Rate-controlling Process Research Articles

Segregation and η phase formation along stacking faults during creep at intermediate temperatures in a Ni-based superalloy

Abstract In this paper, the local compositional and structural changes occurring in association with stacking faults in a Ni-base superalloy are characterized and related to the possible rate-controlling processes during creep deformation at intermediate temperatures. These rate-controlling processes are not presently understood. In order to promote stacking fault shearing, compression creep tests on specially prepared single crystals of an exploratory Ni-base superalloy were conducted at 760 °C in the [0 0 1] orientation. Scanning transmission electron microscopy (STEM) imaging was coupled with state-of-the-art energy dispersive X-ray (EDX) spectroscopy to reveal for the first time an ordered compositional variation along the extrinsic faults inside the γ′ precipitates, and a distinct solute atmosphere surrounding the leading partial dislocations. The local structure and chemistry at the extrinsic fault is consistent with the η phase, a D024 hexagonal structure. Density Functional Theory (DFT) and high angle annular dark field (HAADF)-STEM image simulations are consistent with local η phase formation and indicate that a displacive–diffusive transformation occurs dynamically during deformation.

Thermodynamics, kinetics and modeling studies of KH- RbH- and CsH-doped 2LiNH2/MgH2 hydrogen storage systems

In this study, the effects of several alkali metal hydride dopants on the thermodynamics and kinetics of the 2LiNH2/MgH2 system were determined. The results showed that the stabilities of the doped 2LiNH2/MgH2 system are in the order: KH < RbH < CsH. Kinetics measurements showed that the absorption and desorption rates are in the order: RbH > KH > CsH, with absorption rates being about twice as fast as desorption from the corresponding materials. As expected, the activation energies for the reactions were in the order: RbH < KH < CsH with the activation energies for absorption being less than that for the corresponding desorption reaction. Modeling studies revealed that desorption reactions are controlled by diffusion during the entire process. However, for absorption reactions the rate-controlling process changed during the course of the reactions. The rate-controlling process in the first 70% of the absorption reactions was reaction at the phase boundary whereas diffusion controlled the rate in the latter stages.

High temperature deformation mechanism of 15CrODS ferritic steels at cold-rolled and recrystallized conditions

The ODS ferritic steels realize potentially higher operating temperature due to structural stability by the dispersed nano-size oxide particles. The deformation process and mechanism of 15CrODS ferritic steels were investigated at 1073 K and 1173 K for the cold-rolled and recrystallized conditions. Tensile and creep tests were conducted at the stress in parallel (LD) and perpendicular (TD) directions to the grain boundaries. Strain rate varied from 10−1 to 10−9 s−1. For the LD specimens, deformation in the cold rolled and recrystallized conditions is reinforced by finely dispersed oxide particles. The dominant deformation process for the recrystallized TD specimen is controlled through the grain boundary sliding and stress accommodation via diffusional creep at temperature of 1173 K and lower strain rate less than 10−4 s−1. The grain boundary sliding couldn't be rate-controlling process at 1073 K for the as-cold rolled TD specimen, where a dynamic recovery of the dislocation produced by cold-rolling is related to the deformation process.

Thermal Analysis on Dehydroxylation of Sol-Gel Derived Zinc Doped Calcium Phosphate Powders

The dehydroxylation of Zn free and 4 mol% Zn CaP powder was investigated using thermogravimetric analysis over the range of room temperature to 1200 °C. The kinetic result of dehydroxylation of Zn free and 4 mol% Zn CaP was calculated by means of the Ozawa–Flynn–Wall method. The XRD result indicated that the amount of Zn incorporated in HA lattice influences the phase stability of HA as it decreases with an increase in Zn concentration. According to calculated activation energy and conversion degree, the kinetics of HA dehydroxylation was identified, which included four successive conversion stages kinetically controlled by different rate-controlling processes. The dehydroxylation analysis of TG/DTG data show that Zn incorporation in HA lattice influences the phase stability of HA.

