Phase Equilibrium studies in the system “FeO”–SiO2–Al2O3–CaO–MgO at 1200 °C and Po2 10-8atm

Iron oxides and silica are the major components of copper smelting slag. The oxides of aluminum, calcium and magnesium are also present in the slag that is introduced through copper concentrate, flux and refractories. Liquidus temperatures of the copper smelting slags are usually controlled by Fe/SiO₂. The concentrations of Al₂O₃, CaO and MgO, and FeO/Fe₂O₃ in the slag can also affect the liquidus temperatures where FeO/Fe₂O₃ is a function of oxygen partial pressure. High temperature equilibration under controlled oxygen partial pressure followed by quenching and electron probe microanalysis were used to determine the compositions of the liquid and solid phases at 1200 °C and Po₂ 10^-8 atm. The experimental results are presented in the forms of pseudo-ternary sections “FeO”-CaO-SiO₂ at fixed 2, 4 and 6 wt pct MgO, and 2 + 2, 4 + 4 and 6 + 6 wt pct MgO + Al₂O₃. Spinel and tridymite are the major primary phases in the composition range investigated. In addition, CaSiO₃, pyroxene, olivine, and melilite are also present. The isotherms in the spinel and tridymite primary phase fields move towards higher SiO₂ concentration directions with increasing CaO, Al₂O₃, and MgO concentrations. The experimentally determined results are compared with the FactSage calculations.     

Thermochemical analysis for the reduction behavior of FeO in EAF slag via Aluminothermic Smelting Reduction (ASR) process: Part II. Effect of aluminum dross and lime fluxing on Fe and Mn recovery

We investigated Fe recovery from EAF slag by means of aluminothermic smelting reduction (ASR) at 1773 K with Al dross as the reductant, especially the effect of the added amount of the fluxing agent CaO on the Fe recovery. The maximum reaction temperature calculated using FactSage™ 7.0 decreased with increasing CaO addition, but the experimentally measured maximum temperatures increased with increasing CaO addition. We calculated the amounts of various phases before and after Al dross addition under different conditions of added CaO. FeO and Al₂O₃ contents in molten slag sharply varied within the first 5 min of the reaction, stabilizing soon thereafter. The aluminothermic reduction of FeO appeared to proceed rapidly and in good stoichiometric balance, based upon the mass balance between the consumption of FeO and MnO (ΔFeO and ΔMnO) and the production of Al₂O₃ (∆Al₂O₃). Iron recovery from EAF slag was maximized at about 90% when 40 g of CaO was added to 100 g slag. Furthermore, Mn could also be reduced from the EAF slags by the metallic Al in the Al dross reductant. The solid compounds of spinel (MgO∙Al₂O₃) and MgO were precipitated from the slag during the FeO reduction reaction, as confirmed by means of XRD analysis and thermochemical computations. To maximize Fe recovery from EAF slag, it is crucial to control the slag composition, namely to ensure high fluidity by suppressing the formation of solid compounds.     

Comment on “Thermodynamic modelling of an Al2O3-MnO system using the ionic model” by A.B. Farina and F.B. Neto

The MnO-Al₂O₃ system is very important for the non-metallic inclusion controls in steelmaking and the slag chemistry for high Mn alloy production. Farina and Neto presented their recent thermodynamic assessment on the MnO-Al₂O₃ system using a two-sublattice ionic liquid model for liquid oxide phase and the Compound Energy Formalism for solid phases. However, we found several doubts and mistakes in their assessment in particular related to the Gibbs energy of MnAl₂O₄ spinel phase.     

Combined ab initio/CALPHAD modeling of fcc-based phase-equilibria in the Ir–Nb system

A preliminary thermodynamic assessment of the Ir–Nb system, one of the key binary systems of the Ir-based refractory superalloys, has been performed by combining ab initio calculations and the CALPHAD (CALculation of PHAse Diagrams) technique. The ground-state formation enthalpies have been calculated by the full-potential linearized augmented plane wave method. The free energies at finite temperatures have been estimated using the cluster variation method, where the effective cluster interaction energies have been extracted from the formation enthalpies by the cluster expansion method. The liquid and A1 phases are modeled as substitutional solutions. The L1₀ and L1₂ phases are described using the four-sublattice model with the formula (Ir,Nb)_1/4(Ir,Nb)_1/4(Ir,Nb)_1/4(Ir,Nb)_1/4, while other solid phases are not considered in the present assessment. The obtained parameter set reproduces well the characteristic features of the experimental phase diagram and thermodynamic quantities.     

