Thermo-Calc Software Package Research Articles

Microstructure and phase equilibria in the Ni–Al–W system

Several experimental alloys of the Ni–Al–W ternary system were prepared and their equilibrium microstructures were studied after long term annealing at temperatures 1173, 1273 and 1373 K by means of scanning and transmission electron microscopies with energy-dispersive X-ray microanalysis. Thermodynamic modelling based on CALPHAD approach was performed using software package Thermo-Calc and commercial thermodynamic database for Ni-based superalloys. Experimental results were compared with calculated data and a necessity of modification of selected thermodynamic parameters was stated. Optimization of ternary parameters for thermodynamic description of γ′ phase and binary parameters for intermetallic phases was performed using software package Thermo-Calc. The thermodynamic optimization based on newly obtained experimental results and experimental data available from literature resulted in good agreement of prediction with experiment.

Thermodynamic assessment of B–Zr and Si–Zr binary systems

Abstract Both B–Zr and Si–Zr binary systems have been thermodynamically reassessed with CALPHAD approach. The liquid is treated as substitutional solution phase, of which the excess Gibbs energies are formulated with Redlich–Kister polynomial functions. hcp-Zr and bcc-Zr are treated by interstitial model. Terminal solution β-B is treated as the formula (B) 93 :(B,Zr) 12 using two-sublattice model with B on the first sublattice and B, Zr on the second one. The binary intermetallic compounds, ZrB, ZrB 2 , ZrB 12 , Si 2 Zr, α-SiZr, β-SiZr, α-Si 4 Zr 5 , β-Si 4 Zr 5 , Si 2 Zr 3 , Si 3 Zr 5 , SiZr 2 and SiZr 3 are treated as stoichiometric phases. Subsequently, based on available experimental data from open literatures, optimization is performed using Parrot modules of Thermo-calc software package. And a set of self-consistent thermodynamic parameters describing each phase which appeared in these two systems have been obtained, which can reasonably reproduce the corresponding experimental data.

Computer simulation of W-C-Co-V system diffusion couples

Diffusion couples, WC/Co-1 at.%C, V 8C 7/Co, and WC/Co-0.3wt.%V 8C 7 were constructed and annealed at temperatures ranging from 1100 to 1300°C. The concentration profile of V and W atoms in these couples were measured by EPMA. Aided by Thermo-calc & DICTRA software package, the concentration profiles of W and V in established diffusion couples were calculated by the moving boundary model according to revised dynamic parameters and MOB database, respectively. Then the concentration profile of W in Co binder during sintering WC-Co cemented carbide was simulated by the Cell model. It can provide reference for composition and sintering process design of cemented carbides with high performance. Meanwhile, the validity of revised dynamic parameters of the W-C-Co-V system is verified by the comparison of experimental and simulated results.

Prediction of reoxidation inclusion composition in casting of steel

A model is developed to calculate the composition of reoxidation inclusions that form during pouring of steel castings. The software package Thermo-Calc is used to obtain the inclusion phase fractions and compositions as a function of the temperature and oxygen content of the steel. Oxygen is assumed to be continually absorbed by the steel until the liquidus temperature is reached. Both lever rule and Scheil-type analyses are performed. The model is applied to reoxidation of two carbon steels, one low-alloy steel and one high-alloy steel. The effects of variations in the steel composition and the oxygen absorption rate on the inclusion composition are investigated in a parametric study. The mass fraction of absorbed oxygen is determined by matching predicted with previously measured reoxidation inclusion compositions for the various steels. Good agreement is obtained for most phases present in the inclusions. Interestingly, the agreement in the inclusion compositions occurs for all steel grades when the percentage of absorbed oxygen is equal to 0.9 wt pct. This value is explained using a separate model for the rate of oxygen absorption at the steel-atmosphere interface. Various scenarios are outlined that allow for the 0.9 wt pct of absorbed oxygen to be achieved. The model is then used to calculate the amount of alloy elements consumed and inclusions formed as a function of the oxygen boundary layer thickness in the atmosphere and the integrated free surface area of the liquid steel during pouring. It is found that for unprotected liquid steel transfer operations, such as tapping and ladle filling, the integrated free surface area and exposure time product can reach values of the order of 100 m2s per ton of steel, and that the air-to-steel volume ratio during pouring can be as large as 40. It is concluded that, in order to create a comprehensive tool for simulating reoxidation formation, more detailed models are needed for the external oxygen transfer in the atmosphere, the flow of the liquid steel during pouring, and the internal transport and reactions of chemical species in the steel.

