Modeling of the viscosity in the AL–Cu–Mg–Si system: Database construction

The viscosity database for the Al–Cu–Mg–Si system was constructed using the CALPHAD (CALculation of PHAse Diagram)-type formalism. Viscosities of pure elements were described with the Arrhenius formula based on the experimental data. Subsequently, viscosities of the Al–Cu, Al–Si, Al–Mg and Cu–Si binary systems were assessed via CALPHAD technique and compared with the corresponding experimental data. Due to the lack of experimental data, viscosities in the Mg–Si and Cu–Mg systems were estimated by means of the Hirai's equation. The viscosities of the ternary Al–Cu–Si system were then predicted based on the binary parameters and compared with the experimental results. Using the established viscosity database for the quaternary Al–Cu–Mg–Si system, the viscosities of some commercial aluminum alloys were predicted. The reasonable agreement between calculations and experiments in Al-rich corner indicates that the CALPHAD-type database for the viscosity is valid and the database is suitable for predicting the viscosity of the commercial Al–Cu–Mg–Si based alloys.     

Thermal destabilization of binary and complex metal hydrides by chemical reaction: A thermodynamic analysis

Some alkali and alkali-earth metal hydrides and their complex hydrides have very high hydrogen storage capacities and reversibility. Unfortunately, most of them have decomposition temperatures that are too high. This must be overcome before these hydrides can be considered seriously as practical hydrogen storage materials for on-board applications. In the present study, the CALPHAD approach has been adopted to evaluate thermodynamically the possibility of destabilizing these high temperature binary ionic hydrides and ternary complex hydrides by reacting them with light elements or other hydrides. The MgH₂+Si, LiBH₄+MgH₂, and LiBH₄+Al systems are predicted to show a significant decrease in decomposition temperature. On the other hand, the decrease in the decomposition temperatures of the MgH₂+Al and NaBH₄+Al systems is relatively small. The LiH+Si system also presents a considerable destabilization effect, which is consistent with experiment.     

Alloy composition and process design based on thermodynamic and kinetic simulation: The case of medium Mn steel

This paper presents a design method for alloy materials that can be applied to medium Mn steel containing Al and/or Si. By using thermodynamic calculations based on CALPHAD, a wide range of alloy composition spaces were systematically studied, and the relationship between retained austenite fraction and its stability with intercritical annealing temperature was investigated. Alloys that meet the Process Window (PW) were screened and their various characteristics of alloy elements within the process window were determined. Additionally, the study performs three different kinetic simulations of austenite growth and solute partitioning during isothermal annealing on alloys that meet PW. The prediction of the model is verified by comparing with the experimental data in the literature. The results show that the remote model is more suitable for the actual production of low temperature isothermal intercritical annealing. Therefore, the method provided in this work makes a guiding contribution to alloy development, which can determine the alloy composition and heat treatment process satisfying PW and optimize the overall properties.     

A thermodynamic model for carbon trapping in lattice defects

A thermodynamic model in the framework of the CALPHAD method is proposed to describe the distribution of interstitial solute (carbon) atoms in bcc-iron during tempering of martensite. In the model, the crystal defects are introduced as a separate sublattice, which is a highly favored lattice position for carbon compared to undisturbed interstitial sites or precipitate sites in cementite. With the model, which is convenient for multicomponent, multiphase systems, the fraction of carbon atoms trapped by the defects during tempering can be calculated. It is demonstrated that, when martensite is heavily dislocated, the precipitation of cementite on tempering is greatly suppressed because the high dislocation density provides a sufficient number of stressed sites which can trap most of the carbon atoms. The calculations for an alloy steel show that, unlike cementite, the defect density and carbon trapping phenomenon influence the precipitation of stable carbides only marginally.     

Atomic mobilities and diffusion characteristics for fcc Cu-Ag-Au alloys

CALPHAD kinetics has evolved to be a well-established discipline, which allows complex non-equilibrium processes to be fully explored. Such a success relies on the use of Redlich-Kister polynomials to describe atomic mobilities, with the effect of temperature, composition, magnetic ordering and chemical ordering sufficiently taken into consideration. There is thus an increasing demand to construct high-quality atomic mobility databases for alloys of interest. Based on the CALPHAD framework, the atomic mobilities in fcc Cu-Ag-Au alloys are reported in this work, the results of which can provide fruitful information on alloy design.     