Effect of TiO2 on viscosity and structure for CaO-SiO2-Al2O3-based mold flux

The viscosity of CaO-SiO₂-Al₂O₃-MgO-Na₂O-CaF₂-(TiO₂) was measured by the rotating column method, and the viscosity, break temperature and viscous flow activation energy of the slag were analyzed based on the results. The effect of TiO₂ content on the structure of the slag was studied by Raman spectroscopy. The results show that with the increase of TiO₂ content in CaO-SiO₂-Al₂O₃-based mold flux, the viscosity of mold flux increases, the break temperature increases from 1418 K to 1535 K, and the viscous flow activation energy changes little. At high temperature, TiO₂ can depolymerize the [Si-O] and enter into the network structure as [Ti-O], which increases the frictional resistance to the viscous flow of the mold flux. At the same time, the crystallization temperature and viscosity of the slag is increased.     

Phase equilibria of the Al2O3–CaO–SiO2-(0%, 5%, 10%) MgO slag system for non-metallic inclusions control

Al₂O₃–CaO-(MgO–SiO₂) inclusions are one of the dominant inclusions in Al-deoxidized spring steel, the compositions changes of which are closely related to refining slags and deoxidization process. The Al₂O₃–CaO–SiO₂–MgO system can represent the primary ingredients of the Al₂O₃–CaO inclusions. According to analyzed compositions and predicted liquidus temperature ranges of inclusions and refining slag, equilibra experiments under high temperature, water quenching technique and subsquent electron probe X-ray microanalysis (EPMA) has been conducted to ascertain detailed thermodynamic database for inclusions control. Liquidus temperatures within the dominant phase fields of Ca₃SiO₅, Ca₂SiO₄, CaAl₂O₄, Ca₃Al₄O₉, spinel and MgO with the intervals of 20 °C from 1350 to 1560°C were identified. To further promote inclusions control, the influences of mass ratios of Mass(Al2O3)Mass(Al2O3+SiO2+CaO) and MgO contents on equilibrated phases and liquidus temperature changes have been explored. To further enhance modification levels of Al₂O₃–CaO-(MgO–SiO₂) system inclusions, it is suggested that refining time could be suitably prolonged.     

Thermochemical analysis for the reduction behavior of FeO in EAF slag via Aluminothermic Smelting Reduction (ASR) process: Part Ι. Effect of aluminum on Fe & Mn recovery

We investigated Fe recovery from EAF slag by means of Aluminothermic Smelting Reduction (ASR) at 1773 K, especially the quantitative effect of initial Al/FeO molar ratio upon the Fe recovery. Both calculated and experimentally measured system temperatures continuously increased with increasing initial Al/FeO molar ratio. Furthermore, to predict the reduction behavior we calculated variations in the slag composition by using FactSage™ 7.0 software. FeO and Al₂O₃ contents in molten slag varied sharply within the first 5 min of the reaction and stabilized soon thereafter. The aluminothermic reduction of FeO appeared to proceed rapidly and in good stoichiometric balance, based upon the mass balance between the consumption of FeO and MnO (ΔFeO and ΔMnO) and the production of Al₂O₃ (∆Al₂O₃). Adding an optimal amount of Al (Al/FeO molar ratio ~ 0.8) yielded a Fe recovery of about 90%. Furthermore, the Mn could also be reduced from the EAF slag in the case of excess Al addition (Al/FeO≥0.8). The solid compound spinel (MgO·Al₂O₃) was precipitated from the slag during the FeO reduction, as confirmed by means of XRD analysis and thermochemical computations. Herein, the mechanism of ASR reaction between FeO in molten slag and Al is explained in several steps.     

