arXiv subjects

David Wallis

Publications and source records attributed to David Wallis.

14 recordsLinked to original sources

arXiv2026

Characterizing Dislocation Substructures in Creep-Deformed Olivine Using Electron Channeling Contrast Imaging

Olivine is the dominant mineral in Earth's upper mantle and therefore controls mantle rheology and the mechanics of plate tectonics. The constitutive laws for dislocation-mediated deformation of olivine depend on the nature, density, and arrangements of dislocations within crystals. Hence, imaging and characterizing these defects is important, albeit challenging. Traditional imaging approaches involve (1) transmission electron microscopy (TEM), which samples small areas and requires extensive preparation and (2) oxidation decoration methods that have low spatial resolution and cannot distinguish dislocations of opposite Burgers vectors. Here, we apply electron channeling contrast imaging (ECCI) to unlock insight into the deformation structures within olivine, and combined with electron backscatter diffraction (EBSD) and weighted Burgers vector (WBV) mapping as an informative route to characterize dislocation substructures in bulk materials. Specifically, we have used an ECCI workflow based on selected-area electron channeling patterns (SA-ECPs) and we apply this workflow to a single crystal of San Carlos olivine that was deformed by creep at high temperature. ECCI micrographs reveal subgrain boundaries, surface threading dislocations, and dislocation loops across representative areas. The observations demonstrate that this workflow can reliably reveal the complexity of subgrain boundaries in olivine, which can host multiple dislocation types and exhibit non-planar geometries. Despite the limited number of slip systems in olivine, subgrain boundaries can form complex, mixed assemblies. Overall, such observations can provide a variety of constraints on dislocation types, morphologies, and distributions, which are required to parameterize and calibrate models of transient and steady-state dislocation creep in olivine and other materials.

cond-mat.mtrl-sci↗

arXiv2025

The Effect of Pattern Quality on Measurements of Stress Heterogeneity and Geometrically Necessary Dislocation Density by High-Angular Resolution Electron Backscatter Diffraction

We examine the effect of pattern quality on the output of high-angular resolution electron backscatter diffraction (HR-EBSD) analyses. Band contrast, as a proxy for pattern quality, was varied by adjusting the number of frames averaged per electron backscatter pattern during data collection. The same region in a deformed sample of the mineral olivine was mapped six times varying the number of frames averaged between 1 and 30 between each map. Each data set was analyzed with HR-EBSD, producing maps of intragranular stress heterogeneity and geometrically necessary dislocation (GND) density. As the number of frames averaged increased, the noise in stress and GND calculations decreased, revealing more substructure in the mapped region. The worst pixels, with low band contrast, are the most improved by increased frame averaging, whereas those with high band contrast are largely unaffected. Additionally, the probability distribution of stresses narrows as high-stress noise is reduced with increased pattern quality, which also affects estimates of dislocation density from statistical analysis of the stress distributions. As regions with high stress and/or high GND density are typically of interest in HR-EBSD maps and are often associated with low band contrast, frame averaging may be used as a tool to improve the quality of these analyses. Most importantly, however, is that comparisons are made between HR-EBSD datasets with similar mean band contrast to ensure that observed differences are microstructural in origin and not an artefact of data collection.

cond-mat.mtrl-sci↗

arXiv2024

Microstructural evolution of Carrara marble during semi-brittle deformation

Fifteen marble samples were subjected to semi-brittle deformation through triaxial compression experiments, reaching axial strains of 0.5%, 1.0%, 2.0%, 4.0%, or 7.5% at temperatures of 20C, 200C, or 350C, under a confining pressure of 400 MPa. Deformation twins, lattice curvature, and intragranular microfractures in the samples were quantitatively characterised using forescattered electron images and electron backscatter diffraction. Microstructural analyses revealed that twins accommodate most of the shortening during the first 2% strain, whereas lattice curvature associated with geometrically necessary dislocations predominantly develops in the later stages. Intragranular fracture intensity exhibits an almost linear correlation with strain during the first 2% strain but increases more slowly thereafter. The mechanical data indicate a strong temperature dependence of yield stress, consistent with the temperature dependence of the critical resolved shear stress for dislocation glide. The subsequent strain hardening is likely caused by progressively increasing intensity of interactions among dislocations and between dislocations and twin boundaries. Based on the microstructural data and interpreted hardening mechanisms, we propose a phenomenological model, with microstructural state variables, for semi-brittle deformation at our experimental conditions as a step towards development of a microphysical constitutive model of semi-brittle deformation.

