Publications
520 results found
Leung CLA, Tosi R, Muzangaza E, et 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.
Disney CM, Eckersley A, McConnell JC, et 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.
Autefage H, Allen F, Tang HM, et 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.
Mo J, Groot RD, McCartney G, et 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.
Wang H, Atwood RC, Pankhurst MJ, et 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.
Bhagavath S, Cai B, Atwood R, et al., 2019, Effects of strain rate on hot tear formation in Al-Si-Cu alloys, Joint Conference of 5th International Conference on Advances in Solidification Processes (ICASP) and 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR), Publisher: IOP PUBLISHING LTD, Pages: 1-6, ISSN: 1757-8981
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. Isolated liquid channels were formed under loading, f
Chandler MR, Mecklenburgh J, Rutter E, et 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.
Nommeots-Nomm A, Ligorio C, Bodey AJ, et 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
Cai B, Kao A, Lee PD, et 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.
Leung CLA, Marussi S, Towrie M, et al., 2019, The effect of powder oxidation on defect formation in laser additive manufacturing, Acta Materialia, Vol: 166, Pages: 294-305, ISSN: 1359-6454
Understanding defect formation during laser additive manufacturing (LAM) of virgin, stored, and reused powders is crucial for the production of high quality additively manufactured parts. We investigate the effects of powder oxidation on the molten pool dynamics and defect formation during LAM. We compare virgin and oxidised Invar 36 powder under overhang and layer-by-layer build conditions using in situ and operando X-ray Imaging. The oxygen content of the oxidised powder was found to be ca. 6 times greater (0.343 wt.%) than the virgin powder (0.057 wt.%). During LAM, the powder oxide is entrained into the molten pool, altering the Marangoni convection from an inward centrifugal to an outward centripetal flow. We hypothesise that the oxide promotes pore nucleation, stabilisation, and growth. We observe that spatter occurs more frequently under overhang conditions compared to layer-by-layer conditions. Droplet spatter can be formed by indirect laser-driven gas expansion and by the laser-induced metal vapour at the melt surface. Under layer-by-layer build conditions, laser re-melting reduces the pore size distribution and number density either by promoting gas release from keyholing or by inducing liquid flow, partially or completely filling pre-existing pores. We also observe that pores residing at the track surface can burst during laser re-melting, resulting in either formation of droplet spatter and an open pore or healing of the pore via Marangoni flow. This study confirms that excessive oxygen in the powder feedstock may cause defect formation in LAM.
Evans LM, Margetts L, Lee PD, et 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.
Phillion AB, Shuai S, Guo E, et al., 2019, Synchrotron tomographic quantification of the influence of Zn concentration on dendritic growth in Mg-Zn alloys (vol 156, pg 287, 2018), ACTA MATERIALIA, Vol: 165, Pages: 751-752, ISSN: 1359-6454
Chen Y, Han P, Vandi L-J, et al., 2019, A biocompatible thermoset polymer binder for Direct Ink Writing of porous titanium scaffolds for bone tissue engineering, Materials Science and Engineering: C, Vol: 95, Pages: 160-165, ISSN: 1873-0191
There is increasing demand for synthetic bone scaffolds for bone tissue engineering as they can counter issues such as potential harvesting morbidity and restrictions in donor sites which hamper autologous bone grafts and address the potential for disease transmission in the case of allografts. Due to their excellent biocompatibility, titanium scaffolds have great potential as bone graft substitutes as they mimic the structure and properties of human cancellous bone. Here we report on a new thermoset bio-polymer which can act as a binder for Direct Ink Writing (DIW) of titanium artificial bone scaffolds. We demonstrate the use of the binder to manufacture porous titanium scaffolds with evenly distributed and highly interconnected porosity ideal for orthopaedic applications. Due to their porous structure, the scaffolds exhibit an effective Young's modulus similar to human cortical bone, alleviating undesirable stress-shielding effects, and possess superior strength. The biocompatibility of the scaffolds was investigated in vitro by cell viability and proliferation assays using human bone-marrow-derived Mesenchymal stem cells (hMSCs). The hMSCs displayed well-spread morphologies, well-organized F-actin and large vinculin complexes confirming their excellent biocompatibility. The vinculin regions had significantly larger Focal Adhesion (FA) area and equivalent FA numbers compared to that of tissue culture plate controls, showing that the scaffolds support cell viability and promote attachment. In conclusion, we have demonstrated the excellent potential of the thermoset bio-polymer as a Direct Ink Writing ready binder for manufacture of porous titanium scaffolds for hard tissue engineering.
