Calphad-computer Coupling of Phase Diagrams and Thermochemistry最新文献

In the current work, the phase equilibrium of the CaO-Nb₂O₅-TiO₂ system in different atmospheres was investigated through high-temperature experiments. In air atmosphere, a total of 9 phase fields were determined: 5 three-phase fields(CaO·Nb₂O₅+5Nb₂O₅·2TiO₂+Nb₂O₅·TiO₂, CaNb₂O₆-Nb₂O₅·TiO₂-TiO₂, CaO·Nb₂O₅+3CaO·Nb₂O₅·3TiO₂+TiO₂, CaO·TiO₂+3CaO·Nb₂O₅·3TiO₂+TiO₂, CaNb₂O₆-CaTiO₃-(Ca,Nb,Ti)O), 3 two-phase fields(CaTiO₃+TiO₂, CaTiO₃+4CaO·Nb₂O₅, CaTiO₃+(Ca,Nb,Ti)O), and 1 single phase field (CaO·TiO₂). Some equilibrium experiments were also performed in Ar and H₂ atmospheres, with the phase equilibrium results same as that in air atmosphere. Meanwhile, the Gibbs energy data of Ca₄Ti₃O₁₀ and CaTi₂O₄ were calculated according to the CALPHAD principle. On this basis, the influence of oxygen partial pressure on the phase equilibrium of Ti/Nb containing oxide system was discussed. The research results of this paper are of theoretical significance for the development of the thermodynamic database of Ca-Nb-Ti-O slag system.

Novel alloy concepts enabled via additive manufacturing processes have opened up the possibility of tailoring properties beyond the scope of conventional casting and powder metallurgy processes. The authors have previously presented a novel Al–Mn–Cr–Zr-based alloy system containing three times the equilibrium amounts of Mn and Zr. The alloys were produced via a powder bed fusion-laser beam (PBF-LB) process taking advantage of rapid cooling and solidification characteristics of the process. This supersaturation can then be leveraged to provide high precipitation hardening via direct ageing heat treatments. The hardening is enabled with Zr-rich and Mn-rich precipitates. Literature study confirms that Mn-rich precipitates have a notable solubility of Cr, for example, the Al12Mn precipitate. This study aims to clarify the effect of Cr solubility in the thermodynamics and kinetics simulation and compare the precipitation simulations with samples subject to >1000 h isothermal heat treatment, thus creating an equilibrium-like state. The results show that Cr addition to the precipitates stabilizes the Al12Mn precipitate while slowing the precipitation kinetics thus producing a favourable hardening response. Such observations could be insightful while designing such alloys and optimising heat treatments of the current or even a future alloy system.

Diffusion study of the Ni–Si–V system is significant for the establishment of kinetic database of Ni-based alloys. In this work, the diffusion couple experiment combined with the numerical inverse method was adopted to evaluate the diffusivities and atomic mobilities for the Ni–Si–V fcc phase with high throughput. We prepared 12 fcc Ni–Si–V diffusion couples, which were annealed at 1273, 1373 and 1473 K, and their composition profiles after annealing were then measured by EPMA (Electron Probe Microanalysis). Subsequently, inputting the measured composition profiles as well as the available thermodynamic descriptions into the numerical inverse method incorporated in the CALTPP (CALculation of ThermoPhysical Properties) software, the composition- and temperature-dependent diffusivities and atomic mobilities for the Ni–Si–V fcc phase were simultaneously evaluated. In order to verify the reliability of the present evaluations, the CALTPP-simulated diffusion behaviors such as composition profiles and diffusion paths were compared with the measured ones, demonstrating reasonable agreements with each other. Meanwhile, the high-throughput determinations of diffusivities were confirmed by the ones obtained by the Matano-Kirkaldy method. Furthermore, applying the presently obtained diffusivities and atomic mobilities in combination with thermodynamic descriptions of the Ni–Si–V fcc phase, their diffusion flux, two-dimensional composition profile, activation energy and pre-frequency factor were predicted. It is expected that the presently obtained diffusivities and atomic mobilities of the Ni–Si–V fcc phase can contribute to the establishment of kinetic database of Ni-based alloys for their high-efficiency material design.

Understanding the formation mechanism of core-rim structure in TiC-based cermets is essential for optimizing their mechanical properties, such as strength and toughness. In this work, the formation process of core-rim structure in TiC-WC-Ni cermet at the early sintering stage was investigated by means of the multiphase and phase-concentration modeling framework coupled with CALPHAD (CALculation of PHAse Diagrams) databases. The effect of W content in the Ni binder phase on the rim thickness was examined. The results reveal that W in the Ni binder phase diffuses into the undissolved TiC particles at the early sintering stage, leading to the formation of (Ti,W)C solid solution phase on the particle surface. This solid solution phase serves as the rim which constitutes the core-rim structure together with TiC particles. Furthermore, there is a positive correlation between the rim thickness and the W content in the Ni binder phase. Phase-field simulation exhibits a good agreement with the scanning electron microscopy (SEM) observations of the annealed WC-Ni/TiC–Ni cermet diffusion couple. This study offers a novel approach for exploring the formation mechanism of core-rim structure in TiC-based cermets.

The present work reports on a thermodynamic modeling of the Mo–Pt system using a combined first-principles/CALPHAD approach. First-Principles (FP) calculations are performed to obtain the enthalpies of formation for the four ordered phases of the system (A15, B19, D0₁₉ and MoPt₂) at 0 K. The liquid, bcc, fcc and hcp phases have been modeled as substitutional solutions where the excess term is in the form of the Redlich–Kister polynomial. The four ordered phases are modeled using the NACEF approach taking into account the three order-disorder transitions (B19/A3, D0₁₉/A3 and MoPt₂/A1). FP calculations, experimental phase equilibria and site occupancies data are used to evaluate the model parameters. The thermodynamic description of the Mo–Pt system obtained in this work agree well with all the available data with a limited number of parameters used.

