Publications
130 results found
Britton TB, Jackson CA-L, Wade J, 2020, The reward and risk of social media for academics, Publisher: Center for Open Science
We are three academics who are active on social media. We explore the motivations for and benefits of engaging with social media, as well as its costs and risks. Overall, we believe this engagement to be a net benefit for us, our employers and for wider society.
Kyvelou P, Slack H, Daskalaki Mountanou D, et al., 2020, Mechanical and microstructural testing of wire and arc additively manufactured sheet material, Materials and Design, Vol: 192, ISSN: 0264-1275
Wire and arc additive manufacturing (WAAM) is a method of 3D printing that enables large elements to be built, with reasonable printing times and costs. There are, however, uncertainties relating to the structural performance of WAAM material, including the basic mechanical properties, the degree of anisotropy, the influence of the as-built geometry and the variability in response. Towards addressing this knowledge gap, a comprehensive series of tensile tests on WAAM stainless steel was conducted; the results are presented herein. As-built and machined coupons were tested to investigate the influence of the geometrical irregularity on the stress-strain characteristics, while material anisotropy was explored by testing coupons produced at different angles to the printing orientation. Non-contact measurement techniques were employed to determine the geometric properties and deformation fields of the specimens, while sophisticated analysis methods were used for post processing the test data. The material response revealed a significant degree of anisotropy, explained by the existence of a strong crystallographic texture, uncovered by means of electron backscatter diffraction. Finally, the effective mechanical properties of the as-built material were shown to be strongly dependent on the geometric variability; simple geometric measures were therefore developed to characterise the key aspects of the observed behaviour.
Pedrazzini S, Pek ME, Ackerman AK, et al., 2020, Influence of powder-bed temperature on the microstructure and mechanical properties of Ti-6Al-4V produced by selective laser melting, Publisher: arXiv
Advanced characterisation techniques were used on LPBF Ti-6Al-4V samplesproduced on a heated base plate. When the substrate temperature is 100{\deg}Cthe elongation is 6\%, which increases and peaks at 10\% at 570{\deg}C, thensharply decreases to zero ductility at 770{\deg}C. At 100{\deg}C, a heavilystrained and twinned microstructure, primarily composed of {\alpha}+{\alpha}',was observed and it was comparable to asbuilt microstructures obtained byconventional LPBF methods. At higher temperatures, twins are no longer presentand instead nano-scale {\beta} precipitates are observed within {\alpha}' and{\alpha}, as well as dislocation networks (570{\deg}C) and tangles(770{\deg}C). Solute segregation at crystal defects was observed in allpre-heating conditions. Al and V segregation at microtwins was observed in the100{\deg}C sample, reporting for the first time `selective' and mutuallyexclusive Al- and V-rich regions forming in adjacent twins. V segregation atdislocations was observed in the 570{\deg}C and 770{\deg}C samples, consistentwith the higher preheating temperatures. High O contents were measured in allsamples but with apparent opposing effects. At 100{\deg}C and 570{\deg}C wasestimated to be below the critical threshold for O embrittlement and locallyaids in maintaining a strength high by solid solution strengthening, whereas at770{\deg}C it was above the threshold, therefore failing in a brittle fashion.Based on these observations, the initial increase in ductility from 100{\deg}Cto 570{\deg}C is attributed to a reduction in microtwins and the dislocationnetworks acting as `soft barriers' for slip within a coarser microstructure.The lack of ductility at 770{\deg}C was attributed to local soluteredistribution causing dislocation pinning and an increase of O content in thissample.
