Imperial College London

ProfessorPeterLee

Faculty of EngineeringDepartment of Materials

Visiting Professor
 
 
 
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Contact

 

+44 (0)20 7594 6801p.d.lee

 
 
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Location

 

102Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

447 results found

Cai B, Kao A, Boller E, Magdysyuk OV, Atwood RC, Vo NT, Pericleous K, Lee PDet al., 2020, Revealing the mechanisms by which magneto-hydrodynamics disrupts solidification microstructures, Acta Materialia, Vol: 196, Pages: 200-209, ISSN: 1359-6454

© 2020 A key technique for controlling solidification microstructures is magneto-hydrodynamics (MHD), resulting from imposing a magnetic field to solidifying metals and alloys. Applications range from bulk stirring to flow control and turbulence damping via the induced Lorentz force. Over the past two decades the Lorentz force caused by the interaction of thermoelectric currents and a magnetic field, a MHD phenomenon known as Thermoelectric Magnetohydrodynamics (TEMHD), was also shown to drive inter-dendritic flow altering microstructural evolution. In this contribution, high-speed synchrotron X-ray tomography and high-performance computational simulation are coupled to reveal the evolution, dynamics and mechanisms of solidification within a magnetic field, resolving the complex interplay and competing flow effects arising from Lorentz forces of different origins. The study enabled us to reveal the mechanisms disrupting the traditional columnar dendritic solidification microstructure, ranging from an Archimedes screw-like structure, to one with a highly refined dendritic primary array. We also demonstrate that alloy composition can be tailored to increase or decrease the influence of MHD depending on the Seebeck coefficient and relative density of the primary phase and interdendritic liquid. This work paves the way towards novel computational and experimental methods of exploiting and optimising the application of MHD in solidification processes, together with the calculated design of novel alloys that utilise these forces.

Journal article

Chen Y, Clark SJ, Leung CLA, Sinclair L, Marussi S, Olbinado MP, Boller E, Rack A, Todd I, Lee PDet al., 2020, In-situ Synchrotron imaging of keyhole mode multi-layer laser powder bed fusion additive manufacturing, Applied Materials Today, Vol: 20

© 2020 Elsevier Ltd The keyhole mode in laser powder bed fusion (LPBF) additive manufacturing can be associated with excessive porosity and spatter, however, the underlying physics in multilayer build conditions remain unclear. Here, we used ultra-fast synchrotron X-ray imaging to reveal this phenomena. We in investigated melt pool dynamics, keyhole porosity and spatter formation mechanisms and their impact in all layers of the build. We observed that the transient melt pool dynamics associated with the keyhole include: (I) keyhole initiation, (II) keyhole development, and (III) melt pool recovery. Porosity and spatter were associated with stages (II) and (III). We also discovered that droplet spatter can form due to the collapse of the keyhole recoil zone, causing molten particle agglomeration and ejection during stage (III). Our results clarify the transient dynamics behind the keyhole mode in a multi-layer LBPF process and can be used to guide the reduction in porosity and spatter in additive manufacturing.

Journal article

Prasad A, Yuan L, Lee P, Patel M, Qiu D, Easton M, StJohn Det al., 2020, Towards understanding grain nucleation under Additive Manufacturing solidification conditions, ACTA MATERIALIA, Vol: 195, Pages: 392-403, ISSN: 1359-6454

Journal article

Wang W, Guo E, Phillion AB, Eskin DG, Wang T, Lee PDet al., 2020, Semi-solid compression of nano/micro-particle reinforced Al-Cu composites: An in situ synchrotron tomographic study, Materialia, Vol: 12

© 2020 Four-dimensional fast synchrotron X-ray tomography has been used to investigate the semi-solid deformation of nano- and micro-particle reinforced aluminum-copper composites (Al-10 wt% Cu alloy with ~1.0 wt% Al2O3 nano and ~1.0 wt% Al2O3 micro particles). Quantitative image analysis of the semi-solid deformation behavior of three alloys (base, nano- and micro-particle reinforced) revealed the influence of the particulate size on both microstructural formation and dominant deformation mechanisms. The results showed that initial void closure and incubation period were present in the particle-free and nano-particle reinforced Al-Cu composite during semi-solid compression, while the micro-particle reinforced alloy only showed continual void growth and coalescence into cracks. The results suggest that the nano-particle reinforced composite has the best hot-tearing resistance amongst the three alloys. Improved hot-tear performance with nano-particulate reinforcement was attributed to the small liquid channel thickness, fine grain size which alters the distribution/morphology of the liquid channels, more viscous inter-dendritic liquid, and fewer initial voids.

