arXiv subjects

Vivian Tong

Publications and source records attributed to Vivian Tong.

14 recordsLinked to original sources

arXiv2026

Towards standardisation of average grain size measurement of additively manufactured microstructures using EBSD

Additively manufactured (AM) alloys have heterogeneous microstructures with broad grain size distributions and highly anisotropic and/or non-convex grain shapes. AM components can have complex geometries and porosity which may affect the local microstructure. Currently there is no electron backscatter diffraction (EBSD)-based grain size measurement standard suitable for typical AM materials. An interlaboratory comparison study was conducted to find out what grain size metrics and summary statistics are currently used to describe average grain size. Participants were asked to measure and report the average grain size from the same EBSD map dataset. Detailed reports have been published in Reference [1]. Based on these results, we have tested and propose recommendations for a new standard for measuring average grain size in AM materials. The present work demonstrates the suitability and limitations of the proposal across several different Ni and Al AM components.

physics.app-ph↗

arXiv2026

TrueEBSD in MTEX: automatic image matching for correlative microscopy applications

TrueEBSD is an open-source MATLAB program for image alignment and spatial distortion correction of images and electron backscatter diffraction (EBSD) maps. We have re-implemented TrueEBSD as an add-on to MTEX, an established toolbox for EBSD data analysis. Spatial alignment enables correlative analysis methods, such as augmenting EBSD orientation maps with data from other imaging modes. The augmented EBSD maps can then be analysed further using MTEX. We demonstrate TrueEBSD on two example case studies: one for measuring Co phase fraction and WC contiguity in a WC-Co composite, and another for determining the relative susceptibility of grain boundaries to void formation in a copper polycrystal. In both examples, the EBSD map was augmented with scanning electron microscopy (SEM) image data. This enabled quantitative crystallographic measurements which would not be possible from analysing the EBSD maps and images separately.

cond-mat.mtrl-sci↗

arXiv2023

Curvature-enhanced localised emission from dark states in wrinkled monolayer WSe2 at room temperature

Localised emission from defect states in monolayer transition metal dichalcogenides is of great interest for optoelectronic and quantum device applications. Recent progress towards high temperature localised emission relies on the application of strain to induce highly confined excitonic states. Here we propose an alternative paradigm based on curvature, rather than in-plane stretching, achieved through free-standing wrinkles of monolayer tungsten diselenide (WSe2). We probe these nanostructures using tip-enhanced optical spectroscopy to reveal the spatial localisation of out-of-plane polarised emission from the WSe2 wrinkles. Based on the photoluminescence and Raman scattering signatures resolved with nanoscale spatial resolution, we propose the existence of a manifold of spin-forbidden excitonic states that are activated by the local curvature of the WSe2. We are able to access these dark states through the out-of-plane polarised surface plasmon polariton resulting in enhanced strongly localised emission at room temperature, which is of potential interest for quantum technologies and photonic devices.

cond-mat.mes-hall↗

arXiv2023

An iterative method for reference pattern selection in high resolution electron backscatter diffraction (HR-EBSD)

For high (angular) resolution electron backscatter diffraction (HR-EBSD), the selection of a reference diffraction pattern (EBSP0) significantly affects the precision of the calculated strain and rotation maps. This effect was demonstrated in plastically deformed body-centred cubic and face-centred cubic ductile metals (ferrite and austenite grains in duplex stainless steel) and brittle single-crystal silicon, which showed that the effect is not only limited to measurement magnitude but also spatial distribution. An empirical relationship was then identified between the cross-correlation parameter and angular error, which was used in an iterative algorithm to identify the optimal reference pattern that maximises the precision of HR-EBSD.

cond-mat.mtrl-sci↗

arXiv2021

Micropillar compression of single crystal tungsten carbide, Part 2: Lattice rotation axis to identify deformation slip mechanisms

