or boundary crystallography. Such rotations can occur by diffusion accommodated grain boundary sliding and this mechanism explains best the microstructure and texture of recrystallized grains in some rocks.
Authors: Elisabetta Mariani, Julian Mecklenburgh, David J. Prior, John Wheeler
Authors: John Wheeler, Elisabetta Mariani, Sandra Piazolo, David J. Prior, P.J. Trimby, M.R. Drury
Abstract: The Weighted Burgers Vector (WBV) is defined as the sum, over all types of dislocations, of [(density of intersections of dislocation lines with a map) x (Burgers vector)]. It can be calculated, for any crystal system, solely from orientation gradients in a map view, unlike the full dislocation density tensor, which requires gradients in the third dimension. No assumption is made about gradients in the third dimension and they may be non-zero. The only assumption involved is that elastic strains are small so the lattice distortion is entirely due to dislocations. Orientation gradients can be estimated from gridded orientation measurements obtained by EBSD mapping, so the WBV can be calculated as a vector field on an EBSD map. The magnitude of the WBV gives a lower bound on the magnitude of the dislocation density tensor when that magnitude is defined in a coordinate invariant way. The direction of the WBV can constrain the types of Burgers vectors of geometrically necessary dislocations present in the microstructure, most clearly when it is broken down in terms of lattice vectors. The WBV has five advantages over other measures of local lattice distortion. 1. It is a vector and hence carries more information than any scalar measure of local misorientation. 2. It has an explicit mathematical link to the individual Burgers vectors of dislocations. 3. Since it is derived via tensor calculus, it is not dependent on the map coordinate system, in contrast to existing measures of local misorientation which are not only scalar but dependent on the coordinate system used. 4. Calculation involves no assumptions about energy minimisation. 5. The numerical differentiation involved in calculating the WBV may introduce errors, but there is a direct mathematical link to a contour integral. The net Burgers vector content of dislocations intersecting an area of a map can be simply calculated by an integration round the edge of that area, a method which is fast and complements point-by-point WBV calculations. Errors in orientation measurement will have a much smaller effect here, and dislocations can be detected which are otherwise lost in the noise of any local calculation.
Authors: John Wheeler
Abstract: The Earth deforms dominantly by solid-state creep. Diffusion creep is known to be important. It is less clear whether mechanisms in which grain boundary sliding is accompanied by other processes (dislocation activity), and/or are associated with stress exponents closer to 2 than to 1 are important. Since the mechanisms of superplasticity are themselves not fully resolved, we cannot say for sure whether the Earth deforms superplastically. Models for diffusion creep are relevant for the Earth and possibly for superplastic materials. Modelling shows that large strains may not necessarily obliterate initial textures because grain rotations, although they occur, slow down as microstructures evolve. Modelling also predicts major strength anisotropy induced by grain shape alignment. Models for two-phase diffusion creep can be constructed for when the second phase is inert (insoluble). If both phases are soluble and can participate in diffusion, the basic theory for single phase diffusion creep cannot be applied and new insight is required.
Authors: Angela Halfpenny, David J. Prior, John Wheeler
Abstract: Electron backscatter diffraction (EBSD) is an extremely valuable tool, as it measures full crystallographic orientation information. This technique has been used to measure the statistics of misorientations between original ‘parent’ grains and recrystallised ‘daughter’ grains in a mylonitic quartzite. The angle of misorientation has implications on the controlling recrystallisation mechanism. The sample is a natural mylonitic quartzite collected from the stack of Glencoul, NW Scotland. The sample exhibits a common partially recrystallised microstructure. The data shows the average misorientations between the ‘parent’ and ‘daughter’ grains are 30º, this value seems too high for only subgrain rotation recrystallisation to be taking place. Moreover there is no gradation in the boundary misorientation from the internal substructure of the ‘parent’ grain to the ‘daughter’ grains. The internal substructure size of the ‘parent’ grain is bigger than the size of the ‘daughter’ grains. For subgrain rotation recrystallisation you may expect to see a core and mantle structure and for the ‘daughter’ grains’ to be of similar size to the internal substructure of the ‘parent’ grain. Another mechanism has either controlled the recrystallisation altogether or has become active after subgrain rotation had taken place and modified the microstructure.
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