Coarse Zones Research Articles

Correlation between diffusion coefficient values of chloride ions obtained through column and ion migration tests in cementitious matrices with varying contents of silica fume and mortar

Abstract Corrosion is one of the main phenomena that lead to pathological manifestations in reinforced concrete structures under aggressive environments. with the chloride ion being the most responsible for its occurrence. In this way, understanding the transport mechanisms of this ion through the microstructure of the concrete is of fundamental importance to prevent or delay the penetration of these aggressive agents to guarantee a durable structure. In the literature, there are extensive studies concerning the diffusion of chlorides in concrete and the influence of pozzolanic additions in this mechanism. However, only a few correlate the different methods of analysis. This work aims to determine the chloride ion diffusion coefficients in concrete containing various levels of silica fume (5%, 10%, and 15%) or varying the mortar content (54%, 80%, and 100%), and compares the results obtained through column tests and chloride migration tests. It was observed that, although the techniques used were quite distinct, the diffusion values obtained were similar, contributing to the validation of both techniques. Furthermore, the variation in the mortar ratio causes a reduction in the interfacial transition zone of coarse aggregate/mortars and an increase in the content of aluminates, which promotes a similar effect to the use of silica fume.

Pore-Scale Investigation of Dynamic Immiscible Displacement in Layered Media using Synchrotron X-ray Microtomography.

Understanding the dynamics of immiscible fluid in a porous media is critical in many chemical and environmental engineering processes. However, the geological heterogeneity effect on multiphase flow behavior remains unclear. Here, the dynamics of immiscible fluid displacement and entrapment were experimentally demonstrated at pore-level using time-lapse synchrotron X-ray microtomography. A drainage-imbibition experiment was designed using an unconsolidated layered sand pack that comprised coarse sand and fine sand zones. There were significant differences between the two zones, with regard to the temporal variations in fluid saturation and morphological evolution of nonwetting fluid (oil) during imbibition. Highly connected oil clusters in the coarse zone broke up into many small fragments, whereas the cluster in the fine zone remained connected while spanning multiple pores. To further understand the impacts of pore size and connectivity on multiphase fluid dynamics, a new approach that tracks the temporal variation of immiscible fluid in individual pores was conducted. The surface area at the oil-water interface increased during imbibition, which is expected to facilitate mass transfer and surface interactions. Understanding immiscible fluid displacement in layered porous media at the pore-level could lead to more effective environmental remediation.

Effects of Stabilizing Heat Treatment on Properties of Simulated Microstructures in the Heat-Affected Zone of 347H Stainless Steel

In this paper, two kinds of heat affected zone (HAZ) simulation structures of 347H stainless steel, which are coarse grain zone (CGZ) and unmixed zone (UZ), were prepared by thermal simulator. The material properties of toughness, reheat crack susceptibility and intergranular corrosion susceptibility of the two kinds of HAZ simulation structures were studied by impact test, high temperature tensile test, electrochemical potentiodynamic reactivation (EPR) test and micro morphology test. The result shows that CGHAZ had better toughness. But after the stabilizing heat treatment, it was weakened while that of the UZ was enhanced. The reheat crack susceptibility of the CGZ and UZ both increases after stabilization heat treatment, and the tendency of the UZ are more obvious. Stabilizing heat treatment has a certain effect on the prevention of sensitization process, which can improve the intergranular corrosion resistance of the material. Stabilizing heat treatment is double-edged to 347H HAZ, and it needs combined with the specific situation to used.

