On grain growth kinetics in two-phase polycrystalline materials through Monte Carlo simulation

Monte Carlo Potts model simulation was carried out on a 2D square lattice for various surface fractions of second phase particles for over 50,000 iterations. The observations are in good agreement with known theoretical and experimental results with respect to both growth kinetics as well as grain size distribution. Further, the average grain size and the largest grain size were computed for various surface fractions which have indicated normal grain growth and microstructure homogeneity. The surface fraction of the second phase particles interacting with the grain boundaries (Φ), hitherto not computed through the simulation route, is shown to vary inversely as the average grain size due to Zener pinning.     

Dissolution kinetics of δ phase and its influence on the notch sensitivity of Inconel 718

The dissolution kinetics of δ phase in Inconel 718 at 980 °C, 1000 °C and 1020 °C and its influence on high temperature notch sensitivity have been studied using a quantitative X-ray diffraction (XRD) method and high temperature stress rupture life tests of notched specimens. The amount of δ phase decreases gradually during holding time at 980 °C, 1000 °C and 1020 °C. The δ phase will be fully dissolved in the austenitic matrix at 1020 °C for more than 2 h. A certain amount of δ phase still exists after holding at 980 °C and 1000 °C for times up to 6 h; the amount remaining are 3 wt.% and 0.6 wt.%, respectively. The dissolution rate remains at a high level at the beginning, and then decreases gradually with an increase of holding time. A dynamic equilibrium state can be approached after holding at 980 °C for more than 30 min and at 1000 °C for more than 2 h. The alloy with δ phase amounts higher than 0.62 wt.% does not exhibit notch sensitivity, whereas serious notch sensitivity exists if the concentration is below 0.43 wt.%.     

The influence of Na on metastable defect kinetics in CIGS materials

The electronic properties of matched pairs of Cu(In_xGa_1 − x)Se₂ (CIGS) solar cells, with and without normal sodium levels, were studied by junction capacitance methods including admittance spectroscopy, drive level capacitance profiling (DLCP) and transient photocapacitance spectroscopy (TPC). The capacitance profiling measurements revealed a large deep defect density in the vicinity of the barrier interface that was likely responsible for the lower performance of the reduced Na samples. The metastable properties of CIGS solar cells were also examined, and these revealed marked differences between the two types of samples. These results directly address the predictions of theoretical microscopic models that have been proposed to account for metastable effects in CIGS.     

Dissolution kinetics of iron in liquid zinc

In hot dip galvanizing, steel strip is coated by immersion in a bath of molten zinc. The principal reactions that occur at the steel/liquid zinc interface are (1) dissolution of iron and (2) nucleation and growth of intermetallic compounds. In order to improve the management of industrial galvanizing baths, it is essential to evaluate the flux of dissolved iron that diffuses into the bath from the sheet. For this purpose, a rotating disk device has been developed to study the dissolution and diffusion of iron in pure liquid zinc at the temperature usually employed in galvanizing baths (465°C). Since the dissolution reaction is controlled by diffusion under these conditions, the diffusion coefficient of iron in liquid zinc has been measured and found to be: D _Fe ^Zn(L) = (9.8 ± 0.1) × 10^–10 m²·s^–1     

Modeling of irradiation hardening of polycrystalline materials

High energy particle irradiation of structural polycrystalline materials usually produces irradiation hardening and embrittlement. The development of predictive capability for the influence of irradiation on mechanical behavior is very important in materials design for next-generation reactors. A multiscale approach was implemented in this work to predict irradiation hardening of iron based structural materials. In the microscale, dislocation dynamics models were used to predict the critical resolved shear stress from the evolution of local dislocation and defects. In the macroscale, a viscoplastic self-consistent model was applied to predict the irradiation hardening in samples with changes in texture. The effects of defect density and texture were investigated. Simulated evolution of yield strength with irradiation agrees well with the experimental data of irradiation strengthening of stainless steel 304L, 316L and T91. This multiscale modeling can provide a guidance tool in performance evaluation of structural materials for next-generation nuclear reactors.     

Dissolution of quartz in vitrified ceramic materials

The dissolution of quartz in vitrified ceramic materials was investigated. A mathematical model was derived and compared with published data and experimental data using X-ray diffraction techniques. When compared with published experimental data, the model correlated better than other dissolution models. However, over longer periods of heat treatment, the model becomes less accurate. The model may be of practical use in describing various types of kinetic data used by manufacturers of vitrified ceramic materials. This revised version was published online in November 2006 with corrections to the Cover Date.     

Multi-scale modeling of mechanical behavior of polycrystalline materials

A systematic procedure for multi-scale computational modeling is presented to simulate polycrystalline material behaviors for structural application. The modeling procedure bridged different length scales, from macro-scale to nano-scale, by proper flow of information from one scale to the next scale. The multi-scale model could include material characteristics at different length scales like nano-, micro-, meso-, and macro-scale. Both finite element analysis and molecular dynamics were used in the multi-scale analysis. An example of a large plate with a center hole is given to demonstrate the multi-scale modeling procedure.     