Bhowmik A, Lee J, Adande S, et al., 2020, Investigating spatio-temporal deformation in single crystal Ni-based superalloys using in-situ diffraction experiments and modelling, Materialia, Vol: 9, Pages: 1-14, ISSN: 2589-1529
In this study, we perform a detailed analysis of room temperature deformation of a [100]–orientated singlecrystal Ni-based superalloy, CMSX-4 micropillar, using a combinatorial and complimentary characterisation approach of micro-Laue diffraction coupled with post-deformation microscopy and crystal plasticity modelling.Time-resolved micro-Laue data indicated that deformation was initiated by activation of multiple slip (after 5%engineering strain) which led to the generation of a plastic strain accumulation accompanied by a two-foldincrease in the dislocation density within the micropillar. Subsequent to that, slip occurred primarily on two systems (11̄1)[101] and (111)[1̄01] with the highest Schmid factor in the single crystal micropillar thereby resultingin little accumulation of unpaired GNDs during a major part of the loading cycle, upto 20% strain in this case.Finite element crystal plasticity modelling also showed good agreement with the experimental analyses, wherebysignificant strains were found to develop in the above slip systems with a localisation near the centre of themicropillar. Post-deformation transmission electron microscopy study confirmed that deformation was mediatedthrough a/2<110> dislocations on {111} planes in the 𝛾-phase, while high stress levels led to shearing of the 𝛾′precipitates by a/2<110> partials bounding an anti-phase boundary free to glide on the {111} planes. Duringthe deformation of the single crystal micropillar, independent rotations of the 𝛾 and 𝛾′ phases were quantified byspatially resolved post-deformation micro-Laue patterns. The degree of lattice rotation in the 𝛾-phase was higherthan that in the 𝛾′-phase.
McAuliffe TP, Foden A, Bilsland C, et al., 2020, Advancing characterisation with statistics from correlative electron diffraction and X-ray spectroscopy, in the scanning electron microscope., Ultramicroscopy, Vol: 211, Pages: 1-16, ISSN: 0304-3991
The routine and unique determination of minor phases in microstructures is critical to materials science. In metallurgy alone, applications include alloy and process development and the understanding of degradation in service. We develop a correlative method, exploring superalloy microstructures, which are examined in the scanning electron microscope (SEM) using simultaneous energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). This is performed at an appropriate length scale for characterisation of carbide phases' shape, size, location, and distribution. EDS and EBSD data are generated using two different physical processes, but each provide a signature of the material interacting with the incoming electron beam. Recent advances in post-processing, driven by 'big data' approaches, include use of principal component analysis (PCA). Components are subsequently characterised to assign labels to a mapped region. To provide physically meaningful signals, the principal components may be rotated to control the distribution of variance. In this work, we develop this method further through a weighted PCA approach. We use the EDS and EBSD signals concurrently, thereby labelling each region using both EDS (chemistry) and EBSD (crystal structure) information. This provides a new method of amplifying signal-to-noise for very small phases in mapped regions, especially where the EDS or EBSD signal is not unique enough alone for classification.
Gu T, Xu Y, Gourlay CM, et al., 2020, In-situ electron backscatter diffraction of thermal cycling in a single grain Cu/Sn-3Ag-0.5Cu/Cu solder joint, Scripta Materialia, Vol: 175, Pages: 55-60, ISSN: 1359-6462
The heterogeneous evolution of microstructure in a single grain Cu/SAC305/Cu solder joint is investigated using in-situ thermal cycling combined with electron backscatter diffraction (EBSD). Local deformation due to thermal expansion mismatch results in heterogeneous lattice rotation within the joint, localised towards the corners. This deformation is induced by the constraint and the coefficient of thermal expansion (CTE) mismatch between the β-Sn, Cu6Sn5 and Cu at interfaces. The formation of subgrains with continuous increase in misorientation is revealed during deformation, implying the accumulation of plastic slip at the strain-localised regions and the activation of slip systems (110)[11]/2 and (0)[111]/2.
Guo Y, Collins DM, Tarleton E, et al., 2020, Dislocation density distribution at slip band-grain boundary intersections, Acta Materialia, Vol: 182, Pages: 172-183, ISSN: 1359-6454
We study the mechanisms of slip transfer at a grain boundary, in titanium, using Differential Aperture X-ray Laue Micro-diffraction (DAXM). This 3D characterisation tool enables measurement of the full (9-component) Nye lattice curvature tensor and calculation of the density of geometrically necessary dislocations (GNDs). We observe dislocation pile-ups at a grain boundary, as the neighbour grain prohibits easy passage for dislocation transmission. This incompatibility results in local micro-plasticity within the slipping grain, near to where the slip planes intersect the grain boundary, and we observe bands of GNDs lying near the grain boundary. We observe that the distribution of GNDs can be significantly influenced by the formation of grain boundary ledges that serve as secondary dislocation sources. This observation highlights the non-continuum nature of polycrystal deformation and helps us understand the higher order complexity of grain boundary characteristics.