High Bending Strength Hypereutectic Al-22Si-0.2Fe-0.1Cu-Re Alloy Fabricated by Selective Laser Melting

The objective of the study is to investigate the corresponding microstructure and mechanical properties, especially bending strength, of the hypereutectic Al-Si alloy processed by selective laser melting (SLM). Almost dense Al-22Si-0.2Fe-0.1Cu-Re alloy is fabricated from a novel type of powder materials with optimized processing parameters. Phase analysis of such Al-22Si-0.2Fe-0.1Cu-Re alloy shows that the solubility of Si in Al matrix increases significantly. The fine microstructure can be observed, divided into three zones: fine zones, coarse zones, and heat-affected zones (HAZs). Fine zones are directly generated from the liquid phase with the characteristic of petaloid structures and bulk Al-Si eutectic. Due to the fine microstructure induced by the rapid cooling rate of SLM, the primary silicon presents a minimum average size of ~0.5 μm in fine zones, significantly smaller than that in the conventional produced hypereutectic samples. Moreover, the maximum value of Vickers hardness reaches ~170 HV0.2, and bending strength increases to 687.70 MPa for the as-built Al-22Si-0.2Fe-0.1Cu-Re alloys parts, which is much higher than that of cast counterparts. The formation mechanism of this fine microstructure and the enhancement reasons of bending strength are also discussed.

The influence of porous media heterogeneity on smouldering remediation

Self-sustained Treatment for Active Remediation (STAR) is a thermal remediation technology that uses smouldering, a flameless form of combustion, for destroying organic contaminants in soil. Injected cold air flowing through the soil to the treatment zone supports the release of sufficient energy to maintain a self-sustained reaction and the propagation of the reaction through the contaminated zone as long as the airflow local to the reaction exceeds a minimum value. However, the distribution and magnitude of air flux vectors can be complex in the heterogeneous environment common at contaminated sites. This research presents the first investigation of smouldering remediation under varying degrees and patterns of permeability heterogeneity. Nine experiments examined smouldering remediation in contaminated layers of varying permeability arranged alone and in contrasting layers in series, in parallel, and in two distinct complex patterns. The results suggest that smouldering can successfully propagate across layer boundaries and through layers in series regardless of their permeability (at least down to 1 x 10−12 m2). However, fine layers were not smouldered for layers in parallel with a permeability ratio ≥ 3:1. Numerical modelling of these cases with a published smouldering model revealed that this occurred due to insufficient airflow in the fine layers in some cases, or conductive heat transfer (thermal coupling) between parallel layers in other cases. The more complex heterogeneity patterns underscored the importance of the connected length of the higher permeability pathway on airflow distribution and therefore on smouldering propagation. Disconnected coarse zones supported smouldering in both coarse and fine zones while connected coarse zones kept smouldering in the coarse pathway while bypassing fine zones. Overall, this research provides unique insights into understanding heterogeneous scenarios to ensure the successful application of smouldering remediation.

Research on deposited tracks and microstructures of AlSi10Mg alloy produced by selective laser melting

The selective laser melting (SLM) process parameters significantly affect the bonding of melting powders and the substrate or deposited layer, and the microstructure of end-use components. In this study, single track, double track and cubic sample SLM experiments were carried out to investigate the effect of process parameters on the surface morphology of SLM-fabricated AlSi10Mg alloy. The hierarchical microstructures discriminated by the Si phase are observed in SLM-processed AlSi10Mg samples. The formation mechanism of the hierarchical microstructures is elucidated. The formula proved that the solidification rate (R) increases gradually from the boundary to the center of the melt pool. Coarse zones are formed by the instantaneous existence of an extremely high ratio of thermal gradient (G) and solidification rate at the melt pool boundary, where solidification microstructure grows planar. With the heat propagating, a gradual change of the G/R ratio appears and the microstructure turns to columnar-dendritic growth, creating the fine zones.

Research on the limitations of laser energy density and microstructure characteristics of selective laser melted AlSi10Mg alloy

In this study, deficiencies in using the laser energy density (LED) to combine different process parameters to represent the heat input in selective laser melting (SLM) process are proposed. The change in melt pool shape could not always be clarified by the LED input, since the vary of LED input caused by altering hatch spacing cannot change the melt pool shape. In addition, the microstructure of SLM fabricated AlSi10Mg material was observed, and this paper gives a further elucidation on the formation of the hierarchical microstructures that discriminated by the morphology of Si phase. Microhardness is enhanced by the supersaturated α-Al matrix, and the hardness value of coarse zones and fine zones show different performance.