Crystallography-based prediction of plastic anisotropy of polycrystalline materials

The on-line prediction of metal sheet formability requires that both material characterization (texture identification) and yield loci predetermination be done in very shor time intervals. Of two applicable approaches, i.e., continuum mechanics and crystallography-based methods, only the latter are suitable for this purpose. Several models of plasticity of a polycrystalline material were reviewed, and their applicability to the prediction of plastic anisotropy of face-centered cubic (FCC) metals was evaluated. A tailored set of cold-rolled copper alloy samples was designed and manufactured, representing the wide spectrum of textures and cold work levels typical for the sheet metal industry. The texture was quantitatively described in the form of the orientation distribution functions derived by the inversion of four incomplete pole figures. The Taylor-Bishop-Hill model was applied in order to calculate the planar variation of the plastic strain ratio. The continuum mechanics of textured polycrystals approach was also used for the prediction of the plastic strain-rate ratio for the same set of deformed materials. The theoretical predictions were compared with the plastic strain ratios measured in tensile tests using strain gauges. The applicability of the models for prediction of the plastic anisotropy of FCC metals was discussed in view of the operating deformation mechanisms and other factors such as strain hardening sensitivity and grain size/shape effects.     

六边形纳米CuI及其超疏水性

采用一步液相反应法,在常温常压下,制得六边形纳米CuI.无需任何低表面能物质(硫醉或氟烷基)修饰,该样品就表现出超疏水性,接触角达到157°.最为重要的是样品的超疏水性很稳定,在空气中放置半年后,其接触角无明显变化,具有潜在的应用价值.     

Cutting of polycrystalline superhard materials by jet methods

The authors study the intensity of cutting workpieces of hardmetals, polycrystalline cBN-based material and polycrystalline diamond-on-hardmetal compacts in the following operations: waterjet cutting, laser cutting, liquid-cooled laser cutting, and laser–jet cutting. Special features of cutting two-layered composites that comprise a polycrystalline diamond layer on a hardmetal substrate are analyzed. The paper demonstrates the efficiency of the hybrid machining process that combines the laser–jet and waterjet technologies and provides an effective cutting of workpieces to a required profile.     

An approach for simulating microstructures of polycrystalline materials

In this paper, an approach is identified using concepts in molecular dynamics (MD) and discrete element method (DEM) to generate the microstructure of polycrystalline materials. Using the proposed methods, different types of particles with different grain size and volume fraction in the real material, can be easily generated. It is assumed that the particles can be randomly packed together into a simulation region, by defining artificial interaction forces among them. Such forces may be either adopted from Van der Waals potential energy, or Hooke pair and gravity forces. The proposed method has proved to be fast due to the fact that the algorithm has been implemented on graphical processing units (GPU). Utilizing the Voronoi tessellation method, the set of the generated discrete grains have been altered to space-filling, adjoining polyhedrons with respect to the real geometry. Moreover, as an advantage, the boundary and the interface region of the microstructures were modeled.     

Dielectric permittivity and electrical conductivity of polycrystalline materials

This paper examines the influence of composition, structure, and size factor on the dielectric permittivity and electrical conductivity of polycrystalline materials. We demonstrate that, if the 3D structure of a substance remains unchanged, reducing the grain size increases its permittivity, up to 10⁵–10⁶ for nanocrystalline powders.     

The influence of surface kinetics in modelling chemical vapour deposition processes in porous preforms