Application of CALPHAD to high pressures

The current methods of performing CALPHAD (CALculation of PHAse Diagrams) calculations of high-pressure phase equilibria often lead to spurious predictions of negative thermal expansion or negative heat capacity at high pressures. It is shown that the origin of the problem lies in an incompatibility between the temperature dependence of the widely used SGTE (Scientific Group Thermodata Europe) database and that of typical equations of state of the Mie–Grüneisen type. This inconsistency is also linked to the general problem of describing mechanical instability in CALPHAD. In the present work, a new free energy formulation is developed specifically for inclusion of pressure effects in CALPHAD methodology. It is based on an interpolation between SGTE data at low pressures and the quasiharmonic lattice model at high pressures. The new formulation is constrained to physically credible predictions of the thermophysical properties, while preserving the simplicity of the CALPHAD method. Examples are given of calculations of thermophysical properties and high-pressure phase equilibria in Al, Si, MgO, Fe and the Al–Si binary alloy system.     

A thermodynamic analysis of the calphad approach to phase stability of the transition metals

The phase stability of the transition metals is usually treated by adopting two alternative approaches, a theoretical one, based on (“ab-initio”) quantum mechanical calculations of electron states, and a semiempirical one based on the so-called CALPHAD activity (i.e. Calculation of Phase Diagrams). There is a conflict between the results of these approaches and the purpose was now to investigate whether the causes of the conflict can be clarified by applying thermodynamic methods, and considering various types of available information, melting points, entropy differences, phase diagrams, and the models for solution behaviour used in current CALPHAD work. It is concluded that the reconciliation of these approaches to phase stability of the transition metals cannot be achieved unless a) CALPHAD assumption of a positive melting temperature for all metastable phases is abandoned, and b) the assumption of small deviations from the regular-solution behaviour, which is implicit in the current CALPHAD models for the solution phases, is replaced by treatments accounting for the existence of sharper variations in the thermodynamic properties with the delectron concentration.     

Thermodynamics and its prediction and CALPHAD modeling: Review,state of the art,and perspectives

Thermodynamics is a science concerning the state of a system, whether it is stable, metastable, or unstable, when interacting with its surroundings. The combined law of thermodynamics derived by Gibbs about 150 years ago laid the foundation of thermodynamics. In Gibbs combined law, the entropy production due to internal processes was not included, and the 2^nd law was thus practically removed from the Gibbs combined law, so it is only applicable to systems under equilibrium, thus commonly termed as equilibrium or Gibbs thermodynamics. Gibbs further derived the classical statistical thermodynamics in terms of the probability of configurations in a system in the later 1800's and early 1900's. With the quantum mechanics (QM) developed in 1920's, the QM-based statistical thermodynamics was established and connected to classical statistical thermodynamics at the classical limit as shown by Landau in the 1940's. In 1960's the development of density functional theory (DFT) by Kohn and co-workers enabled the QM prediction of properties of the ground state of a system. On the other hand, the entropy production due to internal processes in non-equilibrium systems was studied separately by Onsager in 1930's and Prigogine and co-workers in the 1950's. In 1960's to 1970's the digitization of thermodynamics was developed by Kaufman in the framework of the CALculation of PHAse Diagrams (CALPHAD) modeling of individual phases with internal degrees of freedom. CALPHAD modeling of thermodynamics and atomic transport properties has enabled computational design of complex materials in the last 50 years. Our recently termed zentropy theory integrates DFT and statistical mechanics through the replacement of the internal energy of each individual configuration by its DFT-predicted free energy. The zentropy theory is capable of accurately predicting the free energy of individual phases, transition temperatures and properties of magnetic and ferroelectric materials with free energies of individual configurations solely from DFT-based calculations and without fitting parameters, and is being tested for other phenomena including superconductivity, quantum criticality, and black holes. Those predictions include the singularity at critical points with divergence of physical properties, negative thermal expansion, and the strongly correlated physics. Those individual configurations may thus be considered as the genomic building blocks of individual phases in the spirit of the materials genome®. This has the potential to shift the paradigm of CALPHAD modeling from being heavily dependent on experimental inputs to becoming fully predictive with inputs solely from DFT-based calculations and machine learning models built on those calculations and existing experimental data through newly developed and future open-source tools. Furthermore, through the combined law of thermodynamics including the internal entropy production, it is shown that the kinetic coefficient matrix of independent internal processes is diagonal with respect to the conjugate potentials in the combined law, and the cross phenomena that the phenomenological Onsager flux and reciprocal relationships are due to the dependence of the conjugate potential of a molar quantity on nonconjugate molar quantities and other potentials, which can be predicted by the zentropy theory and CALPHAD modeling.     

