The coherency strain driving force for CIGM in non-cubic crystals: Comparison with in situ observations in calcite

Chemically induced grain boundary migration (CIGM) was observed in situ in calcite bicrystals in an optical microscope heat-stage. Three high-angle and two low-angle boundaries were observed. The solute was SrCO₃. Migration rates and solute concentrations were compared with boundary orientations. The coherency strain driving force was generalized for plane stress in any crystal system and applied to calcite. Because of the high anisotropy of lattice parameter expansion with Sr addition, a strong dependence of Sr concentration and migration driving force on crystallographic orientation was predicted. However, no correlation between SrCO₃ concentration and boundary orientation was observed, and little evidence for driving force dependence on orientation was found. Some evidence for a correlation between solute concentration and migration rate was found, but the nature of the correlation changed between bicrystals. Migration rates always decreased with time. Near-surface solute concentrations were depressed beneath values expected for stress-free equilibrium and plane stress modified equilibrium, but without obvious orientation dependence, which suggested that the coherency stress had a hydrostatic component. A PV driving force for migration might be present if solute diffusion fields around grain boundaries caused pressure gradients around the boundaries.     

Coupled Consolidation and Contaminant Transport in Compressible Porous Media

This paper presents an experimental and numerical investigation of coupled consolidation and contaminant transport in compressible porous media. Numerical simulations were performed using the CST2 computational model, in which a dual-Lagrangian framework is used to separately follow the motions of fluid and solid phases during consolidation. Diffusion and large strain consolidation-induced transport tests were conducted on composite specimens of kaolinite slurry consisting of an upper uncontaminated layer and a lower layer contaminated with potassium bromide. Assessment of the importance of the consolidation process on solute transport is based on measured and simulated solute breakthrough curves and final contaminant concentration profiles. CST2 simulations closely match the experimental data for three different loading conditions. Diffusion and consolidation-induced advection made important contributions to solute transport and mass outflow in this study. Additional simulations indicate that consolidation-induced contaminant transport may also be affected by specimen boundary drainage and initial concentration conditions.     

Effect of Rare-Earth Additions on the Texture of Wrought Magnesium Alloys: The Role of Grain Boundary Segregation

Magnesium alloys that contain certain rare-earth (RE) additions are known to have improved formability and this can be partly attributed to the different texture they display after recrystallization. Previous experimental work has identified segregation of RE to grain boundaries and dislocations as being potentially important in producing this change in behavior. In the present paper, two classical models (Langmuir–McClean and Cahn–Lücke–Stüwe) are used to explore the likely effect of RE additions on grain boundary solute concentration and drag. It is demonstrated that a wide range of RE elements are predicted to segregate strongly to grain boundaries due to the large atomic size misfit with magnesium. The maximum level of segregation is produced for elements such as Y or Gd that combine a high misfit and high bulk solubility. Segregated Y is predicted to produce a solute drag pressure on migrating boundaries several orders of magnitude greater than that obtained by Al or Zn additions. It is demonstrated that while this drag is predicted to be insufficient to strongly retard static recrystallization under typical annealing conditions, it is expected to suppress dynamic recrystallization by any mechanism requiring boundary migration.     

Investigation of Consolidation-Induced Solute Transport. II: Experimental and Numerical Results

This paper presents an experimental and numerical investigation of consolidation-induced solute transport. Diffusion and large strain consolidation tests were performed on composite specimens of kaolinite clay consisting of an upper uncontaminated layer and a lower layer contaminated with potassium bromide. Experimental measurements of effluent concentration, solute mass outflow, and final concentration profiles were obtained for a variety of initial, boundary, and loading conditions, including unload/reload. Numerical simulations were conducted using a computational model in which solute transport occurs by advection, dispersion, and sorption and is consistent with temporal and spatial variations of porosity and seepage velocity in the consolidating layer. Large strains were taken into account as well as variation of effective diffusion coefficient with porosity and nonlinear nonequilibrium sorption effects. The numerical simulations are in good to excellent agreement with the experimental measurements. Results indicate that, depending on conditions, diffusion and consolidation-induced advection can make important contributions to solute transport and mass outflow. Thus, both mechanisms should be considered for transport analyses involving soft contaminated clays undergoing large volume change. Results also indicate that nonequilibrium sorption effects were not significant for the materials and test conditions used in this study.     

