Centrifuge Modeling of Unstable Infiltration and Solute Transport

Unstable flow can result in the formation of fingers during infiltration into dry soil. Centrifuge modeling is a potentially useful tool to study the relationship between finger size, spacing, and velocity. It can also be used to investigate solute transport in such fingers. Physical properties of the fingers are obtained for three tests conducted at elevated acceleration levels. A fourth test was conducted at 1g. The physical parameters compare well with theoretical predictions. To assess solute transport in fingers, a known concentration of solute was introduced after the fingers had formed. The resulting breakthrough curves were analyzed using the two-region model as well as the advection dispersion equation with appropriate initial and boundary conditions. Although the two-region model is physically more plausible, it was found to match the extensive tailing observed in the breakthrough curves only marginally better than the advection-dispersion equation.     

Precipitate Redistribution during Creep of Alloy 617

Nickel-based superalloys are being considered for applications within advanced nuclear power generation systems due to their high-temperature strength and corrosion resistance. Alloy 617, a candidate for use in heat exchangers, derives its strength from both solid solution strengthening and the precipitation of carbide particles. However, during creep, carbides that are supposed to retard grain boundary motion are found to dissolve and reprecipitate on boundaries in tension. To quantify the redistribution, we have used electron backscatter diffraction (EBSD) and energy-dispersive spectroscopy (EDS) to analyze the microstructure of 617 after creep testing at 900 °C and 1000 °C. The data were analyzed with respect to the location of the carbides (e.g., intergranular vs intragranular), grain boundary character, and precipitate type (i.e., Cr rich or Mo rich). We find that grain boundary character is the most important factor in carbide distribution; some evidence of preferential distribution to boundaries in tension is also observed at higher applied stresses. Finally, the results suggest that the observed redistribution is due to the migration of carbides to the boundaries and not the migration of boundaries to the precipitates.     

Floating Grain Formation and Macrosegregation in a 2024 Al Alloy Prepared by Hot-Top DC Casting with a 2024 Al Alloy Insert

For the first time, the method of inserting a 2024 Al alloy rod into a melt of the same alloy during hot-top direct chill casting was used to verify the origin of floating grains and changes in the shape of the sump. Based on the as-cast structure and measured temperature distribution observed, we found that the 2024 Al alloy insert and the grains formed from melt surfaces and the interior walls of the launder or hot-top melted in the high-temperature region of the upper part of the hot-top; additionally, most of the dendrite fragments were captured by feathery grains. We confirmed that the nuclei in the center part of the liquidus isotherm region were able to form coarse-cell dendritic structures in the center part of the slurry zone due to the broad vertical width of this zone. Then, the coarse dendritic structures settled in the mushy zone, and fine dendrites formed in the periphery. In addition, with the insertion of the 2024 Al rod, a hillock-shaped liquidus isotherm was formed in the center part of the slurry zone, and the size of the floating grains in the billet processed with the insert was larger than that in the billet processed without the insert. However, surprisingly, the application of the 2024 Al rod reduced negative centerline segregation because it reduced the horizontal component of shrinkage flow.

     

Solute Transport Modeling in Overland Flow Applied to Fertigation

A model of solute transport in overland flow is developed and applied to the simulation of surface fertigation. Water flow is simulated using the depth-averaged, 1D shallow water equations. Solute flow is represented by an advection-diffusion model. The resulting set of three partial differential equations is sequentially solved at each time step. First, water flow is computed using the explicit two-step McCormack method. Based on the obtained velocity field, solute transport is explicitly determined from the advection-diffusion equation using the operator split technique. Four field experiments involving fertigation events on an impervious free-draining border were performed to validate the proposed model and to obtain estimates of Kx, the longitudinal dispersion coefficient. A value of Kx = 0.075 m2 s?1 satisfactorily reproduces the field experiments. The model is also applied to the simulation of a fertigation event on a pervious border. A sensitivity analysis is performed to assess the dependence of fertilizer distribution uniformity on the value of Kx. Finally, the proposed model is compared with a previous model based on pure advection.     