Hielscher R, Bartel F, Britton TB, 2019, Gazing at crystal balls: Electron backscatter diffraction pattern analysis and cross correlation on the sphere, Ultramicroscopy, Vol: 207, Pages: 1-11, ISSN: 0304-3991
We present spherical analysis of electron backscatter diffraction (EBSD) patterns with two new algorithms: (1) band localisation and band profile analysis using the spherical Radon transform; (2) orientation determination using spherical cross correlation. These new approaches are formally introduced and their accuracies are determined using dynamically simulated patterns. We demonstrate their utility with an experimental dataset obtained from ferritic iron. Our results indicate that the analysis of EBSD patterns on the sphere provides an elegant method of revealing information from these rich sources of crystallographic data.
Foden A, Collins DM, Wilkinson AJ, et al., 2019, Indexing electron backscatter diffraction patterns with a refined template matching approach, Publisher: ELSEVIER
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- Citations: 23
Wang S, Kalácska S, Maeder X, et al., 2019, The effect of δ-hydride on the micromechanical deformation of Zircaloy-4 studied by in situ high angular resolution electron backscatter diffraction, Scripta Materialia, Vol: 173, Pages: 101-105, ISSN: 1359-6462
Zircaloy-4(Zr-1.5%Sn-0.2%Fe-0.1%Cr wt%)is usedas nuclear fuel cladding materials and hydride embrittlementis amajor failure mechanism. To explore the effect of δ-hydrideon plastic deformation and performance of Zircaloy-4, in situhigh angular resolution electron backscatter diffraction(HR-EBSD)was used to quantify stress andgeometrically necessarydislocation(GND)density during bending tests of hydride-free and hydride-containingsingle crystalZircaloy-4 microcantilevers. Results suggest that while the stress applied was accommodated by plastic slip in the hydride-free cantilever,the hydride-containing cantilever showedprecipitation-induced GND pile-up at hydride-matrix interfacepre-deformation, andconsiderable locally-increasing GNDdensity under tensile stressupon plastic deformation.
Foden A, Previero A, Britton TB, 2019, Advances in electron backscatter diffraction
We present a few recent developments in the field of electron backscatterdiffraction (EBSD). We highlight how open source algorithms and open dataformats can be used to rapidly to develop microstructural insight of materials.We include use of AstroEBSD for single pixel based EBSD mapping andconventional orientation mapping; followed by an unsupervised machine learningapproach using principal component analysis and multivariate statisticscombined with a refined template matching method to rapidly index orientationdata with high precision. Next, we compare a diffraction pattern captured usingdirect electron detector with a dynamical simulation and project this to createa high quality experimental "reference diffraction sphere". Finally, weclassify phases using supervised machine learning with transfer learning and aconvolutional neural network.
Hielscher R, Bartel F, Britton TB, 2019, Gazing at crystal balls - electron backscatter diffraction indexing and cross correlation on a sphere, Microscopy and Microanalysis, Vol: 25, Pages: 1954-1955, ISSN: 1083-0375
Britton B, Jackson C, Wade J, 2019, The reward and risk of social media for academics, Nature Reviews Chemistry, Vol: 3, Pages: 459-461, ISSN: 2397-3358
We are three academics who are active on social media. We explore the motivations for and benefits of engaging with social media, as well as its costs and risks. Overall, we believe this engagement to be a net benefit for us, our employers and for wider society.
