Numerical study of the impact of inoculant and grain transport on macrosegregation and microstructure formation during solidification of an Al-22%Cu alloy

We investigate the impact of the nucleation law for nucleation on Al-Ti-B inoculant particles, of the motion of inoculant particles and of the motion of grains on the predicted macrosegregation and microstructure in a grain-refined Al-22 wt.% Cu alloy casting. We conduct the study by numerical simulations of a casting experiment in a side-cooled 76×76×254 mm sand mould. Macrosegregation and microstructure formation are studied with a volume-averaged two-phase model accounting for macroscopic heat and solute transport, melt convection, and transport of inoculant particles and equiaxed grains. On the microscopic scale it accounts for nucleation on inoculant particles with a given size distribution (and corresponding activation undercooling distribution)and for the growth of globular solid grains. The growth kinetics is described by accounting for limited solute diffusion in both liquid and solid phases and for convective effects. We show that the consideration of a size distribution of the inoculants has a strong impact on the microstructure(final grain size) prediction. The transport of inoculants significantly increases the microstructure heterogeneities and the grain motion refines the microstructure and reduces the microstructure heterogeneities.

Enhancing the high temperature plasticity of a Cu-containing austenitic stainless steel through grain boundary strengthening

The addition of 3 wt% Cu to heat-resistant SUS 304H austenitic steel enhances its high temperature mechanical properties. To further improve the properties, particularly the creep resistance and ductility at high temperatures, a post-solutionizing heat-treatment method that involves an intermediated annealing either at 700 or 800 degrees C after solutionizing for durations up to 180 min was employed. The purpose this heat-treatment is to precipitate planar Cr23C6 at the grain boundaries, which results in the boundaries getting serrated. Detailed microstructural analyses of these `grain boundary engineered' alloys was conducted and their mechanical performance, both at room temperature and at 750 degrees C, was evaluated. While the grain size and texture are unaffected due to the high temperature hold, the volume fraction of Sigma 3 twin boundaries was found to increase significantly. While the strength enhancement was only marginal, the ductility was found to increase significantly, especially at high temperature. A marked increase in the creep resistance was also noted, which is attributed to the reduction of the grain boundary sliding by the grain boundary serrations and the suppression of grain boundary cavitation through the optimization of the volume fraction and spacing of the Cr23C6 precipitates. The special heat-treatment performed with holding time of 3 h at 700 degrees C resulted in the optimum combination of strength, ductility and creep resistance at high temperature. (C) 2014 Elsevier B.V. All rights reserved.

Accurate point matching based on multi-objective Genetic Algorithm for multi-sensor satellite imagery

This paper investigates a novel approach for point matching of multi-sensor satellite imagery. The feature (corner) points extracted using an improved version of the Harris Corner Detector (HCD) is matched using multi-objective optimization based on a Genetic Algorithm (GA). An objective switching approach to optimization that incorporates an angle criterion, distance condition and point matching condition in the multi-objective fitness function is applied to match corresponding corner-points between the reference image and the sensed image. The matched points obtained in this way are used to align the sensed image with a reference image by applying an affine transformation. From the results obtained, the performance of the image registration is evaluated and compared with existing methods, namely Nearest Neighbor-Random SAmple Consensus (NN-Ran-SAC) and multi-objective Discrete Particle Swarm Optimization (DPSO). From the performed experiments it can be concluded that the proposed approach is an accurate method for registration of multi-sensor satellite imagery. (C) 2014 Elsevier Inc. All rights reserved.

Upper Bounds on the Size of Grain-Correcting Codes

In this paper, we revisit the combinatorial error model of Mazumdar et al. that models errors in high-density magnetic recording caused by lack of knowledge of grain boundaries in the recording medium. We present new upper bounds on the cardinality/rate of binary block codes that correct errors within this model. All our bounds, except for one, are obtained using combinatorial arguments based on hypergraph fractional coverings. The exception is a bound derived via an information-theoretic argument. Our bounds significantly improve upon existing bounds from the prior literature.

The contribution of grain boundary sliding in tensile deformation of an ultrafine-grained aluminum alloy having high strength and high ductility

An as-cast Al-7 % Si alloy was processed by high-pressure torsion (HPT) for up to 10 turns at temperatures of 298 or 445 K. The HPT-processed samples had ultrafine-grained structures and they were tested in tension at room temperature at various strain rates in the range from 1.0 x 10(-4) to 1.0 x 10(-2) s(-1). The contributions of grain boundary sliding (GBS) to the total strain were measured directly using atomic force microscopy. Samples simultaneously showing both high strength and high ductility contained the highest fractions of high-angle grain boundaries (HAGB) and exhibited the highest contributions from GBS, whereas samples showing high strength but low ductility gave negligible values for the sliding contributions. It is concluded that high strength and high ductility require both an ultrafine grain size and a high fraction of HAGB.

