Journal of Phase Equilibria and Diffusion最新文献

The quantification of the role of alloying elements on interface migration during phase transformations in steels and selected non-ferrous alloys (e.g., Ti-based) remains an active area of research that was inspired by seminal contributions of Mats Hillert. In previous studies we had introduced atomistically informed solute drag models for simulation of grain growth and recrystallization. In the present study this approach is extended to diffusional phase transformations where a fast-diffusing species (e.g., C in Fe) redistributes between the parent and daughter phases. The proposed methodology is demonstrated with a conceptional analysis of nano- and mesoscale phase field simulations for model binary and ternary alloys. The approach is shown to be consistent with the formulations of the Hillert–Sundman solute drag model. The challenges in applying this simulation approach to experimental data are critically discussed.

Preparation conditions play a key role in tailoring the properties of ceramic materials, including ferrite, to suit efficient industrial and technological applications. This manuscript is concerned with correlating the structural, spectroscopic, optical, transport, and dielectric properties of nickel zinc ferrite nanoparticles (NZNs) to the sintering temperature as an effective preparation condition controlled through the coprecipitation technique. Techniques for studying the various features of the synthesized compounds include x-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, Fourier-transform infrared spectroscopy, ultraviolet-visible spectroscopy, and the LCR bridge. XRD data showed the as-prepared samples' single-phase inverse spinel structure and an increase in crystallinity with increasing sintering temperature. SEM images show the nanosized range with semi-spherical particles for all the NZN fabricated samples. Moreover, Raman spectroscopy results showed that the four distinct active modes (Eg, F2g(2), A1g(2), and A1g(1)) for the NZN compounds have the same lattice strain tendency. The direct and indirect optical energy gap values (2-3.82) eV of the synthesized compounds span a wide range in the visible and ultraviolet spectrum, making them candidates for optoelectronic applications. In general, sintering temperature plays an outstanding role in increasing the values of some features such as crystallite size, optical energy gap, and electrical conductivity, and correspondingly decreasing other features such as unit cell volume dissociation density, absorption bands, and dielectric parameters.

Diffusion is one of the key research interests in developing advanced materials and processes, but acquisition of diffusion coefficients is a bottleneck. Hereby we propose a hybrid scheme that couples the molecular dynamics (MD) simulation for calculating tracer diffusion coefficients, with the CALPHAD method providing experimental benchmarks for pure metals and assessing atomic mobilities. After verifying the scheme by studying the fcc Ni-Co-Fe ternary alloys (including its pure components and binary sub-systems) at 1600 K, for which sufficient amount of experimental data as well as the CALPHAD assessments are available, this scheme demonstrates the ability of establishing a mobility database efficiently with reduced experimental efforts. In order to reconcile the MD simulations with the experiments, a priority is to determine impurity and self-diffusion coefficients for pure metals by CALPHAD assessments of the experimental data from the literature as benchmarks and thus set up a set of elemental bases for pure metals. Then all the MD calculated tracer diffusion coefficients were normalized accordingly, followed by the CALPHAD assessments to develop an atomic mobility database, from which all types of diffusion coefficients can be readily calculated.

The Ga₂O₃-SnO₂-ZnO (GSZO) system has been studied by Knudsen effusion mass spectrometry (KEMS) in the temperature range 1305-1495 K. The incongruent character of vaporization processes in the whole composition range of the GSZO system was observed. The main thermodynamic characteristics of heterogeneous equilibria in the system have been determined, as well as the standard enthalpies of reactions involving Zn₂SnO₄ and ZnGa₂O₄ spinels. A p–x vertical section of the phase diagram of the GSZO system has been plotted along the GaO_1.5 and SnO₂ equimolar ratio line.

