Monitoring grain boundary migration during recrystallisation using topotomography

International audienceThe growth of a single grain during recrystallisation into a mildly deformed {001}100 oriented single crystal has been monitored by synchrotron radiation using the topotomo technique. The formation and migration of individual facets is analysed using a new method which measures distances between grain boundary segments at different time steps along parallel lines normal to the facet plane. One facet is shown to move with a constant rate, while it remains planar and keeps the same boundary plane orientation. The formation of another facet, which is analysed in detail, reveals that first a planar boundary with a different orientation forms before it changes its boundary plane orientation into that of the final facet. It is argued that the local microstructural configuration in front of moving grain boundaries has a considerable influence on the kinetics of individual boundary segments and facets

Potential immunological consequences of pharmacological suppression of gastric acid production in patients with multiple sclerosis

Corticosteroids are standard treatment for patients with multiple sclerosis experiencing acute relapse. Because dyspeptic pain is a common side effect of this intervention, patients can be given a histamine receptor-2 antagonist, proton pump inhibitor or antacid to prevent or ameliorate this disturbance. Additionally, patients with multiple sclerosis may be taking these medications independent of corticosteroid treatment. Interventions for gastric disturbances can influence the activation state of the immune system, a principal mediator of pathology in multiple sclerosis. Although histamine release promotes inflammation, activation of the histamine receptor-2 can suppress a proinflammatory immune response, and blocking histamine receptor-2 with an antagonist could shift the balance more towards immune stimulation. Studies utilizing an animal model of multiple sclerosis indicate that histamine receptor-2 antagonists potentially augment disease activity in patients with multiple sclerosis. In contrast, proton pump inhibitors appear to favor immune suppression, but have not been studied in models of multiple sclerosis. Antacids, histamine receptor-2 antagonists and proton pump inhibitors also could alter the intestinal microflora, which may indirectly lead to immune stimulation. Additionally, elevated gastric pH can promote the vitamin B12 deficiency that patients with multiple sclerosis are at risk of developing. Here, we review possible roles of gastric acid inhibitors on immunopathogenic mechanisms associated with multiple sclerosis

Anisotropic Work-Hardening and Strain Path Effects in an AIMn Alloy (Anisotrope vervormingsversteviging en effecten van het vervormingspad in een AIMn legering)