Wallis D, Hansen LN, Britton TB, et al., 2019, High-angular resolution electron backscatter diffraction as a new tool for mapping lattice distortion in geological minerals, Journal of Geophysical Research. Solid Earth, Vol: 124, Pages: 6337-6358, ISSN: 2169-9356
Analysis of distortions of the crystal lattice within individual mineralgrains is central to the investigation of microscale processes that control andrecord tectonic events. These distortions are generally combinations of latticerotations and elastic strains, but a lack of suitable observational techniqueshas prevented these components being mapped simultaneously and routinely inearth science laboratories. However, the technique of high-angular resolutionelectron backscatter diffraction (HR-EBSD) provides the opportunity tosimultaneously map lattice rotations and elastic strains with exceptionalprecision, on the order of 0.01 degree for rotations and 10-4 in strain, usinga scanning electron microscope. Importantly, these rotations and latticestrains relate to densities of geometrically necessary dislocations andresidual stresses. Recent works have begun to apply and adapt HR-EBSD togeological minerals, highlighting the potential of the technique to provide newinsights into the microphysics of rock deformation. Therefore, the purpose ofthis overview is to provide a summary of the technique, to identify caveats andtargets for further development, and to suggest areas where it offers potentialfor major advances. In particular, HREBSD is well suited to characterising theroles of different dislocation types during crystal plastic deformation and tomapping heterogeneous internal stress fields associated with specificdeformation mechanisms/microstructures or changes in temperature, confiningpressure, or applied deviatoric stress. These capabilities make HR-EBSD aparticularly powerful new technique for analysing the microstructures ofdeformed geological materials.
Gianola DS, Britton TB, Zaefferer S, 2019, New techniques for imaging and identifying defects in electron microscopy, MRS Bulletin, Vol: 44, Pages: 450-458, ISSN: 0883-7694
Defects in crystalline solids control the properties of engineered and natural materials, and their characterization focuses our strategies to optimize performance. Electron microscopy has served as the backbone of our understanding of defect structure and their interactions, owing to beneficial spatial resolution and contrast mechanisms that enable direct imaging of defects. These defects reside in complex microstructures and chemical environments, demanding a combination of experimental approaches for full defect characterization. In this article, we describe recent progress and trends in methods for examining defects using scanning electron microscopy platforms. Several emerging approaches offer attractive benefits, for instance, in correlative microscopy across length scales and in in situ studies of defect dynamics.
Wang S, Giuliani F, Britton TB, 2019, Microstructure and formation mechanisms of δ-hydrides in variable grain size Zircaloy-4 studied by electron backscatter diffraction, Acta Materialia, Vol: 169, Pages: 76-87, ISSN: 1359-6454
Microstructure and crystallography of δ phase hydrides in as-received fine grain and ‘blocky alpha’ large grain Zircaloy-4 (average grain size ∼11 μm and >200 μm, respectively) were examined using electron backscatter diffraction (EBSD). Results suggest that the matrix-hydride orientation relationship is {0001} α ||{111} δ ;<112¯0> α ||<110> δ for all the cases studied. The habit plane of intragranular hydrides and some intergranular hydrides has been found to be {101¯7} of the surrounding matrix. The morphology of intergranular hydrides can vary depending upon the angle between the grain boundary and the hydride habit plane. The misfit strain between α-Zr and δ-hydride is accommodated mainly by high density of dislocations and twin structures in the hydrides, and a mechanism of twin formation in the hydrides has been proposed. The growth of hydrides across grain boundaries is achieved through an auto-catalytic manner similar to the growth pattern of intragranular hydrides. Easy collective shear along <11¯00> makes it possible for hydride nucleation at any grain boundaries, while the process seems to favour grain boundaries with low (<40°) and high (>80°) c-axis misorientation angles. Moreover, the angle between the grain boundary and the adjacent basal planes does not influence the propensity for hydride nucleation.
Wallis D, Hansen L, Britton T, et al., 2019, High-angular resolution electron backscatter diffraction as a new tool for mapping lattice distortion in geological minerals
Moutona I, Breen AJ, Wang S, et al., 2019, Quantification challenges for atom probe tomography of hydrogen and deuterium in zircaloy-4, Microscopy and Microanalysis, Vol: 25, Pages: 481-488, ISSN: 1431-9276
Analysis and understanding of the role of hydrogen in metals is a significant challenge for the future of materials science, and this is a clear objective of recent work in the atom probe tomography (APT) community. Isotopic marking by deuteration has often been proposed as the preferred route to enable quantification of hydrogen by APT. Zircaloy-4 was charged electrochemically with hydrogen and deuterium under the same conditions to form large hydrides and deuterides. Our results from a Zr hydride and a Zr deuteride highlight the challenges associated with accurate quantification of hydrogen and deuterium, in particular associated with the overlap of peaks at a low mass-to-charge ratio and of hydrogen/deuterium containing molecular ions. We discuss possible ways to ensure that appropriate information is extracted from APT analysis of hydrogen in zirconium alloy systems that are important for nuclear power applications.