Journal article

Yuan L, Sabau AS, Stjohn D, Prasad A, Lee PDet al., 2020, Columnar-to-equiaxed transition in a laser scan for metal additive manufacturing, ISSN: 1757-8981

© Published under licence by IOP Publishing Ltd. In laser powder bed fusion additive manufacturing (LPBFAM), different solidification conditions, e.g., thermal gradient and cooling rate, can be achieved by controlling the process parameters, such as laser power and laser speed. Tailoring the behaviour of the columnar to equiaxed transition (CET) of the printed alloy during fabrication can facilitate the production of highly customized microstructures. In this study, effective analytical solutions for both thermal conduction and solidification are employed to model solidifying melt pools. Microstructure textures and solidification conditions are evaluated for numerous combinations of laser power and laser speed under bead-on-plate conditions. This analytical-based high-throughput tool was demonstrated to select specific process parameters that lead to desired microstructures. Two selected process conditions were examined in detail by a highly parallelized microstructural solidification model to reveal both nucleation and grain growth. Both numerical solutions agree well with experiments that are performed based on bead-on-plate conditions, indicating that these numerical models aid evaluation of the nucleation parameters, providing insights for controlling CET during the LPBFAM processing.

Conference paper

Clark SJ, Leung CLA, Chen Y, Sinclair L, Marussi S, Lee PDet al., 2020, Capturing Marangoni flow via synchrotron imaging of selective laser melting, ISSN: 1757-8981

© Published under licence by IOP Publishing Ltd. Marangoni flow has a substantial influence on the quality of components fabricated via laser powder bed fusion (LPBF). However, Marangoni flow in melt pools is rarely quantified due to the opacity of liquid metals and the necessity for in situ evaluation. Here we report the findings of high-temporal-resolution synchrotron x-ray radiography experiments tracking the flow in the melt-pool. Dense, highly attenuating tungsten carbide particles are seeded within an elemental powder blend of aluminium and copper of varying composition. Due to the extremely high temporal resolution of greater than 50 kfps at the 31-ID-B beamline at the Advanced Photon Source, USA, we can track the position of tracer particles from frame to frame. This data provides valuable process guidance for optimising mixing and informs the development and validation of multiphysics models.

Conference paper

Chen Y, Clark S, Leung ACL, Sinclair L, Marussi S, Atwood R, Connoley T, Jones M, Baxter G, Lee PDet al., 2020, Melt pool morphology in directed energy deposition additive manufacturing process, ISSN: 1757-8981

© Published under licence by IOP Publishing Ltd. Directed Energy Deposition Additive Manufacturing (DED-AM) is one of the principal AM techniques being explored for both the repair of high value components in the aerospace industry as well as freeform fabrication of large metallic components. However, the lack of fundamental understanding of the underlying process-structure-property relationships hinders the utilisation of DED-AM for the production or repair of safety-critical components. This study uses in situ and operando synchrotron X-ray imaging to provide an improved fundamental understanding of laser-matter interactions and their influence on the melt pool geometry. Coupled with process modelling, these unique observations illustrate how process parameters can influence the DED-AM melt pool geometry. The calibrated simulation can be used for guidance in an industrial additive manufacturing process for microstructure and quality control.

Conference paper

Golkhosh F, Sharma Y, Martinez DM, Lee PD, Tsai W, Courtois L, Eastwood DS, Phillion ABet al., 2020, 4D synchrotron tomographic imaging of network and fibre level micromechanics in softwood paper, MATERIALIA, Vol: 11, ISSN: 2589-1529

Journal article

Østergaard MB, Zhang M, Shen X, Petersen RR, König J, Lee PD, Yue Y, Cai Bet al., 2020, High-speed synchrotron X-ray imaging of glass foaming and thermal conductivity simulation, Acta Materialia, Vol: 189, Pages: 85-92, ISSN: 1359-6454

Glass foams are attractive thermal insulation materials, thus, the thermal conductivity (λ) is crucial for their insulating performance. Understanding the foaming process is critical for process optimization. Here, we applied high-speed synchrotron X-ray tomography to investigate the change in pore structure during the foaming process, quantifying the foam structures and porosity dynamically. The results can provide guidance for the manufacturing of glass foams. The 3D pore structures were also used to computationally determine λ of glass foams using image-based modelling. We then used the simulated λ to develop a new analytical model to predict the porosity dependence of λ. The λ values of the glass foams when the porosity is within 40% to 95% predicted by the new model are in excellent agreement with the experimental data collected from the literature, with an average error of only 0.7%, which performs better than previously proposed models.