Cellulose based cationic adsorbent fabricated via radiation grafting process for treatment of dyes waste water

A cationized adsorbent was prepared from cellulosic cotton fabric waste via a single step-green-radiation grafting process using gamma radiation source, wherein poly[2-(methacryloyloxy) ethyl]trimethylammonium chloride (PMAETC) was covalently attached to cotton cellulose substrate. Radiation grafted (PMAETC-g-cellulose) adsorbent was investigated for removal of acid dyes from aqueous solutions using two model dyes: Acid Blue 25 (AB25) and Acid Blue 74 (AB74). The equilibrium adsorption data was analyzed by Langmuir and Freundlich isotherms, whereas kinetic data was analyzed by pseudo first order, pseudo second order, intra particle diffusion and Boyd's models. The PMAETC-g-cellulose adsorbent with 25% grafting yield exhibited equilibrium adsorption capacities of ∼540.0mg/g and ∼340.0mg/g for AB25 and AB74, respectively. Linear and nonlinear fitting of adsorption data suggested that the equilibrium adsorption process followed Langmuir adsorption isotherm model, whereas, the kinetic adsorption process followed pseudo-second order model. The multi-linearities observed in the intra-particle kinetic plots suggested that the intraparticle diffusion was not the only rate-controlling process in the adsorption of acid dyes on the adsorbent, which was further supported by Boyd's model. The adsorbent could be regenerated by eluting the adsorbed dye from the adsorbent and could be repeatedly used.

Effects of temperature and strain rate on the tensile properties of potassium-doped tungsten

Tensile tests were performed on pure and K-doped tungsten at temperatures from 25 to 700°C and strain rates between 10−5 and 10−1s−1 in vacuum. The yield strength of both materials increased with increasing strain rate and decreasing temperature. The amount of change in the yield strength decreased with increasing temperature. The determination of activation volumes for plastic deformation highlighted that the rate-controlling process of the deformation behavior at lower temperatures was the same for both materials, namely, kink-pair formation on screw dislocations, and the process was not affected by potassium addition. The fracture strain of both materials increased with increasing strain rate and decreasing temperature, in the temperature range where the materials showed measurable ductility. K-doped W showed higher yield strength and a lower ductile-to-brittle transition temperature than pure W. No negative effect of K addition on strain rate- and temperature-induced changes in tensile properties was found. The analysis also highlighted the effectiveness of K addition, and of the grain refinement induced by it, for improving the mechanical properties of tungsten.

Theoretical investigation of the decarboxylation and decarbonylation mechanism of propanoic acid over a Ru(0 0 0 1) model surface

The hydrodeoxygenation of organic acids is often found to be a rate-controlling process during upgrading of biomass feedstocks into fuels. We developed a microkinetic model based on data obtained from density functional theory calculations for the decarboxylation and decarbonylation mechanisms of propanoic acid (CH3CH2COOH) over a Ru(0001) model surface. The model predicts that the decarbonylation mechanism is two orders of magnitude faster than the decarboxylation mechanism. The most favorable decarbonylation pathway proceeds via removal of the acid –OH group to produce propanoyl (CH3CH2CO) followed by C–CO bond scission of propanoyl to produce CH3CH2 and CO. Finally, CH3CH2 is hydrogenated to CH3CH3. Dehydrogenation reactions that have been observed to be important over Pd catalysts play no role over Ru(0001), and a sensitivity analysis indicates that removal of the acid –OH group is the rate-controlling step in the deoxygenation. Overall, our results suggest that to improve the Ru catalyst performance for the decarbonylation of organic acids, the free site coverage needs to be increased by, for example, adding a catalyst promoter that decreases the hydrogen and CO adsorption strength (without significantly affecting the C–OH bond scission rate), or by raising the reaction temperature and operating at relatively low CO and H2 partial pressures.

