Journal of Phase Equilibria and Diffusion最新文献

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.

The phase equilibria of the La-Co-Zr ternary system at 873 K and 1073 K were studied for the first time using equilibrated alloys employing scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and x-ray diffraction (XRD). The crystal structures of the formed phases in La-Co-Zr alloys annealed at 873 K and 1073 K were identified through Rietveld refinement of the XRD results. The SEM-EDS and XRD results reveal that no ternary intermetallic compounds were detected in the La-Co-Zr ternary system and the binary intermetallic compound La₅Co₁₉ was not observed in the La-Co binary system. The solid solubility of the third element in the binary La-Co and Co-Zr intermetallic compounds was measured by EDS composition measurements. It was found that the maximum solid solubility of Zr in LaCo₅ and LaCo₁₃ is 1.8 and 4.0 at.% at 1073 K and it is 2.3 and 4.7 at.% at 873 K, respectively, while the maximum solid solubility of La in Co₂₃Zr₆ and Co₂Zr is 1.3 and 1.6 at.% at 873 K and it is 3.8 and 6.1 at 1073 K. Furthermore, the solid solubility of Zr and La in the other La-Co and Co-Zr intermetallic compounds is negligible. Finally, the La-Co-Zr ternary isothermal sections at 873 K and 1073 K were established.

Improving the oxygen diffusion capacity of solid electrolyte material is an important goal of researchers in recent decades. For this purpose, the blackening of zirconia was considered as a new strategy to enhance its oxygen diffusion capacity. We comparatively investigated the oxygen transport properties of white (YSZ, yttria stabilized zirconia) and black (ZSZ, Zr³⁺ stabilized zirconia) zirconia by molecular dynamics simulation. The simulation results show that the ZSZ has the same oxygen diffusion mechanism as YSZ. Furthermore, the ZSZ has a much better oxygen diffusion capacity than YSZ, which is confirmed by the lower minimum oxygen diffusion activation energy of ZSZ (0.37 eV) than that of YSZ (0.46 eV). The different oxygen diffusion capacity between YSZ and ZSZ is attributed to their crystal structure difference. The Zr³⁺ ionic radius is much closer to that of Zr⁴⁺ than that of Y³⁺, and it induces smaller lattice distortion in stabilized zirconia to impose a minimal steric blocking effect on the oxygen diffusion process. Therefore, the Zr³⁺ is preferred over Y³⁺to enhance the oxygen diffusion capacity of stabilized zirconia and the black YSZ co-doped by Y³⁺ and Zr³⁺ is proven to be a promising solid electrolyte material with a better oxygen diffusion capacity than YSZ.

In this work, the phase equilibria of the binary Ag-In system were determined using electron probe microanalysis and differential scanning calorimetry on equilibrated alloys and diffusion couples. The compositions of the phase boundaries of solid phases that are highly scattered in the literature have been precisely determined. The temperatures of the invariant reaction ζ ⇄ γ + L and the polymorphic transformation ζ ⇄ γ are substantially lower than previously reported values. In addition, it was found that the In-rich ζ phase phase-separates after months of room-temperature aging.

In non-ideal solutions, partial molar volumes change with composition, which means a diffusion process is always accompanied by change in volume of the system. To account for this change in volume while solving diffusion equation, it is necessary to know the velocity of the local center of volume (({U}^{V})). An expression is derived for ({U}^{V}), using a treatment that is applicable to a multicomponent system. Simulations of multicomponent diffusion profiles with composition dependent partial molar volumes have been absent in the literature so far. The expression derived in this work is also used to generate diffusion profiles in a hypothetical ternary diffusion couple. Significant difference is observed between the concentration profiles obtained with and without the assumption of constant molar volume. Exact calculation of ({U}^{V}) also enables the estimation of the expansion or contraction accompanied by diffusion, which in turn would help in assessing diffusion induced stresses and dimensional changes.

Multicomponent phase space has been shown to consist of an enormous number of materials with different compositions, the vast majority of which have never been made or investigated, with great potential, therefore, for the discovery of exciting new materials with valuable properties. At the same time, however, the enormous size of multicomponent phase space makes it far from straightforward to identify suitable strategies for exploring the plethora of potential material compositions and difficult, therefore, to be successful in discovering desirable new materials. Unfortunately, all our knowhow and understanding has been developed for materials with relatively few components in relatively limited proportions, with most of our scientific theories relying essentially on linear assumptions of component dilution and independence that no longer apply in concentrated multicomponent materials. Trial and error, controlled substitution, parameterisation, thermodynamic modelling, atomistic modelling and machine learning techniques have all been employed as methods of exploring multicomponent phase space, with varying levels of success, but ultimately none of these techniques has proved capable of delivering consistent or guaranteed results. This paper provides an overview of the different techniques that have been used to explore multicomponent phase space, indicates their main advantages and disadvantages, and describes some of their successes and failures.