arXiv subjects

T. Ben Britton

Publications and source records attributed to T. Ben Britton.

34 records · Page 2Linked to original sources

arXiv2021

Correlative statistical microstructural assessment of precipitates and their distribution, with simultaneous electron backscatter diffraction and energy dispersive X-ray spectroscopy

Modern engineering alloys have bespoke microstructures, where features such as precipitates are used to control properties. In many Ni-based alloys, carbo-nitride precipitates are introduced to strengthen and improve performance. These precipitates can be distributed throughout the microstructure and niobium rich carbides are often found at grain boundaries. In this work, we used combined energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) to characterise the population of these precipitates. Processing of the EDS signal is used to label the Mo/Nb-rich precipitates, and their size and location are measured from maps using a circular Hough transform. This label map is combined with the grain boundary network (from EBSD analysis). Statistical analysis, using ANOVA testing, reveals differences in chemistry between carbides found in Ni-rich matrix grain interiors, on random high angle boundaries and on special boundaries (Σ3 and Σ9). These results are compared between wrought and power metallurgy product forms. These distributions are discussed in the context of their performance within demanding environments, such as reactor core internals.

cond-mat.mtrl-sci↗

arXiv2020

The role of lengthscale in the creep of Sn-3Ag-0.5Cu solder microstructures

Creep of directionally solidified Sn-3Ag-0.5Cu wt.% (SAC305) samples with near-<110> orientation along the loading direction and different microstructural lengthscale is investigated under constant load tensile testing and at a range of temperatures. The creep performance improves by refining the microstructure, i.e. the decrease in secondary dendrite arm spacing (λ2), eutectic intermetallic spacing (λe) and intermetallic compound (IMC) size, indicating as a longer creep lifetime, lower creep strain rate, change in activation energy (Q) and increase in ductility and homogeneity in macro- and micro-structural deformation of the samples. The dominating creep mechanism is obstacle-controlled dislocation creep at room temperature and transits to lattice-associated vacancy diffusion creep at elevated temperature (T/T_M > 0.7 to 0.75). The deformation mechanisms are investigated using electron backscatter diffraction (EBSD) and strain heterogeneity is identified between b/-Sn in dendrites and b/-Sn in eutectic regions containing Ag3Sn and Cu6Sn5 particles. The size of the recrystallised grains is modulated by the dendritic and eutectic spacings, however, the recrystalised grains in the eutectic regions for coarse-scaled samples (largest λ2 and λe) is only localised next to IMCs without growth in size.

physics.app-ph↗

arXiv2020

Hydride growth mechanism in Zircaloy-4: investigation of the partitioning of alloying elements

The long-term safety of water-based nuclear reactors relies in part on the reliability of zirconium-based nuclear fuel. Yet the progressive ingress of hydrogen during service makes zirconium alloys subject to delayed hydride cracking. Here, we use a combination of electron back-scattered diffraction and atom probe tomography to investigate specific microstructural features from the as-received sample and in the blocky-alpha microstructure, before and after electrochemical charging with hydrogen or deuterium followed by a low temperature heat treatment at 400C for 5 hours followed by furnace cooling at a rate of 0. 5C per min. Specimens for atom probe were prepared at cryogenic temperature to avoid the formation of spurious hydrides. We report on the compositional evolution of grains and grain boundaries over the course of the sample's thermal history, as well as the ways the growth of the hydrides modifies locally the composition and the structure of the alloy. We observe a significant amount of deuterium left in the matrix, even after the slow cooling and growth of the hydrides. Stacking faults form ahead of the growth front and Sn segregates at the hydride-matrix interface and on these faults. We propose that this segregation may facilitate further growth of the hydride. Our systematic investigation enables us discuss how the solute distribution affects the evolution of the alloy's properties during its service lifetime.

cond-mat.mtrl-sci↗

arXiv2020

Effect of high temperature service on the complex through-wall microstructure of centrifugally cast HP40 reformer tube