physics.geo-ph↗

arXiv2024

Microstructural and Micromechanical Evolution of Olivine Aggregates During Transient Creep

To examine the microstructural evolution that occurs during transient creep, we deformed olivine aggregates to different strains that spanned the initial transient deformation. Two sets of samples with different initial grain sizes of 5 $\mu$m and 20 $\mu$m were deformed in torsion at T = 1523 K, P = 300 MPa, and a constant shear strain rate of 1.5 $\times$ 10$^{-4}$ s$^{-1}$. Both sets of samples experienced strain hardening during deformation. We characterized the microstructures at the end of each experiment using high-angular resolution electron backscatter diffraction (HR-EBSD) and dislocation decoration. In the coarse-grained samples, dislocation density increased from 1.5 $\times$ 10$^{11}$ m$^{-2}$ to 3.6 $\times$ 10$^{12}$ m$^{-2}$ with strain. Although the same final dislocation density was reached in the fine-grained samples, it did not vary significantly at small strains, potentially due to concurrent grain growth during deformation. In both sets of samples, HR-EBSD analysis revealed that intragranular stress heterogeneity increased in magnitude with strain and that elevated stresses are associated with regions of high geometrically necessary dislocation density. Further analysis of the stresses and their probability distributions indicate that the stresses are imparted by long-range elastic interactions among dislocations. These characteristics indicate that dislocation interactions were the primary cause of strain hardening during transient creep. A comparison of the results to predictions from three recent models reveals that the models do not correctly predict the evolution in stress and dislocation density with strain for our experiments due to a lack of previous such data in their calibrations.

cond-mat.mtrl-sci↗

arXiv2024

Transient and steady-state dislocation creep of olivine controlled by dislocation interactions at the isostress endmember

The rheological behaviour of olivine deforming by dislocation creep controls geodynamic processes that involve steady-state flow or transient viscosity evolution. Longstanding rheological models applied to both contexts assume that dislocation creep of olivine aggregates occurs close to the isostrain endmember with each grain deforming to the same strain but supporting different stress. Here, we test this assumption by constructing isostrain and isostress models based on flow laws for single crystals and comparing them to rheological data for aggregates. This analysis reveals that strain rates measured on olivine aggregates agree with those predicted by the isostress model but are an order of magnitude faster than those predicted by the isostrain model. When extrapolated to conditions typical of the shallow upper mantle, the isostress model predicts steady-state viscosities that are one to three orders of magnitude less than those predicted by the isostrain model. Furthermore, deformation close to the isostress endmember implies that transient creep occurs predominantly by dislocation interactions, suggesting viscosity changes that are approximately one order of magnitude greater than those predicted previously based on grain interactions associated with the isostrain model.

cond-mat.mtrl-sci↗

arXiv2024

Mild-to-wild plasticity of Earth's upper mantle

The flow of Earth's upper mantle has long been considered to occur by slow and near-continuous creep. Such behaviour is observed in classical high-temperature deformation experiments and is a fundamental component of geodynamic models. However, the latest generation of high-resolution experiments, capable of sensing temporal heterogeneities in rates of dislocation motion, have revealed that materials ranging from metals to ice exhibit a spectrum of behaviours termed mild-to-wild plasticity. It remains unknown whether olivine, the most abundant mineral in Earth's upper mantle, always exhibits mild continuous flow or can exhibit intermittent wild fluctuations in plastic strain rate. Here, we demonstrate that olivine exhibits measurable wildness, even under conditions at which its behaviour is predicted to be relatively mild. During nanoindentation experiments conducted at room temperature, continuous plastic flow is punctuated by intermittent bursts of displacement with log-normally distrubuted magnitudes, indicating avalanches of correlated dislocation motion that account for approximately 8 +/- 6% of the plastic strain. Remarkably, the framework of mild-to-wild plasticity predicts that wildness should increase with depth in the Earth, with flow of the asthenospheric upper mantle occurring almost entirely by wild fluctuations of deformation at the grain scale. The recognition of intermittent plasticity in geological materials provides new constraints for microphysical models of dislocation-mediated flow and raises questions about the mehcanisms of transient instabilities in otherwise ductile regimes, such as deep earthquakes and slow-slip events.