Thermodynamic modeling of the Si-Y system aided by first-principles and phonon calculations

Thermodynamic modeling of the Si-Y binary system has been performed by the CALPHAD (CALculation of PHAse Diagram) method based on phase diagram and thermochemical data in the literature combined with Gibbs energies of end-members of compounds predicted by first-principles phonon calculations. In particular, non-stoichiometric compounds Si₂Y and Si₃Y₅ are modelled to accommodate their homogeneity ranges in terms of two-sublattice models (Si,Y)₂(Si,Y) and (Si)₃(Si,Y)₅, respectively. Formation of SiY is treated as a peritectic reaction according to experimental results, instead of an eutectic one as described in the previous models. The calculated phase equilibriums and thermodynamic properties are in a satisfactory agreement with available experimental data.     

Computational design of V-CoCrFeMnNi high-entropy alloys: An atomistic simulation study

In the high-entropy alloy (HEA) community, many researchers have been trying to improve the strength of the CoCrFeMnNi HEA by generating a transformation-induced-plasticity (TRIP) effect and/or maximizing the solid solution hardening effect. Adding vanadium (V) to the CoCrFeMnNi HEAs could be an effective way to improve strength, because vanadium stabilizes the body-centered cubic (bcc) phase and its atomic size is larger than Co, Cr, Fe, Mn, and Ni. To design high strength V-added HEAs, we investigated the effect of vanadium on the critical resolved shear stress (CRSS) by utilizing an atomistic simulation, proposing an empirical equation to estimate the relative effect of alloying elements on the CRSS. For this, we first developed the Co-Cr-Fe-Mn-Ni-V hexanary interatomic potential by newly developing the Cr-V, Fe-V, and Mn-V binary interatomic potentials. As a result, two novel V-added HEAs were designed and the designed HEAs show higher strength than the previously developed non-equiatomic CoCrFeMnNi HEAs, as predicted from the empirical equation.     

Phase equilibria and microstructure development in Mg-rich Mg-Gd-Sr alloys: Experiments and CALPHAD assessment

Mg-Sr alloys are promising to fabricate orthopedic implants. The alloying of rare earth elements such as Gd may improve the comprehensive mechanical properties of Mg-Sr alloys. The information on the phase diagram and the microstructure development are required to design chemical composition and microstructure of Gd alloyed Mg-Sr alloys. The phase equilibria and the microstructure development in Mg-rich Mg-Gd-Sr alloys (Gd, Sr < 30 at. %) are experimentally investigated via phase identification, chemical analysis, and microstructure observation with respect to the annealed ternary alloys. The onset temperatures of liquid formation are measured by differential scanning calorimetry. A thermodynamic database of the Mg-rich Mg–Gd–Sr ternary system is developed for the first time via CALPHAD (CALculation of PHAse Diagram) approach assisted by First-Principles calculations. The thermodynamic calculations with the developed database enable a well reproduction of the experimental findings and the physical-metallurgical understanding of the microstructure formation in solidification and annealing.     

Optimization of heat treatment process of Al–Mg–Si cast alloys with Zn additions by simulation and experimental investigations

With increasing demands of the high strength and light weight suspension components from automotive industry, aluminum alloys with higher strength than traditional alloys must be developed. In order to meet this requirement for cast Al alloys, higher solute concentration than traditional A356 alloys is needed to enhance the solute and later precipitation strengthening effects. Considering Zn as one of the alloying elements with high solubility in Al, we investigate the effects of Zn addition and related heat treatment parameters on the mechanical properties of Al–Si–Mg alloys by using a combination of experimental and thermodynamic methods. With the addition of Zn, the yield strength (YS) and ultimate tensile strength (UTS) of Al–Si–Mg alloys are significantly improved by up to 52.2% and 8.5% compared with those without Zn additions. After a proper heat treatment, the YS and UTS of Al–Si–Mg–Zn alloys can be further increased by up to 35% and 28%, respectively, compared to the as-cast state. Based on the study, optimal heat treatment parameters for Al–Si–Mg alloys with different Zn contents are proposed.     

催化剂对煤着火特性影响规律的计算机辅助研究（Ⅰ）碱金属、碱土金属盐对煤的催化着火规律

应用计算机高速采集煤在燃烧过程中的各瞬时温度，通过特殊软件的数据处理，可以直接得到表征煤着火特性的着火温度，并应用该种计算机辅助研究方法，研究碱金属、碱土金属盐类对煤的催化着火影响。研究表明：碱金属、碱土金属盐的催化活性，随对应的金属的第一电离能的减弱而递增，但带结晶水的碱土金属盐例外，其催化活性，明显优于该属不带结晶水的的盐。