The anisotropy of the flow stress is an important material property which strongly influences the strength and forming limits of a workpiece in various industrial forming processes. It has been demonstrated in the literature that the intragranular substructure contributes considerably to this anisotropy. Mechanical tests that realise an abrupt change of the strain path after a certain amount of prestraining provide a direct way to assess the evolution of the anisotropy with strain. The mechanical response that follows after such a strain path change (SPC) is moreover very informative about the substructure, since the observed transient hardening behaviour is thought to be determined by the same micro scale mechanisms that lead to the formation of the substructure. A clear understanding of the involved microstructural mechanisms is however still missing. Furthermore, changes of the strain path often occur in modern forming processes.The mechanical behaviour of an aluminium-manganese alloy during strain path changes is first characterised in an experimental study. In order to obtain a comprehensive data set of different types of SPC tests at different amounts of prestrain, a device to perform tests in simple shear has been developed. Various SPC tests are realised by combining deformations to different levels of prestrain through cold rolling or simple shear and subsequent straining in simple shear. The contributions to the anisotropy of the substructure and of the texture are separated through a multilevel approach which takes the measured evolution of the texture into account. Observed trends for the different SPC are discussed in the light of the possible microstructural mechanisms that are generally considered to determine work hardening and the formation of the substructure.A new multiscale model is developed which accounts for the different contributions to work hardening and anisotropy on their most appropriate length scales. The contributions to the anisotropy stemming from the substructure are modelled at the meso scale through the values of the critical resolves shear stresses of the different slip systems of the individual grain orientations. The transient work hardening effects after strain path changes are modelled by the gradual softening of slip systems that become less active after an SPC and a two stage hardening regime for active slip systems that were active in the opposite slip direction prior to the SPC. The capabilities of the model and the influence of the model parameters are discussed by examples of an individual orientation subjected to specific deformation modes.The mechanical behaviour for different strain path changes as predicted by the model is compared to experimental results. Since the used texture model does not capture the texture evolution of the alloy reasonably accurate, the comparison between model and experiments is based on properties that characterise the influence of the substructure. For the extreme cases of strain path changes the model results correspond very well to the experimental results. For intermediate strain path changes, such a correspondence is however not always observed. The evolution with prestrain can only be captured within a limited strain interval, indicating that the evolution equation within the model for the substructural anisotropy during monotonic straining is not yet advanced enough to capture the true behaviour. The capabilities of the model are summarised and compared to those of earlier models aimed at predicting the hardening behaviour after strain path changes.Voorwoord i Abstract v Samenvatting vii List of symbols ix List of abbreviations xvii Table of contents xix 1 General introduction 1 1.1 Length scales in plastic deformation 3 1.2 Concepts in plasticity 4 1.3 Strain path changes 7 1.4 Approach 8 1.5 Overview of text 9 2 Literature survey 11 2.1 General framework: Anisotropy and work hardening in metallic materials 13 2.1.1 Phenomenological description of an anisotropic flow surface 13 2.1.2 The influence of texture on anisotropy 14 2.1.3 Work hardening as result of dislocation interaction processes 19 2.1.4 Anisotropy as a result of slip system interactions 23 2.2 Formation of intragranular substructure 28 2.2.1 Experimental observations of substructure 29 2.2.2 Models for substructure formation 34 2.2.3 Internal stress state, work hardening and anisotropy of dislocation substructures 41 2.2.4 Conclusions 46 2.3 Substructure during strain path changes 47 2.3.1 Mechanical behaviour after strain path changes 49 2.3.2 Substructural evolution during strain path changes 59 2.3.3 Proposed mechanisms during strain path changes 63 2.4 Existing models for strain path changes 68 2.4.1 Teodosiu-Hu model 69 2.4.2 Peeters model 74 2.4.3 Holmedal model 82 2.5 Summary 84 3 Experimental observations 87 3.1 Material 88 3.1.1 Microstructure 88 3.1.2 Texture and anisotropy 90 3.2 Tests in simple shear 92 3.2.1 Dimensions of the test specimen 92 3.2.2 Shear tester 95 3.2.3 Strain mapping system 97 3.3 Mechanical work hardening behaviour 98 3.3.1 Monotonic hardening behaviour 99 3.3.2 Cross tests 106 3.3.3 Strain Reversal tests 111 3.3.4 Other strain path changes 113 3.3.5 Comparison of the different strain path changes 119 3.4 Summary 125 4 Multiscale model for cold deformation 127 4.1 Modelling framework 128 4.1.1 Overview 128 4.1.2 Texture model 134 4.1.3 Micro scale model 140 4.2 Meso scale model for work hardening anisotropy 141 4.2.1 Mathematical formulation 142 4.2.2 Influence of HX, H and n 144 4.2.3 Relation to experimental observations 148 4.3 Work hardening during strain path changes 153 4.3.1 Extra state variables for each slip system 153 4.3.2 Softening if slip system becomes much less active 159 4.3.3 Hardening if slip system is reversed 169 4.4 Summary 181 5 Assessment of the model 183 5.1 Work hardening predictions for different strain paths 184 5.1.1 Considerations regarding texture and the presence of second phase dispersoids 184 5.1.2 Consideration regarding hardening differences between slip systems of individual orientations 188 5.1.3 Considerations regarding hardening differences between orientations 188 5.1.4 Model predictions of strain path changes at different prestrains 190 5.1.5 Model predictions for intermediate Schmitt parameters 197 5.1.6 Implementation of substructural anisotropy in the A-Lamel model 200 5.1.7 Conclusions 202 5.2 Comparison with other SPC models 203 5.2.1 Comparison with the model of Teodosiu-Hu 203 5.2.2 Comparison with the models of Peeters and Rauch 206 5.2.3 Comparison with the model of Holmedal 208 5.3 Discussion 209 6 Conclusions 213 References 217 Appendix A: The Taylor factor in multiple slip with different tau 237 Appendix B: Microscopic work hardening models 245 Appendix C: Texture components for fcc materials 253 List of publications 259 Curriculum vitae 263nrpages: 263status: publishe

