Journal of Phase Equilibria and Diffusion最新文献

In the present work, phase equilibria in the Li(_2)O–Al(_2)O(_3) system were experimentally studied and calorimetric measurements were performed. Based on obtained results and data from literature, thermodynamic parameters of the system were assessed. The solid solution phases were modeled using Compound Energy Formalism (CEF) and liquid phase was described by two-sublattice partially ionic liquid model. The experimental investigations for selected compositions of isothermally heat-treated samples were performed using x-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) were used to measure the temperature of the reactions as well as the heat capacities, respectively. After DTA, the microstructure was analyzed using SEM. Temperature of peritectic melting of h-LiAl(_5)O(_8) was determined to be 2222 K and temperature of eutectic reaction Liq (leftrightarrow) (gamma)-LiAlO(_2) + h-LiAl(_5)O(_8) to be 1965 K. Heat capacity of LiAlO(_2) and LiAl(_5)O(_8) was measured in the temperature range of 100-1300 K. The degree of inversion of spinel phase (Al(^{+3}), Li(^{+1}))(_1^T):(Al(^{+3}), Li(^{+1}), Va)(_2^O):O(_4) was modelled assuming Al(^{+3}) and Li(^{+1}) can occupy tetrahedral (T) and octahedral (O) cationic sublattices while its composition extension in Al(_2)O(_3) enriched region was described by introducing vacancies in octahedral sites. Thermodynamic description derived in the present study reproduces the degree of inversion close to that of the high-temperature spinel phase, which means that Al(^{+3}) ions preferentially occupy the tetrahedral sites. The calculated phase diagram satisfactorily agrees with the experimental results. Available experimental thermodynamic data are also reproduced within uncertainty limits.

Abstract

Systematic studies on the lattice expansion and order-disorder phase equilibria are attempted for the A-B binary alloy on the two-dimensional square lattice. The atomic pair potentials are described by the Morse potential and the configurational entropy is formulated within the pair approximation of the Cluster Variation Method (CVM). The lattice expansion of the uniformly deformable lattice is enhanced by introducing lattice vibration effects through the Debye–Grüneisen model. The introduction of the local lattice distortion by continuous displacement CVM (CDCVM) further increases the lattice expansion. The transition temperature obtained for a uniformly deformable lattice is reduced by the thermal vibration effects, which is interpreted as the curvature effects of atomic pair potentials. The local lattice relaxation further reduced the transition temperature, which is ascribed to the additional freedom of distributing atomic pairs over a wide range of distances.

Many different types of phases can form within alloys, from highly-ordered intermetallic compounds, to structurally-ordered but chemically-disordered solid solutions, and structurally-disordered (i.e. amorphous) metallic glasses. The different types of phases display very different properties, so predicting phase formation is important for understanding how materials will behave. Here, we review how first-principles data from the AFLOW repository and the aflow++ software can be used to predict phase formation in alloys, and describe some general trends that can be deduced from the data, particularly with respect to the importance of disorder and entropy in multicomponent systems.

Abstract

During powder-pack boronizing, an alloy is enveloped by a mixture comprising a source of boron, a diluent and an activator. If the activity of B in boriding media exceeds that in alloys, then this difference is a driving force, which, at elevated temperatures, may result in single- or multi-layered protective coatings. If it is intended to model and optimize this diffusion-controlled process, then an ability to calculate and control boron activity in both currently employed and prospective diluent-source-activator blends would be indispensable. In literature, such calculations are seldom performed, which likely reflects a reluctance or trepidation to deal with systems routinely containing five or more components. Consequently, it is commonly yet mistakenly believed that the activity of B can be smoothly changed by gradually changing a fraction of its source. In reality, a multiphase nature of boronizing powders is manifested in intricate concentration dependencies of boron activity with intervals where it remains constant. Another puzzling feature is the constancy of boron activity in phase fields which ostensibly have a non-zero number of degrees of freedom. In this work, a seeming inconformity with Gibbs' phase rule is addressed. Although it is not unreasonable to expect that an equilibrium phase assemblage at high temperatures would differ from ingredients mixed at room temperature, it is instructive to realize how drastically dissimilar they can be in some cases. If a boriding medium is utilized several times (a routine industrial practice), then its evolution caused by multiple heating and cooling cycles should not be overlooked.