physics.geo-ph↗

arXiv2023

Grain-size effects during semi-brittle flow of calcite rocks

We study the role of grain size in the rheological behaviour of calcite aggregates in the semi-brittle regime. We conduct triaxial deformation tests on three rocks, Solnhofen limestone, Carrara marble and Wombeyan marble, with average grain sizes of 5-10 $\mu$m, 200 $\mu$m and 2 mm, respectively, at pressures in the range 200-800 MPa and temperatures in the range 20-400 $^\circ$C. At all conditions, both strength and hardening rate increase with decreasing grain size. Flow stress scales with the inverse of grain size to a power between 1/3 and 2/3. Hardening rate decreases linearly with the logarithm of grain size. In-situ ultrasonic monitoring reveals that P-wave speed tends to decrease with increasing strain, and that this decrease is more marked at room temperature than at 200 and 400 $^\circ$C. The decrease in wave speed is consistent with microcracking, which is more prevalent at low temperature and low pressure. Microstructural observations reveal high twin densities in all deformed samples. Twin density increases with stress, consistent with previous datasets. Spatial distributions of intragranular misorientation indicate that twins are sometimes obstacles to dislocation motion, but this effect is not ubiquitous. Computed slip-transfer statistics indicate that that twins are typically weaker barriers to dislocation glide than grain boundaries, so that their effect on dislocation accumulation and hardening rates is likely smaller than the effect of grain size. Indeed, our data reveal that grain size exerts a first-order control on flow stress and hardening in calcite, whereas twinning may only have a secondary impact on these behaviours.

physics.geo-ph↗

arXiv2020

Biominerals with Texture Gradients are Functionally Graded Bioceramics Toughened by Stress Delocalization

Biomineralizing organisms are widely noted and extensively studied due to their ability to generate structures exhibiting exceptional crystallographic control. Primarily, it is the organisms, such as sea-urchins or bivalves, that generate nearly single-crystalline biocrystals that have attracted attention. In contrast, biomineralizing organisms with seemingly disordered polycrystalline bio-armor have been left relatively unstudied. However, the crystalline ordering in the black-lipped pearl oyster, Pinctada margaritifera, reveals that biominerals with varying crystal textures are an unrecognized class of functionally graded materials. Changing crystal textures inevitably cause a variation in Young modulus due to the orientation-dependent mechanical properties of crystals. The case of Pinctada margaritifera demonstrates that bioceramics with such crystallographical gradients are toughened by stress delocalization and reduced stress intensity factors, outperforming non-graded counterparts. These findings suggest that a multitude of biominerals, which are perceived as poorly ordered because of their polycrystallinity and changing crystal textures, may be considered as graded materials with hitherto unidentified emergent mechanical properties. The underlying design principle is remarkably simple and applicable to a wide range of crystalline material classes and may thus serve as a blueprint for future bioinspired functional materials.

cond-mat.mtrl-sci↗

arXiv2020

Dislocation interactions in olivine control postseismic creep of the upper mantle

Changes in stress applied to mantle rocks, such as those imposed by earthquakes, induce a period of evolution in viscosity and microstructure. This transient creep is often modelled based on stress transfer among slip systems due to grain interactions. However, recent experiments have demonstrated that the intragranular accumulation of stresses among dislocations is the dominant cause of strain hardening in olivine at low temperatures, raising the question of whether the same process contributes to transient creep at higher temperatures. Here, we demonstrate that olivine samples deformed at 25{\deg}C or 1150 to 1250{\deg}C both contain stress heterogeneities of ~1 GPa that are imparted by dislocations and have correlation lengths of ~1 micrometre. The similar stress distributions formed in both temperature regimes indicate that accumulation of stresses among dislocations also provides a contribution to transient creep at high temperatures. The results motivate a new generation of models that capture these intragranular processes and may refine predictions of evolving mantle viscosity over the earthquake cycle.