CALPHAD/first-principles re-modeling of the Co–Y binary system

The thermodynamic properties and phase diagram of the Co–Y system have been analyzed by the CALPHAD technique using a computational optimization procedure via the ThermoCalc software package. Based on the experimental data, nine intermetallic phases were taken to be stoichiometric compounds and two phases, Co 5Y and Co 17Y 2 were described with three-sublattice models. First-principles calculations were obtained for the enthalpy of formation of five stable Co–Y compounds and the calculated end members of the Co 5Y phase. The model accounts for the magnetic contribution of Co 7Y 2, Co 5Y and Co 17Y 2. The thermochemical and equilibrium information in the CoY system has been accurately reproduced and is shown to be in agreement with all experimentally validated phase reactions.

The dependence of the β-AlFeSi to α-Al(FeMn)Si transformation kinetics in Al–Mg–Si alloys on the alloying elements

An homogenisation process is applied to as cast billets Al–Mg–Si alloys in order to improve the extrudability. During this homogenisation, plate-like β-AlFeSi phase transforms to a more rounded α-Al(FeMn)Si phase which are more favourable for the extrusion process. In this paper, the influence of the alloying elements on the rate of the intermetallic β-to-α transformation is studied. A Finite Element Model (FEM) predicts the kinetics of the β-to-α transformation for various Mn and Si concentrations. The software package Thermo-Calc is used to derive the equilibrium solubilities for various alloy compositions. These solubilities are used in the Finite Element Method as boundary conditions for the interfacial concentrations of the intermetallics. Subsequently, the results of the Finite Element Model are validated with experimental data.

Determination of isothermal transformation diagrams for sigma-phase formation in cast duplex stainless steels CD3MN and CD3MWCuN

Work was undertaken to establish reliable time-temperature-transformation (TTT) diagrams for the cast duplex stainless steels CD3MN and CD3MWCuN. The latter contains higher Cr, Ni, and Mo contents compared to the former. The TTT diagrams for sigma-phase formation in both alloys were developed based on quantitative metallography results from optical and scanning electron microscopy. The kinetics of sigma-phase were found to be much faster in CD3MWCuN alloys than in CD3MN. An Avrami analysis was employed to assess transformation characteristics and to refine the TTT diagram determination. Theoretical prediction of phase equilibria using the thermodynamic software package ThermoCalc was compared to the amount of sigma phase observed experimentally from long-term heat treatments. The constructed TTT diagrams of the cast alloys were compared to wrought counterpart alloys.

High-speed thermodynamic calculations for kinetic simulations

Simulation of phase transformations may be divided into two parts: the simulation of kinetics and the calculation of thermodynamic quantities. A number of softwares for thermodynamic calculations are available and it is thus convenient to use such software to obtain an accurate description of thermodynamic properties and couple them to a software simulating the kinetics. A major problem then is that the computational work to evaluate the thermodynamic quantities is often too heavy unless very simplified thermodynamic models are used. That problem may be solved by calculating the thermodynamic quantities in selected points and time steps only and apply an artificial neural network to obtain the values in all other points. This approach has successfully been applied to diffusion in alloys and has enabled us to incorporate thermodynamic data from the Thermo-Calc software package into simulations of diffusion in a very efficient way.