The isothermal chemical vapour infiltration (ICVI) process is a well known technique for the production of composites and the surface modification of porous preforms. Mathematical modelling of the process can provide a better understanding of the influence of individual process parameters on the deposition characteristics such as final porosity or deposition profiles in the pore network. The influence of different rate expressions for several binary compounds on the ICVI process is discussed. Experimental work is used to validate the importance of correct kinetic expressions in a continuous ICVI model for cylindrical pores. The predicted infiltration characteristics are compared with experimental results. The final densification and Thiele modulus, i.e. a number which is a measure for the diffusion limitations in a pore, are used for the evaluation of the presented model, and conditions are given for an optimal densification of a porous preform by the ICVI process for several binary compounds. The deposition profiles as predicted by the model calculations are in agreement with the experimentally determined deposition profiles of TiN and TiC in small tubes. Moreover, it can be concluded that the shape of the deposition profiles is determined by the heterogeneous reaction kinetics. There is only a qualitative agreement between the predicted densification and measured densification for the synthesis of TiN and TiB₂ in sintered porous alumina. This mismatch can be explained in terms of a complexity of the pore network and differences in reaction kinetics. Model calculations reveal that there is a scattering for the predicted residual porosity as a function of the Thiele modulus for TiN. Moreover, this Thiele modulus can not fully account for the changes in densification at different temperatures. Given these uncertainties it is likely that a residual porosity of less than one percent can be obtained if the Thiele modulus is smaller than 1 × 10^–4. However, a CVI process with such a small Thiele modulus will not be practical, because of the concomitant long process times. Therefore, more precise conditions for the individual process parameters, i.e. concentration, reactor pressure, and temperature are deduced from the model calculations.Nomenclature a, b, c reaction order constants - C _i(x, t) concentration of species i at axial position x and time t (mole m^–3) - C _i ^o bulk concentration of species i (mole m^–3) - C _i ^* (x, t) dimensionless concentration of species i at axial position x and time t - D _e(x, t) effective diffusion coefficient at axial position x and time t (m²s^–1) - D _ij(x, t) binary diffusion coefficient (m²s^–1) - D _K(x, t) Knudsen diffusion coefficient at position x and time t (m²s^–1) - F correction factor for effective diffusion coefficient - k growth rate constant (ms^–1(m³mole^–1)^a+b-1) - K _i adsorption-desorption equilibrium constant (m³mole^–1) - L length of a pore (m) - M _i molecular weight of species i (g mole^–1) - M _ij harmonic mean of the molecular weights of species i andj (g mole^–1) - M _s molecular weight of deposit (g mole^–1) - m _t measured mass increase (g) - n _i stoichiometric number - P reactor pressure (Pa) - R(C _i) growth rate (mole(m^–2s^–1)) - r(x, t) pore radius at position x and time t (m) - r ^o initial pore radius (m) - r ^* dimensionless pore radius - S geometrical surface area (m²) - s ^t fraction of free titanium sites at the surface of TiN - s ⁿ fraction of free nitrogen sites at the surface of TiN - T temperature (K) - t time (s) - t _p process time (s) - U K _HCl/(K _{H 2} C _{H 2})^1/2 (m³ mole^–1) - V volume of alumina substrate (m³) - W K _TiCl3(m³ mole^–1) - X volume of infiltrated deposit relative to initial pore volume - x axial distance (m) - x ^* dimensionless axial distance - z number of time steps - dummy variable for integration - porosity of sintered porous alumina substrate - ratio of the volume over the surface area perpendicular to the flux (m) - density deposit (kg m^–3) - _ij a characteristic length (Å) - tortuosity factor of substrate - Thiele modulus - _D collision integral     

Dissolution kinetics evaluation of controlled-release tablets containing propranolol hydrochloride

In the present research, controlled-release propranolol hydrochloride tablets were prepared for twice-daily administration, allowing more uniform plasmatic levels of the drug. Pharmaceutical formulations were prepared with hydrophobic Eudragit® RSPO. The physical properties of the tablets were determined. Dissolution tests were performed in capsules containing the raw material using the following dissolution media: (A) distilled water, (B) simulated gastric juice without enzymes, and (C) simulated enteric juice without enzymes. A dissolution test was also performed for simulated samples (tablets) using distilled water as the dissolution medium.     

Surface modification methodologies for polycrystalline alumina: effects on morphology and frictional coefficients

Several methodologies of surface modifications were applied to polycrystalline alumina (PCA) samples to study their effects on surface morphology and frictional coefficients. Modified surfaces were first tested in a specially designed frictional apparatus against wires of stainless steel (SS) and beta-titanium (-Ti) alloys and then evaluated by scanning electron microscopy. Techniques included ion implantation of chromium ions (Cr⁺), ion beam assisted deposition of diamond-like carbon (DLC), coatings of gamma-irradiated polymers (PEO), and electroless nickel plating of a composite material of polytetrafluoroethylene dispersed in a nickel phosphorus matrix, Niflor NT^® (NF). Implanting ions into the bulk material had no effect on surface morphology. Although covering the surface, the DLC coating mimicked the underlying topography. The coatings of PEO and NF obliterated and smoothed over the normally rough and faceted PCA surfaces. When compared to control samples, neither the Cr⁺ or DLC process reduced the friction normally seen against SS and -Ti wires. When tested in both the dry and wet states, the PEO coated samples retained their traditional levels of frictional resistance. Only the composite material, NF, successfully reduced the friction when compared with controls. Although this composite coating is not recommended for oral use, the results show that simply smoothing over the rough surface is inadequate for friction reduction; the surface must somehow also be made lubricious.     

A calculation of the bulk modulus of polycrystalline materials

It is shown that the bulk modulus of a polycrystalline material, composed of cubic single crystals, is the same as that of the constituent single crystal. The bulk modulus of the aggregate is independent of the distribution of the individual single crystals. The same results apply also to other polycrystalline systems, whose constituent single crystals undergo a pure uniform contraction when subjected to hydrostatic pressure.