Yufit V, Tariq F, Biton M, et al., 2019, Operando visualisation and multi-scale tomography studies of dendrite formation and dissolution in zinc batteries, Joule, Vol: 3, Pages: 485-502, ISSN: 2542-4351
Alternative battery technologies are required to meet growing energy demands and address the limitations of present technologies. As such, it is necessary to look beyond lithium-ion batteries. Zinc batteries enable high power density while being sourced from ubiquitous and cost-effective materials. This paper presents, for the first time known to the authors, multi-length scale tomography studies of failure mechanisms in zinc batteries with and without commercial microporous separators. In both cases, dendrites were grown, dissolved, and regrown, critically resulting in different morphology of dendritic layer formed on both the electrode and the separator. The growth of dendrites and their volume-specific areas were quantified using tomography and radiography data in unprecedented resolution. High-resolution ex situ analysis was employed to characterize single dendrites and dendritic deposits inside the separator. The findings provide unique insights into mechanisms of metal-battery failure effected by growing dendrites.
Wang Y, Liu B, Yan K, et al., 2019, Corrigendum to ‘Probing deformation mechanisms of a FeCoCrNi high-entropy alloy at 293 and 77 K using in situ neutron diffraction’ [Acta Mater. 154C (2018) 79–89], Acta Materialia, Vol: 163, Pages: 240-242, ISSN: 1359-6454
The authors regret that there were errors in Figs. 3 and 4, which in turn meant there were errors in Table 2. In Figs. 3 and 4, the lattice strain ((d-d0)/d0, where d is the lattice spacing) as a function of strain/stress should have plotted. Please find below the corrected versions of the figures and table. The authors would like to apologise for any inconvenience caused. Fig. 3. The evolution of elastic lattice strains along the axial and radial directions in grain families having {111}, {200}, {220}, {311} and {222} crystallographic planes during tensile loading at (a) 77 K and (b) 293 K.[Figure presented] Fig. 4. The (111) first order and (222) second order reflections together with the stacking fault probability as a function of true strain at (a) 77 K, (b) 293 K. [Figure presented] Table 2. Uniaxial materials properties of FeCoCrNi HEA at 77 and 293 K. [Table presented]
Hollis C, Al Hajri A, Van Boxel S, et al., 2019, Characterization of porosity within a microporous reservoir, shuaiba formation of Oman, Carbonate Pore Systems: New Developments and Case Studies, Editors: McNeill, Harris, Rankey, Hsieh, Publisher: Society for Sedimentary Geology, Pages: 162-182
Although carbonate reservoirs often have high total pore volumes, permeability often doesn’t show astrong correlation to total porosity. Carbonate pore networks are also widely recognized as beinghighly heterogeneous, with marked variability in pore size (from sub-micron to millimetre scale andabove) within an individual core plug. It is perhaps for this reason that there has been relatively littlequantification of carbonate pore size and shape, despite significant advances in our ability to imagenaturally porous media using electron microscopy and advanced X-ray imaging.This study focuses on four samples of limestone from the uppermost Shuaiba Formation in northernOman. These samples were selected for X-ray CT and ESEM imaging and quantitative analysisfollowinga detailed reservoir quality evaluation of the study interval across seven fields. This interval has beenwell-studied sedimentologically but the processes and timing of diagenetic modification, and thenature of the resultant pore network, are less well understood. The samples represent a range oflithofacies associations that occur immediately beneath the Shuaiba - Nahr Umr unconformity, withinan interval that is recognized for possessing higher permeability than the underlying reservoir. Thesamples were imaged at multiple scales and their pore network analyzed.Within the sample set, over 70% of the total pore volume is < 1 m diameter. The 3D equivalent poreradii within individual samples ranges from <0.1m to >100 m, the size of the X-ray imaged samplesbeing limited to 1 mm3. The average aspect ratios of all pores was < 2, and was highest in micropores(<1 m pore radii). Mean co-ordination number was < 3 in all samples, and was highest withinmicropores. Since most pore throat radii are < 1 m, this most likely reflects the higher resolutionneeded to image micropores. Multivarient analysis shows that permeability prediction is improvedwhen pore topological parameters are known. The
Yuan L, Prasad A, Lee PD, et al., 2019, Numerical simulation of wave-like nucleation events, Joint Conference of 5th International Conference on Advances in Solidification Processes (ICASP) and 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR), Publisher: IOP PUBLISHING LTD, ISSN: 1757-8981
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.