Microstructural and phase equilibria study in the Ni–Al–Cr–W system at 1173 and 1273 K

Several alloys of the Ni–Al–Cr–W quaternary system were prepared and their microstructure was studied quantitatively after long-term annealing at 1173 and 1273 K using scanning and transmission electron microscopy with energy-dispersive X-ray analysis. Thermodynamic modelling of the system was done by means of the Thermo-Calc software package. The thermodynamic parameters for the calculations were taken from commercial database for Ni-base superalloys. Two sections of quaternary phase diagrams at 1173 and 1273 K with constant Ni level of 71 and 76 at.% were discussed. Based on the comparison of our new experimental data with the results of thermodynamic modelling, a necessity of adjustment of some thermodynamic parameters was stated. Two of the interaction parameters describing the Ni–Al–W ternary subsystem were adjusted and better agreement of calculations with experiments was attained.

Microstructural Changes of a γ-TiAl Based Alloy with a Fully Lamellar Microstructure Due to Annealing at Elevated Temperatures

ABSTRACTThe changes of a fully lamellar microstructure caused by annealing at 700 and 800°C for up to 10,000 hours in air have been investigated for a Ti-46.5at.%Al-4at.%(Cr,Nb,Ta,B) sheet material. Changes in morphology as well as in chemical composition and volume fraction of constitutional phases were recorded by means of transmission electron microscopy. Thermodynamic equilibrium calculations performed with the software package ThermoCalc provide baseline information upon both the chemical composition and the amount of constitutional phases of the investigated multi-component alloy. It could be demonstrated that due to high cooling rates which are required to adjust a “defect-free” undisturbed fully lamellar microstructure, composition and amount of the α2 phase are far from the thermodynamic equilibrium. However, during high temperature exposure the composition as well as the amount of α2 phase move towards the values predicted by the calculation. Furthermore, former perfectly shaped γ/α2 lamellae interfaces are disturbed, α2 lamellae decompose and new phases appear instead. Dislocations emitted from dissolving α2 lamellae move into the γ lamellae. Mechanisms responsible for decomposition of lamellae and formation of new phases are explained by thermodynamic equilibrium calculations and diffusion considerations. Finally, results of tensile tests conducted at room temperature and 700°C on long-term annealed specimens are presented and discussed. This work is a basis for understanding microstructural changes and their impact on mechanical properties of γ-TiAl based alloys with a fully lamellar microstructure during service at elevated temperatures.

Thermodynamic assessment of the BaO - TiO 2 quasibinary system

Experimental data from the literature on the BaO-TiO 2 quasibinary system have been evaluated to assess the thermodynamic parameters of all the stable phases by means of the CALPHAD (CALculation of PHAse Diagram) method. The two-sublattice ionic solution model was used to describe the liquid phase, while the compound energy model was applied to describe the high-temperature forms of BaTiO 3. Other compounds were treated as stoichiometric phases. The assessment was conducted using the Thermo-calc software package and a set of parameters was given. Calculated equilibrium phase diagrams and thermochemical properties compared with the experimental data are presented. There is a good agreement between the calculations and the experimental data. Some contradictions are pointed out and discussed.

Reassessment of the Fe-Gd (Iron-Gadolinum) system

The Fe-Gd binary system was reassessed in order to obtain a consistent thermodynamic description. Compared to previous work, some new experimental information was used. It was established that not all the experimental data are interconsistent. The assessment was done by means of the software package THERMO-CALC, using models for the Gibbs energy of individual phases.

PT Paths from modal proportions: application to the Koralm Complex, Eastern Alps

Thermodynamic pseudosections portray those parts of a petrogenetic grid that are relevant to a given bulk composition and the reactions appearing on them can therefore be used directly to infer the PT path that the rock followed. However, for many ‘normal’ bulk compositions the use of pseudosections is hampered by the fact that they display only few large fields of high thermodynamic variance in the PT range of interest. Here it is discussed how modal information on reaction progress within these fields can be used to determine PT path information for thermodynamically high variant metamorphic assemblages. We use this information on reaction progress to contour pseudosections for modal proportions of minerals using the software package THERMOCALC. The approach is applied to di- tri- and quadrivariant assemblages from the Koralm complex in the eastern Alps. A PT path for these rocks is derived from modal considerations and compared with interpretations of mineral composition contours on the same pseudosection and with conventional thermobarometry. It is shown that at least part of the complex must have cooled initially near isobarically from prevalent peak conditions around 700°C and 14 kbar before the rocks commenced a Barroviantype decompression path.