A 3D Granular Model Of Equiaxed-Granular Solidification

The solidification of an aluminum-copper alloy has been simulated in 3D using a granular model. Compared to previous similar 2D approaches for where only one phase is continuous, the extension to 3D allows for concurrent continuity of the solid and liquid phases. This concurrent continuity is a key factor in the formation of the solidification defect known as hot tearing. In this 3D model, grains are modeled as polyhedrons based on a Voronoi tessellation of a pseudo-random set of nucleation centers. Solidification within each polyhedron is calculated using a back-diffusion model. By performing a series of simulations over a range of grain sizes and cooling rates, the percolation of the solid grains is determined. The results, which indicate that the grain size and cooling rates play an important role in hot tear formation, constitute a basis on which feeding and deformation calculations will be carried out further

Three-dimensional phase-field simulation of micropore formation during solidification: Morphological analysis and pinching effect

A three-dimensional (3-D) multiphase-field model has been developed in order to study the formation of a micropore constrained to grow in a solid network (i.e. pinching effect). The model accounts for the pressure difference due to capillarity between liquid and gas, the equilibrium condition at triple (solid-liquid-pore) lines, and the partitioning and diffusion of dissolved gases such as hydrogen. From the predicted 3-D morphology of the pore, entities such as the interfacial shape distribution are plotted and analyzed. It is shown that the mean curvature of the pore-liquid surface, and thus also the pressure inside the pore, is uniform. The results are then compared with analytical pinching models. While predicting a similar trend, analytical models tend to underestimate the pore curvature at high solid fractions. Despite the complex morphology of pores reconstructed using high-resolution X-ray tomography, the present phase-field results suggest that a simple pinching model based on a spherical tip growing in between remaining liquid channels is a fairly good approximation. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Phase Field Modelling Of Twinned Dendrite Growth

Twinned dendrite growth has been found to occur in aluminum alloys when critical thermal conditions (G approximate to 100 K/cm, nu(s) approximate to 1 mm/s) and a slight convection in the melt are present during directional solidification. Split in their trunk by a coherent (111) twin plane, such dendrites grow along directions with a complex branch structure of , but also sometimes secondary arms. To explain the twinned dendrite growth kinetics advantage, Eady and Hogan suggested that the Young Laplace equation involving the solid-liquid interfacial energy gamma(sl) and the twin energy gamma(t) at the triple junction stabilizes a grooved tip(1). Wood et al proposed instead that torque terms associated with the anisotropy of gamma(sl) stabilize a sharp pointed tip(2). Finally, Henry suggested the possibility of the existence of a doublon, initiated precisely by a grooved tip, that would evolve depending on the solute content(3). In a recent experimental work, we have shown that the doublon conjecture is probably not valid for high solute content aluminum alloys, whereas it could be valid at low, composition. In the present work, the twinned dendrite tip morphology and growth kinetics have been investigated using a 3D phase field method implemented on a massively parallel computer. The twin boundary energy has been imposed via an appropriate boundary condition fixing the angle of the phase field gradient with respect to the boundary. Besides this angle, various experimental conditions such as thermal gradient, gradient direction, velocity of the isotherms and compositions have been investigated. The growth kinetics obtained under such conditions has been compared with that of regular dendrites

Study of the twinned dendrite tip shape I : Phase-field modeling

The growth kinetics advantage of twinned aluminum dendrites over regular ones is still an unsolved problem of solidification. Although it is linked to the tip geometry, the influence of a coherent (1 1 1) twin plane on a twinned dendrite tip is unclear, despite several past experimental observations. In the present contribution, a three-dimensional phase field model implemented on a cluster of parallel computers has been used to simulate the growth of a twinned dendrite under various directional solidification conditions. Only half a dendrite was modeled by replacing the coherent twin plane by a boundary with an appropriate condition on the phase parameter that is equivalent to the Young-Laplace equilibrium condition along the triple line between twinned solid, untwinned solid and liquid. It is found that the small liquid cusp present at the tip rapidly evolves into a doublon-type morphology, i.e. a dendrite split in its center by a deep and thin liquid pool with the triple line at the root. At high growth rates, the two sides of the doublon tend to coalescence and form small isolated liquid droplets. The positive concentration gradient near the doublon root appears to be rapidly smeared out by back-diffusion in the solid, thus making difficult its quantification through experimental methods. These simulation results are correlated with new experimental evidence presented in a companion paper. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Application of Inverse Methods to the Estimation of Boundary Conditions and Properties

Inverse methods can be used in solidification and related processes for the estimation of boundary conditions or physical properties of materials. For heat flow problems, these methods are based upon a minimisation of the errors between calculated and measured temperatures at given locations and times of the space-time domain, the calculated values being obtained from a numerical solution of the heat flow equation. In the present case, a maximum a posteriori technique has been implemented into a finite element code. This method is then applied to several situations for the determination of : i) the time-dependent heat-transfer coefficient at the surface of a steel rod which has been water-cooled after induction heating (non-stationary situation) ; ii) the space-dependent heat flow at the surface of a direct chill cast aluminium slab (stationary situation) ; and iii) the temperature-dependent thermal conductivity of aluminium-silicon alloys. In this latter case, the influence of the silicon concentration on the thermal conductivity is clearly revealed