Journal article

Nelson T, Cai B, Warnken N, Lee PD, Boller E, Magdysyuk OV, Green NRet al., 2020, Gravity effect on thermal-solutal convection during solidification revealed by four-dimensional synchrotron imaging with compositional mapping, Scripta Materialia, Vol: 180, Pages: 29-33, ISSN: 1359-6462

The effect of gravity on thermo-solutal convection and its impact on solidification dynamics of an Al-15 wt%Cu alloy were studied using high speed synchrotron tomography. A method for mapping the composition of the solidifying samples was developed, enabling three-dimensional quantification of the time evolved solute concentration and dendrite morphology. Differences in solute segregation, dendrite morphology and fragmentation between upwards and downwards solidification were identified, which were attributed to buoyancy-modulated thermal-solutal convection.

Journal article

Evans L, Lee P, Javaheri B, Felder A, Hopkinson M, Pitsillides Aet al., 2020, Identifying novel osteoarthritis (OA) biomarkers via anatomical discrepancies between OA-predisposed and OA-protected murine knee joints., Publisher: WILEY, Pages: 143-144, ISSN: 0021-8782

Conference paper

Madi K, Staines KA, Bay BK, Javaheri B, Geng H, Bodey AJ, Cartmell S, Pitsillides AA, Lee PDet al., 2020, In situ characterization of nanoscale strains in loaded whole joints via synchrotron X-ray tomography, Nature Biomedical Engineering, Vol: 4, Pages: 343-354, ISSN: 2157-846X

Imaging techniques for quantifying changes in the hierarchical structure of deforming joints are constrained by destructive sample treatments, sample-size restrictions and lengthy scan times. Here, we report the use of fast low-dose pink-beam synchrotron X-ray tomography in combination with mechanical loading at nanometric precision for in situ imaging, at resolutions below 100 nm, of the mechanical strain in intact untreated joints under physiologically realistic conditions. We show that in young, older and osteoarthritic mice, hierarchical changes in tissue structure and mechanical behaviour can be simultaneously visualized, and that the tissue structure at the cellular level correlates with the mechanical performance of the whole joint. We also use the tomographic approach to study the colocalization of tissue strains to specific chondrocyte lacunar organizations within intact loaded joints and to explore the role of calcified-cartilage stiffness on the biomechanics of healthy and pathological joints.

Journal article

Javaheri B, Razi H, Gohin S, Wylie S, Chang Y-M, Salmon P, Lee PD, Pitsillides AAet al., 2020, Lasting organ-level bone mechanoadaptation is unrelated to local strain, Science Advances, Vol: 6, Pages: 1-13, ISSN: 2375-2548

Bones adapt to mechanical forces according to strict principles predicting straight shape. Most bones are, however, paradoxically curved. To solve this paradox, we used computed tomography–based, four-dimensional imaging methods and computational analysis to monitor acute and chronic whole-bone shape adaptation and remodeling in vivo. We first confirmed that some acute load-induced structural changes are reversible, adhere to the linear strain magnitude regulation of remodeling activities, and are restricted to bone regions in which marked antiresorptive actions are evident. We make the novel observation that loading exerts significant lasting modifications in tibial shape and mass across extensive bone regions, underpinned by (re)modeling independent of local strain magnitude, occurring at sites where the initial response to load is principally osteogenic. This is the first report to demonstrate that bone loading stimulates nonlinear remodeling responses to strain that culminate in greater curvature adjusted for load predictability without sacrificing strength.

Journal article

Bhartiya A, Madi K, Disney CM, Courtois L, Jupe A, Zhang F, Bodey AJ, Lee P, Rau C, Robinson IK, Yusuf Met al., 2020, Phase-contrast 3D tomography of HeLa cells grown in PLLA polymer electrospun scaffolds using synchrotron X-rays, Journal of Synchrotron Radiation, Vol: 27, Pages: 158-163, ISSN: 0909-0495

Advanced imaging is useful for understanding the three-dimensional (3D) growth of cells. X-ray tomography serves as a powerful noninvasive, nondestructive technique that can fulfill these purposes by providing information about cell growth within 3D platforms. There are a limited number of studies taking advantage of synchrotron X-rays, which provides a large field of view and suitable resolution to image cells within specific biomaterials. In this study, X-ray synchrotron radiation microtomography at Diamond Light Source and advanced image processing were used to investigate cellular infiltration of HeLa cells within poly L-lactide (PLLA) scaffolds. This study demonstrates that synchrotron X-rays using phase contrast is a useful method to understand the 3D growth of cells in PLLA electrospun scaffolds. Two different fiber diameter (2 and 4 µm) scaffolds with different pore sizes, grown over 2, 5 and 8 days in vitro, were examined for infiltration and cell connectivity. After performing visualization by segmentation of the cells from the fibers, the results clearly show deeper cell growth and higher cellular interconnectivity in the 4 µm fiber diameter scaffold. This indicates the potential for using such 3D technology to study cell-scaffold interactions for future medical use.