Guo E, Phillion AB, Chen Z, et al., 2019, In Situ Tomographic Observation of Dendritic Growth in Mg/Al Matrix Composites, Light Metals Symposium at the 148th TMS Annual Meeting, Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 1561-1567, ISSN: 2367-1181
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Chandler MR, Fauchille A-L, Kim HK, et al., 2018, Correlative optical and X-ray imaging of strain evolution during double-torsion fracture toughness measurements in shale, Journal of Geophysical Research. Solid Earth, Vol: 123, Pages: 10517-10533, ISSN: 2169-9356
Mode‐I Fracture Toughness, KIc, was measured in six shale materials using the double‐torsion technique. During loading, crack propagation was imaged both using twin optical cameras, and with fast X‐ray radiograph acquisition. Samples of Bowland, Haynesville, Kimmeridge, Mancos, Middlecliff, and Whitby shales were tested in a range of orientations. The measured fracture toughness values were found to be in good agreement with existing literature values. The two imaging techniques improve our understanding of local conditions around the fracture‐tip, through in situ correlation of mechanical data, inelastic zone size, and fracture‐tip velocity. The optical Digital Image Correlation technique proved useful as a means of determining the validity of individual experiments, by identifying experiments during which strains had developed in the two “rigid” specimen halves. Strain maps determined through Digital Image Correlation of the optical images suggest that the scale of the inelastic zone is an order of magnitude smaller than the classically used approximation suggests. This smaller damage region suggests a narrower region of enhanced permeability around artificially generated fractures in shales. The resolvable crack‐tip was tracked using radiograph data and found to travel at a velocity around 470 μm/s during failure, with little variation in speed between materials and orientations. Fracture pathways in the bedding parallel orientations were observed to deviate from linearity, commonly following layer boundaries. This suggests that while a fracture traveling parallel to bedding may travel at a similar speed to a bedding perpendicular fracture, it may have a more tortuous pathway, and therefore access a larger surface area.
Leung CLA, Marussi S, Towrie M, et al., 2018, Laser-matter interactions in additive manufacturing of stainless steel SS316L and 13-93 bioactive glass revealed by in situ X-ray imaging, Additive Manufacturing, Vol: 24, Pages: 647-657, ISSN: 2214-8604
Laser-matter interactions in laser additive manufacturing (LAM) occur on short time scales (10−6–10−3s) and have traditionally proven difficult to characterise. We investigate these interactions during LAM of stainless steel SS316L and 13-93 bioactive glass powders using a custom built LAM process replicator (LAMPR) with in situ and operando synchrotron X-ray real-time radiography. This reveals a wide range of melt track solidification phenomena as well as spatter and porosity formation. We hypothesise that the SS316L powder absorbs the laser energy at its surface while the trace elements in the 13-93 bioactive glass powder absorb and remit the infra-red radiation. Our results show that a low viscosity melt, e.g. 8 mPa s for SS316L, tends to generate spatter (diameter up to 250 μm and an average spatter velocity of 0.26 m s−1) and form a melt track by molten pool wetting. In contrast, a high viscosity melt, e.g. 2 Pa s for 13-93 bioactive glass, inhibits spatter formation by damping the Marangoni convection, forming a melt track via viscous flow. The viscous flow in 13-93 bioactive glass resists pore transport; combined with the reboil effect, this promotes pore growth during LAM, resulting in a pore size up to 600 times larger than that exhibited in the SS316L sample.
