Phase diagrams offer various areas of materials science and technology indispensable information for the comprehension of the properties of materials. The microstructure of solid materials is generally classified according to the size of the constituents – for example, at the electron, atomic, or granular level (Sect. 1.3). Accordingly, fundamental principles like quantum mechanics, statistical mechanics, or thermodynamics are applied individually to describe the physical properties. Phases are important features of material because they characterize homogeneous aggregations of matter with respect to chemical composition and uniform crystal structure. The various functions of a material are closely related to the phases and structures of the materialʼs composition. Therefore, to develop a material with a maximum level of desired functions, it is essential to undertake design of the structure in advance.
Phase diagrams are composed by means of experimental measurements, as well as statistical thermodynamic analysis. The construction of phase diagram calculations based on experiments and thermodynamic analysis are generally referred to as the calculation of phase diagrams (CALPHAD) approach [20.1
]. This method provides a very accurate understanding of the properties originating in the macroscopic character of the material under study.
This chapter is organized in three parts:
In the first part, a brief outline of the CALPHAD method is summarized.
In the second part, the method for deriving the Gibbs free energies incorporating the ab initio calculations is presented in order to clarify the uncertainty of thermodynamic properties for metastable solution phases, taking the Fe–Be-based bcc phase as an example. Some results
for metastable phase equilibria in the Fe–Be,
and Co–Al binary systems are shown.
In the third part the application to predict thermodynamic properties of compound phases is discussed. The thermodynamic modeling for the Perovskite carbide with an E21-type structure in the Fe–Al–C, Co–Al–C and Ni–Al–C ternary systems is illustrated, and constructions of phase diagrams are performed.