As we all know, phase diagrams can be tested experimentally. However, experimental phase diagrams require a lot of manpower and material resources. Under the conditions of high temperature, high pressure, and corrosive gases involved in the reaction, they will also face difficulties in composition control, container selection, and high temperature measurement, and the experimental determination is always Limited, one-sided, unable to make a complete and comprehensive understanding of the phase diagram and thermodynamic properties of the system.Figure 1: Fe-C phase diagramThen, the phase diagram calculation is a convenient solution. He calculated the phase equilibrium of the system by using thermodynamic principles and plotted the phase diagram.After the phase diagram calculation is introduced, only the thermodynamic data of some key regions and some key phases of the phase diagram of the system can be experimentally measured to optimize the parameters of the Gibbs free energy model, and the whole phase diagram can be extrapolated to build the system. Complete phase diagram thermodynamic database. As a result, the workload of the phase diagram study is greatly reduced, and it is possible to avoid possible experimental difficulties.In simple terms, phase diagram calculations can have 6 important advantages or meanings(1) It can be used to distinguish actual phase diagram data and thermochemical data itself and their consistency, so as to make reasonable evaluation of different experimental results obtained, and to provide users with accurate and reliable phase diagram information;(2) The metastable part of the phase diagram can be extrapolated and predicted to obtain a metastable phase diagram;(3) It can extrapolate and predict multi-phase diagrams, calculate multi-phase balance, and provide reference for the design of actual materials and processing technology;(4) By calculating the Gibbs free energy curve, the component range of the diffusion-free phase transition can be predicted;(5) It can provide important information such as phase change driving force and activity required for phase transition kinetics research;(6) It is convenient to obtain various phase diagrams with different thermodynamic variables as coordinates for the study and control of material preparation and use processes under different conditions.So, why can we get the phase diagram by calculating the phase diagram?The essence of the phase diagram calculation is to establish the thermodynamic models of each phase according to the crystal structure, magnetic order, and chemical order transition of each phase in the target system, and to construct the Gibbs free energy expression of each phase from these models. Finally, the phase diagram is calculated by the equilibrium condition. Among them, the undetermined parameters in each phase thermodynamic model are obtained based on phase balance and thermodynamic property data reported in the literature and optimized by means of phase diagram calculation software. Based on the obtained thermodynamic parameters of low component systems (generally binary and ternary systems), the phase diagram and thermodynamic information of the multicomponent system can be obtained by extrapolation or by adding a small number of multivariate parameters.In general, we control the temperature, pressure, and composition during the material processing. Therefore, we choose the Gibbs free energy as a model function in the calculation of the phase diagram.For a system with a certain material but energy exchange with the outside world (closed system), the constant temperature and pressure process is always in the direction of Gibbs free energy reduction, and the total Gibbs free energy of the system is the lowest in equilibrium. The chemical positions of the constituent elements in the phases are equal. If we know the free energy-composition curve at all temperatures, we can calculate the phase diagram by finding the minimum free energy or equivalent solution chemical bit.In order to calculate the phase diagram we need to know the metastable part of the free energy curve, the free energy of the metastable configuration of the pure element and the metastable phase transition point.The phase diagram optimization and calculation process can be simply divided into five steps:(1) Collection and evaluation of experimental data. The purpose of the assessment is to judge the accuracy of the experimental data based on the experimental methods used by the authors, and to select experimental data that are consistent with thermodynamic principles and are relatively more reasonable.(2) Selection of free energy model. According to the structure of the phase, a reasonable model is selected and can be verified by extrapolating the multivariate system.(3) Using the measured phase diagrams and thermochemical data to optimize the undetermined parameters in the Gibbs free energy expression; then use the appropriate algorithm and the corresponding computer program to calculate the phase diagram on the computer according to the phase equilibrium conditions.(4) Comparison and analysis of calculation results and experimental data. If there is a large difference between the two, adjust the parameters to be determined or reselect the thermodynamic model, and then perform an optimization calculation until the calculation results are consistent with most of the phase diagram data and thermochemical data within the experimental error range.