GCr15轴承钢连铸冷却速率下凝固原位观察

GCr15轴承钢在连铸凝固过程中的组织生长与溶质偏析是碳化物液析的重要诱因,成为产品质量提升的关键.为此,针对国内某钢厂240 mm×240 mm GCr15轴承钢的连铸过程,选取方坯表面下方40、80和120 mm位置处的坯样为研究对象,首先建立二维凝固传热模型,结合红外测温试验,求解它们在糊状区的平均冷却速率,然后...     

Long-Term Tritium Transport through Field-Scale Compacted Soil Liner

A 13-year study of tritium transport through a field-scale earthen liner was conducted by the Illinois State Geological Survey to determine the long-term performance of compacted soil liners in limiting chemical transport. Two field-sampling procedures (pressure-vacuum lysimeter and core sampling) were used to determine the vertical tritium concentration profiles at different times and locations within the liner. Profiles determined by the two methods were similar and consistent. Analyses of the concentration profiles showed that the tritium concentration was relatively uniformly distributed horizontally at each sampling depth within the liner and thus there was no apparent preferential transport. A simple one-dimensional analytical solution to the advective–dispersive solute transport equation was used to model tritium transport through the liner. Modeling results showed that diffusion was the dominant contaminant transport mechanism. The measured tritium concentration profiles were accurately modeled with an effective diffusion coefficient of 6×10?4?mm2/s, which is in the middle of the range of values reported in the literature.     

A mass transport theory for interphase precipitation with application to vanadium steels

A mass transport theory is developed for interphase precipitation reactions in binary and pseudobinary systems where constant concentrations of solute are maintained at the interphase boundary. The theory presents exact solutions to the problem of diffusion-controlled growth involving the coupled motion of two boundaries. It is generally applicable to interphase pre-cipitation in both solid-solid and solid-liquid systems. Aspects of the theory are discussed with reference to the interphase precipitation reaction in model iron-carbon-vanadium alloys.     

Solute segregation and boundary structural change during grain growth

It is shown that the residual resistivity of dilute aluminum and copper alloys at liquid helium temperature systematically falls below that of the single crystal value during grain growth annealing. The resistivity undergoes a sudden upward perturbation at intermediate anneal times or temperatures, and again declines before finally returning to the single crystal value at very large grain sizes. Comparing the observed resistivity change with that attributable to the changing density of grain boundaries permits this electronic purification to be interpreted in terms of a redistribution of solute to and from migrating boundaries. Calculation of boundary solute content levels during the grain growth process suggests that the initial resistivity decline corresponds to an adsorption stage of segregation that terminates upon attainment of a common boundary solute density approximating that of full saturation. It is proposed that the observed resistivity perturbation occurs concomitantly with attainment of full saturation and reflects the onset of a grain boundary structural transformation. Support for this interpretation is provided by experimental evidence that grain boundaries in the same aluminum material can exist in different states having different electronic scattering probabilities and different calculated levels of solute at full adsorption. The results of this study suggest that existing models describing solute-influenced grain boundary phenomena may require revision to incorporate the contribution of boundary motion to segregate levels.     

Solute Transport through Unsaturated Soil due to Evaporation

A new method for predicting upward solute movement in unsaturated sand soil due to evaporation is developed and tested. Laboratory experiments were conducted in an unsaturated uniformly packed sand column with a cross section of 1.20 × 0.50 m2 and a constant shallow ground-water table. Evaporation was measured by a new ventilated chamber system. Solute movement from the ground water upward was monitored. Water and solute movement could be accurately reproduced by numerically solving Richards' equation and the convection-dispersion equation in one-dimension. The experimentally measured dispersivity for the unsaturated homogeneous sand agreed closely with the values which are available in the literature. This paper offers a new approach for investigating dispersion phenomena in unsaturated porous media exposed to evaporation.     