Kinetics of the (gamma) /(gamma ^{prime}) phase transformation and the interdiffusion phenomena in single-crystal Ni-based superalloys, under isothermal annealing and composition gradient, is investigated through the phase-field and continuum diffusion models. The employed models in the present work exploit CALPHAD-based thermodynamics and kinetics databases, in order to perform realistic simulations. We specifically predict the interdiffusion of elements for a hypothetical alloy AlCoCrTaNi/Ni diffusion couple, equivalent to the CMSX-10/Ni diffusion couple, at 1323 K. Accordingly, the phase fraction and morphology of (gamma ^{prime}) precipitations in the (gamma) matrix is simulated as well. The implemented multi-component diffusion model takes into account vacancies and pore formation, reflecting Kirkendall effect. Furthermore, the time evolution of morphology parameters of the precipitate-depleted zone in the diffusive region (i.e., the position and the size) are estimated.

In various steel families, aluminum nitride plays an important role in the definition of steel behavior. It is now known that the effects are related to submicron particles, so it is not surprising that there was significant debate about the observed effects for a considerable time even as some of its benefits were already taken advantage of, in some steels. Currently, there is considerable knowledge and understanding of the effects of these particles and it is now possible, in many cases, to use computational thermodynamics to understand and properly tailor these effects. In the present work we briefly review how this knowledge developed and how the combination of thermodynamic and kinetic computational modeling makes it possible to take the most advantage of AlN in combination with the steel processing cycles. The results of the simulation examples presented should help engineers involved in alloy and process design and encourage the more extensive use of computational thermodynamic simulation in these design stages.

Hillert often used the combined first and second law of thermodynamics to discuss and derive the principles of equilibrium as well as non-equilibrium thermodynamics. His method will now be reviewed and applied to reactions in homogeneous systems as well as transport processes As an example of homogeneous systems undercooled liquids and glasses is discussed.

Two methods were used to investigate the Y₂O₃-Ta₂O₅ system: the equilibration method, which covered temperatures from 1573 to 1973 K, and the DTA method, which reached up to 2473 K. Phase identification was carried out using x-ray diffraction and scanning electron microscopy with energy-dispersive x-ray spectroscopy (SEM/EDX). The temperature of the eutectic reaction, L → YTa₃O₉ (P) + Ta₂O₅, was determined to be 2019 K, with the corresponding eutectic composition being 78 mol.% Ta₂O₅. The study presents evidence that contradicts the existence of the eutectic reaction L → YTaO₄ (T) + YTa₃O₉ (P). Instead, it identifies a peritectic reaction L + YTaO₄ (T) → YTa₃O₉ (P), which was observed at a temperature of around 2075 K. Additionally, the heat capacity of the YTa₃O₉ (P) phase was measured using differential scanning calorimetry (DSC) over the temperature range from 240 to 1300 K. The results of this experimental investigation will lead to the development of a thermodynamic database for the Y₂O₃-Ta₂O₅ system.

Nickel-base superalloys contain high amounts of solutes like Cr, Mo, W, Nb, Ti, etc. These solutes promote the formation of different types of carbide and boride phases that may contain multiple elements. Researchers have mostly discussed the roles of primary elements responsible for the formation of a given carbide/boride phase, often ignoring the role of other solutes on its stability. In the present work, thermodynamic stability of carbide and boride phases in seven commercial superalloys, namely, Alloy 625, Alloy 690, Alloy 718, MAR M246, Rene 100, Udimet 710 and Nimonic 80A, has been studied using the CALPHAD based Thermo-Calc software. The aim of the study was to understand the role of different alloying elements on temperature stability and chemical compositions of equilibrium phases in superalloys. As the accuracy of CALPHAD based predictions depends upon the database used, a detailed examination of its inadequacies has also been carried out to ascertain the limitations of the predicted data. From the calculated equilibrium chemical compositions, major and minor constituents promoting the formation of carbides and borides have been identified. The individual effect of a given solute as well as the synergistic effect of two solutes on the relative thermodynamic stability of carbide/boride phases has been identified using property diagrams and isothermal sections of the temperature-composition diagrams. Most of the simulated results have been found to be consistent with the experimental data available in the literature. From a comparison of the experimental literature and the simulated data of the stable carbide and boride phases in the studied alloys, the interplay of different solutes has been deduced to define conditions under which these phases form, within the limitations of the database used. This study has helped in better understanding of general tendencies of solutes to form different carbide and boride phases in nickel-based superalloys.