Journal article

Wang YQ, Clark SJ, Cai B, Venero DA, Yun K, Gorley M, Surrey E, McCartney DG, Sridhar S, Lee PDet al., 2020, Small-angle neutron scattering reveals the effect of Mo on interphase nano-precipitation in Ti-Mo micro-alloyed steels, Scripta Materialia, Vol: 174, Pages: 24-28, ISSN: 1359-6462

Ti-containing micro-alloyed steels are often alloyed with molybdenum (Mo) to reduce nano-precipitate coarsening, although the mechanism is still disputed. Using small angle neutron scattering we characterised the precipitate composition and coarsening of Ti-alloyed and Ti-Mo-alloyed steels. The results demonstrate ~25 at.% of Ti is substituted by Mo in the (Ti, Mo)C precipitates, increasing both the precipitate volume percent and average size. Mo alloying did not retard precipitation coarsening, but improved lattice misfit between precipitate and matrix, contributing to better ageing resistance of the Ti-Mo-alloyed steel. This new understanding opens opportunities for designing ageing-resistant micro-alloyed steels with lean alloying elements.

Journal article

Arzilli F, La Spina G, Burton MR, Polacci M, Le Gall N, Hartley ME, Di Genova D, Cai B, Vo NT, Bamber EC, Nonni S, Atwood R, Llewellin EW, Brooker RA, Mader HM, Lee PDet al., 2019, Magma fragmentation in highly explosive basaltic eruptions induced by rapid crystallization, Nature Geoscience, Vol: 12, Pages: 1023-1028, ISSN: 1752-0894

Basaltic eruptions are the most common form of volcanism on Earth and planetary bodies. The low viscosity of basaltic magmas inhibits fragmentation, which favours effusive and lava-fountaining activity, yet highly explosive, hazardous basaltic eruptions occur. The processes that promote fragmentation of basaltic magma remain unclear and are subject to debate. Here we used a numerical conduit model to show that a rapid magma ascent during explosive eruptions produces a large undercooling. In situ experiments revealed that undercooling drives exceptionally rapid (in minutes) crystallization, which induces a step change in viscosity that triggers magma fragmentation. The experimentally produced textures are consistent with basaltic Plinian eruption products. We applied a numerical model to investigate basaltic magma fragmentation over a wide parameter space and found that all basaltic volcanoes have the potential to produce highly explosive eruptions. The critical requirements are initial magma temperatures lower than 1,100 °C to reach a syn-eruptive crystal content of over 30 vol%, and thus a magma viscosity around 105 Pa s, which our results suggest is the minimum viscosity required for the fragmentation of fast ascending basaltic magmas. These temperature, crystal content and viscosity requirements reveal how typically effusive basaltic volcanoes can produce unexpected highly explosive and hazardous eruptions.

Journal article

Bhagavath S, Cai B, Atwood R, Li M, Ghaffari B, Lee PD, Karagadde Set al., 2019, Combined deformation and solidification-driven porosity formation in aluminum alloys, Metallurgical and Materials Transactions A, Vol: 50, Pages: 4891-4899, ISSN: 1073-5623

In die-casting processes, the high cooling rates and pressures affect the alloy solidification and deformation behavior, and thereby impact the final mechanical properties of cast components. In this study, isothermal semi-solid compression and subsequent cooling of aluminum die-cast alloy specimens were characterized using fast synchrotron tomography. This enabled the investigation and quantification of gas and shrinkage porosity evolution during deformation and solidification. The analysis of the 4D images (3D plus time) revealed two distinct mechanisms by which porosity formed; (i) deformation-induced growth due to the enrichment of local hydrogen content by the advective hydrogen transport, as well as a pressure drop in the dilatant shear bands, and (ii) diffusion-controlled growth during the solidification. The rates of pore growth were quantified throughout the process, and a Gaussian distribution function was found to represent the variation in the pore growth rate in both regimes. Using a one-dimensional diffusion model for hydrogen pore growth, the hydrogen flux required for driving pore growth during these regimes was estimated, providing a new insight into the role of advective transport associated with the deformation in the mushy region.