Wang S, Giuliani F, Britton T, 2019, Variable temperature micropillar compression to reveal <a> basal slip properties of Zircaloy-4, Scripta Materialia, Vol: 162, Pages: 451-455, ISSN: 1359-6462
Zircaloy-4 is widely used as nuclear fuel cladding materials, where it is important to understand the mechanical properties between room temperature and reactor operating temperatures (around 623 K). To aid in this understanding, we have performed compression tests on micropillars aligned to activate <a> basal slip across this temperature range. Engineering analysis of the results indicates that the plastic yield follows a thermally activated constitutive law. We also observe the nature of the slip bands formed on the side surface of our pillars and see characteristic ‘bulging’ that tends to localise as temperature increases.
Winkelmann A, Ben Britton T, Nolze G, 2019, Constraints on the effective electron energy spectrum in backscatter Kikuchi diffraction, Publisher: AMER PHYSICAL SOC
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- Citations: 14
Jun T-S, Maeder X, Bhowmik A, et al., 2019, The role of β-titanium ligaments in the deformation of dual phase titanium alloys, Materials Science and Engineering: A, Vol: 746, Pages: 394-405, ISSN: 0921-5093
Multiphase titanium alloys are critical materials in high value engineeringcomponents, for instance in aero engines. Microstructural complexity isexploited through interface engineering during mechanical processing to realisesignificant improvements in fatigue and fracture resistance and strength. Inthis work, we explore the role of select interfaces using in-situmicromechanical testing with concurrent observations from high angularresolution electron backscatter diffraction (HR-EBSD). Our results aresupported with post mortem transmission electron microscopy (TEM). Usingmicro-pillar compression, we performed in-depth analysis of the role of select{\beta}-titanium (body centred cubic) ligaments which separate neighbouring{\alpha}-titanium (hexagonal close packed) regions and inhibit the dislocationmotion and impact strength during mechanical deformation. These results shedlight on the strengthening mechanisms and those that can lead to strainlocalisation during fatigue and failure.
Tong VS, Knowles AJ, Dye D, et al., 2019, Rapid electron backscatter diffraction mapping: Painting by numbers, Materials Characterization, Vol: 147, Pages: 271-279, ISSN: 1044-5803
Microstructure characterisation has been greatly enhanced through the use of electron backscatter diffraction (EBSD), where rich maps are generated through analysis of the crystal phase and orientation in the scanning electron microscope (SEM). Conventional EBSD analysis involves raster scanning of the electron beam and serial analysis of each diffraction pattern in turn. For grain shape, crystallographic texture, and microstructure analysis this can be inefficient. In this work, we present Rapid EBSD, a data fusion approach combining forescatter electron (FSE) imaging with static sparse sampling of EBSD patterns. We segment the FSE image into regions of similar colour (i.e. phase and crystal orientation) and then collect representative EBSD data for each segmented region. This enables microstructural assessment to be performed at the spatial resolution of the (fast) FSE imaging whilst including orientation and phase information from EBSD analysis of representative points. We demonstrate the Rapid EBSD technique on samples of a cobalt based superalloy and a strained dual phase titanium alloy, comparing the results with conventional analysis. Rapid EBSD is advantageous for assessing grain size distributions in time-limited experiments.
Gu T, Gourlay C, Britton T, 2019, Evaluating creep deformation in controlled microstructures of Sn-3Ag-0.5Cu solder, Journal of Electronic Materials, Vol: 48, Pages: 107-121, ISSN: 0361-5235
The reliability of solder joints is affected significantly by thermomechanical properties such as creep and thermal fatigue. In this work, the creep of directionally solidified (DS) Sn-3Ag-0.5Cu wt.% (SAC305) dog-bone samples (gauge dimension: 10 × 2 × 1.5 mm) with a controlled <110> or <100> fibre texture is investigated under constant load testing (stress level: ∼ 30 MPa) at a range of temperatures from 20°C to 200°C. Tensile testing is performed and the secondary creep strain rate and the localised strain gradient are studied by two-dimensional optical digital image correlation (2-D DIC). The dominating creep mechanisms and their temperature dependence are explored at the microstructural scale using electron backscatter diffraction (EBSD), which enables the understanding of the microstructural heterogeneity of creep mechanisms at different strain levels, temperatures and strain rates. Formation of subgrains and the development of recrystallization are observed with increasing strain levels. Differences in the deformation of β-Sn in dendrites and in the eutectic regions containing Ag3Sn and Cu6Sn5 are studied and related to changes in local deformation mechanisms.
