Theoretical and experimental thermodynamics: case study - thermophysical properties of liquid alloys

The most important source of information regarding the microstructure evolution of a certain alloy is its phase diagram. Sometimes, the real microstructure is out of equilibrium, but the equilibrium conditions represent a starting point to deduce non-equilibrium microstructures. All chemical reactions in an alloy system are governed by thermodynamics and, if the thermodynamic properties of all competing phases are known, it is possible to calculate the chemical equilibrium for any set of conditions, expressed by pressure, temperature and chemical composition. Since the late 1970s, the CALPHAD method is widely used making it possible the calculations of chemical equilibria by minimization of the total Gibbs energy of binary and/or multicomponent alloy systems as well as having the capability of providing the information in a number of convenient ways. The CALPHAD method combines the theoretical models used to describe free energy functions of all phases present in an alloy system that are derived, in the main, using experimental measurements thus maintaining a strong connection with reality. In practice, the CALPHAD method is an iterative method that evolves as a result of a continuous interplay between theory and experiment. Indeed, more recent applications are related to its extension to nano-sized alloys and calculations of their phase diagrams taking into account that the size dependence of the properties is the competition between the surface and bulk energies. Also in this case, the theoretical models are substantiated by the experimental determination of density and surface properties as well as of the size dependent melting temperature depression. The thermophysical properties of alloy melts are related to the thermodynamics. The determination of both, the thermodynamic and the thermophysical properties of high melting liquid alloys is often hampered by the experimental difficulties related to high temperature measurements. In particular, concerning the data on the surface properties of multicomponent alloys, only the surface tension data of the corresponding pure components can be found in literature. The same is observed for the viscosity. Therefore, it is only possible to estimate the missing property values in terms of theoretical framework. The energetics of mixing and structural arrangement in regular, compound forming and phase separating liquid alloys can be analyzed through the study of basic properties (molar volume/density, ultrasound velocity, isothermal compressibility), surface properties (surface tension and surface segregation), dynamic properties (chemical diffusion, viscosity and electrical resistivity) and microscopic functions (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) in the framework of classical thermodynamic and quantum statistical models and theories. An overview of different models widely used to predict the thermophysical properties of liquid alloys, a comparison of calculated property data with experimental datasets and the problems related to modelling will be also presented.