Modelling of the toughening mechanisms in rubber-modified epoxy polymers

A mathematical model has been developed to quantify the relationships between the microstructure and fracture properties of multiphase rubber-toughened epoxy polymers. Good agreement between predictions from the model and experimental results have been found. The model also reveals that localized plastic shear banding in the epoxy matrix, running between the rubbery particles, is the dominating mechanism under all testing conditions. Plastic void growth in the epoxy matrix is the other main toughening mechanism. This latter mechanism is initiated by internal cavitation of the rubbery particle, or by debonding at the particle-matrix interface, and is particularly significant at higher test temperatures.     

Role of adsorbed fluoride ions on the dissolution of synthetic hydroxyapatite

The dissolution kinetics at 37°C and for constant pH values of 5.0 and 6.0 are studied for calcium hydroxyapatite (HAp) equilibrated at pH 7.0 in the presence of 1 or 10 ppm of F^-. Experiments are realized using an automatic setup which allows a continuous record of proton uptake and calcium or fluoride activities in solution using specific electrodes. It is shown that the presence of fluoride ions affects the dissolution kinetics in two ways. Their presence at the HAp interface reduces the final solubility of HAp, but accelerates the initial dissolution rates. Both observations are fully interpreted using the HAp dissolution model recently developed. According to this model, apatite dissolution is auto-inhibited by the adsorption at the solid interface of calcium ions forming a cationic semi-permeable layer. Adsorbed fluoride ions interact with this natural cationic layer, decrease its ionic capacity and accelerate the dissolution process.     

An analytical model for secondary phase dissolution kinetics

An analytical model for dissolution kinetics of secondary phase particles upon isothermal annealing has been proposed. Considering the interactions of solute diffusion fields in front of the secondary phase/matrix interface upon dissolution, a Johnson–Mehl–Avrami type equation, subjected to necessary modification, was derived, in combination with a classic dissolution model for single-particle system. Compared with the semiempirical dissolution models, which are used to fit the experimental results and phase-field method simulation, the current model follows an analogous form, but with the time-dependent kinetic parameters. Distinct from the model fitting work published recently, the current model is derived from the diffusion-controlled transformation theory, while the modeling quality is guaranteed by the physically realistic model parameters. On this basis, the current model calculation leads to a clear relationship between the secondary phase volume fraction and the time. Accordingly, model predictions for isothermal θ′ dissolution in Al–3.0wt%–Cu alloy and silicon dissolution in Al–0.8wt%–Si alloy were performed; good agreement with the published experimental data has been achieved.     

Prediction for debonding damage process and effective elastic properties of glass-bead-filled modified polyphenylene oxide

A three-dimensional three-phase finite element unit cell model has been applied to predict the debonding damage process of particulate polymer composites (PPC) during tensile deformation. The model consists of a particle, an interface and a polymer matrix. The particle-matrix debonding process has been simulated by using a particle-matrix debonding criterion and a vanishing finite element technique. For verification of the predicted results, the results obtained in the in situ SEM experiments of our previous study on glass beads reinforced modified polyphenylene oxide (GB/PPO) were used The predicted results are in good agreement with the experimental micro-scale tensile debonding-damage process. Anisotropic damage in materials like PPC is very difficult to measure by conventional experimental methods. In order to provide anisotropic damage information of the damaged composites, this was amongst the first time to predict the effective elastic properties of PPC in different principal directions by performing a virtual load–unload test on the partially debonded cell model. Some predicted results would be compared with those of experimental load–unload test.     

