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Hydrogen accommodation in Zr second phase particles: implications for H pick-up and hydriding of zircaloy-2 and zircaloy-4
No full textAb-initio computer simulations have been used to predict the energies associated with the accommodation of H atoms at interstitial sites in α, β-Zr and Zr–M intermetallics formed with common alloying additions (M = Cr, Fe, Ni). Intermetallics that relate to the Zr2(Ni,Fe) second phase particles (SPPs) found in Zircaloy-2 exhibit favourable solution enthalpies for H. The intermetallic phases that relate to the Zr(Cr,Fe)2 SPPs, found predominantly in Zircaloy-4, do not offer favourable sites for interstitial H. It is proposed that Zr(Cr,Fe)2 particles may act as bridges for the migration of H through the oxide layer, whilst the Zr2(Ni,Fe)-type particles will trap the migrating H until these are dissolved or fully oxidised. © 2012, Elsevier Ltd
Pressure-induced polyamorphism and formation of 'aragonitic' amorphous calcium carbonate
No full textAmorphous calcium carbonate (ACC) is a precursor to the crystalline phases of CaCO3, commonly found in the earliest stages of biomineral development and as one of the metastable states formed during the inorganic precipitation of calcium carbonate crystalline polymorphs.1 Its isotropic and hydrous moldable character allows many organisms to form very complex conformations of their shells or skeletons by taking advantage of these unique properties.2 At least two different phases of biogenic ACC have been described to date: a highly hydrated phase with one water molecule per CaCO3 unit, and an anhydrous phase that forms as a transient phase prior to crystallization to vaterite or calcite. Recently, the existence of polyamorphism (the existence of a substance in different amorphous modifications, akin to polymorphism in crystalline materials) in synthetic hydrated ACC has been suggested based mainly on X-ray absorption spectroscopy (XAS) and nuclear magnetic resonance (NMR) data that show different local structures of ACC precipitated from solutions at different pH values: calcite-like ACC is obtained at pH≈8.75 and vaterite-like ACC precipitates from solutions of pH≈9.8 and higher. In addition to these two amorphous polymorphs, other studies have shown hints of aragonite local order in ACC from shells of freshwater snails, based on XAS data that show Ca-O coordination numbers of approximately 9, the theoretical value of aragonite.These results were reproduced in synthetic samples of ACC doped with Mg2+, suggesting a role of this cation in the selection of the ACC amorphous polymorph. © 2013, Wiley-Vch Verlag
The validation of synthetic spectra used in the performance evaluation of radionuclide identifiers
No full textThis work has evaluated synthetic gamma-ray spectra created by the RASE sampler using experimental data. The RASE sampler resamples experimental data to create large data libraries which are subsequently available for use in evaluation of radionuclide identification algorithms. A statistical evaluation of the synthetic energy bins has shown the variation to follow a Poisson distribution identical to experimental data. The minimum amount of statistics required in each base spectrum to ensure the subsequent use of the base spectrum in the generation of statistically robust synthetic data was determined. A requirement that the simulated acquisition time of the synthetic spectra was not more than 4% of the acquisition time of the base spectrum was also determined. Further validation of RASE was undertaken using two different radionuclide identification algorithms. © 2013, Elsevier Ltd
Temperature dependence of electrical properties and crystal structure of 0.29Pb(In1/2Nb1/2)O-3-0.44Pb(Mg1/3Nb2/3)O-3-0.27PbTiO(3) single crystals
No full textWe characterized the temperature dependent ( ∼ 25–200°C) electromechanical properties and crystal structure of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals using in situ electrical measurement and neutron diffraction techniques. The results show that the poled crystal experiences an addition phase transition around 120°C whereas such a transition is absent in the unpoled crystal. It is also found that the polar order persists above the maximum dielectric permittivity temperature at which the crystal shows a well-defined antiferroelectric behavior. The changes in the electrical properties and underlying crystal structure are discussed in the paper. © 2013, Hindawi Publishing Corporation
A solution for estimating the tensile yield strength from small specimens
No full textThe small punch test is an innovative test that utilises small disc-shaped specimens to assess the mechanical behaviour of materials. The main advantage is the relatively small specimen size. In this article, a modified analytical solution for the small punch maximum bend strength is proposed that is based on classical plate theory. A clear linear relationship is observed between the tensile yield strength σYS and the small punch maximum bend strength σy for both alloys and metal matrix composites. © ASTM International
Neutron residual stress measurements in rails