Visualization of grain subdivision by analysing the misorientations within a grain using electron backscatter diffraction

The misorientation relative to the average orientation of a grain and the point-to-point relative misorientation along a line across a moderately cold deformed grain, calculated from an electron backscatter diffraction (EBSD) dataset, are analysed in detail by visualizing both the misorientation angle and the misorientation axis. The significance of monitoring the misorientation axis is illustrated by an example of a grain subdivided into a misorientation band structure. A new technique to visualize the subdivision structure by assigning colours to misorientations in such a way that the contrast is maximized within a grain is introduced and discussed. Furthermore, some methods for grain boundary reconstruction from EBSD datasets are compared with the map of the confidence index in order to provide a validation of the accuracy of these methods.status: publishe

Gradient matrix method to image crystal curvature by processing of EBSD data and trial recognition of low-angle boundaries in IF steel

A gradient matrix has been introduced to derive the lattice orientation gradient, indicative of crystal substructures, from EBSD orientation data. A trial mapping of this term around a grain junction in a low deformed IF steel proves to reveal a distinct low-angle boundary extending from the junction line. The background dislocation substructure, however, is imaged rather poorly because of the mapping noise. (C) 2007 Elsevier B.V. All rights reserved.status: publishe

Characterisation of porosity within a microporous reservoir, Shuaiba Formation of Oman

Although carbonate reservoirs often have high total pore volumes, permeability often does not show a strong correlation to total porosity. Carbonate pore networks are also widely recognized as being highly heterogeneous, with marked variability in pore size (from submicron to millimeter scale and above) within an individual core plug. It is perhaps for this reason that there has been relatively little quantification of carbonate pore size and shape, despite significant advances in our ability to image naturally porous media using electron microscopy and advanced X-ray imaging. This study focuses on four samples of limestone from the uppermost Shuaiba Formation in northern Oman. These samples were selected for X-ray computerized tomography (CT) and environmental scanning electron microscope (ESEM) imaging and quantitative analysis following a detailed reservoir quality evaluation of the study interval across seven fields. This interval has been well studied sedimentologically, but the processes and timing of diagenetic modification, and the nature of the resultant pore network are less well understood. The samples represent a range of lithofacies associations that occur immediately beneath the Shuaiba-Nahr Umr unconformity, within an interval that is recognized for possessing higher permeability than the underlying reservoir. The samples were imaged at multiple scales, and their pore network was analyzed. Within the sample set, over 70% of the total pore volume was ,1 lm in diameter. The three-dimensional (3D) equivalent pore radii within individual samples ranged from ,0.1 lm to .100 lm, with the size of the X-ray imaged samples being limited to 1 mm3. The average aspect ratios of all pores was ,2, and it was highest in micropores (,1 lm pore radii). Mean coordination number was ,3 in all samples, and it was highest within micropores. Since most pore throat radii are ,1 lm, this most likely reflects the higher resolution needed to image micropores. Multivariant analysis shows that permeability prediction is improved when pore topological parameters are known. The highest measured permeability within the data set occurred in the sample with the highest volume of resolved porosity, highest aspect ratio, and highest coordination number. However, average permeability overall was highest in those facies associations with abundant macropores, where the representative elemental volume is greater than the sample size required for X-ray CT analysis and even routine core analysis. In these samples, high permeability is facilitated by the connectivity of a low volume of large (.30 lm) pores embedded within a network of micropores. In these samples, sweep efficiency during hydrocarbon production is likely to be poor. The results of this study provide one of the first detailed data sets of 3D pore shape and size within this volumetrically important reservoir and insight into pore connectivity within microporous reservoirs on the Arabian Plate. The results provide good evidence that the .1 lm fraction of these rocks contributes to single-phase flow, but they demonstrate the complexity of pore shape even at the micron scale

Detection and micromechanical reconstruction of junction disclinations in low deformed IF steel based on EBSD data

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Equal channel angular drawing of aluminium sheet

A method of equal channel angular drawing of sheet metals (ECADS) is presented and results of its application to a thin sheet of commercially pure aluminium are reported. The obtained mechanical properties, textures and microstructures are compared to those resulting from closely related processing techniques: (a) ECAD of rods, and (b) continuous shearing of sheets (CSS).status: publishe