Theoretical analysis of the adsorption properties of methyl violet dye on iron-doped mesoporous silica microspheres

The optimum adsorption conditions of methyl violet (MV) dye from aqueous solutions by iron-doped mesoporous silica microspheres (Fe-MSM) were investigated through adsorption experiments to improve the effect of advanced treatment of dye wastewater. The novel adsorbents were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, N2 adsorption–desorption isotherms, and scanning electron microscopy. The effects of adsorbent dosage, adsorption time, initial dye concentration, and pH on the adsorption performance were analyzed. Given these effects, the kinetics equation, related parameters, and the adsorption mechanisms were obtained. Results indicate that the different Fe contents had a slight influence on the adsorption capacity of the samples. The adsorption efficiency reached 98% for an MV solution with an initial concentration of 50 mg/L, adsorbent dosage of 10 mg, equilibrium time for adsorption of 30 min, and an alkaline pH. The adsorption process followed the pseudo-second-order kinetic model. The intra-particle diffusion of dye molecules into interior surface was achieved through rate-controlled process, and further adsorption mechanisms were involved except for ion exchange. Fe-MSM could be an ideal material for the treatment of dyestuff wastewater because of its relatively high performance and rapid sorption velocity.

On the identification of superdislocations in the γ′-phase of single-crystal Ni-base superalloys – An application of the LACBED method to complex microstructures

Ni-base superalloys are used for turbine blades, which operate in the creep range at temperatures above 1000°C. One of the objectives of modern materials science is to analyze the combination of elementary deformation and microstructural coarsening processes and to identify physically based micromechanical models which allow one to predict the mechanical behavior on the macroscale. High-temperature creep of single-crystal Ni-base superalloys is governed by dislocation plasticity in the well-known γ/γ′-microstructure. For a comprehensive description of plasticity, it is important to understand the nucleation, glide and climb of superdislocations in the γ′-phase. The rate-controlling dislocation processes have to be identified and therefore a reliable Burgers vector analysis of superdislocations is essential. Superdislocations exhibit complex dislocation cores, typically comprising superpartial dislocations and planar defects. Therefore, conventional Burgers vector analysis based on the invisibility criterion often fails, due to the presence of pronounced residual contrast. In the present work, large-angle convergent-beam electron diffraction (LACBED) is employed for Burgers vector determination of two characteristic superdislocations, of the standard <110> and the more complex <100> type. LACBED results are compared with results obtained using the conventional invisibility analysis. While both techniques work for the standard superdislocation, the conventional analysis fails to analyze the <100> superdislocation, which shows pronounced residual contrast even under conditions of g·b=0 and g·b×u=0. In contrast, the LACBED technique allows for an unambiguous determination of the Burgers vector, including its magnitude and absolute sense. In the present study, the use of LACBED to identify dislocations in the complex microstructure of an Ni-base superalloy is outlined and the better performance of LACBED as compared to the conventional gb-analysis is discussed.

Potassium, rubidium and cesium hydrides as dehydrogenation catalysts for the lithium amide/magnesium hydride system

In this study, the effectiveness of several alkali metal hydrides (KH, RbH and CsH) for improving the hydrogen desorption properties of a 2LiNH2/MgH2 mixture was studied. Results showed that the relative effectiveness of these additives in decreasing the hydrogen desorption temperature, lowering the activation energy and increasing desorption rates from the mixtures is in the order: RbH > KH > CsH > Un-catalyzed. Modeling studies showed that diffusion through a Li2Mg(NH)2 product layer is the rate-controlling process. It is believed that the alkali elements: K, Rb and Cs partially replace the Li in the product layer. This may have an inductive effect in which the N–H bond is weakened thus leading to lower desorption enthalpies. The lattice expansion caused by substitution of the larger alkali elements for Li may also allow for faster diffusion and increased desorption rates.

Kinetics of Reactive Diffusion in the (Sn&amp;ndash;Cu)/Ni System at Solid-State Temperatures