Symmetrical boundaries with tilt axes <100> , <110> and <111> in nickel have been studied using atomistic modeling methods. The energies of grain boundaries (GBs), as well as the energies of point defects formation in high-angle GBs, have been calculated by the molecular statics method. Dependences of the GB energy on the misorientation angle are given for each of the three axes listed above. The energies of point defects formation are given as a function of energy depending on the distance between the position of the defect and the GB plane. Simulation of self-diffusion in high-angle GBs has been carried out in two versions–for vacancy and interstitial mechanisms of mass transfer. Temperature dependencies of self-diffusion coefficients have been constructed and the activation energies have been estimated. Based on the comparison of activation energies, the dominating diffusion mechanisms have been established. Anisotropy and the dominating diffusion mechanism are completely determined by the type of individual boundary. Based on calculations of the GB energy and self-diffusion simulation data, the Borisov correlation for high-angle GBs is considered, and it is shown that this correlation is quite poor for these grain boundaries.

The phase relationships of the Ti-Mn-Mo ternary system on the Ti-Mn side at 900 and 1000 °C were experimentally studied based on microstructure and phase constituents from the equilibrated alloys using electron probe microanalysis, scanning electron microscopy, and X-ray diffraction. The solubilities of Mo in the βTiMn, TiMn₂, TiMn₃ and TiMn₄ phases of the Ti-Mn system were measured. A three-phase region and seven two-phase regions were experimentally determined. No ternary compounds were found. Based on the experimental data and the thermodynamic descriptions of the three binary sub-systems available in the literature, a set of thermodynamic parameters of the Ti-Mn-Mo ternary system was obtained. The calculated isothermal sections and vertical sections agree well with the experimental results. The calculated liquidus projection and invariant reaction scheme of the Ti-Mn-Mo ternary system are also presented. The present work can provide essential experimental and thermodynamic data for the design of biocompatible medical titanium alloys.

Abstract

Proper descriptions of Topologically Closed-Packed (TCP) phases in thermodynamic databases are essential to adequately design new alloys. Thus, the recently introduced Effective Bond Energy Formalism (EBEF) is used in this work to describe the sigma (σ) phase in the Co-Cr-Ni-Re system. The EBEF is applied to a five-sublattice (5-SL) thermodynamic model consistent with its crystal structure and its implementation was supported by new data from Density Functional Theory (DFT). The Matrix Inversion Method is described and used to automate the generation of the EBEF parameters. Good descriptions of the ternary systems are obtained even without any ternary parameters for any of the phases. This is the first time that an EBEF description of a quaternary TCP phase is established using the SGTE descriptions for the pure elements.

The isothermal sections of the Al-Cr-Mo ternary system at 1200 and 1000 °C were experimentally determined based on microstructure and phase constituents from the equilibrated alloys employing electron probe microanalysis, scanning electron microscopy, and x-ray diffraction. A new ternary compound phase named η with AlTi₃-type crystal structure covering a composition range with ~ 75 at.% Al was detected in these two investigated isothermal sections in the present work. Based on the experimental results and the published data of the three binary sub-systems, a thermodynamic description for the Al-Cr-Mo system was carried out using the CALPHAD (CALculation of PHAse Diagrams) method. The newly detected ternary phase η was modeled by a two-sublattice model of (Al)₃(Al,Cr,Mo)₁. A set of reliable thermodynamic parameters of the Al-Cr-Mo system was obtained, which are in satisfactory agreement with the experimental data. Based on the obtained thermodynamic parameters, much information related to the stable vertical sections, isothermal sections, liquidus projection, and miscibility gap of the bcc phase were predicted in the present work. The present work can provide essential experimental and thermodynamic data for the establishment of the Ni-Al-Cr-Mo based alloy database.

This work aims to computationally explore the diffuse prerequisites for σ phase precipitation in high-alloyed duplex steels. The diffusion control for the early precipitation stage of the σ phase was studied by the Calphad thermodynamic modeling combined with the DICTRA software. Different nucleation sites, α/γ, and α/α and γ/γ grain-boundaries, were studied using thermodynamic and mobility databases developed by the present authors in previous works for high-alloyed duplex stainless steel. Simulation results were compared to experimentally obtained microstructures of aged samples. The study of the concentration profiles of the elements Cr, Mo, and Ni revealed that the precipitation is most favorable within the α phase due to fast mobility and strong thermodynamic driving force favoring the formation of the σ phase. Based on the present results, the role of the different heterogeneous sites for the precipitation of the σ phase in hyper duplex steel is discussed.