Prediction of Austenitization and Homogenization of Q235 Plain Carbon Steel during Reheating Process

In this paper,the austenitization and homogenization process of Q235 plain carbon steel during reheating is predicted using a two-dimensional model which has been developed for the prediction of diffusive phase transformation(e,g.αto γ).The diffusion equations are solved within each phase(αand γ) and an explicit finite volume technique formulated for a regular hexagonal grid are used.The discrete interface is represented by special volume elements α/γ,an volume element α undergoes a transition to an interface state before it becomes γ.The procedure allows us to handle the displacement of theinterface while respecting the flux condition at the interface.The simulated microstructure shows the dissolution of ferrite particles in the austenite matrix is presented at different stages of the phase transflrmation.Specifically,the influence of the microstructure scale and the hwating rate on the phase transformation kinetics has been investigated.The experimental results agree well with the simulated ones.     

Selective etching and dissolution of BaF2 crystals

The selective etching and dissolution of BaF₂ crystals have been studied in aqueous solutions of inorganic salts, inorganic and organic acids. The effect of additives on selective etch rates is demonstrated. It is observed that chemical exchange reactions determine the selective etching and dissolution of BaF₂ crystals. The form of etch pits corresponds to the real dissolution form of BaF₂ crystals (rhombododecahedron). It has been established that the process of dissolution in HNO₃ and HCl is diffusion controlled. The activation energy is independent of acid concentration and has the same value for both HNO₃ and HCl. The pre-exponential factor is concentration dependent. An empirical equation for dissolution kinetics has been obtained.     

Pre-treatment process of B<Subscript>4</Subscript>C particles to improve incorporation into molten AA2014 alloy

The properties of particle-reinforced aluminum alloy composites depend on the microstructure and evolved uniformity distribution of particles in the matrix, wettability of particles by the melt and chemical reactions between particles and matrix. The wettability of B₄C particles by the molten aluminum alloys is generally poor below 1100 °C making their incorporation difficult. In this study, in order to improve incorporation of the B₄C particles by AA2014 alloy melt a novel pre-treatment process was presented and B₄C-2014 Al composites containing B₄C particles up to 12 vol.% were manufactured by using stir casting. SEM observations have shown that uniformity distribution of B₄C in the matrix was satisfactory and there was no reaction at the particle-matrix interface due to low processing temperature. However clustering of B₄C particles was observed in relatively high particle-containing composites, which is commonly associated with the casting route.     

Control of the structure and mechanical properties of thick vacuum condensates using dispersed particles

This work summarizes the results of investigations of the structural and mechanical properties of two-phase condensates obtained by the simultaneous electron beam evaporation from independent sources of metals (nickel and iron) and of non-metallic materials (oxides, carbides and borides). The mechanical properties are considered in relation to the substrate temperature, the particle diameter, the second phase content, interphase interactions at the particle-matrix material interface and the grain size of the metallic matrix. An extension of the mechanism that controls the strength and ductility of two-phase condensates of the dispersion-strengthened type is presented.     

Dissolution Kinetics and Rate-Controlling Mechanisms

The kinetics and mechanisms of dissolution in a newly designed diffusion cell were investigated. It was found that benzoic acid is an ideal compound to calibrate the fluid hydrodynamics in the diffusion cell when the mass transfer at the boundary layer is the primary contributor for its dissolution process. The dissolution of a very lipophilic drug, progesterone was also studied and the mechanisms of its dissolution process appear to be diffusion controlled. Using the dimensionless Sh-Re-Sc relationship developed earlier, it became possible to study the kinetics and mechanisms of dissolution of a solid drug. A reciprocal plot was established to obtain the kinetic constant of dissolution at the interface. This approach could be applied to investigate the kinetics of dissolution and diffusional resistance of other drugs     

Calorimetric evaluation of the effects of SiC concentration on precipitation processes in SiC particulate-reinforced 7091 aluminium

