Computational Thermodynamics in Materials Science

"The CALPHAD method (CALculation of PHAse Diagrams) has been extensively developed in the last 25 years and became a useful tool in materials science. Thermodynamic descriptions for the functions of state of unary, binary and ternary phases are derived by the optimization of phase diagrams. These functions are stored in computerized thermochemical databases and can be combined to extrapolate to multi component systems. Depending on special demand, the actually most advantagous presentations of the calculated results can be chosen: tables or in a clear graphical form as phase diagrams, phase amount diagrams or volatility diagrams. The method will be desribed by the system Ti-Si-C-N-O including the ceramic subsystem Si3N4-SiC-TiCi,N1_1,-C-N-O which is very important for the development of ceramic composites. The calculations give the thermodynamic stabilities of the condensed phases as function of temperature, concentration and partial pressure of different gas species. Adjusting the sintering conditions to these results the formation of unfavourable compounds or liquid phases can be suppressed.IntroductionThe CALculation of PHAse Diagrams (CALPHAD-method) of multicomponent systems provides the possibility to plan experiments very efficiently and avoid the experimental trial and error approach for the development of materials. The main problem for best use of calculated results is their interpretation with regard to the specific applications under consideration. Tables give the exact quantitative data for the treated phenomena. However, most important to make best use of these thermodynamic data is their graphical representation especially designed for the solution of specific problems, scientific as well as application oriented. Only well chosen graphical representations support a comprehensive understanding of the thermodynamic interrelationships and the transfer to the problem."