Journal article

Ostergaard MB, Cai B, Petersen RR, Konig J, Lee PD, Yue Yet al., 2019, Impact of pore structure on the thermal conductivity of glass foams, Materials Letters, Vol: 250, Pages: 72-74, ISSN: 0167-577X

The thermal conductivity (λ) of glass foams is thought to depend on pore size. We report on the impact of pore size, determined using X-ray microtomography, and percentage porosity on the λ of glass foams. Glass foams were prepared by heating powder mixtures of obsolete cathode ray tube (CRT) panel glass, Mn3O4 and carbon as foaming agents, and K3PO4 as additive, to a suitable temperature above Tg, and subsequent cooling. Here, we report for the first time a correlation between λ and pore size in the range 0.10–0.16 mm showing a decrease from 57 to 49 mW m−1 K−1 with increasing the pore size for glass foams with porosities of 87–90%. This indicates that the pore structure should be optimized in order to improve the insulating performance of glass foams.

Journal article

Ma L, Dowey PJ, Rutter E, Taylor KG, Lee PDet al., 2019, A novel upscaling procedure for characterising heterogeneous shale porosity from nanometer-to millimetre-scale in 3D, Energy, Vol: 181, Pages: 1285-1297, ISSN: 0360-5442

Microstructures and pore systems in shales are key to understanding the role of shale in many energy applications. This study proposes a novel multi-stage upscaling procedure to comprehensively investigate the heterogeneous and complex microstructures and pore systems in a laminated and microfractured shale, utilising 3D multi-scale imaging data. Five imaging techniques were used for characterisation from sub-nanoscale to macroscale (core-scale), spanning four orders of magnitude. Image data collected using X-ray tomography, Focused Ion Beam, and Electron Tomography techniques range in voxel size from 0.6 nm to 13 μm. Prior to upscaling, a novel two-step analysis was performed to ensure sub-samples were representative. Following this, a three-step procedure, based on homogenising descriptors and computed volume coefficients, was used to upscale the quantified microstructure and pore system. At the highest resolution (nanoscale), four distinct pore types were identified. At the sub-micron scale equations were derived for three pore-associated phases. At the microscale, the volume coefficients were recalculated to upscale the pore system to the millimetre- scale. The accuracy of the upscaling methodology was verified, predicting the total porosity within 7.2% discrepancy. The results provide a unique perspective to understand heterogeneous rock types, breaking though prior scale limitations in the pore system.

Journal article

Leung CLA, Tosi R, Muzangaza E, Nonni S, Withers PJ, Lee PDet al., 2019, Effect of preheating on the thermal, microstructural and mechanical properties of selective electron beam melted Ti-6Al-4V components, Materials and Design, Vol: 174, Pages: 1-10, ISSN: 0264-1275

Two-stage preheating is used in selective electron beam melting (SEBM) to prevent powder spreading during additive manufacturing (AM); however, its effects on part properties have not been widely investigated. Here, we employed three different preheat treatments (energy per unit area, EA) to a Ti-6Al-4V powder bed. Each standalone build, we fabricated a large block sample and seven can-shaped samples containing sintered powder. X-ray computed tomography (XCT) was employed to quantify the porosity and build accuracy of the can-shaped samples. The effective thermal conductivity of the sintered powder bed was estimated by XCT image-based modelling. The microstructural and mechanical properties of the block sample were examined by scanning electron microscopy and microhardness testing, respectively. The results demonstrate that increasing EA reduces the anisotropy of tortuosity and increases the thermal conductivity of the sintered powder bed, improving the heat transfer efficiency for subsequent beam-matter interaction. High preheat has a negligible effect on the porosity of large AM components; however, it decreases the microhardness from 330 ± 7 to 315 ± 11 HV0.5 and increases the maximum build error from 330 to 400 μm. Our study shows that a medium EA (411 kJ m−2) is sufficient to produce components with a high hardness whilst optimising build accuracy.

Journal article

Disney CM, Eckersley A, McConnell JC, Geng H, Bodey AJ, Hoyland JA, Lee PD, Sherratt MJ, Bay BKet al., 2019, Synchrotron tomography of intervertebral disc deformation quantified by digital volume correlation reveals microstructural influence on strain patterns, Acta Biomaterialia, Vol: 92, Pages: 290-304, ISSN: 1742-7061