Birch R, Wang S, Tong VS, et al., 2019, The effect of cooling rate and grain size on hydride microstructure in Zircaloy-4, Journal of Nuclear Materials, Vol: 513, Pages: 221-225, ISSN: 0022-3115
We explore the distribution, morphology and structure of zirconium hydrides formed using different cooling rates through the solid state Zr+[H] → Zr + hydride transus, in fine and blocky alpha Zircaloy-4. We observe that cooling rate and grain size control the phase and distribution of hydrides formed. The blocky alpha (coarse grain, > 200 μm) Zircaloy-4, has a smaller grain boundary area to grain volume ratio and this significantly affects nucleation and growth of hydrides as compared to fine grain size (∼11 μm) material.
Britton TB, Tong V, Hickey J, et al., 2018, AstroEBSD: exploring new space in pattern indexing with methods launched from an astronomical approach, Journal of Applied Crystallography, Vol: 51, Pages: 1525-1534, ISSN: 0021-8898
Electron backscatter diffraction (EBSD) is a technique used to measure crystallographic features in the scanning electron microscope. The technique is highly automated and readily accessible in many laboratories. EBSD pattern indexing is conventionally performed with raw electron backscatter patterns. These patterns are software processed to locate the band centres (and sometimes edges) from which the crystallographic index of each band is determined. Once a consistent index for many bands is obtained, the crystal orientation with respect to a reference sample and detector orientation can be determined and presented. Unfortunately, because of challenges related to crystal symmetry, there are limited available pattern-indexing approaches and this has probably hampered open development of the technique. In this article, a new method of pattern indexing is presented, based upon a method with which satellites locate themselves in the night sky, and its effectiveness is systematically demonstrated using dynamical simulations and real experimental patterns. The benefit of releasing this new algorithm as open-source software is demonstrated when this indexing process is utilized, together with dynamical solutions, to provide some of the first accuracy assessments of an indexing solution. In disclosing a new indexing algorithm, and software processing toolkit, the authors hope to open up EBSD developments to more users. The software code and example data are released alongside this article for third party developments.
Breen AJ, Mouton I, Lu W, et al., 2018, Atomic scale analysis of grain boundary deuteride growth front in Zircaloy-4, Scripta Materialia, Vol: 156, Pages: 42-46, ISSN: 1359-6462
Zircaloy-4 (Zr-1.5%Sn-0.2%Fe-0.1%Cr wt%) was electrochemically charged with deuterium to create deuterides and subsequently analysed with atom probe tomography and scanning transmission electron microscopy to understand zirconium hydride formation and embrittlement. At the interface between the hexagonal close packed (HCP) α-Zr matrix and a face centred cubic (FCC) δ deuteride (ZrD1.5–1.65), a HCP ζ phase deuteride (ZrD0.25–0.5) has been observed. Furthermore, Sn is rejected from the deuterides and segregates to the deuteride/α-Zr reaction front.
Lan B, Britton TB, Jun T-S, et al., 2018, Direct volumetric measurement of crystallographic texture using acoustic waves, Acta Materialia, Vol: 159, Pages: 384-394, ISSN: 1359-6454
Crystallographic texture in polycrystalline materials is often developed as preferred orientation distribution of grains during thermo-mechanical processes. Texture dominates many macroscopic physical properties and reflects the histories of structural evolution, hence its measurement and control are vital for performance optimisation and deformation history interogation in engineering and geological materials. However, exploitations of texture are hampered by state-of-the-art characterisation techniques, none of which can routinely deliver the desirable non-destructive, volumetric measurements, especially at larger lengthscales. Here we report a direct and general methodology retrieving important lower-truncation-order texture and phase information from acoustic (compressional elastic) wave speed measurements in different directions through the material volume (avoiding the need for forward modelling). We demonstrate its deployment with ultrasound in the laboratory, where the results from seven representative samples are successfully validated against measurements performed using neutron diffraction. The acoustic method we have developed includes both fundamental wave propagation and texture inversion theories which are free from diffraction limits, they are arbitrarily scalable in dimension, and can be rapidly deployed to measure the texture of large objects. This opens up volumetric texture characterisation capabilities in the areas of material science and beyond, for both scientific and industrial applications.
