Glassy materials: thermodynamic and kinetic quantities

Modern technology makes it possible to obtain different kinds of glassy material. In the as-prepared state most of these show relaxation effects, glass transition and crystallization under heat treatment, with very similar phenomenological peculiarities. The thermodynamic treatment of glassy materials is complicated by the fact that the glassy state is not always a metastable state but is sometimes an unstable state. Evaluation of the entropy of the glass, with respect to the corresponding stable crystalline phase at the same temperature and pressure, from heat capacity measurements is discussed. The “ideal glass” concept is introduced to calculate the Gibbs free energy of formation of the glass. The residual entropy is presented as a measure of the glass deviation from ideal behaviour. The treatment is extended to alloy glass. The thermodynamic and kinetic parameters which control the crystallization process are reviewed. Polymorphous and primary crystallization are considered, assuming, either homogeneous or inhomogeneous nucleation, followed by interface-controlled crystal growth.     

Deformation of wrought uranium: Experiments and modeling

The room temperature deformation behavior of wrought polycrystalline uranium is studied using a combination of experimental techniques and polycrystal modeling. Electron backscatter diffraction is used to analyze the primary deformation twinning modes for wrought alpha-uranium. The {1 3 0}〈3 1 0〉 twinning mode is found to be the most prominent twinning mode, with minor contributions from the ‘{1 7 2}’〈3 1 2〉 and {1 1 2}‘〈3 7 2〉’ twin modes. Because of the large number of deformation modes, each with limited deformation systems, a polycrystalline model is employed to identify and quantify the activity of each mode. Model predictions of the deformation behavior and texture development agree reasonably well with experimental measures and provide reliable information about deformation systems.     

Plasma deposition of catalytic thin films: Experiments,applications, molecular modeling

Plasma deposition of catalytic thin films is reviewed in view of highlighting some interesting features. Plasma sputtering, plasma enhanced chemical vapor deposition and plasma enhanced metalorganic chemical vapor deposition and their preferential use in various kinds of catalytic films are described. Fuel cell electrodes, gas sensors and photocatalytic films are emphasized as significant applications. As example, magnetron sputtering deposition is successfully used for growing fuel cell electrodes with high performances. Doping doped TiO2 photocatalysts are deposited using various kinds of plasma depending on the expected film morphology. Gas sensors are well designed when using plasma deposition. Plasma treatment of catalysts offers a suitable alternative to thermal treatments. Finally, associated simulations, especially recent progress in molecular dynamic simulations of catalytic film growth are surveyed. This is a suitable way to understand basic mechanisms of catalytic film growth.     

On the definitions of paraequilibrium and orthoequilibrium

Hultgren’s terminology of paraferrite and paracementite in alloy steels and his definition of paraequilibrium are reviewed. They are not completely compatible due to the possibility of forming ferrite with the initial alloy content (paraferrite) even under full local equilibrium (local orthoequilibrium). That has caused some confusion.     

The thermodynamic modeling of precious-metal-modified nickel-based superalloys

Thermodynamic modeling of precious-metal-modified Ni-based super-alloys (PMMS) was performed in this study using the CALPHAD approach. With this approach, the effects of platinum-group metals (PGMs) such as platinum, iridium, and ruthenium on the properties of nickel-based superalloys and their interplay with other alloying elements were understood from a thermodynamic and phase equilibrium point of view. Thermodynamic database containing PGMs was developed on the basis of the PanNi¹ database for multi-component nickel alloys. The database was first validated with available experimental data. It was then used to understand phase stability and phase transformation temperatures, such as liquidus, solidus, and γ′ precipitation temperature, of PGM modified nickel-based superalloys. The effects of alloying elements on the formation of strengthening γ′ precipitate and their partitioning in γ and γ′ were also discussed.     

Thermodynamic and kinetic modeling of the Cr-Ti-V system

A synergistic approach of thermodynamic and kinetic modeling is applied to the Cr-Ti-V system. To assist the design of (α+β) and β titanium alloys for structural applications and vanadium alloys for fusion reactor applications, a set of self-consistent and optimized thermodynamic model parameters is presented to describe the phase equilibria of the Cr-Ti, Cr-V, Ti-V, and Cr-Ti-V systems. The Laves phases, α-Cr₂Ti, β-Cr₂Ti, and γ-Cr₂Ti, are described by a two-sublattice model assuming antistructure atoms on both sublattices. The calculated thermodynamic quantities and phase diagrams are in good accord with the corresponding experimental data. To assist the simulation of the kinetics of diffusional transformations in bodycentered cubic (bcc) alloys, the atomic mobilities of Cr, Ti, and V are modeled. A set of optimized mobility parameters is given. Very good agreement between the calculated and experimental diffusivities was found.     

Thermodynamic and kinetic modeling of the Cr-Ti-V system

A synergistic approach of thermodynamic and kinetic modeling is applied to the Cr-Ti-V system. To assist the design of (α+β) and β titanium alloys for structural applications and vanadium alloys for fusion reactor applications, a set of self-consistent and optimized thermodynamic model parameters is presented to describe the phase equilibria of the Cr-Ti, Cr-V, Ti-V, and Cr-Ti-V systems. The Laves phases, α-Cr₂Ti, β-Cr₂Ti, and γ-Cr₂Ti, are described by a two-sublattice model assuming antistructure atoms on both sublattices. The calculated thermodynamic quantities and phase diagrams are in good accord with the corresponding experimental data. To assist the simulation of the kinetics of diffusional transformations in bodycentered cubic (bcc) alloys, the atomic mobilities of Cr, Ti, and V are modeled. A set of optimized mobility parameters is given. Very good agreement between the calculated and experimental diffusivities was found.     