Magnitude and Origin of Electrical Noise at Individual Grain Boundaries in Graphene

Grain boundaries (GBs) are undesired in large area layered 2D materials as they degrade the device quality and their electronic performance. Here we show that the grain boundaries in graphene which induce additional scattering of carriers in the conduction channel also act as an additional and strong source of electrical noise especially at the room temperature. From graphene field effect transistors consisting of single GB, we find that the electrical noise across the graphene GBs can be nearly 10 000 times larger than the noise from equivalent dimensions in single crystalline graphene. At high carrier densities (n), the noise magnitude across the GBs decreases as proportional to 1/n, suggesting Hooge-type mobility fluctuations, whereas at low n close to the Dirac point, the noise magnitude could be quantitatively described by the fluctuations in the number of propagating modes across the GB.

Exceptional resistance to grain growth in nanocrystalline CoCrFeNi high entropy alloy at high homologous temperatures

Nanocrystalline CoCrFeNi high entropy alloy, synthesized by mechanical alloying followed by spark plasma sintering, demonstrated extremely sluggish grain growth even at very high homologous temperature of 0.68 T-m (900 degrees C) for annealing duration of 600 h. Mechanically alloyed powder had carbon and oxygen as impurities, which in turn led to the formation of two-phase mixture of FCC and Cr-rich carbide with fine distribution of Cr-rich oxide during spark plasma sintering. Sluggish grain growth is attributed to the Zener pinning effect from the fine dispersion of oxide, mutual retardation of grain boundaries in the presence of two phases, and sluggish diffusivity because of cooperative diffusion of multi-principle elements. (C) 2015 Elsevier B.V. All rights reserved.

Dislocations And Twins In Nanocrystalline Ni After Severe Plastic Deformation: The Effects Of Grain Size

Deformation microstructures have been investigated in nanocrystalline (nc) Ni with grain sizes in the 50-100 nm range. It was found that deformation twinning started to occur in grains of similar to 90 nm, and its propensity increased with decreasing grain size. In most of the nc grains dislocations were observed as well, in the form of individual dislocations and dipoles. It is concluded that dislocation-mediated plasticity dominates for grain sizes in the upper half, i.e. 50-100 nm, of the nanocrystalline regime. (C) 2007 Published by Elsevier B.V.

Molecular dynamics simulation of interaction of a dislocation array from a crack tip with grain boundaries

The interaction of a dislocation array emitted from a crack tip under mode II loading with asymmetric tilt grain boundaries (GBs) is analysed by the molecular dynamics method. The GBs can generally be described by planar and linear matching zones and unmatching zones. All GBs are observed to emit dislocations. The GBs migrated easily due to their planar and linear matching structure and asymmetrical type. The diffusion induced by stress concentration is found to promote the GB migration. The transmissions of dislocations are either along the matched plane or along another plane depending on tilt angle theta. Alternate processes of stress concentration and stress relaxation take place ahead of the pileup. The stress concentration can be released either by transmission of dislocations, by atom diffusion along GBs, or by migration of GBs by formation of twinning bands. The simulated results also unequivocally demonstrate two processes, i.e. asymmetrical GBs evolving into symmetrical ones and unmatching zones evolving into matching ones during the loading process.

Mechanical modeling of grain boundary sliding of polycrystals

Using a variational method, a general three-dimensional solution to the problem of a sliding spherical inclusion embedded in an infinite anisotropic medium is presented in this paper. The inclusion itself is also a general anisotropic elastic medium. The interface is treated as a thin interface layer with interphase anisotropic properties. The displacements in the matrix and the inclusion are expressed as polynomial series of the cartesian coordinate components. Using the virtual work principle, a set of linear algebraic equations about unknown coefficients are obtained. Then the general sliding spherical inclusion problem is accurately solved. Based on this solution, a self-consistent method for sliding polycrystals is proposed. Combining this with a two-dimensional model of an aggregate polycrystal, a systematic analysis of the mechanical behaviour of sliding polycrystals is given in detail. Numerical results are given to show the significant effect of grain boundary sliding on the overall mechanical properties of aggregate polycrystals.