Characterization and Modeling of Precipitation Kinetics in a Fe-Si-Ti Alloy

In this article, we present a systematic study of precipitation kinetics in a Fe-Si-Ti alloy in the temperature range 723?K to 853?K (450?°C to 580?°C), combining complementary tools (transmission electron microscopy (TEM), atom probe tomography (APT), and small-angle neutron scattering (SANS)). We show that the Heusler phase Fe₂SiTi dominates the precipitation process in the investigated time and temperature range, regardless of the details of the initial temperature history. A numerical model based on the evolution of precipitate size classes gives very good agreement with the experimental results, and its application to different alloy compositions provides directions for future alloy optimization.     

铜基合金中溶质元素的Lnr0i的模型计算

采用自由体积理论和Ｍｉｅｄｅｍａ生成热模型，导出了液态合金中溶质元素的ｌｎｒｉ０的计算公式；计算了１２７３Ｋ下铜液中溶质元素的ｌｎｒｉ０。结果表明，计算值和实验值的正负号一致率为９０％，并基本处于同一数量级。     

Microstructure and Cyclic Deformation Behavior of a Friction-Stir-Welded 7075 Al Alloy

Microstructural changes and cyclic deformation characteristics of friction-stir-welded 7075 Al alloy were evaluated. Friction stir welding (FSW) resulted in significant grain refinement and dissolution of η′ (Mg(Zn,Al,Cu)₂) precipitates in the nugget zone (NZ), but Mg₃Cr₂Al₁₈ dispersoids remained nearly unchanged. In the thermomechanically affected zone (TMAZ), a high density of dislocations was observed and some dislocations were pinned, exhibiting a characteristic Orowan mechanism of dislocation bowing. Two low-hardness zones (LHZs) between the TMAZ and the heat-affected zone (HAZ) were observed, with the width decreasing with increasing welding speed. Cyclic hardening and fatigue life increased with increasing welding speed from 100 to 400 mm/min, but were only weakly dependent on the rotational rate between 800 and 1200 rpm. The cyclic hardening of the friction-stir-welded joints exhibiting a two-stage character was significantly stronger than that of the base metal (BM) and the energy dissipated per cycle decreased with decreasing strain amplitude and increasing number of cycles. Fatigue failure occurred in the LHZs at a lower welding speed and in the NZ at a higher welding speed. Fatigue cracks initiated from the specimen surface or near-surface defects in the friction-stir-welded joints, and the initiation site exhibited characteristic intergranular cracking. Crack propagation was characterized by typical fatigue striations along with secondary cracks.     

半固态铝合金的制备工艺研究

采用自制的电磁搅拌装置，制备出具有球形初生相微粒的半固态Ａｌ６．６％Ｓｉ合金。研究了冷却速率和搅拌强度对半固态合金的球形初生相微粒形成的影响。球形初生相颗粒的生成必须具有适当的冷却速率，而高的磁感应强度则有利于初生相颗粒的细化、球化和均匀化。提出了球形初生相微粒演化机理，认为剧烈的搅拌使初生相在长大过程中，不断发生枝晶臂的弯曲融合、熔断和机械断裂。     

Depth-Dependent Dispersion Coefficient for Modeling of Vertical Solute Exchange in a Lake Bed under Surface Waves

Variable pressure at the sediment/water interface due to surface water waves can drive advective flows into or out of the lake bed, thereby enhancing solute transfer between lake water and pore water in the lake bed. To quantify this advective transfer, the two-dimensional (2D) advection-dispersion equation in a lake bed has been solved with spatially and temporally variable pressure at the bed surface. This problem scales with two dimensionless parameters: a “dimensionless wave speed” (W) and a “relative dispersivity” (λ). Solutions of the 2D problem were used to determine a depth-dependent “vertically enhanced dispersion coefficient” (DE) that can be used in a 1D pore-water quality model which in turn can be easily coupled with a lake water quality model. Results of this study include a relationship between DE and the depth below the bed surface for W>50 and λ ? 0.1. The computational results are compared and validated against a set of laboratory measurements. An application shows that surface waves may increase the sediment oxygen uptake rate in a lake by two orders of magnitude.     