Luan Q, Britton TB, Jun T, 2018, Strain rate sensitivity in commercial pure titanium: the competition between slip and deformation twinning, Materials Science and Engineering: A, Vol: 734, Pages: 385-397, ISSN: 0921-5093
Titanium alloys are widely used in light weight applications such as jet engine fans, where their mechanical performance under a range of loading regimes is important. Titanium alloys are mechanically anisotropic with respect to crystallographic orientation, and remarkably titanium creeps at room temperature. This means that the strain rate sensitivity (SRS) and stress relaxation performance are critical in predicting component life. In this work, we focus on systematically exploring the macroscopic SRS of Grade 1 commercially pure titanium (CP Ti) with varying grain sizes and texture using uniaxial compression. Briefly, we find that Ti samples had positive SRS and samples compressed along the sheet rolling direction (RD) (i.e. soft grains dominant) were less rate sensitive than bars compressed along the sheet normal direction (ND) (i.e. hard grains dominant). We attribute this rate sensitivity to the relative activity of slip and twinning. Within the grain size range of ~ 317 ±7 μm, we observe an increase in the rate sensitivity, where volume fraction of {101 ̅2}<101 ̅1> T1 tensile twins was low, and the twin width at different strain rates were similar. These observations imply that the macroscopic rate sensitivity is controlled by the ensemble behaviour of local deformation processes: the amount of slips accumulated at grain boundaries affects the SRS, which is grain size and texture dependent. We hope that this experimental study motivates mechanistic modelling studies using crystal plasticity, including strain rate sensitivity and twinning, to predict the performance of titanium alloys.
Niania M, Podor R, Britton TB, et al., 2018, In situ study of strontium segregation in La<inf>0.6</inf>Sr<inf>0.4</inf>Co<inf>0.2</inf>Fe<inf>0.8</inf>O<inf>3- δ</inf>in ambient atmospheres using high-temperature environmental scanning electron microscopy, Journal of Materials Chemistry A, Vol: 6, Pages: 14120-14135, ISSN: 2050-7496
Samples of the solid oxide fuel cell cathode material La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) were annealed using High-Temperature Environmental Scanning Electron Microscopy (HT-ESEM) from room temperature to 1000 °C in atmospheres of pure oxygen, pure water and ambient lab air. Image series of each heat treatment were taken where microstructural changes were observed and compared between samples. Strontium segregation rate was found to be significantly increased in the presence of pure water as compared to pure O2and ambient air. Electron backscattered diffraction (EBSD) was performed in order to assess the effect of crystal orientation on particle formation and surface sensitive chemical analysis techniques were used to determine the chemical changes at the grain surface as a result of the different heat treatments. It was shown that crystal orientation affected the nature and growth rate of strontium-based particles, however, due to the pseudo-symmetry of La0.6Sr0.4Co0.2Fe0.8O3-δ, precise crystal orientation relationships could not be determined. The chemical composition of the grain surface was found to be approximately equal under each atmosphere.
Britton TB, Goran D, Tong VS, 2018, Space rocks and optimising scanning electron channelling contrast, Materials Characterization, Vol: 142, Pages: 422-431, ISSN: 1044-5803
Forescatter electron imaging is a popular microscopy technique, especially for scanning electron microscopes equipped with an electron backscatter diffraction detector. In principal, this method enables qualitative imaging of microstructure but quantitative assessment can be limited due to limited information about the contrast afforded. In this work, we explore forescatter electron imaging and demonstrate that imaging can be optimised for topographic, phase, and subtle orientation contrast imaging through appropriate sample and detector positioning. We demonstrate the relationship between imaging modes using systematic variation in detector positioning and compare this with pseudo-forescatter electron images, obtained from image analysis of diffraction patterns, to explore and confirm image contrast modes. We demonstrate these contrast mechanisms on a map obtained from a sample of the Gibeon meteorite.
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