1,721,018 research outputs found
<a> prismatic, <a> basal, and <c+a> slip strengths of commercially pure Zr by micro-cantilever tests
No full textSlip strengths of <a> basal, <a> prism, and <c+a> pyramidal systems in commercially pure zirconium have been determined using micro-cantilever testing. A range of single crystal cantilevers 0.5 µm to 10 µm wide, oriented for single slip were prepared using focused ion beam (FIB) machining and subsequently deflected using a nanoindenter. The critical resolved shear stress (Ïcrss) was found by fitting a crystal plasticity finite element model to the experimental load-displacement data for these micro-bending tests. All the three slip systems in alpha-Zr show a marked size effect in bending described well by CRSS(W)=Tau0 + AWn, where W is the cantilever width, Tau0 is the CRSS at the macro scale and n=-1. The exponent, n, of near -1 is in good accord with hardening caused by the back stress generated by dislocations piling up at a diffuse barrier caused by the reduction of stress near the neutral axis. The macro scale CRSS values were used to successfully simulate deformation of a conventional macroscopic compression test
The interactions between slip band, deformation twins and grain boundaries in commercial purity titanium
Get PDFThis thesis apply High Resolution Electron Back Scatter Diffraction (HR-EBSD) technique to a variety of microstructure features and their interactions in pure h.c.p polycrystals. By correlating high quality Kikuchi patterns with a reference pattern, the relative state and distribution of strain, stress, and geometrically necessary dislocation (GND) density can be obtained with high strain sensitivity (10-4) and angular resolution (10 radian). This technique is companied by a further investigation of subsurface features using Differential Aperture X-ray Micro-diffraction (DAXM) technique. The two technique have shown excellent agreement in capturing the magnitude and distribution of stress and GND. Stress field and GND distribution induced by slip band and grain boundary interactions, including blocked slip band with no observable slip transfer in SEM and slip transfer, were characterised. It was found that some blocked slip bands lead to high and localised stress concentration in the neighbouring grain while others did not, and no stress concentration were correlated with transferred slip bands. These three categories of interactions were rationalised using a slip transfer criteria (called LRB criteria) by investigating the geometric alignments between the impinging slip system and all possible slip systems in the neighbouring grain. The level of stress concentration were quantified into a stress intensity factor K, following the Frank, Eshelby, and Nabarro (FEN) model. It was found that the level of stress intensity correlates well with the number of dislocations within the pile up plane. The slip band and grain boundary interaction case that led to the highest magnitude of stress intensity factor was further investigated using DAXM experiments. The 3D data set informed us additional information hidden below the sample surface. The distribution of stress concentration in 3D is a ribbon conforming to the line of intersection between slip plane and grain boundary. Stress intensity factor calculation along this ribbon have shown large variations which led to a concern that sometimes 2D results might not be conclusive. For example, if damage is observed in sample surface, there is a possibility that large populations of damage already exist below sample surface as a result of the stress fluctuations. The level of stress concentration and distribution measured by both HR-EBSD and DAXM agree with each other and 3D lattice rotation gradient used in DAXM GND calculation was found to affect the range of GND distribution and how fast it decays away from grain boundary. Twinning is a deformation mechanism in HCP metal that is equally important as dislocation slip. The stress concentrations associated with twin propagation, approaching grain boundary, and thickening were characterised using HR-EBSD, from which the calculated stress tensor were used to generate a local Schmid factor (LSF) map. It was found that during twin propagation, local positive shear provides a favourable LSF condition that promote twin tip extension while supress it from thicken. When twin tip is approaching the grain boundary, the positive shear stress field no longer favour twin propagation, a narrow positive LSF field still exist at the tip of twin, promoting it to grow thick. During propagation and thickening process, the LSF seem to only affect the tip of twins and therefore these processes are possibly tip controlled
The effect of pattern overlap on the accuracy of high resolution electron backscatter diffraction measurements
No full textData examining the spatial resolution of electron backscatter diffraction. EBSD patterns were obtained from a Zr alloy (zircaloy-4) using a Bruker eFlashHR detector at full 1600x1152 pixel resolution on an Zeiss Auriga SEM at 20keV and 10nA.
Cross correlation analysis carried out within Matlab was used to measure the degree of pattern overlap and its effects on errors in strain measurements. High resolution, cross-correlation based, electron backscatter diffraction (EBSD) measures the variation of elastics trains and lattice rotations from a reference state. Regions near grain boundaries are often of interest but overlap of patterns from the two grains could reduce accuracy of the cross-correlation analysis. To explore this concern, patterns from the interior of two grains have been mixed to simulate the interaction volume crossing a grain boundary so that the effect on the accuracy of the cross-correlation results can be tested. It was found that the accuracy of HR-EBSD strain measurements performed in a FEGSEM on zirconium remains good until the incident beam is less than 18nm from a grain boundary. A simulated microstructure was used to measure how often pattern overlap occurs at any given EBSD step size, and a simple relation was found linking the probability of overlap with step size
HR-EBSD Measurements near Twins in Zicaloy-2
No full textHR-EBSD measurements made on a Zircaloy-2 sample deformed to 2.7% in tension to generate multiple deformation twins.
HR-EBSD measurements were made using A Bruker HReFlash detector mounted on a Zeiss Merlin SEM. Sample preparation used Ar ion beam milling on a Gatan PECS II system as a final step.