The plastic deformation mechanisms of tungsten carbide at room and elevated temperatures influence the wear and fracture properties of WC-Co hardmetal composite materials. The relationship between residual defect structures, including glissile and sessile dislocations and stacking faults, and the slip deformation activity, which produce slip traces, is not clear. Part 1 of this study showed that {10-10} was the primary slip plane at all measured temperatures and orientations, but secondary slip on the basal plane was activated at 600 °C, which suggests that dislocations can cross-slip onto the basal plane at 600 °C. In the present work, Part 2, lattice rotation axis analysis of deformed WC micropillar mid-sections was used to discriminate prismatic slip from multiple prismatic slip in WC, which enabled the dislocation types contributing to plastic slip to be distinguished, independently of TEM residual defect analysis. Prismatic-oriented micropillars deformed primarily by multiple prismatic slip at room temperature, but by prismatic slip at 600 °C. Deformation in the near-basal oriented pillar at 600 °C can be modelled as prismatic slip along constrained by the indenter face and pillar base. Secondary basal slip, which was observed near the top of the pillar, was activated to maintain deformation compatibility with the indenter face. The lattice rotations, buckled pillar shape, mechanical data, and slip traces observed in the pillar are all consistent with this model.

cond-mat.mtrl-sci↗

arXiv2021

Micropillar compression of single crystal tungsten carbide, Part 1: temperature and orientation dependence of deformation behaviour

Tungsten carbide cobalt hardmetals are commonly used as cutting tools subject to high operation temperature and pressures, where the mechanical performance of the tungsten carbide phase affects the wear and lifetime of the material. In this study, the mechanical behaviour of the isolated tungsten carbide (WC) phase was investigated using single crystal micropillar compression. Micropillars in two crystal orientations, 1-5 $μ$m in diameter, were fabricated using focused ion beam (FIB) machining and subsequently compressed between room temperature and 600 °C. The activated plastic deformation mechanisms were strongly anisotropic and weakly temperature dependent. The flow stresses of basal-oriented pillars were about three times higher than the prismatic pillars, and pillars of both orientations soften slightly with increasing temperature. The basal pillars tended to deform by either unstable cracking or unstable yield, whereas the prismatic pillars deformed by slip-mediated cracking. However, the active deformation mechanisms were also sensitive to pillar size and shape. Slip trace analysis of the deformed pillars showed that {10-10} prismatic planes were the dominant slip plane in WC. Basal slip was also activated as a secondary slip system at high temperatures.

cond-mat.mtrl-sci↗

arXiv2020

TrueEBSD: correcting spatial distortions in electron backscatter diffraction maps

Electron backscatter diffraction (EBSD) in the scanning electron microscope is routinely used for microstructural characterisation of polycrystalline materials. Maps of EBSD data are typically acquired at high stage tilt and slow scan speed, leading to tilt and drift distortions that obscure or distort features in the final microstructure map. In this paper, we describe TrueEBSD, an automatic postprocessing procedure for distortion correction with pixel-scale precision. Intermediate images are used to separate tilt and drift distortion components and fit each to a physically-informed distortion model. We demonstrate TrueEBSD on three case studies (titanium, zirconium and hydride containing Zr), where distortion removal has enabled characterisation of otherwise inaccessible microstructural features.

cond-mat.mtrl-sci↗

arXiv2020

In-situ study of creep in Sn-3Ag-0.5Cu solder

The creep behaviour and microstructural evolution of a Sn-3Ag-0.5Cu wt.% sample with a columnar microstructure have been investigated through in-situ creep testing under constant stress of 30 MPa at 298 K. This is important, as 298 K is high temperature within the solder system and in-situ observations of microstructure evolutions confirm the mechanisms involved in deformation and ultimately failure of the material. The sample has been observed in-situ using repeat and automatic forescatter diode and auto electron backscatter diffraction imaging. During deformation, polygonisation and recrystallisation are observed heterogeneously with increasing strain, and these correlate with local lattice rotations near matrix-intermetallic compound interfaces. Recrystallised grains have either twin or special boundary relationships to their parent grains. The combination of these two imaging methods reveal one grain (loading direction, LD, 10.4 ° from [100]) deforms less than the neighbour grain 2 (LD 18.8° from [110]), with slip traces in the strain localised regions. In grain 1, (1-10)[001] slip system are observed and in grain 2, (1-10)[-1-11]/2 and (110)[-111]/2 slip systems are observed. Lattice orientation gradients build up with increasing plastic strain and near fracture recrystallisation is observed concurrent with fracture.

cond-mat.mtrl-sci↗

arXiv2019

Understanding plasticity in zirconium using in-situ measurement of lattice rotations