The kinetics of solid-state reactive diffusion in the (Pd-Cu)/Sn system was experimentally observed to examine how adding Cu into Pd influenced the growth behavior of compounds at the junction between the Sn-base solder and the multilayer Pd/Ni/Cu conductor during energization heating. In our experiment, Sn/(Pd-Cu)/Sn diffusion couples with Cu mole fractions (y) of 0.243, 0.497, and 0.735 were isothermally annealed at temperatures of 433 K to 473 K for various times up to 385 h. Annealing caused an intermetallic layer of rather uniform thickness to form between the Sn and Pd-Cu specimens in the diffusion couple. The intermetallic layer consisted of PdSn4, PdSn3, PdSn2, and Cu6Sn5 for y = 0. 243. However, PdSn3 and PdSn2 were not clearly detected for y = 0.497 or 0.735. The total thickness of the intermetallic layer was proportional to a power function of the annealing time, with the exponent of the power function being 0.33 to 0.53. Cases where the exponent was smaller than 0.5 indicated that boundary diffusion controlled the growth of the intermetallic layer. Cases where the exponent was equal to 0.5 implied that volume diffusion was the rate-controlling process. Even for exponents close to 0.5, however, the grain growth behavior of the intermetallic layer suggested that the layer growth was governed by boundary diffusion. For the annealing times tested in this work, the overall growth rate of the intermetallic layer was insensitive to the Cu concentration at y 0.75. As a consequence, the growth of compounds at the Pd junction in the multilayer Pd/Ni/Cu conductor was markedly decelerated by adding Cu when y > 0.75.

Optimizing niobium dealloying with metallic melt to fabricate porous structure for electrolytic capacitors

The transition behavior from a Nb–Ni precursor to porous Nb and the coarsening of ligaments by a dealloying reaction in a Mg melt are investigated. Based on these results, the kinetics of the reaction are discussed. When a Nb25Ni75 (at.%) disk was immersed in a Mg melt, the Ni content decreased the most at the surface of the disk and less so towards the interior of the disk. In this disk, it was found that transition layers of body-centered cubic-Nb and Ni6Nb7 formed. These transition layers grew, following a parabolic law, and the activation energy of such growth was close to values reported for the diffusion of solute atoms in liquids, suggesting that diffusion of Ni in the Mg melt was the rate-controlling process of the dealloying reaction. The ligament size depended on time and temperature, following a power law with an exponent of 4, suggesting that surface diffusion was a key part of coarsening. These results agree well with those reported for conventional dealloying in an aqueous solution. Based on the kinetics analyses, the dealloying conditions were optimized to maximize the specific surface area, allowing a Nb electrolytic capacitor to be produced with a maximum mass-specific capacitance of 650,000μFVg−1, about three times larger than the highest value previously reported.

Grain refinement and high strain rate superplasticity in alumunium 2024 alloy processed by high-pressure torsion

Abstract An Al-2024 alloy was processed by high-pressure torsion (HPT) to produce an ultrafine-grained structure with a grain size of ~240 nm. A maximum elongation of ~750% was attained with an initial strain rate of 1×10−2 s−1 at 673 K, demonstrating the advent of high strain rate superplasticity through grain refinement by the HPT processing. Evaluation of the strain-rate sensitivity and the activation energy for the deformation confirmed that grain boundary sliding through grain boundary diffusion is the rate-controlling process for the superplastic deformation of the HPT-processed Al-2024 alloy.

The Use of Size Distributions in Determining Growth Mechanisms: The Growth of Grain Boundary Precipitates in Cobalt-20 Iron

Accurate prediction of microstructural stability in an alloy depends not only on a sound knowledge of the thermodynamics of the system but also of the kinetics of the phase changes involved. Conventionally, precipitate growth mechanisms have been inferred from the variation with aging time of various single parameters such as the mean, mode or maximum of the precipitate size distribution, which has then been compared to theoretical models of growth of an individual precipitate. In the present study, the development, with aging time at 1003 K (730 °C), of the size and shape distributions of grain boundary precipitates in Co-20Fe has been examined to determine the rate-controlling processes, and the conclusions compared to those from conventional analysis. The growth of the precipitates was well described by the grain boundary-dependent collector plate mechanism of Brailsford and Aaron. As the precipitates grew, low-energy facets were formed, which could move only by the propagation of ledges, and thickening was inhibited. The precipitates’ diffusion fields in the grain boundary overlapped and the size distributions of the longest aged specimens showed that local coarsening occurred under partial interface control.