Aluminum hydroxide colloid facilitated transport of 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47) in porous media

In this study, aluminum hydroxide colloids, which widely exist in soils, were selected to investigate their effect on the infiltration of an abundant congener of PBDEs (BDE-47) to groundwater. The batch and column experiments were conducted to study the co-migration of aluminum hydroxide colloid and BDE-47 in two sand media with particle sizes of 2–4.75 and 0.15 mm. The results indicated that the colloid significantly increased the transport of BDE-47 to 24.32% and 65.84% in the vadose zone of coarse and fine sand columns, respectively. The adsorption and blocking effect were found to be the two main functions during the co-migration of aluminum hydroxide colloids with BDE-47. Specifically, BDE-47 that adsorbed on colloids moved faster in the coarse porous media, and the breakthrough peak of BDE-47 appeared early in the media at an approximate pore volume of 0.15. In comparison, colloids that adsorbed onto the fine porous media formed a layer that blocked the adsorption of BDE-47 onto the fine porous media. This weakened the protection capacity of the vadose zone and led to a greater than 80% amount of BDE-47 breakthrough to the vadose zone.

Research on metallurgical bonding of selective laser melted AlSi10Mg alloy

The densification behavior, surface morphology and attendant microstructural characteristics of the selective laser melting (SLM) processed AlSi10Mg alloy affected by the processing parameters were systematically investigated. Increasing the laser scanning speed or hatch spacing will deteriorate the metallurgical bonding between melt pools, resulting in the increase of irregular pores. Scanning speed and hatch spacing affect the liquid metallurgical bonding of melt pools in different ways. By manipulating scanning speed, the shape of the melt pool changes, resulting in different extents of metallurgical bonding. Whereas hatch spacing influences the resultant metallurgical bonding by controlling the overlapping rate between neighboring scan tracks simply. The formations of the hierarchical microstructures which discriminated by the Si phase are elucidated. Coarse zones are formed by the instantaneous existence of extremely high ratio of thermal gradient (G) and solidification rate (R) at the melt pool boundary, where solidification microstructure grows planar. Fine zones are formed by columnar-dendritic growth of microstructure. During the solidification process, the contraction forces that generated by the trapped gas in the pores and gravity are applied to the liquid around irregular pores and, forms the porous microstructures different from that in dense areas eventually. The tensile tests reveal that the tensile properties of SLM-processed samples are significantly affected by the formation of porosity.

Microstructure and mechanical properties of TiC/AlSi10Mg alloy fabricated by laser additive manufacturing under high-frequency micro-vibration

TiC/AlSi10Mg alloys were successfully fabricated by laser additive manufacturing under high-frequency micro-vibration. The application of high-frequency micro-vibration during laser processing could accelerate the melt flow, promote the floating of gas and slag in the molten pool and significantly reduce the pores in the alloys, which could obtain refined and homogenous microstructure with superior alloy density as well as good performances with relatively high production efficiency. The effects of different vibrational frequencies on the microstructure and properties of the alloys have been systematically investigated. The microstructure, chemical composition, phase constituent, hardness distribution and tensile properties of the TiC/AlSi10Mg alloys were investigated by optical microscope (OM), scanning electron microscope (SEM), energy dispersive X-ray analysis (EDAX), X-ray diffractometer (XRD), Vickers micro-hardness tester, and high-precision electronic universal testing machine, respectively. The results shows that the structures of the TiC/AlSi10Mg alloys are refined by high-frequency micro-vibration. The size of the cellular grain is decreased from 4 to 13 μm to 2–4 μm. The overlap region of the molten pool could be divided into three parts: fine zone, coarse zone and heat affected zone (HAZ). The number of pores in the alloys is reduced by high-frequency micro-vibration and the density of the alloys is effectively improved. When the vibration frequency is 969 Hz, the structure is more compact and the defect in the alloy is the lowest, providing a good density (99.1%). In addition, the maximum average hardness is 183.3 HV0.05 at the vibration frequency of 969 Hz, and the maximum tensile strength and elongation are 314.7 MPa and 8.81% respectively at this vibration frequency, which are higher than those at other vibrational frequencies.