Substrate-based atom waveguide using guided two-color evanescent light fields

We propose a dipole-force linear waveguide which confines neutral atoms up to lambda/2 above a microfabricated single-mode dielectric optical guide. The optical guide carries far blue-detuned light in the horizontally-polarized TE mode and far red-detuned light in the vertically-polarized TM mode, with both modes close to optical cut-off. A trapping minimum in the transverse plane is formed above the optical guide due to the differing evanescent decay lengths of the two modes. This design allows manufacture of mechanically stable atom-optical elements on a substrate. We calculate the full vector bound modes for an arbitrary guide shape using two-dimensional non-uniform finite elements in the frequency-domain, allowing us to optimize atom waveguide properties. We find that a rectangular optical guide of 0.8um by 0.2um carrying 6mW of total laser power (detuning +-15nm about the D2 line) gives a trap depth of 200uK for cesium atoms (m_F = 0), transverse oscillation frequencies of f_x = 40kHz and f_y = 160kHz, collection area ~ 1um^2 and coherence time of 9ms. We discuss the effects of non-zero m_F, surface interactions, heating rate, the substrate refractive index, and the limits on waveguide bending radius.Comment: 12 pages, 4 figures, revtex, submitted to Phys. Rev. A Replaced: final version accepted by PRA v.61 Feb 2000. (2 paragraphs added

Observation of radiation pressure exerted by evanescent waves

We report a direct observation of radiation pressure, exerted on cold rubidium atoms while bouncing on an evanescent-wave atom mirror. We analyze the radiation pressure by imaging the motion of the atoms after the bounce. The number of absorbed photons is measured for laser detunings ranging from {190 MHz} to {1.4 GHz} and for angles from {0.9 mrad} to {24 mrad} above the critical angle of total internal reflection. Depending on these settings, we find velocity changes parallel with the mirror surface, ranging from 1 to {18 cm/s}. This corresponds to 2 to 31 photon recoils per atom. These results are independent of the evanescent-wave optical power.Comment: 6 pages, 4 figure

Power Test of the First Two HL-LHC Insertion Quadrupole Magnets Built at CERN

The High-Luminosity project (HL-LHC) of the CERN Large Hadron Collider (LHC), requires low β* quadrupole magnets in Nb$_3$Sn technology that will be installed on each side of the ATLAS and CMS experiments. After a successful shortmodel magnet manufacture and test campaign, the project has advanced with the production, assembly, and test of full-size 7.15- m-long magnets. In the last two years, two CERN-built prototypes (MQXFBP1 and MQXFBP2) have been tested and magnetically measured at the CERN SM18 test facility. These are the longest accelerator magnets based on Nb$_3$Sn technology built and tested to date. In this paper, we present the test and analysis results of these two magnets, with emphasis on quenches and training, voltage-current measurements and the quench localization with voltage taps and a new quench antenna

Rhizoctonia solani AG8: New breakthroughs in control and management

The next generation of control options for Rhizoctonia solani (AG8), the causal agent of Rhizoctonia root rot, is in-furrow liquid injection. The efficacy of banding two new fungicides, Uniform® and EverGol® Prime, was evaluated as an alternative to seed treatments across three years of trials conducted in Western Australia (WA) by DAFWA and in South Australia (SA) by SARDI

Dual-source computed tomography in patients with acute chest pain: feasibility and image quality

The aim of this study was to determine the feasibility and image quality of dual-source computed tomography angiography (DSCTA) in patients with acute chest pain for the assessment of the lung, thoracic aorta, and for pulmonary and coronary arteries. Sixty consecutive patients (32 female, 28 male, mean age 58.1±16.3 years) with acute chest pain underwent contrast-enhanced electrocardiography-gated DSCTA without prior beta-blocker administration. Vessel attenuation of different thoracic vascular territories was measured, and image quality was semi-quantitatively analyzed by two independent readers. Image quality of the thoracic aorta was diagnostic in all 60 patients, image quality of pulmonary arteries was diagnostic in 59, and image quality of coronary arteries was diagnostic in 58 patients. Pairwise intraindividual comparisons of attenuation values were small and ranged between 1±6 HU comparing right and left coronary artery and 56±9 HU comparing the pulmonary trunk and left ventricle. Mean attenuation was 291±65 HU in the ascending aorta, 334±93 HU in the pulmonary trunk, and 285±66 HU and 268±67 HU in the right and left coronary artery, respectively. DSCTA is feasible and provides diagnostic image quality of the thoracic aorta, pulmonary and coronary arteries in patients with acute chest pain