The intervertebral disc (IVD) has a complex and multiscale extracellular matrix structure which provides unique mechanical properties to withstand physiological loading. Low back pain has been linked to degeneration of the disc but reparative treatments are not currently available. Characterising the disc’s 3D microstructure and its response in a physiologically relevant loading environment is required to improve understanding of degeneration and to develop new reparative treatments. In this study, techniques for imaging the native IVD, measuring internal deformation and mapping volumetric strain were applied to an in situ compressed ex vivo rat lumbar spine segment. Synchrotron X-ray micro-tomography (synchrotron CT) was used to resolve IVD structures at microscale resolution. These image data enabled 3D quantification of collagen bundle orientation and measurement of local displacement in the annulus fibrosus between sequential scans using digital volume correlation (DVC). The volumetric strain mapped from synchrotron CT provided a detailed insight into the micromechanics of native IVD tissue. The DVC findings showed that there was no slipping at lamella boundaries, and local strain patterns were of a similar distribution to the previously reported elastic network with some heterogeneous areas and maximum strain direction aligned with bundle orientation, suggesting bundle stretching and sliding. This method has the potential to bridge the gap between measures of macro-mechanical properties and the local 3D micro-mechanical environment experienced by cells. This is the first evaluation of strain at the micro scale level in the intact IVD and provides a quantitative framework for future IVD degeneration mechanics studies and testing of tissue engineered IVD replacements.

Journal article

Autefage H, Allen F, Tang HM, Kallepitis C, Gentleman E, Reznikov N, Nitiputri K, Nommeots-Nomm A, Young G, Lee PD, Pierce BF, Wagermaier W, Fratzl P, Goodship A, Jones JR, Blunn G, Stevens Met al., 2019, Multiscale analyses reveal native-like lamellar bone repair and near perfect bone-contact with porous strontium-loaded bioactive glass, Biomaterials, Vol: 209, Pages: 152-162, ISSN: 0142-9612

The efficient healing of critical-sized bone defects using synthetic biomaterial-based strategies is promising but remains challenging as it requires the development of biomaterials that combine a 3D porous architecture and a robust biological activity. Bioactive glasses (BGs) are attractive candidates as they stimulate a biological response that favors osteogenesis and vascularization, but amorphous 3D porous BGs are difficult to produce because conventional compositions crystallize during processing. Here, we rationally designed a porous, strontium-releasing, bioactive glass-based scaffold (pSrBG) whose composition was tailored to deliver strontium and whose properties were optimized to retain an amorphous phase, induce tissue infiltration and encourage bone formation. The hypothesis was that it would allow the repair of a critical-sized defect in an ovine model with newly-formed bone exhibiting physiological matrix composition and structural architecture. Histological and histomorphometric analyses combined with indentation testing showed pSrBG encouraged near perfect bone-to-material contact and the formation of well-organized lamellar bone. Analysis of bone quality by a combination of Raman spectral imaging, small-angle X-ray scattering, X-ray fluorescence and focused ion beam-scanning electron microscopy demonstrated that the repaired tissue was akin to that of normal, healthy bone, and incorporated small amounts of strontium in the newly formed bone mineral. These data show the potential of pSrBG to induce an efficient repair of critical-sized bone defects and establish the importance of thorough multi-scale characterization in assessing biomaterial outcomes in large animal models.

Journal article

Mo J, Groot RD, McCartney G, Guo E, Bent J, van Dalen G, Schuetz P, Rockett P, Lee PDet al., 2019, Ice crystal coarsening in ice cream during cooling: a comparison of theory and experiment, Crystals, Vol: 9, Pages: 1-14, ISSN: 2073-4352

Ice cream is a complex multi-phase structure and its perceived quality is closely related to the small size of ice crystals in the product. Understanding the quantitative coarsening behaviour of ice crystals will help manufacturers optimise ice cream formulations and processing. Using synchrotron X-ray tomography, we measured the time-dependent coarsening (Ostwald ripening) of ice crystals in ice cream during cooling at 0.05 °C/min. The results show ice crystal coarsening is highly temperature dependent, being rapid from ca. −6 to −12 °C but significantly slower at lower temperatures. We developed a numerical model, based on established coarsening theory, to calculate the relationship between crystal diameter, cooling rate and the weight fraction of sucrose in solution. The ice crystal diameters predicted by the model are found to agree well with the measured values if matrix diffusion is assumed to be slowed by a factor of 1.2 due to the presence of stabilizers or high molecular weight sugars in the ice cream formulation.