Critical assessment and thermodynamic modeling of the Cu–Fe–O–Si system

The present study is the first Calphad-type assessment of the Cu–Fe–O–Si system. All relevant thermodynamic and phase equilibrium data have been critically evaluated to produce a thermodynamic database describing the Gibbs energies of all phases in the system. The predictive range of the database covers all conditions of pyrometallurgical production of copper in terms of temperature and oxygen partial pressure. Liquid oxide slag and liquid metal phases have been described using two separate solution models, both developed within the framework of the Modified Quasichemical Formalism. Slag model is expressed as [Cu⁺, Fe²⁺, Fe³⁺, Si⁴⁺][O^2-] and metal model is expressed as (Cu^I, Fe^II, O^II). They are internally consistent with the models for fcc–Cu, fcc–Fe, bcc–Fe, spinel, wüstite, CuFeO₂, Cu₂O, Fe₂SiO₄, Fe₂O₃ and SiO₂ obtained in the previous optimizations of the Cu–O, Fe–O, Cu–Fe, Cu–Fe–O, Cu–O–Si, Fe–O–Si sub-systems.     

Critical thermodynamic re-evaluation and re-optimization of the CaO–FeO–Fe2O3–SiO2 system

Comprehensive literature review, critical re-assessment and thermodynamic re-optimization of phase diagrams and thermodynamic properties of all phases have been carried out for the CaO–FeO–Fe₂O₃–SiO₂ system. Thermodynamic assessments of this system were previously published, however some of them were incomplete or described only limited range of compositions. In addition, more recent important experimental data have been available after previous assessments. The Modified Quasichemical model is used to describe the Gibbs energy of the liquid slag. The monoxide, wollastonite, α-Ca₂SiO₄ and α’-Ca₂SiO₄ solid solutions are described using the random mixing Bragg-Williams model. Spinel, olivine, pyroxene and melilite solid solutions are modelled using sublattice model based on the Compound Energy Formalism. A set of optimized parameters for the thermodynamic models has been obtained which reproduces all available experimental data within the experimental uncertainties from sub-solidus to above the liquidus temperatures at all compositions and atmosphere conditions.     

Characterization of Al–Ni intermetallics around 30–60 at% Al for TLPB application

Interest on the Al–Ni equilibrium diagram along the latest years is associated with the attractive properties of its intermetallic phases, such as high thermal stability, high corrosion resistance and high strength to density ratio. The Transient Liquid Phase Bonding (TLPB) is a technological process which can be applied to manufacture new pieces and to perform reparations. Morphology, composition profiles, growth kinetic and hardness as a function of temperature and composition of the Intermetallic Layers (ILs) were analyzed, especially focused on solid–solid interactions during isothermal annealing in reactive diffusion couples Ni/Al (800–1170 °C). The study yields to the following association of the Al–Ni Intermetallic Phases (IPs) to the ILs: L₁ (Al₃Ni), L₂ (Al₃Ni₂), L₃ (Ni-poor AlNi), L₄ (Ni-rich AlNi) and L₅ (AlNi₃). The composition ranges of L₃ and L₄ are 36–46 and 53–58 at% Al, respectively. Martensitic transformation was found in the half thickness of L₄ (L_4M and L_4S) at 1170 °C. Kinetics show diffusion controlled growth for L₂ and L₅ and interface reaction control for L₄ at 800–1170 °C, while L₃ revealed a mixed kinetic behavior: parabolic at 800–1000 °C and linear at 1170 °C. The growth rate constants presented temperature dependence according to the Arrhenius model. Vickers microhardness values decrease with annealing temperature and Ni concentration for ILs, and put in evidence different mechanical properties of L₃, L_4M and L_4S.     

Thermodynamic optimization of the binary PbO-“Cu2O”, “Cu2O”-SiO2 and ternary PbO-“Cu2O”-SiO2 systems

Liquidus phase equilibrium data obtained in the recent study by the authors for the binary PbO–“Cu₂O”, “Cu₂O”–SiO₂ and the ternary PbO–“Cu₂O”–SiO₂ systems in equilibrium with metallic Cu or Pb–Cu alloy (as a part of research program on the characterization of the multicomponent PbO–ZnO–FeO–Fe₂O₃–“Cu₂O”–CaO–SiO₂ system), combined with phase equilibrium and thermodynamic data from the literature, have been used to obtain a self-consistent set of parameters of the thermodynamic models for all phases. The modified quasichemical model is used for the liquid slag phase. From these model parameters, the optimized ternary phase diagram is back calculated. Liquidus surface with cristobalite, tridymite and quartz (SiO₂), two immiscible liquids, cuprite (Cu₂O), lead silicates (PbSiO₃, Pb₂SiO₄, Pb₁₁Si₃O₁₇, Pb₅SiO₇), massicot (PbO) and copper plumbite Cu₂PbO₂ primary phase fields has been constructed. Available experimental data are described within uncertainties. Oxygen partial pressures and distribution of lead between slag and metal have been calculated.     