Predictions of titanium alloy properties using thermodynamic modeling tools

Thermodynamic modeling tools have become essential in understanding the effect of alloy chemistry on the final microstructure of a material. Implementation of such tools to improve titanium processing via parameter optimization has resulted in significant cost savings through the elimination of shop/laboratory trials and tests. In this study, a thermodynamic modeling tool developed at CompuTherm, LLC, is being used to predict β transus, phase proportions, phase chemistries, partitioning coefficients, and phase boundaries of multicomponent titanium alloys. This modeling tool includesPandat, software for multicomponent phase equilibrium calculations, andPanTitanium, a thermodynamic database for titanium alloys. Model predictions are compared with experimental results for one α-β alloy (Ti-64) and two near-β alloys (Ti-17 and Ti-10-2-3). The alloying elements, especially the interstitial elements O, N, H, and C, have been shown to have a significant effect on the β transus temperature, and are discussed in more detail herein. This paper was presented at the Beta Titanium Alloys of the 00’s Symposium sponsored by the Titanium Committee of TMS, held durig the 2005 TMS Annual Meeting & Exhibition, February 13–16, 2005 in San Francisco, CA.     

Experiments and modeling of rapid solidification of plasma-sprayed yttria-stabilized zirconia

Thermal barrier coatings (TBCs) are widely used to increase the working temperature and improve the high-temperature corrosion resistance of base materials. Thermal spraying methods such as air plasma spraying (APS) are convenient techniques to deposit TBCs. This work examines the rapid solidification of APS-deposited yttria-stabilized zirconia (YSZ), by modeling the non-equilibrium solidification of a single molten particle. The model solves the so-called hyperbolic equations for heat and mass transfer to predict interface undercooling and velocity as a function of time, and also predicts the morphology of the solidification front (and thus the microstructural characteristics of rapidly solidified YSZ) as a function of interface velocity. Results are presented of a single particle solidifying onto a steel substrate, and onto a previously deposited particle. The numerical results are also compared to experimental data of the microstructure of a YSZ splat deposited by APS, to validate the model.     

Recent advances in kinetic and thermodynamic regulation of magnesium hydride for hydrogen storage

Developing safer and more efficient hydrogen storage technology is a pivotal step to realizing the hydrogen economy. Owing to the lightweight, high hydrogen storage density and abundant reserves, MgH₂ has been widely studied as one of the most promising solidstate hydrogen storage materials. However, defects such as stable thermodynamics, sluggish kinetics and rapid capacity decay have seriously hindered its practical application. This article reviews recent advances in catalyst dopin...     

On the kinetic and thermodynamic studies on the Cr-S system at high temperature

In order to elucidate the sulphidation behaviour of chromium at high temperatures, kinetic and thermodynamic investigations were carried out. Firstly, the P-T diagram of Cr₂S₃ (r.h.), Cr₂S₃ (tr), and Cr₃S₄ phases was determined. Then, kinetic studies and structural examination of the scales were performed. The sulphidation obeys a parabolic rate law, but Arrhenius plots of the rate constants show no straight lines. Scales were composed of many Cr sulphides and their distributions in the scales changed depending upon the corrosion conditions. A similar variation was observed in the surface morphologies. The mechanism of scale growth is discussed based on kinetic factors and lattice defects.     

螺旋藻粉生物质回收稀土元素Yb(Ⅲ):吸附等温线、动力学和热力学

研究以螺旋藻粉作为吸附剂从废水溶液中回收Yb(Ⅲ)的吸附特性和机理.利用SEM和XPS对吸附剂的表面结构以及元素价态进行分析,探究螺旋藻粉对Yb(Ⅲ)的吸附机理;通过吸附等温线、动力学和热力学模型分析螺旋藻粉对Yb(Ⅲ)的吸附特性.Langmuir模型表明,当温度为318 K时,螺旋藻粉对Yb(Ⅲ)的最大吸附量为72....     

Coupled thermodynamic/kinetic analysis of diffusional transformations during laser hardening and laser welding

Several important industrial material processes, such as welding and surface treatments with high energy beams, incorporate rapid thermal cycles characterized by high heating/cooling rates and short dwell times. Computational simulation of the evolution of microstructure under these extreme conditions has received rather limited attention. With the advent of modern computational tools regarding alloy thermodynamics and kinetics, it is possible to simulate the progress of diffusional phase transformations and thus to predict microstructural development. In the present work, moving boundary diffusion problems have been simulated for two cases. In the first case the rapid austenitization during laser transformation hardening of a hypoeutectoid steel was examined. The effects of heating rate, maximum temperature, dwell time and initial microstructure fineness were analyzed. In the second case the aging, dissolution and coarsening of strengthening precipitates in the heat affected zone of laser welds in Al–Mg–Si alloys was examined. The simulation provided the variation of the volume fraction and average size of the strengthening phase during the weld thermal cycle. In both cases the calculations were performed by applying the coupled thermodynamics and kinetics approach, incorporated in the DICTRA program. This kind of simulation provides useful information for the design of the above processes.     

Size dependency of melting point of crystalline nano particles and nano wires: A thermodynamic modeling

A semi-empirical thermodynamic model for size dependency of melting point of nano particles and wires has been proposed by introducing a size dependency of surface energy. The model predicts the size dependency of melting point of nano particles and wires for a wide range of elements: fcc (Au, Pt, Ni), hcp (Mg), and bcc (W), all in good agreement with experimental data and/or molecular dynamics simulations. Since the model is free from adjustable parameters, it is applicable to a wider range of materials.