Understanding deformation in polycrystalline metals is critical to use them in high-value high-risk applications. We present in-situ characterisation of plastic deformation of zirconium, a hexagonal closed packed (HCP), metal. Analysis of plastic deformation is performed using electron backscatter diffraction (EBSD) to reveal changes in lattice orientation. Through implementation of TrueEBSD, we can relate the lattice rotations back to the undeformed reference frame. This enables us to explore which slip systems are active and the degree of homogeneous (i.e. deformation with respect to the external load) and heterogeneous (i.e. deformation with respect to the local grain neighbourhood). Additionally, from our analysis, we notice that lattice rotations consistent with a significant fraction pyramidal slip are found. These results are placed in the context of deformation and performance of HCP alloys and zirconium alloys used as nuclear fuel cladding.

cond-mat.mtrl-sci↗

arXiv2018

The effect of cooling rate and grain size on hydride formation in Zircaloy-4

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. The blocky alpha (coarse grain, > 200 micrometer) 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 micrometer) material.

cond-mat.mtrl-sci↗

arXiv2018

AstroEBSD: exploring new space in pattern indexing with methods launched from an astronomical approach

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 (EBSPs). 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 are obtained, the crystal orientation with respect to a reference sample & detector orientation can be determined and presented. Unfortunately, due to challenges related to crystal symmetry, there are limited available pattern indexing approaches and this has likely hampered open development of the technique. In this manuscript, we present a new method of pattern indexing, based upon a method with which satellites locate themselves in the night sky, and systematically demonstrate its effectiveness using dynamical simulations and real experimental patterns. The benefit of releasing this new algorithm is demonstrated as we utilise this indexing process, 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 tool-kit, we hope this opens up EBSD developments to more users. The software code and example data is released alongside this article for 3rd party developments.

physics.comp-ph↗

arXiv2018

Space rocks and optimising scanning electron channelling contrast

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.

cond-mat.mtrl-sci↗

arXiv2018

Characterisation of slip and twinning in high rate deformed zirconium with electron backscatter diffraction

Zirconium alloys are used in the nuclear industry as structural materials, and can be subject to high strain rate loading conditions during forming and in the case of a reactor accident. In this context, the relationship between strain rate dependent mechanical properties, crystallographic texture and deformation modes, such as slip and deformation twinning, are explored in this work. Commercially pure zirconium is deformed to 10 % engineering strain under quasi-static and high strain rate loading, and post-mortem analysis of the samples is performed using electron backscatter diffraction (EBSD) to observe different twin and slip systems activated. Twin types are identified from local intergranular misorientation maps, and active slip systems are identified from long range intragranular misorientation maps. We link characterisation of the mechanical responses, twin types and morphologies, and relative slip system activation as a function of loading mode. We find that variations in strength and hardening can be related to the relative propensity of twinning and the number of active slip systems.

cond-mat.mtrl-sci↗

arXiv2017

Using transmission Kikuchi diffraction to characterise α variants in an α + β titanium alloy

Two phase titanium alloys are important for high performance engineering components, such as aeroengine discs. The microstructures of these alloys are tailored during thermomechanical processing to precisely control phase factions, morphology and crystallographic orientations. In bimodal two phase (α + β) Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloys there are often three microstructural lengthscales to consider: large (~10 μm) equiaxed primary α; >200 nm thick plate α with a basketweave morphology; and very fine scaled (>50 nm plate thickness) secondary α that grows between the larger α plates surrounded by retained β. In this work, we utilise high spatial resolution transmission Kikuchi diffraction (TKD, also known as transmission based electron backscatter diffraction, t-EBSD) and scanning electron microscopy (SEM) based forward scattering electron imaging to resolve the structures and orientations of basketweave and secondary α in Ti-6242. We analyse the α variants formed within one prior β grain, and test whether existing theories of habit planes of the phase transformation are upheld. Our analysis is important in understanding both the thermomechanical processing strategy of new bimodal two-phase titanium alloys, as well the ultimate performance of these alloys in complex loading regimes such as dwell fatigue. Our paper champions the significant increase in spatial resolution afforded using transmission techniques, combined with the ease of SEM based analysis using conventional electron backscatter diffraction (EBSD) systems and forescatter detector (FSD) imaging, to study the nanostructure of real-world engineering alloys.

cond-mat.mtrl-sci↗