(5) After optimization, all the phase diagrams and thermodynamic data are connected by a thermodynamic model into a self-consistent whole, and finally stored in a model parameter to form a phase diagram thermodynamic database.Now, there are sophisticated phase diagram calculation software that can perform phase diagram calculations.Phase diagram calculation software is essentially a combination of thermodynamic models and computational principles with large-scale numerical calculations and powerful computer processing functions. It can not only achieve multivariate and multiphase equilibrium calculations, but also give various forms of stable and metastable phase diagrams. Other parameters that are closely related to the preparation and use of the material can be obtained.The main functions and features of commonly used phase diagram thermodynamic calculation software (Thermo-Calc, Fact Sage, Pandat, Jmatpro) are as follows. Here’s a brief list of their respective characteristics1, Thermo-Calc softwareThermo-Calc software has become a complete data system, powerful function, and a relatively complete structural calculation system. It is a thermodynamic calculation software that enjoys a good reputation in the world.Thermo-Calc software allows phase equilibrium calculations (such as liquidus and solidus temperature, composition and proportion of each phase, etc.), phase diagram calculations, and thermodynamic calculations. Thermodynamic data can also be tabulated and calculated. The thermodynamic function of the reaction changes and the driving force, the phase balance of the evaluation chemical system, and the phase transition, and the various phase diagrams are drawn by an automatic drawing program.2, Fact Sage softwareFact Sage software is a combination of ChemSage/SOLGA-SMIX two thermochemical software packages. Has the advantages of rich database content, powerful computing functions, and easy operation under the Windows platform.In addition to the multivariate multiphase balance calculation, Fact Sage software can also calculate and draw phase diagrams, dominant area maps, potential-pH diagrams, thermodynamic optimization, and mapping processing.Fact Sage software applications include materials science, pyrometallurgy, hydrometallurgy, electrometallurgy, corrosion, glass industry, combustion, ceramics, geology and so on.The Fact Sage 5.5 database includes:(1) A pure substance database containing 4,517 compounds;(2) Oxide database containing 20 elements;(3) Molten salt data containing 20 cations and 8 anions(4) Comprehensive database containing common alloy systems such as Pb, Sn, Fe, Cu, Zn, etc.(5) Specific databases for specific industrial processes such as electrolytic aluminum, paper industry, and high purity silicon.In addition, Fact Sage can also use other well-known international databases such as SGTE, and provide users with the ability to create private databases.3,Pandat softwareThe biggest advantage of the Pandat package is that even if the free energy function has multiple lowest points within a certain range of components, users who do not have phase diagram calculation expertise and calculation skills can do without setting initial values. Using the Pandat software, it can also automatically search for the stable balance of multi-phased multi-phase systems.The main features of Pandat’s software include computing, editing, and advanced features. The calculation function mainly covers:(1) Computation of phase diagram: Computation of binary phase, ternary and multicomponent equilibrium phase diagrams (isothermal section, equivalent section, user-defined section);(2) Calculation of liquidus: The liquidus (melting point) and primary precipitated phase can be automatically calculated, and the isotherm can be drawn.(3) Solidification calculation: Output information includes the curve of solid fraction, density, specific heat, enthalpy, etc. as a function of temperature;(4) Phase diagram optimization: used to evaluate a series of phase diagrams and thermochemical data, and to obtain thermodynamic model parameters that can be manipulated in the Windows interface to optimize phase diagrams.The main features of Pandat software are: friendly operation interface, easy to learn and use; stable and reliable calculation result; no need for user to input initial value and estimated value; software automatically finds balance points; supports user-defined database, and calculates for various phase diagrams and thermodynamics. Provides a powerful computing platform.Figure 2: The components of the PanGUI4, Jmatpro softwareJMatPro is based on powerful and stable thermodynamic models and thermodynamic data as the core technology and calculation. All physical models have been extensively validated to ensure the accuracy of material performance calculations. JMatPro’s calculation speed is very fast, usually can be completed within one minute. The most immediate advantage of fast calculations is that users can quickly experiment with their own material formulas and complete the desired calculations at their own computer.The main features include:(1) Stable and metastable phase diagram calculations. The user can calculate component planes such as phase diagrams of multiple alloy systems, and can also calculate phase diagrams of multicomponent alloys that change with temperature or vary with composition.(2) Calculation of physical properties – used for material CAE simulation. The relationship between the properties of the material and the temperature can be calculated. The performance data for each phase in the alloy can also be calculated at the same time and the phase diagram in the solidification process can be calculated.(3) Mechanical properties. The mechanical properties of the material can be calculated at room temperature and high temperature conditions.(4) Phase change calculation: Martensitic transformation, steel welding thermal cycle and multi-pass hot rolling calculation, TTT/CCT curve, etc.
Source: Meeyou Carbide