Dislocations and twins in nanocrystalline Ni after severe plastic deformation: the effects of grain size

Deformation microstructures have been investigated in nanocrystalline (nc) Ni with grain sizes in the 50-100 nm range. It was found that deformation twinning started to occur in grains of similar to 90 nm, and its propensity increased with decreasing grain size. In most of the nc grains dislocations were observed as well, in the form of individual dislocations and dipoles. It is concluded that dislocation-mediated plasticity dominates for grain sizes in the upper half, i.e. 50-100 nm, of the nanocrystalline regime. (C) 2007 Published by Elsevier B.V.

Framework for the Shape Optimization of Aerodynamic Profiles Using Genetic Algorithms

This study developed a framework for the shape optimization of aerodynamics profiles using computational fluid dynamics (CFD) and genetic algorithms. Agenetic algorithm code and a commercial CFD code were integrated to develop a CFD shape optimization tool. The results obtained demonstrated the effectiveness of the developed tool. The shape optimization of airfoils was studied using different strategies to demonstrate the capacity of this tool with different GA parameter combinations.

The Effects of Grain Size Distribution and Structure on Mechanical Behavior of Silt

Abstract: In order to investigate the effects of the grain size distribution and the micro-structure of soils on the mechani- cal characteristics, some static triaxial compression tests were carried out, and then the relationship of stress-strain and the strength behavior of silty sand were compared among undisturbed samples with different grain size distribution, undis- turbed and remolded samples with the same grain size distribution, and reconstituted samples (or called mixed samples) with different grain size distribution. The effects of grain size distribution and structure on the mechanic behavior of silty sands were mainly analyzed. It is shown that the obvious differences of the mechanical characteristics between undis- turbed soils and remolded soils are caused by the differences of soil structures. Although the grain size distribution are different between two soil samples, their mechanical characteristics may be close to each other, or may have obvious differences because of the effects of micro-structure.

Engineering nucleic acid mechanisms for regulation and readout of gene expression : conditional Dicer substrate formation and sensitive multiplexed northern blots

Nucleic acids are most commonly associated with the genetic code, transcription and gene expression. Recently, interest has grown in engineering nucleic acids for biological applications such as controlling or detecting gene expression. The natural presence and functionality of nucleic acids within living organisms coupled with their thermodynamic properties of base-pairing make them ideal for interfacing (and possibly altering) biological systems. We use engineered small conditional RNA or DNA (scRNA, scDNA, respectively) molecules to control and detect gene expression. Three novel systems are presented: two for conditional down-regulation of gene expression via RNA interference (RNAi) and a third system for simultaneous sensitive detection of multiple RNAs using labeled scRNAs.

RNAi is a powerful tool to study genetic circuits by knocking down a gene of interest. RNAi executes the logic: If gene Y is detected, silence gene Y. The fact that detection and silencing are restricted to the same gene means that RNAi is constitutively on. This poses a significant limitation when spatiotemporal control is needed. In this work, we engineered small nucleic acid molecules that execute the logic: If mRNA X is detected, form a Dicer substrate that targets independent mRNA Y for silencing. This is a step towards implementing the logic of conditional RNAi: If gene X is detected, silence gene Y. We use scRNAs and scDNAs to engineer signal transduction cascades that produce an RNAi effector molecule in response to hybridization to a nucleic acid target X. The first mechanism is solely based on hybridization cascades and uses scRNAs to produce a double-stranded RNA (dsRNA) Dicer substrate against target gene Y. The second mechanism is based on hybridization of scDNAs to detect a nucleic acid target and produce a template for transcription of a short hairpin RNA (shRNA) Dicer substrate against target gene Y. Test-tube studies for both mechanisms demonstrate that the output Dicer substrate is produced predominantly in the presence of a correct input target and is cleaved by Dicer to produce a small interfering RNA (siRNA). Both output products can lead to gene knockdown in tissue culture. To date, signal transduction is not observed in cells; possible reasons are explored.

Signal transduction cascades are composed of multiple scRNAs (or scDNAs). The need to study multiple molecules simultaneously has motivated the development of a highly sensitive method for multiplexed northern blots. The core technology of our system is the utilization of a hybridization chain reaction (HCR) of scRNAs as the detection signal for a northern blot. To achieve multiplexing (simultaneous detection of multiple genes), we use fluorescently tagged scRNAs. Moreover, by using radioactive labeling of scRNAs, the system exhibits a five-fold increase, compared to the literature, in detection sensitivity. Sensitive multiplexed northern blot detection provides an avenue for exploring the fate of scRNAs and scDNAs in tissue culture.