Centrifugally cast reformer tubes are used in petrochemical plants for hydrogen production. Due to the conditions of hydrogen production, reformer tubes are exposed to high temperature which causes creep damage inside the microstructure. In this study, two different ex-service HP40 alloy reformer tubes which come from the same steam reformer unit have been compared by microstructural characterisation performed at a range of length scales from mm to um. Analyses performed by EBSD (Electron Backscatter Diffraction), EDS (Energy Dispersive X-ray Spectroscopy) and PCA (Principal Component Analysis) show that both tubes have similar microstructural constituents, with the presence of an austenitic matrix and M23C6, G phase and M6C carbides at the grain boundaries. Even if both tubes have a similar microstructure, one tube due to it localisation inside the steam reformer unit presents a region with more micro cracks which may indicate that this tube have accumulated more creep damage than the other one.

physics.app-ph↗

arXiv2020

Slip-hydride interactions in Zircaloy-4: Multiscale mechanical testing and characterisation

The interactions between δ-hydrides and plastic slip in a commercial zirconium alloy, Zircaloy-4, under stress were studied using in situ secondary electron microscope (SEM) micropillar compression tests of single crystal samples and ex situ digital image correlation (DIC) macroscale tensile tests of polycrystalline samples. The hydrides decorate near basal planes in orientation, and for micropillars orientated for basal slip localised shear at the hydride-matrix interface is favoured over slip in α-Zr matrix due to a lower shear stress required. In contrast, for pillars oriented for prismatic slip the shear stress needed to trigger plastic slip within the hydride is slightly higher than the critical resolved shear stress (CRSS) for the prismatic slip system. In this case, slip in the hydride is likely achieved through <110>-type shear which is parallel to the activated -type shear in the parent matrix. At a longer lengthscale, these results are used to inform polycrystalline samples analysed using high spatial resolution DIC. Here localised interface shear remains to be a significant deformation path which can both cause and be caused by matrix slip on planes closely-oriented to the phase boundaries. Matrix slip on planes nearly perpendicular to the adjacent hydride-matrix interfaces can either result in plastic slip within the hydrides or get arrested at the interfaces, generating local stress concentration. Through these mechanisms, the presence of δ-hydrides leads to enhanced strain localisation in Zircaloy-4 early in the plastic regime.

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

Dislocation density distribution at slip band-grain boundary intersections

We study the mechanisms of slip transfer at a grain boundary, in titanium, using Differential Aperture X-ray Laue Micro-diffraction (DAXM). This 3D characterization 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 neighbor 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.

cond-mat.mtrl-sci↗

arXiv2019

In-situ electron backscatter diffraction of thermal cycling in a single grain Cu/Sn-3Ag-0.5Cu/Cu solder joint

The heterogeneous evolution of microstructure in a single 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, localised towards the corners of the joint and decreases towards the centre of the joint. This deformation is induced by the constraint from the CTE between the \b{eta}-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 (1-10)[111]/2 and (11-0)[111]/2.

cond-mat.mtrl-sci↗

arXiv2019

High-angular resolution electron backscatter diffraction as a new tool for mapping lattice distortion in geological minerals

Analysis of distortions of the crystal lattice within individual mineral grains is central to the investigation of microscale processes that control and record tectonic events. These distortions are generally combinations of lattice rotations and elastic strains, but a lack of suitable observational techniques has prevented these components being mapped simultaneously and routinely in earth science laboratories. However, the technique of high-angular resolution electron backscatter diffraction (HR-EBSD) provides the opportunity to simultaneously map lattice rotations and elastic strains with exceptional precision, on the order of 0.01 degree for rotations and 10-4 in strain, using a scanning electron microscope. Importantly, these rotations and lattice strains relate to densities of geometrically necessary dislocations and residual stresses. Recent works have begun to apply and adapt HR-EBSD to geological minerals, highlighting the potential of the technique to provide new insights into the microphysics of rock deformation. Therefore, the purpose of this overview is to provide a summary of the technique, to identify caveats and targets for further development, and to suggest areas where it offers potential for major advances. In particular, HREBSD is well suited to characterising the roles of different dislocation types during crystal plastic deformation and to mapping heterogeneous internal stress fields associated with specific deformation mechanisms/microstructures or changes in temperature, confining pressure, or applied deviatoric stress. These capabilities make HR-EBSD a particularly powerful new technique for analysing the microstructures of deformed geological materials.