Bjerre MK, Azeem MA, Tiedje NS, et al., 2018, A graphite nodule growth model validated by in situ synchrotron x-ray tomography, Modelling and Simulation in Materials Science and Engineering, Vol: 26, ISSN: 0965-0393
An accurate prediction of ductile cast iron (DCI) microstructures is crucial for a science-based optimisation of cast component design. The number density and distribution of graphite nodules critically influence the mechanical performance of a component in service. Although models predicting nodule growth have been researched for many years, recent improvements have been impeded by lack of detailed experimental data on nodule growth kinetics for validation. This data has now been made available through in situ observations of the solidification of DCI using synchrotron x-ray tomography in combination with a high temperature environmental cell. In the present investigation, a new sphere of influence (SoI) model for spheroidal graphite growth is proposed. It inherently incorporates the competition for carbon between neighbouring nodules and the depletion of carbon in the matrix. Comparing simulation results to the in situ observations of graphite growth, the SoI model successfully predicts both growth of individual nodules as well as the size distribution of a large nodule population during solidification.
Disney CM, Lee PD, Hoyland JA, et al., 2018, A review of techniques for visualising soft tissue microstructure deformation and quantifying strain Ex Vivo, Journal of Microscopy, Vol: 272, Pages: 165-179, ISSN: 1365-2818
Many biological tissues have a complex hierarchical structure allowing them to function under demanding physiological loading conditions. Structural changes caused by ageing or disease can lead to loss of mechanical function. Therefore, it is necessary to characterise tissue structure to understand normal tissue function and the progression of disease. Ideally intact native tissues should be imaged in 3D and under physiological loading conditions. The current published in situ imaging methodologies demonstrate a compromise between imaging limitations and maintaining the samples native mechanical function. This review gives an overview of in situ imaging techniques used to visualise microstructural deformation of soft tissue, including three case studies of different tissues (tendon, intervertebral disc and artery). Some of the imaging techniques restricted analysis to observational mechanics or discrete strain measurement from invasive markers. Full-field local surface strain measurement has been achieved using digital image correlation. Volumetric strain fields have successfully been quantified from in situ X-ray microtomography (micro-CT) studies of bone using digital volume correlation but not in soft tissue due to low X-ray transmission contrast. With the latest developments in micro-CT showing in-line phase contrast capability to resolve native soft tissue microstructure, there is potential for future soft tissue mechanics research where 3D local strain can be quantified. These methods will provide information on the local 3D micromechanical environment experienced by cells in healthy, aged and diseased tissues. It is hoped that future applications of in situ imaging techniques will impact positively on the design and testing of potential tissue replacements or regenerative therapies.
Pankhurst MJ, Vo NT, Butcher AR, et al., 2018, Quantitative measurement of olivine composition in three dimensions using helical-scan X-ray micro-tomography, AMERICAN MINERALOGIST, Vol: 103, Pages: 1800-1811, ISSN: 0003-004X
Olivine is a key constituent in the silicate Earth; its composition and texture informs petrogenetic understanding of numerous rock types. Here we develop a quantitative and reproducible method to measure olivine composition in three dimensions without destructive analysis, meaning full textural context is maintained. The olivine solid solution between forsterite and fayalite was measured using a combination of three-dimensional (3D) X-ray imaging techniques, 2D backscattered electron imaging, and spot-analyses using wavelength-dispersive electron probe microanalysis. The linear attenuation coefficient of natural crystals across a range of forsterite content from ∼73–91 mol% were confirmed to scale linearly with composition using 53, 60, and 70 kV monochromatic beams at I12-JEEP beamline, Diamond Light Source utilizing the helical fly-scan acquisition. A polychromatic X-ray source was used to scan the same crystals, which yielded image contrast equivalent to measuring the mol% of forsterite with an accuracy of <1.0%. X-ray tomography can now provide fully integrated textural and chemical analysis of natural samples containing olivine, which will support 3D and 3D+time petrologic modeling. The study has revealed >3 mm domains within a large crystal of San Carlos forsterite that varies by ∼2 Fo mol%. This offers a solution to an outstanding question of inter-laboratory standardization, and also demonstrates the utility of 3D, non-destructive, chemical measurement. To our knowledge, this study is the first to describe the application of XMT to quantitative chemical measurement across a mineral solid solution. Our approach may be expanded to calculate the chemistry of other mineral systems in 3D, depending upon the number, chemistry, and density of end-members.
