A physically consistent method to implement volume properties in Calphad-type thermodynamic descriptions of alloys

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The impact of volume changes in alloys is of utmost importance in optimizing performance and density of materials, the development of joints and in the study of materials behavior during processing when phase transformations occur, such as casting and heat treatment. The development of a new method is proposed to generalize the Calphad approach in such a way that physically consistent thermodynamic databases can be constructed with the non-trivial characteristic that volume properties are derived from them. A correct self-consistent thermodynamic description of volume related properties, such as thermal expansivity and compressibility, along with the pressure dependence of thermodynamic functions, benefits after the successful completion of this project.So far serious attempts to develop a thermodynamic database including the derivation of volume properties have been limited to the fields of mineral physics and geophysics. In these fields the accent in the thermodynamic modelling for the development of databases of silicate and oxide systems shifts to vibrational models coupled with ab initio methods to prevent unrealistic behavior in thermodynamic properties and to achieve better predictions of properties at conditions where experimental data are missing. Although it has been shown that these methods are quite successful to develop thermodynamic databases, it is impracticable at this stage of research to remake thermodynamic databases, which are coupled to Gibbs energy formulations based on the Calphad approach. It is the goal of the proposed research to develop a Gibbs energy formalism from which all thermodynamic properties, including the volume related properties, can be derived. This formalism serves as an interim solution that can be accommodated in Calphad-type modelling. The development of this formalism is assisted by vibrational formalisms and ab initio methods to assure that physically unrealistic behavior in thermodynamic properties is prevented. Additionally they are used to predict properties for which presently no experimental data are available. They will also be used to gain more insight in the effect of composition on vibrational and static lattice properties to obtain a partitioning of physical effects into the excess properties. To enhance the application of the proposed method, a thermodynamic dataset including volume properties is developed for selected binary and ternary systems in the system Ag-Au-Cu-Ni-Pd-Pt. Vibrational formalisms are employed to achieve a physically self-consistent description of phases in this system. This description consists of a dataset containing vibrational input parameters, analogous to those developed in mineral physics and geophysics. These parameters could be used in these fields for future studies of materials at the extreme conditions prevailing in the solid mantle and core of the earth. The better constrained thermodynamic properties serve as constraints in the interim solution, which can be accommodated in present day Calphad-type modelling. That results in a dataset generally applicable in the Calphad approach to derive thermodynamic properties in pressure-temperature-composition space. Because the developed Gibbs energy formalism is self-consistent and independent of the chosen system it is additionally applicable in a general way to make predictions of these properties in the low pressure regime, which are important for applications in industrial processes.