A comprehensive differential scanning calorimetric (DSC) investigation has been conducted on the precipitation and dissolution behaviour of SiC particulate-reinforced 7091 aluminium. DSC is shown to be a particularly attractive experimental technique for developing new thermal and thermomechanical processes for aluminium-based metal matrix composites. These new processes are necessitated due to the deleterious effects that the SiC reinforcement causes on the aluminium matrix precipitation behaviour and the resultant ductility and fracture toughness properties. In this study the effects of SiC concentration and ageing temperatures, times, and sequences were evaluated. In addition to the unreinforced 7091 alloy, composites containing 10, 20 and 30 vol% particulate SiC were characterized. Identifications of specific phases involved in reactions detected with DSC were achieved by making direct correlations to previously published transmission electron microscopy studies. It was found that the presence of SiC in the aluminium significantly affects the solid state transformation kinetics of the 7091 aluminium matrix. Specifically, increasing the SiC concentration was found to decrease the temperatures at which GPI and GPII zones precipitate at their maximum rates and to increase the temperature at which GPI zones revert at maximum rate. The transition phase,, and equilibrium phase,, formation kinetics were observed to be insensitive to SiC concentration. Also, increasing the SiC concentration was seen to decrease the temperature at which the equilibrium phase dissolution occurred at its maximum rate. Transformation mechanics have been proposed which are consistent with these observations.     

The role of interlayers in diffusion bonded joints in metal-matrix composites

Solid state bonded joints in a particulate metal-matrix composite containing 17 vol% SiC in Al-Li 8090 alloy had planar bond interfaces with particle-particle (P-P) interfaces and particle-matrix (P-M) interfaces aligned in and parallel to the bond interface. The insertion of a matrix interlayer into the MMC-MMC bond interface changed the type of particle interface and interface area fractions in the two new bond planes created. In these planes P-P interfaces became P-M interfaces and the P-M area fraction either increased or fell to zero depending on the particle symmetry with respect to the bond plane. The implications for the mechanical properties of diffusion bonded joints in MMCs are discussed.     

Theoretical model of the interfacial reactions between solid iron and liquid zinc-aluminium alloy

Steel strip is often coated with a layer of zinc in order to protect it against corrosion. One of the most commonly used coating processes is continuous hot dip galvanizing. In this process, the steel strip is immersed in a molten zinc bath containing small amounts of aluminium (less than 1 wt%). A model has been developed describing the kinetics of the galvanizing reactions that occur at the steel/liquid zinc interface (dissolution of iron, heterogeneous nucleation and growth of the intermetallic phase designated Fe₂Al₅Zn _x ). The model has been validated using experimental data available in the literature for a classical galvanizing treatment that lasts three seconds.     

The kinetics of the discontinuous precipitation and dissolution in Mg-rich Al alloys

The morphology and growth kinetics of the discontinuous precipitation and dissolution reactions in supersaturated Mg-Al solid solutions containing 7.3, 9.1 and 10.9 at % Al have been investigated by optical and scanning electron microscopy and X-ray measurements. The volume fraction of regions transformed by the discontinuous precipitation reaction, the reaction front velocity, the interlamellar spacing and the average composition of the solute-depleted lamellae were determined as a function of the temperature. For the first time, the kinetics of the discontinuous dissolution reaction has been studied in the Mg-Al system. It has been shown that the transport of the solute atoms during both reactions is governed by grain boundary diffusion.     

Dissolution Kinetics and Rate-Controlling Mechanisms

Abstract

The kinetics and mechanisms of dissolution in a newly designed diffusion cell were investigated. It was found that benzoic acid is an ideal compound to calibrate the fluid hydrodynamics in the diffusion cell when the mass transfer at the boundary layer is the primary contributor for its dissolution process. The dissolution of a very lipophilic drug, progesterone was also studied and the mechanisms of its dissolution process appear to be diffusion controlled. Using the dimensionless Sh-Re-Sc relationship developed earlier, it became possible to study the kinetics and mechanisms of dissolution of a solid drug. A reciprocal plot was established to obtain the kinetic constant of dissolution at the interface. This approach could be applied to investigate the kinetics of dissolution and diffusional resistance of other drugs     