Dynamic Adsorption of Cd2+ Onto Acid-Modified Attapulgite From Aqueous Solution

AbstractIn the present study, acid-modified attapulgite was used, as an adsorbent, to remove as much Cd2+ as possible from aqueous solution. Static adsorption experiments using powdered acid-modified attapulgite, and dynamic adsorption using granular acid-modifed attapulgite, were conducted to explore the practical application of modified attapulgite in the adsorption of Cd2+. The modified attapulgite had a larger specific surface area and thinner fibrous crystals than the unmodified version. No obvious differences were noted, in terms of the crystal structure, between the natural attapulgite and the modified version. The effects of initial concentration, pH, contact time, and ionic strength on the adsorption of Cd2+ were investigated, and the results showed that the adsorption capacity of the modified attapulgite was increased with increasing pH and the initial Cd2+ concentration. The adsorption properties were analyzed by means of dynamic adsorption tests with respect to various Cd2+ concentrations and flow rates. The maximum adsorption capacity of 8.83 mg/g occurred at a flow rate of 1 mL/min and at an initial concentration of 75 mg/L. Because there was better accord between the data and a pseudo-second order model than a pseudo-first-order model, external mass transfer is suggested to be the rate-controlling process. The experimental data were also fitted for the intraparticle diffusion model, implying that the intraparticle diffusion of Cd2+ onto the modified attapulgite was also important for controlling the adsorption process. The Bohart-Adams model was more suitable than the Thomas model for describing the dynamic behavior with respect to the flow rate and the initial Cd2+ concentration. This research provided the theoretical basis for the dynamic adsorption of Cd2+ on the modified attapulgite. Compared to the powdered modified attapulgite, the dynamic adsorption by granular modified attapulgite appeared more favorable in terms of practical application.

Fabrication and thermo-mechanical behavior of ultra-fine porous copper.

Porous materials with ligament sizes in the submicrometer to nanometer regime have a high potential for future applications such as catalysts, actuators, or radiation tolerant materials, which require properties like high strength-to-weight ratio, high surface-to-volume ratio, or large interface density as for radiation tolerance. The objective of this work was to manufacture ultra-fine porous copper, to determine the thermo-mechanical properties, and to elucidate the deformation behavior at room as well as elevated temperatures via nanoindentation. The experimental approach for manufacturing the foam structures used high pressure torsion, subsequent heat treatments, and selective dissolution. Nanoindentation at different temperatures was successfully conducted on the ultra-fine porous copper, showing a room temperature hardness of 220 MPa. During high temperature experiments, oxidation of the copper occurred due to the high surface area. A model, taking into account the mechanical properties of the copper oxides formed during the test, to describe the measured mechanical properties in dependence on the proceeding oxidation was developed. The strain rate sensitivity of the copper foam at room temperature was ∼0.03 and strongly correlated with the strain rate sensitivity of ultra-fine grained bulk copper. Although oxidation occurred near the surface, the rate-controlling process was still the deformation of the underlying copper. An increase in the strain rate sensitivity was observed, comparably to that of ultra-fine-grained copper, which can be linked to thermally activated processes at grain boundaries. Important insights into the effects of oxidation on the deformation behavior were obtained by assessing the activation volume. Oxidation of the ultra-fine porous copper foam, thereby hindering dislocations to exit to the surface, resulted in a pronounced reduction of the apparent activation volume from ~800 to ~50 b 3, as also typical for ultra-fine grained materials.

Modelling of As3+ adsorption from aqueous solution using Azadirachta indica by artificial neural network

An intensive study has been made on the removal efficiency of As3+ from aqueous solution by zinc oxide nanoparticle entrenched on activated silica (ZnO-NPs-AS) using aqueous leaf extract of Azadirachta indica. ZnO-NPs-AS is characterized using SEM-EDX, FT-IR and XRD. The effect of various parameters such as initial concentration of As3+, adsorbent dosage, contact time, pH and agitation is studied systematically. The maximum adsorption of As3+ is found to be 98.31% at pH 5, equilibrium time of 50 min using adsorbent of 3 g/L and initial concentration of 0.06 mg/L at agitation speed of 250 rpm. Adsorption parameters for the Langmuir, Freundlich, Tempkin and BET isotherms were determined. The equilibrium data were best described by Langmuir isotherm model and fits quite well with the experimental data with good correlation coefficient of 0.974. The results of intraparticle diffusion model suggested that intraparticle diffusion was not the rate-controlling process. From the values, it is accomplished that the maximum adsorption corresponds to a saturated monolayer of As3+ molecules on the adsorbent surface with constant energy. The data were analysed using kinetics models akin to pseudo-first and second order. All the findings presented in this study suggested following pseudo-second-order equation for the adsorption of As3+ on to Zno-NPs-AS. The data collected from laboratory-scale experimental set up are used to train a feed forward back propagation learning algorithm having 5-20-1 three-layered architecture. The model uses tangent sigmoid transfer function at input to hidden layer whereas a linear purelin function is used at output layer. The data are divided into training (70%), testing (15%), and validation (15%) sets. The network is found to be working satisfactorily as absolute mean square percentage error of 0.0014 is obtained during training phase. Comparison between the model results and experimental data gives a high degree of correlation (R2 = 0.986) indicating that the matlab nntool 2010a neural network model is able to predict the sorption efficiency with reasonable accuracy.