Formation of CaZrO3 at the interface between CaO–SiO2–MgO–CaF2(–ZrO2) slags and magnesia refractories: Computational and experimental study

The formation behaviour of calcium zirconate (CaZrO₃) at the interface between the CaO–SiO₂–MgO–CaF₂(–ZrO₂) slags (B$B$(=(mass% CaO) /(mass% SiO₂)) = 2.0) used in the AOD converter and the MgO refractories has been computed by employing a commercial thermodynamic software. The solubility of zirconia (ZrO₂) in the liquid CaO–SiO₂-7 mass% MgO slag phase is relatively small, viz. about 2–3 mass% and the CaZrO₃ phase is formed at about B>1.5$B>1.5$. The region of fully liquid phase extends to the composition saturated by dicalcium silicate (Ca₂SiO₄) and cubic ZrO₂ (B∼1.2$B\sim 1.2$). The effect of 7 mass% CaF₂ addition on the solubility of ZrO₂ in the liquid slag phase was computed to be negligible, while the liquid phase exists through the entire compositions. In addition, the region of fully liquid phase extends to the more basic composition range (B∼1.5$B\sim 1.5$), where the saturating phases are the Ca₂SiO₄ and CaZrO₃. The thermodynamic calculations indicate CaZrO₃ not to form at (mass% ZrO₂) /(mass% MgO) ((=Z/M))<0.6. In the “ Liquid+Ca₂SiO₄+MgO” region, the activity of SiO₂ in the liquid phase is nearly fixed because the activity of CaO in the liquid phase is unaffected by the activity of ZrO₂. However, with higher ZrO₂ activity from the increase in the Z/M ratio, the activity of CaO in the liquid phase is expected to decrease due to the formation of CaZrO₃. The formation behaviour of CaZrO₃ in the slags, computed based on the Gibbs energy minimization principles, could experimentally be confirmed by employing the XRD and SEM–EDS analysis.     

Thermodynamic re-assessment of the Al–Ir system

The thermodynamic assessment of the Al–Ir binary system was performed using the CALPHAD technique. The B2-AlIr phase was described, using the two sublattice model with the formula (Al,Ir,V a)_1/2(Al,Ir,V a)_1/2, while Al₉Ir₂, Al₃Ir, Al₁₃Ir₄, Al₄₅Ir₁₃, Al₂₈Ir₉, and Al_2.7Ir compounds were treated as stoichiometric compounds. The fcc-based phases (L1₀-AlIr, L1₂-Al₃Ir, L1₂-AlIr₃ and A1) were described using the four sublattice model with the formula, (Al,Ir)_1/4(Al,Ir)_1/4(Al,Ir)_1/4(Al,Ir)_1/4. From ab initio calculations (VASP) the formation enthalpies of the stable/metastable intermetallic phases involved in the Al–Ir system were estimated. The thermodynamic quantities, such as the phase equilibria, invariant reactions, and formation enthalpies of the intermetallic phases, were calculated using the obtained parameter set, and agree well with experimental data.     

Thermodynamic modeling of fcc order/disorder transformations in the Co-Pt system

The present work reports on a thermodynamic modeling of the Co-Pt system with ordered fcc phases of L1₀ and L1₂ structures by means of the CALPHAD method. The liquid, hcp and fcc phases have been modeled as substitutional solutions where the interaction parameters are composition dependent in the form of the Redlich-Kister polynomial. The disordered and ordered fcc phases have been modeled in terms of the compound energy formalism with a single Gibbs energy function. The obtained phase equilibria and activities of Co and Pt agree well with the available experimental data. First-principles calculations are performed to obtain the enthalpies of formation for the ordered fcc phases at 0 K. These calculated enthalpies of formations for the ordered phases are less negative than the enthalpies of the disordered state at low temperatures determined from the CALPHAD modeling. The Fe-Pt and Ni-Pt systems exhibit the same feature as that in the Co-Pt system, which is discussed in terms of the total magnetic moment of ordered fcc phases.     