Tiedje NS, Bjerre MK, Azeem MA, et al., 2018, Analysis of local conditions on graphite growth and shape during solidification of ductile cast iron, Transactions of the Indian Institute of Metals, Vol: 71, Pages: 2699-2705, ISSN: 0972-2815
3D X-ray tomography recordings have been used to study graphite growth during solidification of ductile cast iron. Using data from such recordings, it is shown how local growth conditions influence growth rate and morphology of nodules during solidification. Experiments show that it is common for nodules to gradually change shape during solidification so that sphericity decreases. It is also found that different shaped nodules can evolve in direct contact with liquid iron and also after they are encapsulated in austenite. It is observed that a significant proportion of originally complete spherical nodules become less spherical via formation of protrusions on the surface; these new surfaces are observed to grow relatively faster. It is shown that encapsulation of the graphite nodule by austenite may be incomplete and that at the end of solidification, partial encapsulation and the effect of the number of nearest graphite nodules play a crucial role in determining the final graphite morphology.
Guo E, Kazantsev D, Mo J, et al., 2018, Revealing the microstructural stability of a three-phase soft solid (ice cream) by 4D synchrotron X-ray tomography, Journal of Food Engineering, Vol: 237, Pages: 204-214, ISSN: 0260-8774
Understanding the microstructural stability of soft solids is key to optimizing formulations and processing parameters to improve the materials' properties. In this study, in situ synchrotron X-ray tomography is used to determine the temperature dependence of ice-cream's microstructural evolution, together with the underlying physical mechanisms that control microstructural stability. A new tomographic data processing method was developed, enabling the features to be segmented and quantified. The time-resolved results revealed that the melting-recrystallization mechanism is responsible for the evolution of ice crystal size and morphology during thermal cycling between −15 and −5 °C, while coalescence of air cells is the dominant coarsening mechanism controlling air bubble size and interconnectivity. This work also revealed other interesting phenomena, including the role of the unfrozen matrix in maintaining the ice cream's microstructural stability and the complex interactions between ice crystals and air structures, e.g. the melting and recrystallization of ice crystals significantly affect the air cell's morphology and the behavior of the unfrozen matrix. The results provide crucial information enhancing the understanding of microstructural evolution in multi-phase multi-state complex foodstuffs and other soft solids.
Mo J, Guo E, Graham McCartney D, et al., 2018, Time-resolved tomographic quantification of the microstructural evolution of ice cream, Materials, Vol: 11, ISSN: 1996-1944
Ice cream is a complex multi-phase colloidal soft-solid and its three-dimensional microstructure plays a critical role in determining the oral sensory experience or mouthfeel. Using in-line phase contrast synchrotron X-ray tomography, we capture the rapid evolution of the ice cream microstructure during heat shock conditions in situ and operando, on a time scale of minutes. The further evolution of the ice cream microstructure during storage and abuse was captured using ex situ tomography on a time scale of days. The morphology of the ice crystals and unfrozen matrix during these thermal cycles was quantified as an indicator for the texture and oral sensory perception. Our results reveal that the coarsening is due to both Ostwald ripening and physical agglomeration, enhancing our understanding of the microstructural evolution of ice cream during both manufacturing and storage. The microstructural evolution of this complex material was quantified, providing new insights into the behavior of soft-solids and semi-solids, including many foodstuffs, and invaluable data to both inform and validate models of their processing.
Tallia F, Russo L, Li S, et al., 2018, Bouncing and 3D printable hybrids with self-healing properties, Materials Horizons, Vol: 5, Pages: 849-860, ISSN: 2051-6355
Conventional composites often do not represent true synergy of their constituent materials. This is particularly evident in biomaterial applications where devices must interact with cells, resist cyclic loads and biodegrade safely. Here we propose a new hybrid system, with co-networks of organic and inorganic components, resulting in unprecedented mechanical properties, including “bouncy” elasticity and intrinsic ability to self-heal autonomously. They are also developed as new ‘inks’ that can be directly 3D printed. A hybrid is different from a nanocomposite because the components are indistinguishable from each other at the nanoscale and above. The properties are generated by a novel methodology that combines in situ cationic ring-opening polymerisation with sol–gel, creating silica/poly(tetrahydrofuran)/poly(ε-caprolactone) hybrids with molecular scale interactions and covalent links. Cartilage is notoriously difficult to repair and synthetic biomaterials have yet to mimic it closely. We show that 3D printed hybrid scaffolds with pore channels of ∼200 μm mimic the compressive behaviour of cartilage and provoke chondrocytes to produce markers integral to articular cartilage-like matrix. The synthesis method can be applied to different organic sources, leading to a new class of hybrid materials.