Journal article

Wang H, Atwood RC, Pankhurst MJ, Kashyap Y, Cai B, Zhou T, Lee PD, Drakopoulos M, Sawhney Ket al., 2019, High-energy, high-resolution, fly-scan X-ray phase tomography, Scientific Reports, Vol: 9, Pages: 1-11, ISSN: 2045-2322

High energy X-ray phase contrast tomography is tremendously beneficial to the study of thick and dense materials with poor attenuation contrast. Recently, the X-ray speckle-based imaging technique has attracted widespread interest because multimodal contrast images can now be retrieved simultaneously using an inexpensive wavefront modulator and a less stringent experimental setup. However, it is time-consuming to perform high resolution phase tomography with the conventional step-scan mode because the accumulated time overhead severely limits the speed of data acquisition for each projection. Although phase information can be extracted from a single speckle image, the spatial resolution is deteriorated due to the use of a large correlation window to track the speckle displacement. Here we report a fast data acquisition strategy utilising a fly-scan mode for near field X-ray speckle-based phase tomography. Compared to the existing step-scan scheme, the data acquisition time can be significantly reduced by more than one order of magnitude without compromising spatial resolution. Furthermore, we have extended the proposed speckle-based fly-scan phase tomography into the previously challenging high X-ray energy region (120 keV). This development opens up opportunities for a wide range of applications where exposure time and radiation dose are critical.

Journal article

Yuan L, Prasad A, Lee PD, Stjohn Det al., 2019, Numerical simulation of wave-like nucleation events, ISSN: 1757-8981

© Published under licence by IOP Publishing Ltd. The Interdependence model [1] predicted that nucleation would occur in waves of events with regions of no nucleation in between each wave. The waves continue to form until nucleation covers the sample. The cause of this phenomenon was attributed to the formation of a nucleation-free zone which incorporates solute suppressed nucleation and inhibited nucleation zones. Recent real-time synchrotron x-ray studies by Prasad et al [2], Liotti et al [3] and Xu et al [4] have confirmed this hypothesis showing nucleation occurs in a step-wise fashion with a number of events occurring followed by little or no nucleation for a short period before another set of events occurs. A microscale solidification model that predicts diffusion-controlled dendritic growth has successfully shown the effect of the developing constitutional supercooling on the selection of nucleation events. In this study, we use this model to predict the solidification behaviour under the conditions experienced during these real-time synchrotron studies.

Conference paper

Bhagavath S, Cai B, Atwood R, Lee PD, Karagadde Set al., 2019, Effects of strain rate on hot tear formation in Al-Si-Cu alloys, ISSN: 1757-8981

© Published under licence by IOP Publishing Ltd. The alloy casting process is one of the major manufacturing processes to produce near net shape components. The casing process is prone to a wide variety of defects, with hot tear being one of the most detrimental. The two main factors generally recognized as the primary cause for formation of hot tears are the mechanical response of the mush (which effects its permeability), and the solidification range (solidification time). The response of the mushy zone under deformation is mainly affected by the solid fraction, strain rate and grain morphology. Even though the science behind the formation of hot tear is understood, there is no general criterion to quantify the hot tear formation under varying casting conditions. The development of ultra-fast X-ray imaging has facilitated the means to quantify the effects of the critical parameters in-situ and develop better correlations for hot tear prediction. The in situ experiments will also provide insights into mush rheology, which has significant influence on hot tear formation. In this study, isothermal semi solid compression studies of Al-Si-Cu alloys were carried out using specially built thermo-mechanical rig. We studied the effects of the strain rate in the range of 2 × 10-4-0.02/s and solid fraction (∼0.6-0.9) on the mechanical response of the mushy zone. The sample were characterized before and after deformation using X-ray micro tomography. The data was subjected to an image processing routine and the amount of porosity and hot tear was quantified. The stress-strain curve of the semisolid alloys showed a characteristic strain softening behaviour for semi solid samples with ∼0.6-0.7 solid fraction, irrespective of loading rates, whereas the behaviour at higher fractions were that of constant flow stress. Additionally, in situ compression experiments were carried out, wherein the liquid channel thickness at various strain values were measured. Isola

Conference paper

Nommeots-Nomm A, Ligorio C, Bodey AJ, Cai B, Jones JR, Lee PD, Poologasundarampillai Get al., 2019, Four-dimensional imaging and quantification of viscous flow sintering within a 3D printed bioactive glass scaffold using synchrotron X-ray tomography, Materials Today Advances, Vol: 2, ISSN: 2590-0498