Microstructure of selective laser melted AlSi10Mg alloy

The influence of laser power during selective laser melting (SLM) on the grain morphology and texture component in AlSi10Mg alloy has been investigated, using electron backscattered diffraction (EBSD). Both equiaxed and columnar grains were observed. The formation of equiaxed grains was attributed to the huge thermal gradient on the border of melt pool and the columnar to equiaxed transition (CET) occurred in front of the columnar grains. The grain size of low laser power sample was found smaller than that of higher ones. A fine pseudoeutectic structure, in which Si existed as fibrous, was observed because of the high cooling rate. This paper, from a new angle, explained the formations of three different zones across the melt pool, which were differentiated by the morphology of Si phase. The three zones correspond to the three temperature zones, which were divided by liquidus and solidus temperature, during the heating by laser beam. The coarse zones are formed by reheating the basic metal to semi-solid state when the temperature is lower than the liquidus temperature but higher than the solidus temperature.

Spatial non-stationarity analysis to estimate dwelling units in Riyadh, Saudi Arabia

ABSTRACTDwelling unit (population) data at finer zones are important for different applications. The census data are not available every year and published at coarse zones. While the remotely sensed data have addressed this limitation, analysis of spatial non-stationarity in the relationship between dwelling and detailed urban remote-sensing covariates for dwelling estimates is almost novel. Riyadh, Saudi Arabia, is chosen as a case study. The remote-sensing variables have been derived from QuickBird data using an object-based image analysis. To analyse the spatial non-stationarity, ordinary least squares (OLS) and geographically weighted regression (GWR) approaches are applied. The OLS models suggest that utilizing three-residential classes provides more accurate results than built area and residential built area. The GWR models are more accurate than the OLS model. This research proves that the GWR approach cannot completely handle the spatial non-stationarity problem without efficient explanatory variables. For example, the GWR model that uses three-residential classes is the best model to estimate dwelling compared with the GWR model that uses residential built area. This article confirms that the application of the OLS approach with efficient explanatory variables can successfully account the spatial non-stationarity and the results are relatively comparable with the GWR model.

Relationship between spacing of eutectic colonies and tensile properties of transient directionally solidified Al-Ni eutectic alloy

Eutectic Al-Ni alloys are widely faced as materials to be considered for advanced structural components. Nevertheless, still there is a lack of research on microstructural aspects of the eutectic Al-6.3 wt%Ni alloy and important points need to be addressed, such as, the morphology of the unsteady solidified eutectic, size and distribution of phases under different solidification cooling rates and the effects of the resulting microstructural features on mechanical properties. As such, in the present study, an Al-6.3 wt%Ni alloy is directionally solidified under a wide range of cooling rates and the resulting microstructure is shown to be formed by eutectic colonies. Three general microstructural features characterize the colony: a fine central zone composed of fibrous α-Al + Al3Ni eutectic, a boundary coarse zone formed by lamellar eutectic, and an Al-rich zone delimiting the colony. A quantitative analysis relating solidification thermal parameters to Al3Ni and colony spacings is outlined. Furthermore, the evolution of tensile properties as a function of these spacings was examined, and the highest strength and elongation of 160 MPa and 15%, respectively, are associated with an ultrafine bimodal structure formed by eutectic colonies with 55 μm in spacing containing very fine fibers (300 nm in spacing) and lamellae with 750 nm in spacing.

Experimental Study on Characteristics of the Plastic Zone in Coarse Grained Soil Slope

In order to study the development of plastic zone of cohesionless coarse grained soil slope under external load, the plane strain tests of cohesionless soil in the different average particle sizes were carried out by using the same slope model size. The results show that the beginning of bifurcation of strain in soil is a sign of plastic zone trigger, and with the average particle size increasing, the required time for the development of shaping belt is long; With the increase of the average particle size, the plastic zone width has varying degrees of growth; At the same time, the plastic zone of sliding position is affected by the load positions to a certain extent, and with the extension of the loading position to the slope, the relative position of the plastic belt also extends to the depth of the slope.