Transport processes at quartz–water interfaces: constraints from hydrothermal grooving experiments

Abstract. We performed hydrothermal annealing experiments on quartzite samples at temperatures of 392 to 568 °C and fluid pressures of 63 to 399 MPa for up to 120 h, during which hydrothermal grooves developed on the free surfaces of the samples. An analysis of surface topology and groove characteristics with an atomic force microscope revealed a range of surface features associated with the simultaneous and successive operation of several processes partly depending on crystal orientation during the various stages of an experiment. Initially, dissolution at the quartzite-sample surface occurs to saturate the fluid in the capsule with SiO2. Subsequently, grooving controlled by diffusion processes takes place parallel to dissolution and precipitation due to local differences in solubility. Finally, quench products develop on grain surfaces during the termination of experiments. The average groove-root angle amounts to about 160°, varying systematically with misorientation between neighboring grains and depending slightly on temperature and run duration. The grooving is thermally activated, i.e., groove depth ranging from 5 nm to several micrometers for the entire suite of experiments generally increases with temperature and/or run time. We use Mullins' classical theories to constrain kinetic parameters for the transport processes controlling the grooving. In the light of previous measurements of various diffusion coefficients in the system SiO2–H2O, interface diffusion of Si is identified as the most plausible rate-controlling process. Grooving could potentially proceed faster by diffusion through the liquid if the fluid were not convecting in the capsule. Characteristic times of healing of microfractures in hydrous environments constrained from these kinetic parameters are consistent with the order of magnitude of timescales over which postseismic healing occurs in situ according to geophysical surveys and recurrence intervals of earthquakes.

Mathematical modeling of CO2 uptake by concrete during accelerated carbonation curing

Accelerated CO2 curing is a promising carbon capture and storage technology that can provide durable, pre-cast concrete products. A mathematical framework for predicting CO2 uptake and distribution during carbonation curing is presented, incorporating equations describing CO2 gas transport, dissolution in concrete pore water and reaction with cement compounds. The numerical simulations show that carbonation reaction of tricalcium silicate, the most abundant and reactive compound in cement, was the rate-controlling process for CO2 uptake. The specific surface area of compounds available for reaction determines the rate and extent of uptake. The 30-minute carbonation efficiency increased from 16.5% to 23% with a two-fold increase in the total specific surface area. The rate of CO2 uptake by cement doubled with a two-fold increase in CO2 gas partial pressure, but the extent of carbonation did not change significantly due to the formation of solid carbonation products on reactive surfaces and in the pore space.

Autoignition behavior of a spherical cluster consisted of a center fine droplet and surrounding twelve fine droplets

Autoignition behavior of spherical clusters of pure droplets was experimentally examined near the ignition limit. The fuel with the melting points little above room temperatures was selected as the test fuel. The three-dimensional droplet clusters having hexagonal closest packing structure were selected as the test clusters, which consisted of a center droplet and surrounding twelve droplets (thirteen droplets in total). The initial diameters of the droplets were much reduced from the authors’ previous studies. The results showed that the ignition delay of the three-dimensional droplet clusters also had a minimum at a certain droplet spacing, which has been predicted by Niioka and coworkers as a result of rate-controlling process transition between reaction and evaporation. Autoignition of the droplet clusters could be achieved for the center fine droplet, which is out of ignition limit when isolated, in combination with finer surrounding droplets.