Bioglass® was the first material to form a stable chemical bond with human tissue. Since its discovery, a key goal was to produce three-dimensional (3D) porous scaffolds which can host and guide tissue repair, in particular, regeneration of long bone defects resulting from trauma or disease. Producing 3D scaffolds from bioactive glasses is challenging because of crystallization events that occur while the glass particles densify at high temperatures. Bioactive glasses such as the 13–93 composition can be sintered by viscous flow sintering at temperatures above the glass transition onset (Tg) and below the crystallization temperature (Tc). There is, however, very little literature on viscous flow sintering of bioactive glasses, and none of which focuses on the viscous flow sintering of glass scaffolds in four dimensions (4D) (3D + time). Here, high-resolution synchrotron-sourced X-ray computed tomography (sCT) was used to capture and quantify viscous flow sintering of an additively manufactured bioactive glass scaffold in 4D. In situ sCT allowed the simultaneous quantification of individual particle (local) structural changes and the scaffold's (global) dimensional changes during the sintering cycle. Densification, calculated as change in surface area, occurred in three distinct stages, confirming classical sintering theory. Importantly, our observations show for the first time that the local and global contributions to densification are significantly different at each of these stages: local sintering dominates stages 1 and 2, while global sintering is more prevalent in stage 3. During stage 1, small particles coalesced to larger particles because of their higher driving force for viscous flow at lower temperatures, while large angular particles became less faceted (angular regions had a local small radius of curvature). A transition in the rate of sintering was then observed in which significant viscous flow occurred, resulting in large reduction of surfac

Journal article

Chandler MR, Mecklenburgh J, Rutter E, Lee Pet al., 2019, Fluid injection experiments in shale at elevated confining pressures: determination of flaw sizes from mechanical experiments, JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, Vol: 124, Pages: 5500-5520, ISSN: 2169-9313

Triaxial experiments and direct fluid injection experiments have been conducted at confining pressures up to 100 MPa on Mancos shale, Whitby mudstone, Penrhyn slate, and Pennant sandstone. Experiments were conducted with sample axes lying both parallel and perpendicular to layering in the materials. During triaxial failure Penrhyn slate was stronger for samples with cleavage parallel to maximum principal stress, but the two orientations in the shales displayed similar failure stresses. Initial flaw sizes of around 40 μm were calculated from the triaxial data using the wing crack model, with the shales having shorter initial flaws than the nonshales. During direct fluid injection, breakdown was rapid, with no discernible gap between fracture initiation and breakdown. Breakdown pressure increased linearly with confining pressure but was less sensitive to confining pressure than expected from existing models. A fracture mechanics‐based model is proposed to determine the initial flaw size responsible for breakdown in injection experiments. Flaw sizes determined in this way agree reasonably with those determined from the triaxial data in the nonshales at low confining pressures. As confining pressure rises, a threshold is reached, above which the fluid injection experiments suggest a lower initial flaw length of around 10 μm. This threshold is interpreted as being due to the partial closure of flaws. In the shales an initial flaw length of around 10 μm was determined at all confining pressures, agreeing reasonably with those determined through the triaxial experiments.

Journal article

Cai B, Kao A, Lee PD, Boller E, Basevi H, Phillion AB, Leonardis A, Pericieous Ket al., 2019, Growth of β intermetallic in an Al-Cu-Si alloy during directional solidification via machine learned 4D quantification, Scripta Materialia, Vol: 165, Pages: 29-33, ISSN: 1359-6462

Fe contamination is a serious composition barrier for Al recycling. In Fe-containing Al-Si-Cu alloy, a brittle and plate-shaped β phase forms, degrading the mechanical properties. Here, 4D (3D plus time) synchrotron X-ray tomography was used to observe the directional solidification of Fe-containing Al-Si-Cu alloy. The quantification of the coupled growth of the primary and β phase via machine learning and particle tracking, demonstrates that the final size of the β intermetallics were strongly influenced by the solute segregation and space available for growth whereas the β orientation was controlled by the temperature gradient direction. The work can be used to validate predictive models.

Journal article

Evans LM, Margetts L, Lee PD, Butler CAM, Surrey Eet al., 2019, Image based in silico characterisation of the effective thermal properties of a graphite foam, Carbon, Vol: 143, Pages: 542-558, ISSN: 0008-6223

Functional materials’ properties are influenced by microstructures which can be changed during manufacturing. A technique is presented which digitises graphite foam via X-ray tomography and converts it into image-based models to determine properties in silico. By simulating a laser flash analysis its effective thermal conductivity is predicted. Results show ∼1% error in the direction the foam was ‘grown’ during manufacturing but is significantly less accurate in plane due to effective thermal conductivity resulting from both the foam's microstructure and graphite's crystalline structure. An empirical relationship is found linking these by using a law of mixtures. A case study is presented demonstrating the technique's use to simulate a heat exchanger component containing graphite foam with micro-scale accuracy using literature material properties for solid graphite. Compared against conventional finite element modelling there is no requirement to firstly experimentally measure the foam's effective bulk properties. Additionally, improved local accuracy is achieved due to exact location of contact between the foam and other parts of the component. This capability will be of interest in design and manufacture of components using graphite materials. The software used was developed by the authors and is open source for others to undertake similar studies.

Journal article

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