Effect of arc distance on HAZ thermal cycles and microstructural evolution 10CrNi3MoV steel

Double-sided double GTAW and GMAW (DSGTAW and DSGMAW) have been proposed to reduce hydrogen induced crack (HIC) susceptibility in recent years. This paper is mainly to investigate the effect of arc distance on thermal cycles and microstructural evolution of HAZ. To clarify this, DSGTAW and single gas tungsten arc welding (SGTAW) were employed in backing welding of 10CrNi3MoV low-alloy high-strength steel. The thermal processes were studied by numerical simulation in this paper. The heat input of each pass was 21.7 kJ/cm. The thermal cycles of SGTAW and DSGTAW with different arc distances (d) were compared and analyzed. The microstructural evolution of coarse grained heat affected zone (CGHAZ) of SGTAW and unaltered coarse grained heat affected zone (UACGHAZ) of DSGTAW was observed by confocal laser scanning microscope (CLSM). The results show that DSGTAW can significantly decrease the cooling rate of HAZ. As the arc distance increases, the trough temperature (T t ) between the two peak temperatures induced by the fore and rear arc will decrease and the HAZ temperature gradient of rear pass will increase which can lead to the change of thermal cycles. When arc distance increases to 60 mm, brittle intercritically reheated CGHAZ (IRCGHAZ) will form.

Transport of Escherichia coli through a Thick Vadose Zone

Livestock manure applications on fields can be a source of contamination in water resources, including groundwater. Although fecal indicators like have often been detected in tile drainage systems, few studies have monitored groundwater at depth after manure treatments, especially at sites with a deep, heterogeneous vadose zone. Our hypothesis was that microbial transport through a thick vadose zone would be limited or nonexistent due to attenuation processes, subsurface thickness, and heterogeneity. This study tested this hypothesis by monitoring concentrations beneath a 12-m-thick vadose zone of coarse, heterogeneous glacial sediments after surface application of liquid swine manure. was detected on all 23 sample dates over the 5-mo period (4 Apr. 2012-13 Aug. 2012), with particularly elevated concentrations 1 wk after application and lasting for 5 wk. Variable low-level concentrations before and after the elevated period suggest remobilization and delayed transport of microorganisms to the water table without additional loadings within the flow field. These findings suggest preferential flow pathways allowing deep infiltration of manure bacteria as well as a continued source of bacteria, with variable retention and travel times, over several months. Preferential flow pathways at this site include soil macropores, depression focused infiltration, and pathways related to subsurface heterogeneity and/or fracture flow through finer-grained diamict beds. Further research is needed to confirm the relative contribution of sources, constrain travel times, and define specific transport pathways.

Simulations of nanoscale Ni/Al multilayer foils with intermediate Ni2Al3 growth

Nanoscale multilayers of binary metallic systems, such as nickel/aluminum, exhibit self-propagating exothermic reactions due to the high formation enthalpy of the intermetallic compounds. Most of the previous modeling approaches on the reactions of this system rely on the use of mass diffusion with a phenomenological derived diffusion coefficient representing single-phase (NiAl) growth, coupled with heat transport. We show that the reaction kinetics, temperatures, and thermal front width can be reproduced more satisfactorily with the sequential growth of Ni2Al3 followed by NiAl, utilizing independently obtained interdiffusivities. The computational domain was meshed with a dynamically generated bi-modal grid consisting of fine and coarse zones corresponding to rapid and slower reacting regions to improve computational efficiency. The PDEPE function in MATLAB was used as a basis for an alternating direction scheme. A modified parabolic growth law was employed to model intermetallic growth in the thickness direction. A multiphase enthalpy function was formulated to solve for temperatures after discrete phase growth and transformations at each time step. The results show that the Ni2Al3 formation yields a preheating zone to facilitate the slower growth of NiAl. At bilayer thicknesses lower than 12 nm, the intermixing layer induces oscillating thermal fronts, sharply reducing the average velocities.

