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Abstract: 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 the 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 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.

Open Access

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Abstract: Soil organic carbon (SOC) is of key importance for soil functioning. It strongly impacts soil fertility, greenhouse gas emissions, nutrient retention, and contaminant degradation. The soil pore network determines how oxygen, water and nutrients are transported and exchanged in soil, and the architecture of the soil is therefore equally fundamental to soil functions. For a thorough understanding of the microbial habitat, the soil pore network architecture needs to be evaluated alongside with the spatial distribution of SOC, but the challenge ahead is the 3-D visualization of organic carbon at the micro-scale. At present, such visualizations are undertaken using staining agents, but their non-specific binding to other features in the soil aggravates evaluation of organic carbon at the micro-scale. In the present study, we investigated the potential and limitations of using joint white-beam neutron and X-ray imaging for mapping the 3-dimensional organic carbon distribution in soil. This approach is viable because neutron and X-ray beams have complementary attenuation properties. Soil minerals consist to a large part of silicon and aluminium, elements which are relatively translucent to neutrons but attenuate X-rays. In contrast, attenuation of neutrons is strong for hydrogen, which is abundant in SOC, while hydrogen barely attenuates X-rays. When considering dried soil samples, the complementary attenuation for neutrons and X-rays may be used to quantify the fractions of air, SOC and minerals for any imaged voxel in a bi-modal 3-dimensional image, i.e. a combined neutron and X-ray image. We collected neutron data at the IMAT beamline at the ISIS facility and X-ray data at the I12 beamline at the Diamond Light source, both located within the Rutherford Appleton Laboratory, Harwell, UK. The neutron image clearly showed variations in neutron attenuation within soil aggregates at approximately constant X-ray attenuations. This indicates a constant bulk density with varying organic matter and/or mineralogy. For samples with identical mineral composition, neutron attenuation data of sieved and repacked soil samples exhibited a large coefficient of determination (R2) in a regression between volumetric SOC content and neutron attenuation (0.9). Even larger R2 (0.93) were obtained when the volumetric clay content was also included into the regression. However, when comparing soil samples with different mineralogy, R2 dropped to 0.24 and 0.37, depending whether the clay content was considered or not. To improve the method, it is necessary to include specifics of the soil mineralogy. Here, analysing the time-of-flight neutron attenuation data collected at the IMAT beamline will provide further insights. In summary, our approach yielded promising results. We anticipate that quantitative 3-D imaging of organic carbon contents in soil will be possible in the near future.

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Abstract: In this study, the polymorphic transitions of mefenamic acid (MA) were studied by synchrotron X-ray powder diffraction combined with differential scanning calorimetry (XRD-DSC). The initial material was found to be phase-pure form I which, when heated, produces two endotherms that can be observed by DSC at 162.72 and 219.55°C. The former was found to correspond to a solid–solid enantiotropic transition from form I to a mixture of forms II and III. The latter is the melting point of form II. As form I is heated, significantly greater unit-cell expansion is seen in the a direction than in b and c, which can be explained by the stronger intermolecular interactions in the bc plane. Refinements of the reported MA structures against the patterns collected during heating revealed that at 175°C there exists a mixture of forms I, II and III, whereas only forms II and III remain at 205°C. However, reflections are observed at both temperatures which cannot be fitted with the known forms of MA. It is hypothesized that a new form of MA is produced upon heating. The stability of MA after the enantiotropic transition temperature is II > III > I, which differs from the previously reported II > I > III.

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Abstract: Nature materials, such as bones and nacre, achieve excellent balance of toughness and strength via a hierarchical “brick-and-mortar” microstructure, which is an attractive model for engineering materials design. Here, we produced nacre-like Ti-Ta metallic composites via a powder metallurgy process, during which mixed powders were sintered by spark plasma sintering, followed by hot and cold rolling and then annealing. The structure consists of soft Ta-enriched regions and hard Ti-enriched regions in a hierarchical and laminated fashion. The microstructural heterogeneity spans several scales due to the diffusion between Ti and Ta. This yields a novel metal-metal composite with a balanced combination of strength and ductility (1226 MPa ultimate tensile strength and 20.8% elongation), outperforming most of conventional Ti based alloys and composites. Via the complementary in situ synchrotron X-ray diffraction and electron microscopies, it is found out that multiple micromechanisms are active, including nano-particle and dislocation localized strengthening as well as phase transformation induced plasticity. The manufacturing route developed here is versatile, capable of making high performance bio-mimic metallic composites.

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Abstract: Tb for Ca substituted hydroxyapatite ceramic samples with composition Ca10−xTbx(PO4)6(OH1−x/2−δ)2, where x = 0.1, 0.5, were synthesized by solid-state reaction at 1300 °C in air, and their crystal structure, vibrational spectra, luminescence, and magnetic properties were studied. Implanting Tb3+ into the calcium apatite crystal lattice results in formation of an effective TbO+ ion which displays a short terbium–oxygen bond of 2.15 Å and a stretching vibration at 534 cm−1. The Tb3+ electronic structure has been revealed by analyzing the luminescence spectra and dc/ac magnetization data. Accordingly, the ground state represents a pseudo doublet with MJ = ±6 and the first exited level is by 112 cm−1 higher in energy. The ion exhibits field induced magnetic bistability with the magnetization reversing over the first exited state. Three paths of magnetization relaxation with field-temperature controlled switching between the paths have been identified.