Kinetic study of the magnesium oxychloride cement cure reaction

Magnesium oxychloride (MOC) is a ceramic material with significant fire-resistant properties and growing potential as an alternative building material for passive fire protection systems. The present study examined the magnesium oxychloride 5-phase cure reaction at temperatures from 35 to 55 °C using time-resolved quantitative X-ray diffraction and differential scanning calorimetry to monitor kinetics. The reaction was characterized as a two-step process: dissolution of magnesium oxide followed by crystallization of magnesium oxychloride from the solvated state. With stoichiometric proportions, 37% of the MgO dissolves before the onset of crystallization at a critical amorphous concentration. A maximum crystallinity of 82–84% was achieved for each temperature. Assuming first-order kinetics for both MgO dissolution and MOC crystallization, a kinetic model predicts 42.4 and 26.1 kJ/mol for dissolution and crystallization activation energies, respectively. This model was applied to pilot-scale production and accurately predicts the cure reaction time as a function of cure temperature. In an alternative approach to modeling the cure reaction, the Avrami nucleation and growth model was fit to calorimetric measurements. This model predicts diffusion-controlled, one-dimensional growth with an activation energy of 72.4 kJ/mol, which accounts for both dissolution and crystallization.     

Diffusion mechanisms for the growth of Nb3Sn intermetallic layers

The analysis of compound formation in a diffusion couple previously developed by the authors has been extended to include the effect of interface kinetics. When interface kinetics dominate, a linear rate law may obtain rather than the parabolic rate law expected if simple bulk diffusion were the rate-limiting process. Additional data for the rate of formation of Nb₃Sn at a bronze-niobium interface have been presented. Our results imply that interface kinetics are important when the concentration of Sn in the bronze matrix exceeds about 8 at.% or when certain metallic impurities are present in the niobium.     

Excellent flexural properties of aminimide-cured epoxy resin as a matrix for mica-dispersed polymer composites

The flexural behaviour of mica-dispersed epoxy resin composites has been examined. The flexural strength and flexural modulus have been determined as a function of the volume fraction of mica flakes (V _f) for both aminimide-cured epoxy resin matrix and a conventional epoxy resin reference matrix. On the basis of microscopic observation of fractured surfaces, the effect of improving the particle-matrix interface has been analysed using the modulus reduction factor (MRF) in a modified form. It is found that there is a steady increase in the flexural modulus with the volume fraction of mica flake for the aminimide-cured epoxy resin matrix. In contrast, the increase in flexural modulus levels off at a high content of filler for the reference samples. It is noteworthy that the intact mica flakes without surface treatment exhibit a substantial reinforcing effect on the flexural strength in the case of aminimide-cured epoxy resin composites. A further surprise is the difference among the curing agents used. The reference epoxy resins behave just like conventional matrix resins, exhibiting 30 to 40% reduction in the flexural strength when a small fraction of mica is added. These superior properties of the matrix resin for the composites are ascribed to the characteristics of aminimide-cured epoxy resins such as hardness, toughness, and excellent adhesivity.     

Polymerization of 16-heptadecenoic acid monolayers

Monolayers of heptadecenoic acid (vinyl acid) were spread at a gas-water interface. The terminal double bond of the vinyl acid enabled it to be polymerized on exposure to UV radiation. Pressure-area isotherms for both monomer and polymer vinyl acid were then recorded and compared with isotherms from similar compounds.Preliminary studies were conducted to minimize the degree of dissolution of the fatty acid. The polymerization kinetics were examined by the use of UV spectroscopy and measurements of the reduction in the area occupied by the film at a constant surface pressure. The reaction appeared to follow zero-order kinetics after an initial acceleration period. The rate constant for the reaction increased with increasing surface pressure.A dynamic mechanical test was performed to determine the mechanical properties of monolayers at the gas-water interface. Films were polymerized to various conversions and then examined to determine the relation between mechanical properties and conversion. A simple model is proposed to correlate the monolayer's structure and the dynamic mechanical data.