Shuai S, Guo E, Wang J, et al., 2018, Synchrotron tomographic quantification of the influence of Zn concentration on dendritic growth in Mg-Zn alloys, Acta Materialia, Vol: 156, Pages: 287-296, ISSN: 1359-6454
Dendritic microstructural evolution during the solidification of Mg-Zn alloys was investigated as a function of Zn concentration using in situ synchrotron X-ray tomography. We reveal that increasing Zn content from 25 wt% to 50 wt% causes a Dendrite Orientation Transition (DOT) from a six-fold snow-flake structure to a hyper-branched morphology and then back to a six-fold structure. This transition was attributed to changes in the anisotropy of the solid-liquid interfacial energy caused by the increase in Zn concentration. Further, doublon, triplon and quadruplon tip splitting mechanisms were shown to be active in the Mg-38 wt%Zn alloy, creating a hyper-branched structure. Using the synchrotron tomography datasets, we quantify, for the first time, the evolution of grain structures during the solidification of these alloys, including dendrite tip velocity in the mushy zone, solid fraction, and specific surface area. The results are also compared to existing models. The results demonstrate the complexity in dendritic pattern formation in hcp systems, providing critical input data for the microstructural models used for integrated computational materials engineering of Mg alloys.
Azeem MA, Bjerre MK, Atwood RC, et al., 2018, Synchrotron quantification of graphite nodule evolution during the solidification of cast iron, Acta Materialia, Vol: 155, Pages: 393-401, ISSN: 1359-6454
In cast iron, graphite develops in conjunction with the metallic matrix during solidification. The morphology and distribution of the embedded graphite is pivotal for mechanical properties from yield strength to fatigue. A novel high temperature environmental cell was developed and combined with in situ synchrotron tomography to investigate and quantify microstructural evolution, including graphite nodule nucleation and growth rates in ductile cast iron. The mechanisms of degenerate graphite nodule formation were also revealed. The formation of a coherent primary gamma phase dendritic network before the graphite nucleation is demonstrated. The graphite nodule nucleation rate, mobility and growth rates are compared to classical models, highlighting the limitations in these models. The results provide unique insights to tune the temperature pathways during cast iron solidification to achieve desired uniform rounded graphite morphologies and size distributions.
Ma L, Slater T, Dowey PJ, et al., 2018, Hierarchical integration of porosity in shales, Scientific Reports, Vol: 8, ISSN: 2045-2322
Pore characterization in shales is challenging owing to the wide range of pore sizes and types present. Haynesville-Bossier shale (USA) was sampled as a typical clay-bearing siliceous, organic-rich, gas-mature shale and characterized over pore diameters ranging 2 nm to 3000 nm. Three advanced imaging techniques were utilized correlatively, including the application of Xe+ plasma focused ion beam scanning electron microscopy (plasma FIB or PFIB), complemented by the Ga+ FIB method which is now frequently used to characterise porosity and organic/inorganic phases, together with transmission electron microscope tomography of the nano-scale pores (voxel size 0.6 nm; resolution 1–2 nm). The three pore-size scales each contribute differently to the pore network. Those <10 nm (greatest number), 10 nm to 100 nm (best-connected hence controls transport properties), and >100 nm (greatest total volume hence determines fluid storativity). Four distinct pore types were found: intra-organic, organic-mineral interface, inter-mineral and intra-mineral pores were recognized, with characteristic geometries. The whole pore network comprises a globally-connected system between phyllosilicate mineral grains (diameter: 6–50 nm), and locally-clustered connected pores within porous organic matter (diameter: 200–800 nm). Integrated predictions of pore geometry, connectivity, and roles in controlling petrophysical properties were verified through experimental permeability measurements.
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