cond-mat.mtrl-sci↗

arXiv2020

Tracking European bat species with passive acoustic directional monitoring

We have developed a method of animal localisation that detects the angle from a sensor towards the direction of an animal call. The method is as simple to use as deploying a conventional static sound recorder, but provides tracking information as well as sound recordings. The principal of operation is to detect the phase difference between microphones positioned closely together. The phase is detected by converting the signals to their analytic form with a Hilbert transform. The angle is then calculated from the phase difference, frequency and microphone separation. Angular measurements provide flight paths above the sensor, and can give details of activity and behaviour that are not possible with a single channel static recorder. We recorded flight paths for 5 bat species on a single night at a site in Denmark (Pipistrellus nathusii, Pipistrellus pygmaeus, Eptesicus serotinus, Myotis daubentonii and Nyctalus noctula). The median error in angular measurement for the species was between 3 and 7 degrees. Calls at high angles from normal, corresponding with a poor signal-to-noise ratio, had larger errors compared to calls recorded in the centre of the field of view. Locations in space could be estimated by combining angular measurements from two or more sensors.

q-bio.PE↗

arXiv2019

Insight into the microphysics of antigorite deformation from spherical nanoindentation

The mechanical behavior of antigorite strongly influences the strength and deformation of the subduction interface. Although there is microstructural evidence elucidating the nature of brittle deformation at low pressures, there is often conflicting evidence regarding the potential for plastic deformation in the ductile regime at higher pressures. Here, we present a series of spherical nanoindentation experiments on aggregates of natural antigorite. These experiments effectively investigate the single-crystal mechanical behavior because the volume of deformed material is significantly smaller than the grain size. Individual indents reveal elastic loading followed by yield and strain hardening. The magnitude of the yield stress is a function of crystal orientation, with lower values associated with indents parallel to the basal plane. Unloading paths reveal more strain recovery than expected for purely elastic unloading. The magnitude of inelastic strain recovery is highest for indents parallel to the basal plane. We also imposed indents with cyclical loading paths, and observed strain energy dissipation during unloading-loading cycles conducted up to a fixed maximum indentation load and depth. The magnitude of this dissipated strain energy was highest for indents parallel to the basal plane. Subsequent scanning electron microscopy revealed surface impressions accommodated by shear cracks and a general lack of lattice misorientation around indents, indicating the absence of dislocations. Based on these observations, we suggest that antigorite deformation at high pressures is dominated by sliding on shear cracks. We develop a microphysical model that is able to quantitatively explain the Young's modulus and dissipated strain energy data during cyclic loading experiments, based on either frictional or cohesive sliding of an array of cracks contained in the basal plane.

physics.geo-ph↗

arXiv2019

Dislocation interactions during low-temperature plasticity of olivine strengthen the lithospheric mantle

The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, models of lithospheric flexure reveal differences in lithospheric strength that are difficult to reconcile based on such flow laws. Recent rheological data from low-temperature deformation experiments on olivine suggest that this discrepancy may be largely explained by strain hardening. Details of the mechanical data, specifically the effects of temperature-independent back stresses stored in the samples, indicate that strain hardening in olivine occurs primarily due to long-range elastic interactions between dislocations. These interpretations provided the basis for a new flow law that incorporates hardening by development of back stress. Here, we test this hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of < 4.3 GPa or in a nanoindenter at applied contact stresses of > 10.2 GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 10$^{14}$ m$^{-2}$ and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned along slip bands and interacting with dislocations of other types that act as obstacles. The stress heterogeneities and accumulations of dislocations along their slip planes are consistent with strain hardening resulting from long-range back-stresses acting between dislocations. These results corroborate the mechanical data in supporting the form of the new flow law for low-temperature plasticity and provide new microstructural criteria for identifying the operation of this deformation mechanism in natural samples.

physics.geo-ph↗

arXiv2019

Thick adherent diamond films on AlN with low thermal barrier resistance

Growth of $>$100 $μ$m thick diamond layer adherent on aluminium nitride is presented in this work. While thick films failed to adhere on untreated AlN films, hydrogen/nitrogen plasma treated AlN films retained the thick diamond layers. Clear differences in zeta potential measurement confirms the surface modification due to hydrogen/nitrogen plasma treatment. Areal Raman maps showed an increase in non-diamond carbon in the initial layers of diamond grown on pre-treated AlN. The presence of non-diamond carbon has minimal effect on the interface between diamond and AlN. The surfaces studied with x-ray photoelectron spectroscopy (XPS) revealed a clear distinction between pre-treated and untreated samples. The surface aluminium goes from nitrogen rich environment to an oxygen rich environment after pre-treatment. Cross section transmission electron microscopy shows a clean interface between diamond and AlN. Thermal barrier resistance between diamond and AlN was found to be in the range of 16 m$^2$K/GW which is a large improvement on the current state-of-the-art.

physics.app-ph↗

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↗