Solute transport in low‐heterogeneity sandboxes: The role of correlation length and permeability variance

AbstractThis work examines how heterogeneity structure, in particular correlation length, controls flow and solute transport. We used two‐dimensional (2D) sandboxes (21.9 cm × 20.6 cm) and four modeling approaches, including 2D Advection‐Dispersion Equation (ADE) with explicit heterogeneity structure, 1D ADE with average properties, and nonlocal Continuous Time Random Walk (CTRW) and fractional ADE (fADE). The goal is to answer two questions: (1) how and to what extent does correlation length control effective permeability and breakthrough curves (BTC)? (2) Which model can best reproduce data under what conditions? Sandboxes were packed with the same 20% (v/v) fine and 80% (v/v) coarse sands in three patterns that differ in correlation length. The Mixed cases contain uniformly distributed fine and coarse grains. The Four‐zone and One‐zone cases have four and one square fine zones, respectively. A total of seven experiments were carried out with permeability variance of 0.10 (LC), 0.22 (MC), and 0.43 (HC). Experimental data show that the BTC curves depend strongly on correlation length, especially in the HC cases. The HC One‐zone (HCO) case shows distinct breakthrough steps arising from fast advection in the coarse zone, slow advection in the fine zone, and slow diffusion, while the LCO and MCO BTCs do not exhibit such behavior. With explicit representation of heterogeneity structure, 2D ADE reproduces BTCs well in all cases. CTRW reproduces temporal moments with smaller deviation from data than fADE in all cases except HCO, where fADE has the lowest deviation.

Microstructure and Microhardness of 17-4PH Deposited with Co-based Alloy Hardfacing Coating

Abstract Hardfacing is widely used to improve the performance of components exposed to severe service conditions. In this paper, the surface modification was evaluated for precipitation hardening martensitic stainless steel 17-4PH deposited with Co-based alloy stellite12 by the plasma-transferred arc welding (PTAW). The microstructure and microhardness of coating and heat affected zone(HAZ) of base metal were characterized by optical microscope (OM), scanning electron scanning microscope (SEM), X-ray diffractometer and hardness tester. The results show that the interface between weld metal and base metal is favorable without pore and crack, at the same time elements diffusion is observed in the fusion area. However, as the distance from the interface increases, HAZ comprises three different microstructural zones, namely, zones of coarse overheated structures, quenching martensite and martensite, ferrite. The microhardness decreases gradually from the HAZ near interface to the base metal, except the zone of coarse overheated structures. The microhardness of the coating improves a lot and fluctuates in a definitive range, and microstructural gradient is observed including the fusion area (the planar region and the bulky dendrite in a direction perpendicular to the weld interface), the transition zone (the dendrite in a multi-direction way) and the fine grain zone near the surface in the coating (fine equiaxial structure).

Developing an Empirical Equation for Modeling Particle Deposition Velocity onto Inclined Surfaces in Indoor Environments

For the purpose of modeling indoor particle dispersion with an Eulerian drift flux model or analyzing indoor particle deposition onto various surfaces accurately, it may take considerable time to calculate the deposition velocity for each surface as numerical integration or calculation is usually needed. In this article, a modified three-layer model is presented to calculate indoor particle deposition velocities for surfaces with different inclinations and for different friction velocities. Then, 1020 cases, covering the common indoor scenarios, were modeled to obtain a database of indoor particle deposition velocities. Based on the results of the 1020 cases, an empirical equation was generated to determine indoor particle deposition velocities. The empirical equation was divided into four parts, named the Fine zone, Coarse zone, Zero zone, and Transition zone. In the Fine zone, the friction velocity decides the particle deposition velocity, while in the Coarse zone, the inclination angle of the surface i...