The strain, stress and dislocation density variations available here were then calculated using in-house Matlab code (XEBSD) described in the following publications:
High resolution electron backscatter diffraction measurements of elastic strain variations in the presence of larger lattice rotations
TB Britton and AJ Wilkinson
Ultramicroscopy, (2012), vol. 114, 82-95
doi:10.1016/j.ultramic.2012.01.004
Measurement of residual elastic strain and lattice rotations with high resolution electron backscatter diffraction
TB Britton and AJ Wilkinson
Ultramicroscopy, (2011) vol. 111, 1395-1404
doi:10.1016/j.ultramic.2011.05.007
Determination of elastic strain fields and geometrically necessary dislocation distributions near nanoindents using electron back scatter diffraction
AJ Wilkinson and D Randman
Philosophical Magazine, (2010), vol. 90, 1159-1177
doi:10.1080/14786430903304145
Crystal Plasticity Analysis of Micro-Deformation, Lattice Rotation and Geometrically Necessary Dislocation Density
FPE Dunne, R Kiwanuka, AJ Wilkinson
Proc. Royal Society A, (2012), vol. 468, 2509-2531
doi:10.1098/rspa.2012.0050 Stress fields close to twin tips and the associated local neighbourhoods of a hexagonal close-packed (HCP) polycrystal were studied in details. For this purpose, a coarse grain textured Zircaloy-2 sample was firstly strained uniaxially in a macroscopic direction that favours tensile twin formation. The sample was then unloaded and residual elastic strains and lattice rotations measured using the high-resolution electron backscatter diffraction (HR-EBSD) technique. Measured elastic strain maps of various clusters of grains including parent and twin pairs were then analysed. Stress, dislocation density, and their associated concentrations close to twin tips, within twins, in the immediate neighbouring grain, at the intersection of two twins, and within parent grains were investigated. It is shown that the stress field at the twin tips varies as a function of local neighbourhood. High stress, lattice rotation, and dislocation density concentrations were generally observed close to twin tips both within twins and within the immediate neighbouring grains. It is shown dislocation density concentration is maximum at the intersection of two twins which can potentially provide susceptible site for crack nucleation
Deformation studies near hard particles in a superalloy
Get PDFSuperalloys have performed well as blade and disc materials in turbine engines due to their exceptional elevated temperature strength, high resistance to creep, oxidation and corrosion as well as good fracture toughness. This study explores the use of a relatively new technique of strain measurement, high resolution electron backscatter diffraction (HR-EBSD) to measure local deformation fields. The heart of the HR-EBSD technique lies in comparing regions in EBSD patterns from a strained region of a sample to those in a pattern from an unstrained region. This method was applied to study the elastic strain fields and geometrically necessary dislocation density (GND density) distribution near hard carbide particles in a nickel-based superalloy MAR-M-002. Significant thermal strains were initially induced by thermal treatment, which included a final cooling from the ageing temperature of 870°C. Elastic strains were consistent with a compressive radial strain and tensile hoop strain that was expected as the matrix contracts around the carbide. The mismatch in thermal expansion coefficient of the carbide particles compared to that of the matrix was sufficient to have induced localized plastic deformation in the matrix leading to a GND density of 3 × 1013 m–2 in regions around the carbide. These measured elastic strain and GND densities have been used to help develop a crystal plasticity finite element model in another research group and some comparisons under thermal loading have also been examined. Three-point bending was then used to impose strain levels within the range ±12% across the height of a bend bar sample. GND 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 carbide particles. The higher GND densities near the carbides (order of 1014 per m2) are generated by the large strain gradients produced around the plastically rigid inclusion during monotonic mechanical deformation with some minor contribution from the pre-existing residual deformation from thermal loading. A method was developed of combining the local EBSD measurements with FE modelling to set the average residual strains within the mapped region even when a good strain free reference point was unavailable. Cyclic loading was then performed under four point loading to impose strain levels of about ±8% across the height of bend bar samples. Similar measurements as in the case of monotonic deformation were made at several interruptions to fatigue loading. Observations from the cyclic loading such as slip features, carbide cracking, GND density accumulation have been explored around carbide particles, at regions away from them and near a grain boundary
Grain Boundary Sliding in Tin
No full textGrain boundary sliding is an important deformationmechanism at higher temperatures (relative to the melting point) and can make significant contributions in creep and superplastic deformation regimes.
Here we make available data from compression tests on micro-pillars machined by focused ion beam into single and bi-crystals of pure tin. The data demonstrate marked size effects with the strength increasing for single and bi-crystals as the sample size is reduced. For the bu-crystals the fraction of the overal deformation contributed by grain boundary sliding increases strongly as the sample size is reduced. Grain boundary sliding is an important deformation mechanism that contributes to creep and superplastic forming. In tin-based lead-free solders grain boundary sliding can make significant contributions to in service performance. Novel microcompression tests designed to isolate individual grain boundaries and assess their mechanical resistance to sliding were conducted on tin. The boundary sliding deformation was more obvious for smaller sample cross-sections and made a larger contribution to the overall deformation. As with dislocation and twinning mediated plasticity there was a significant size effect in which the resistance to grain boundary sliding increases as the sample size is reduced
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