Sulfur segregation to grain boundaries in Ni3Al and Ni3(AI,Ti) alloys

The segregation of S to grain boundaries in Ni₃Al and Ni₃(Al, Ti) has been studied using Auger electron spectroscopy. The S concentration at the grain boundaries decreases more slowly with increasing temperature than would be predicted by segregation models based on a single solute binding energy to the grain boundaries. This behavior, which can be interpreted as an increase in the effective solute binding energy for a grain boundary as a function of temperature, is consistent with predictions of a model based on the existence of a spectrum of solute binding energies for grain boundaries. Formerly Professor and Head of the Department of Metallurgical Engineering, Michigan Technological University     

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.     

Radial Pore Diffusion with Nonuniform Intraparticle Porosities

Effects of radially dependent intraparticle pore sizes on solute fate and transport are examined for batch systems with spherical particles using a recently developed numerical model. The model can accommodate multiple particles distributed in size, mass transfer resistance at particle boundaries, intraparticle reversible sorption kinetics, and first-order decays. Two applications are examined. In the first application, random or deterministic intraparticle porosities across a spherical particle are considered. In the second application, multiple particles distributed in sizes with particle size-dependent intraparticle porosities are studied. Results from these applications indicate that concentration profiles are largely determined by interplays between B, η, and ε that incorporate the effects of intraparticle pore structures. Steady-state concentration values in both applications are determined by the volume-averaged intraparticle porosities. These results could be useful for understanding solute tailing behavior in natural porous media and the design of synthetic sorbents for treatment of contaminated waters.     

The stress corrosion susceptibility of a quenched and tempered 12 pct crmov martensitic stainless steel

The stress corrosion susceptibility of a martensitic 12 pct Cr 1 pct MoV stainless steel in alkaline chloride solution has been measured as a function of tempering heat treatment. The microstructures produced during tempering have been characterized by transmission electron microscopy and related to measured hardness values. In addition, scanning transmission electron microscopy combined with energy dispersive X-ray microanalysis has allowed the distribution of alloying elements within the microstructure to be examined. Electron energy loss spectroscopy was used to establish fully precipitate compositions, and the microanalysis results have been explained in terms of a diffusion controlled growth of grain boundary precipitates. The overall stress corrosion cracking susceptibility has been correlated with the development of chromium solute depletion profiles about prior austenite grain boundaries.     

Predicting Solute Flux through a Clay Membrane Barrier

Measured solute flux breakthrough curves (FBCs) from column tests performed on a semipermeable clay membrane subjected to KCl solutions are compared with predicted FBCs using independently measured flow and transport properties. The predicted FBCs are based on three scenarios: (1) Advective–dispersive transport that neglects membrane behavior; (2) advective–dispersive transport that accounts for the concentration dependency of the effective salt-diffusion coefficient (Ds?) resulting from membrane behavior, referred to as partially coupled transport; and (3) fully coupled transport that includes both the explicit coupling terms (e.g., hyperfiltration, chemico-osmosis) associated with clay membrane behavior and the concentration dependency of Ds?. The FBCs predicted by fully coupled transport agree best with the measured FBCs. However, for the diffusion-controlled conditions of the column tests, the steady-state solute fluxes predicted by partially coupled transport are only 23–69% higher than the measured steady-state fluxes. The results imply that the advective–dispersive transport theory can be used to provide reasonably accurate, albeit somewhat conservative, estimates of steady-state solute flux through clays that behave as semipermeable membranes, provided diffusion is a significant, if not dominant, solute transport process and the concentration dependency of Ds? are taken into account.     