Solute Diffusion Characteristics of a Rapid Hardening Al-Cu-Mg Alloy during the Early Stages of Age Hardening

The heat-transfer characteristics of a rapid hardening Al-1.1Cu-1.7Mg at. pct alloy during the early stages of age hardening have been tested, both by experiment and finite element modeling, for a typical laboratory-scale sample, and subsequently a maximum diffusion distance by random walk has been calculated for the solute atom species. It is found that due to small diffusion distances compared to the average dislocation loop interspacing, the dislocation-locking hardness mechanism is not likely, and rather, the cluster hardening model is more accurate.     

Modeling Al enrichment in galvanized coatings

Aluminum enrichment in galvanized coatings was shown to be due to the formation of an inhibition layer, consisting of Fe₂Al₅,at the substrate/coating interface. The formation of the inhibition layer is a two-stage process. The first stage, associated with a high rate of Al uptake, is nucleation controlled, and the successive stage is diffusional growth controlled. The critical nucleus size is approximately one molecule of the compound Fe₂Al₅, and the energy barrier for the heterogeneous nucleation is 0.94 eV. Aluminum uptake increases with increasing strip-entry temperature and thickness, because both work to increase the effective temperature for the nucleation and growth processes. A model was proposed in which Al enrichment was shown as a function of bath Al content, bath temperature, strip-entry temperature, strip thickness, immersion time, and coating weight. The model is in good agreement with experimental results available in the open literature.     

Modeling of Cell/Dendrite Transition During Directional Solidification of TiAl Alloy Using Cellular Automaton Method

 Solute diffusion controlled solidification model was used to simulate the initial stage cellular to dendrite transition of Ti44Al alloys during directional solidification at different velocities. The simulation results show that during this process, a mixed structure composed of cells and dendrites was observed,where secondary dendrites are absent at facing surface with parallel closely spaced dendrites, which agrees with the previous experimental observation. The dendrite spacings are larger than cellular spacings at a given rate, and the columnar grain spacing sharply increases to a maximum as solidification advance to coexistence zone. In addition, simulation also revealed that decreasing the numbers of the seed causes the trend of unstable dendrite transition to increase. Finally, the main influence factors affecting cell/dendrite transition were analyzed, which could be the change of growth rates resulting in slight fluctuations of liquid composition occurred at growth front. The simulation results are in reasonable agreement with the results of previous theoretical models and experimental observation at low cooling rates.     

Effect of Solute Contents of Al-Ni-Sr Alloy on Glass Forming Ability and Thermal Stability

Glass forming ability (GFA) and structural characterization of a new amorphous Al-Ni-Sr alloy melt spun were investigated on the theory of the factors for the glass formation. The GFA was varied with the ratio of Ni to Sr in detail and successful amorphous phase dispersed with 2 ～ 5 nm precipitates was prepared from Al-Ni5-Sr3. Thermal stability and crystallization behavior of the amorphous phase were determined by continuous and isothermal heating technique on DCS, resulting that the crystallization occurs by the growth of fcc Al nanocrystals on the preexistent nuclei.     

Modeling Uncoupled Solute Transport in Natural Organic Matter Size Fractionation by Ultrafiltration

Physicochemical separation of organic macrosolutes and colloidal particles is routinely required during the analysis of natural, waste, and process waters derived from aquatic and terrestrial environmental samples. This study was conducted to demonstrate the utility of a two-parameter nonlinear permeation coefficient model (PCM) in describing the uncoupled transport of solutes in dilute heterogeneous solutions subjected to batch ultrafiltration (UF). The PCM was used in conjunction with natural organic matter (NOM) permeate data for a natural water and six hydrophobic and hydrophilic subfractions to determine permeation coefficients p and NOM concentrations Cr0 with apparent molecular weight less than membrane specific cutoff values of moderately hydrophilic YC/YM series Amicon? UF membranes. Experimentally measured permeation coefficients p determined for the whole water were found to correlate well with composite permeation coefficients p? calculated using a mass-fraction weighted average of individual NOM subfraction permeation coefficient values. Correlation of experimentally measured and calculated permeation coefficient values (p and p?) indicated that the PCM can adequately describe uncoupled transport of chemically distinct solute fractions during batch UF of heterogeneous dilute solutions.