No full textRails were among the first objects of study by neutron diffraction strain measurement and the first experiments were done as early as the late 1980s [1, 2]. This interest is easy to explain: the problem of rail fracturing is critical from the public safety point of view and the penetrating ability of neutrons suggested the possibility of breakthrough experiments and fast progress in this field. It was well-established that residual stresses, both near-surface and interior, played a signifi cant role in the development of defects which led to rail failure. This suggested three distinct approaches of neutron diffraction strain measurement that could contribute to various problems of the rail industry. The first method was to map the complete triaxial stress distribution non-destructively in the interior of an intact rail, ideally before and after significant service. Another approach was to use slices, for example to characterize how different processing methods produce favourable or detrimental stress distributions in rails. A third technique was to make non-destructive measurements, but in critical and not very deep portions of rails, for example, to examine defects and their relation to rail failure in the top running surface of rails, e.g. "white layer" formation. © 2013, Taylor & Francis
Indefinite media based on wire array metamaterials for the THz and Mid-IR
No full textFibre drawing is used to fabricate indefinite media based on wire array metamaterials. Fibres containing arrays of indium wires embedded in polymer are drawn using an optical fibre draw tower, a technique that is intrinsically scalable to larger-volume fabrication. During drawing, the surface tension of the liquid indium can result in fluctuations to the wire diameter through the Plateau–Rayleigh instability. This is investigated and minimised through a modification of the draw process to achieve wire diameters as low as 1 micrometer. Such wire array fibres are assembled and characterised as electric metamaterials through the resulting high-pass filtering behaviour. By controlling the draw ratio, the fibre drawing technique is shown to produce electric metamaterials over a wide range of frequencies, from the THz through to the edge of the mid-IR.© 2013 Wiley-Vch Verlag
An isotopic and modelling study of flow paths and storage in Quaternary calcarenite, SW Australia: implications for speleothem paleoclimate records
No full textWe investigated the distinctive shallow sub-surface hydrology of the southwest Western Australia (SWWA) dune calcarenite using observed rainfall and rainfall δ18O; soil moisture, cave drip rate and dripwater δ18O over a six-year period: August 2005–March 2012. A lumped parameter hydrological model is developed to describe water fluxes and drip δ18O. Comparison of observed data and model output allow us to assess the critical non-climatic karst hydrological processes that modify the precipitation δ18O signal and discuss the implications for speleothem paleoclimate records from this cave and those with a similar karst setting. Our findings include evidence of multiple reservoirs, characterised by distinct δ18O values and recharge responses (‘low’ and ‘high’ flow sites). Dripwaters exhibit δ18O variations in wet versus dry years at low-flow sites receiving diffuse seepage from the epikarst with an attenuated isotopic composition that approximates mean rainfall. Recharge from high-magnitude rain events is stored in a secondary reservoir which is associated with high-flow dripwater that is 1‰ lower than our monitored low-flow sites (δ18O). One drip site is characterised by mixed-flow behaviour and exhibits a non-linear threshold response after the cessation of drainage from a secondary reservoir following a record dry year (2006). Additionally, our results yield a better understanding of the vadose zone hydrology and dripwater characteristics in Quaternary age dune limestones. We show that flow to our monitored sites is dominated by diffuse flow with inferred transit times of less than one year. Diffuse flow appears to follow vertical preferential paths through the limestone reflecting differences in permeability and deep recharge into the host rock. © 2013, Elsevier Ltd
High-resolution strain mapping through time-of-flight neutron transmission diffraction
No full textThe spatial resolution of time of flight neutron transmission diffraction was recently improved by the extension of photon/electron counting technology to imaging of thermal and cold neutrons. The development of novel neutron sensitive microchannel plates enables neutron counting with spatial resolution of ~55 um and time-of-flight accuracy of ~1 us, with efficiency as high as 70% for cold and ~40% for thermal neutrons. The combination of such a high resolution detector with a pulsed collimated neuron beam provides the opportunity to obtain a 2-dimensional map of neutron transmission spectra in one measurement. The results of our neuron transmission measurements demonstrate that maps of strains integrated along the beam propagation direction can be obtained with ~100 microstrain accuracy and spatial resolution of ~100 um providing there are sufficient neutron events collected. In this paper we describe the capabilities of the MCP neutron counting detectors and present the experimental results of 2-dimensional strain maps within austenitic steel compact tension (CT) crack samples measured at the ENGIN-X beamline of the ISIS pulsed neutron source. © 2014, Trans Tech Publications
Molecular dynamics simulations of radiation damage in D0(19) Ti3Al intermetallic compound
No full textMolecular dynamics (MD) has been applied to simulate the radiation damage created in displacement cascades in D019 Ti3Al structural intermetallics. Collision cascades formed by the recoil of either Al or
Ti primary knock-on atoms (PKA) with energy EPKA = 5, 10, 15 or 20 keV were considered in Ti3Al single
crystals at T = 100, 300, 600 and 900 K. At least 24 different cascades for each (EPKA, T, PKA type) set were
simulated. A comprehensive treatment of the modelling results has been carried out. We have evaluated
the number of Frenkel pairs, fraction of Al and Ti vacancies, self-interstitial atoms and anti-sites as a function
of (EPKA; T, PKA type). Preferred formation of both Al vacancies and self-interstitial atoms in D019 Ti3Al exposed to irradiation has been detected.© 2013, Elsevier B.V