Thermodynamic optimization of the binary CaO–ZnO and ternary CaO–ZnO–SiO2 systems

Liquidus phase equilibrium data of the present authors for the CaO–ZnO–SiO₂ system (as a part of research program on the characterization of the multicomponent PbO–ZnO–FeO–Fe₂O₃-“Cu₂O”-CaO-SiO₂ system), combined with phase equilibrium and thermodynamic data from the literature, have been used to obtain a self-consistent set of parameters of the thermodynamic models for all phases. The modified quasichemical model is used for the liquid slag phase; lime (Ca,Zn)O, zincite (Zn,Ca)O, α- and α′-dicalcium silicate (Ca,Zn)₂SiO₄ and tricalcium silicate (Ca,Zn)₃SiO₅ are described within Bragg-Williams formalism; tridymite, cristobalite SiO₂, wollastonite, pseudowollastonite CaSiO₃, rankinite Ca₃Si₂O₇, willemite Zn₂SiO₄, melilite (hardystonite) Ca₂ZnSi₂O₇ and Ca–Zn feldspar CaZnSi₃O₈ are treated as stoichiometric compounds. From these model parameters, the optimized ternary phase diagram is back calculated.     

Liquidus projection of the Al–Nb–V system

In the present work, the liquidus projection of the Al–Nb–V system is proposed for the first time. It corresponds to important data for the design of innovative low-density Al-containing refractory high-entropy alloys. Experimental investigation was carried out via microstructural characterization of fifty-six alloys in as-cast state using scanning electron microscopy (SEM), electron dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD). Results showed no signs of ternary phases and only primary precipitation fields from binary phases were observed. The BCC primary precipitation field is dominant, while V₅Al₈ primary precipitation field is restricted to a small region close to Al–V binary. Three class II (U_I, U_II and U_III) ternary invariant reactions were proposed.     

Thermodynamic re-assessment of the Re–X (X=Al,Co, Cr,Ta) binary systems

Rhenium was one of important alloying elements in the Ni-based superalloys. Based on the molar Gibbs energy of the pure Re updated in SGTE Pure 5 database, the Re–X(X=Al, Co, Cr, Ta) systems were re-optimized by means of CALPHAD (CALculation of PHAse Diagrams) technique. In the present work, the phases liquid, fcc, bcc and hcp were described using a substitutional solution model. The phases AlRe, Al₃Re, Al₆Re, Al₁₂Re, AlRe₂ and Al₁₁Re₄ in the Re–Al system were described as stochiometric compound. The Al₄Re_H and Al₄Re_L instead of Al₄Re were evaluated in the present work. The phases σ in the Re–Cr and Re–Ta systems and χ in the Re–Ta system were modeled as (X, Re)₁₀(X, Re)₂₀ (X=Cr or Ta) and Re₂₄(Re, Ta)₁₀(Re, Ta)₂₄, respectively. A set of self-consistent thermodynamic parameters of the Re–X systems were obtained and the optimized results were in good agreement with the experimental data.     

CALPHAD assessment of bio-oriented Ti–Zr–Sn system and experimental validation in Ti/Zr-rich alloys

The development of CALPHAD-type thermodynamic database for Ti or Zr based biomedical alloys has been spurred by the increased interest in efficiently tailoring an alloy composition to obtain high stability of β_bcc, low Young's modulus, and free of detrimental phases. However, the thermodynamic prediction is not adequate to be performed without the information of key sub-ternary Ti–Zr–Sn system. In present work, the thermodynamic assessment of Ti–Zr–Sn system is performed via a critical evaluation of phase equilibria and microstructure development in this ternary system. The partial isothermal sections at 1323 K and 1473 K with Sn content below 40 at. % are obtained by analyzing chemical compositions and crystal structures of individual phases in the annealed alloys. The composition homogeneity range of most phases is validated to favor a ternary extension paralleling to the Ti–Zr axis. Particularly, β_bcc and η phases (with the chemical composition (Ti, Zr)₅Sn_3+x) show complete solubility of Ti and Zr from Ti–Sn edge to Zr–Sn edge. With the database, negligible ternary solubility of Zr₄Sn phase, microstructure development in the as-cast samples, and the controversial conclusions in literature are discussed. Most of the experimental findings, including equilibrium phase constitution, solidification sequence, DSC signals, projections of liquidus, are reproduced in a self-consistent way. The work moves towards the completeness of multi-component Ti/Zr thermodynamic database. It can be used for composition design of novel metastable β-type biomedical alloys.