cond-mat.mtrl-sci↗

arXiv2019

The effect of δ-hydride on the micromechanical deformation of Zircaloy-4 studied by in situ high angular resolution electron backscatter diffraction

Zircaloy-4 is used extensively as nuclear fuel cladding materials and hydride embrittlement is a major failure mechanism. To explore the effect of δ-hydride on plastic deformation and performance of Zircaloy-4, in situ high angular resolution electron backscatter diffraction (HR-EBSD) was used to quantify stress and geometrically necessary dislocation (GND) density during bending tests of hydride-free and hydride-containing single crystal Zircaloy-4 microcantilevers. Results suggest that while the stress applied was accommodated by plastic slip in the hydride-free cantilever, the hydride-containing cantilever showed precipitation induced GND pile-up at hydride-matrix interface pre-deformation, and considerable locally increasing GND density under tensile stress upon plastic deformation.

cond-mat.mtrl-sci↗

arXiv2019

New techniques for imaging and identifying defects in electron microscopy

Defects in crystalline materials control the properties of 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, where several emerging approaches offer attractive benefits, for instance in correlative microscopy across length scales and in situ studies of defect dynamics.

cond-mat.mtrl-sci↗

arXiv2019

Quantification challenges for atom probe tomography of hydrogen and deuterium in Zircaloy-4

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 challenges associated to accurate quantification of hydrogen and deuterium, in particular associated to the overlap of peaks at 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 a zirconium alloy systems that is important for nuclear power.

cond-mat.mtrl-sci↗

arXiv2019

The role of β-titanium ligaments in the deformation of dual phase titanium alloys

Multiphase titanium alloys are critical materials in high value engineering components, for instance in aero engines. Microstructural complexity is exploited through interface engineering during mechanical processing to realise significant improvements in fatigue and fracture resistance and strength. In this work, we explore the role of select interfaces using in-situ micromechanical testing with concurrent observations from high angular resolution electron backscatter diffraction (HR-EBSD). Our results are supported with post mortem transmission electron microscopy (TEM). Using micro-pillar compression, we performed in-depth analysis of the role of select β-titanium (body centred cubic) ligaments which separate neighbouring α-titanium (hexagonal close packed) regions and inhibit the dislocation motion and impact strength during mechanical deformation. These results shed light on the strengthening mechanisms and those that can lead to strain localisation during fatigue and failure.

cond-mat.mtrl-sci↗

arXiv2018

Microstructure and formation mechanisms of δ-hydrides in variable grain size Zircaloy-4 studied by electron backscatter diffraction

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. Results suggest that the the matrix-hydride orientation relationship is {0001}α||{111}δ;<11-20>α||<110>δ for all the cases studied. The habit plane of intragranular hydrides and some intergranular hydrides has been found to be {10-17} 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 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 <1-100> 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.

cond-mat.mtrl-sci↗

arXiv2018

Constraints on the Effective Electron Energy Spectrum in Backscatter Kikuchi Diffraction

Electron Backscatter Diffraction (EBSD) is a technique to obtain microcrystallographic information from materials by collecting large-angle Kikuchi patterns in the scanning electron microscope (SEM). An important fundamental question concerns the scattering-angle dependent electron energy distribution which is relevant for the formation of the Kikuchi diffraction patterns. Here we review the existing experimental data and explore the effective energy spectrum that is operative in the generation of backscatter Kikuchi patterns from silicon. We use a full pattern comparison of experimental data with dynamical electron diffraction simulations. Our energy-dependent cross-correlation based pattern matching approach establishes improved constraints on the effective Kikuchi pattern energy spectrum which is relevant for high-resolution EBSD pattern simulations and their applications.

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↗