Phase equilibria knowledge on the Al–V–Zr system has an important role for designing low-density Al-containing refractory multi-principal element alloys. In order to contribute to the literature data related to this system, the liquidus projection of the Al–V–Zr ternary system was experimentally investigated in this work by microstructural characterization of as-cast alloys. Sixty alloys were produced in an arc furnace, the microstructures of the as-cast alloys were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) for the determination of the chemical composition of the phases and microconstituents as well as X-ray diffractometry (XRD). The liquidus projection of the Al–V–Zr system is presented in this work for the first time in the literature. Through microstructural analysis and thermodynamic extrapolation, three Class I, eleven Class II and one Class III ternary invariant reactions are proposed. The results showed that the ternary compound Zr_0.9V_0.4Al_2.7 is formed from the liquid through the following reaction: L + ZrAl₂ → Zr_0.9V_0.4Al_2.7. The limits of the 12 primary solidification regions are established and the nature of the monovariant and ternary invariant reactions are determined. The ZrAl₂ primary solidification region significantly extends into the liquidus projection, participating in most reactions involving the other phases in equilibrium with the liquid.

In this work, the composition dependent tracer and interdiffusion coefficient with phase change in γ/γ′ Ir/Ir₃Nb superalloy are studied by means of first-principles calculation together with nudged elastic band, and quasi−harmonic thermodynamics. The formulae scattered in the literature for composition dependence tracer diffusion coefficient are summarized and executed using first principles calculation. Each term in the newly derived formulae, such as energy barrier, jump frequency, defect concentration, correlation, solvent enhancement factor, effective escape frequency, etc. is meticulously calculated. We find the composition dependent tracer diffusion coefficient of Nb in L1₂ γ′-Ir₃Nb phase is contributed mainly from the antisite bridge rather than five-frequency model proposed in FCC γ-Ir dilute solid solution. The faster diffuser is Nb in γ-Ir, while Ir in γ′-Ir₃Nb. Based on the tracer diffusion coefficient, the interdiffusion coefficient is obtained using Darken-Manning equation. The composition profile and phase boundary movement of γ/γ′ Ir/Ir₃Nb superalloy are evaluated through an analytical solution of Matano-Boltzmann equations. The calculated diffusion coefficients and composition profile are in good agreement with experiments. Our findings not only serve as a successful example for the quantitative calculation of composition dependent tracer diffusion coefficient, but also shed lights on the physics behind the diffusion in intermetallics.

We performed ab initio molecular dynamic simulations to investigate the thermodynamic, dynamic, and structural properties of Al–Ni melts at 2033 K. The calculated enthalpies of mixing exhibit a strong compositional dependence, and the relationship between chemical short-range order and thermodynamic mixing quantities is effectively described by the modified regular solution model. Furthermore, the self-diffusion coefficients with a strong non-linear compositional dependence in Al-rich melts are closely related to the structural evolution of Al–Ni melts. Examination of local ordering evolutions reveals that the strong chemical affinity between dissimilar species in Al–Ni melts gives rise to the formation of a heterogeneous structure characterized by string-like or solute-solvent network medium-range order. Finally, the coupling of chemical short-range order and string-like medium-range order demonstrates a clear correlation between structural features and dynamic properties. These aspects provide deeper insight into the structure-dynamics relationship.

The ternary phase diagram Ti–Co–Sn was studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Isothermal sections at 1000 and 1200 °C have been determined experimentally for the first time in the entire range of compositions. Thirteen and ten three-phase regions were found at 1000 °C and 1200 °C, respectively. Vertical sections at 10, 20 and 30 at.% Sn were plotted. The most striking feature of the ternary Ti–Co–Sn phase diagram is formation of a ternary compound TiCo₂Sn (Heusler phase, τ), which was found at both investigated temperatures. The TiCoSn compound (half-Heusler phase) was not found. Among binary compounds, Ti₅Sn₃ and TiCo have the widest homogeneity regions. The Ti₅Sn₃ phase dissolves 10.0 and 9.8 at.% Co at 1200 and 1000 °C, respectively, forming an interstitial solid solution. The solubility of Sn in TiCo is more than 10 at.% at both 1200 and 1000 °C. The remaining binary intermetallic phases hardly dissolved the third component. The liquid phase at 1000 °C mainly exists in the Sn-rich corner, while at 1200 °C it stretches along Co–Sn side spreading from the Sn corner and is also present on the Ti-rich side. In addition, two four-phase invariant transition type reactions TiCo₂ (h) + (αCo) ⇄ τ + TiCo₃ and TiCo + TiCo₂ (h) ⇄ τ + TiCo₂ (c) were deduced.

The phase transformations and pressure-volume dependencies of the Ti–45Al alloy with respect to pressure have been investigated by means of in-situ observation using multi anvil-type high-pressure devices and synchrotron radiation. Under hydrostatic compression from 0 to 10.1 GPa, about 2.3 vol % of γ transforms continuously to α₂. Lattice parameters as well as volume fractions of these two phases have been determined as functions of pressure. Bulk moduli estimated using Birch-Murnaghan's equation of state are 146.2 GPa for the γ phase, 136.7 GPa for the α₂ phase, and 145.6 GPa for their two-phase mixture of Ti–45Al alloy. First-principles have also applied to investigate bulk moduli of two single phases, and the deviation between calculations and measurements is discussed and attributed to mainly phase transformation. The present study provides useful insights into thermodynamics of α₂ and γ phases under high pressure.