Effects of shear flow and anisotropic kinetics on the morphological stability of a binary alloy

The effect of a parallel shear flow and anisotropic interface kinetics on the onset of instability during the directional solidification of a binary alloy at constant velocity is calculated. The model for anisotropy is based on the motion of steps. A shear flow (linear Couette flow or asymptotic suction profile), parallel to the crystal-melt interface in the same direction as the step motion, decreases interface stability in that the critical solute concentration decreases. A shear flow counter to the step motion enhances stability for small shear rates; for larger shear rates, the neutral curve develops a bimodal structure, and the critical solute concentration slowly decreases with shear rate.     

The effect of macroscopic solute diffusion in the liquid upon surface macrosegregation

An existing one-dimensional mathematical model that predicts the macrosegregation formation due to solidification shrinkage has been modified to also account for the macroscopic diffusion of solute in the liquid. It is shown both numerically and analytically that such solute diffusion has an almost negligible influence on the predicted solute profile, except in a very thin layer near the chill surface, where severe solute depletion develops. This layer is related to a discontinuity in the diffusive solute flux at the surface, and for a moderately cooled Al-4.5 pct Cu alloy, the layer thickness is of the order 100 μm. When the lever rule is imposed, the solute concentration at the surface becomes equal to the partition coefficient multiplied by the nominal alloy concentration, and the boundary layer solidifies completely once the temperature drops below the liquidus temperature of the (initial) melt. This indicates that assuming local thermodynamical equilibrium at a domain boundary when simultaneously accounting for the macroscale solute diffusion should be reconsidered in macrosegregation modeling.     

Effect of Sand Movement on a Cohesive Substrate

Flume experiments investigated the effect of mobile sand on the erosion of cohesive beds. The fluid-induced stress alone was not enough to cause erosion, and sand motion as bed load was needed. Erosion rates and suspended sediment concentration were found to increase with increasing sand transport and to decrease with increasing median grain size. The erosion rate was found to be at a maximum during saltation, intermediate during creep, and lowest during suspension.     

Solute-atom segregation at (002) twist boundaries in dilute NiPt alloys: Structural/chemical relations

Monte Carlo simulations, utilizing embedded atom method (EAM) potentials, are used to investigate solute-atom segregation behavior at symmetrical (002) twist boundaries, at T = 850 K, in Pt-3 at.% Ni and Ni-3 at.% Pt alloys. The results show that, unlike the previously investigated AuPt system, the boundaries are enhanced in solute atoms on both sides of the phase diagram. For low-angle boundaries on the Pt-rich side the atomic sites enhanced in solute concentration are arranged in hourglass-like structures centered on the square grid of primary grain boundary dislocations. While for the same boundaries on the Ni-rich side the atomic sites enhanced in solute concentration are located in bipyramidal regions based on the square cells of the same grain boundary dislocations. Thus, the atomic sites that are enhanced on one side of the phase diagram are not affected on the other side and vice versa.     

Mechanism for diffusion induced grain boundary migration

Grain boundaries are found to migrate under certain conditions when solute atoms are diffused along them. This phenomenon, termed diffusion induced grain boundary migration (DIGM), has now been found in six systems. The observed phenomenon and empirical data are used to discard certain concepts for the driving force and the mechanism. We propose a mechanism in which differences in the diffusion coefficients of the diffusing species along the grain boundary cause a self-sustaining climb of grain boundary dislocations and motion of their associated grain boundary steps.     

An AEM investigation of the formation of precipitate-free zones in an AI-16 Wt Pct Ag alloy: Determination of equilibrium and metastable solvus lines

Precipitation of the equilibrium y-phase at grain boundaries in an aged Al-16 wt pct (～4.5 at. pct) Ag alloy is responsible for the formation of a solute depleted region, free of metastable Guinier-Preston (GP) zones, in the immediate vicinity of the grain boundary. Solute concentration profiles have been obtained across these grain boundary regions using Analytical Electron Microscopy (AEM) in thin foils of the alloy aged at low temperatures. The Ag concentrations at the grain boundary (between γ precipitates) and at the edge of the precipitate-free zone have been used to determine the solid solubility of Ag in Al (the α/(α + γ) solvus) and the metastable GP zone solvus line, respectively.