Determination of the thermodynamic functions and calculation of phase diagrams for metastable phases

The formation of metastable phases in non-equilibrium processing techniques is still not understood quantitatively. This is mainly a result of the lack of detailed information on kinetic constraints and the thermodynamics of the systems investigated. In this paper a thermodynamic approach for the interpretation of metastable phase formation is illustrated on the NiTi system which is based on the determination of free energy curves by electromotive force measurements and the calculation of phase diagrams for metastable phases by applying the CALPHAD method. Special emphasis is placed on the thermodynamics of the undercooled liquid and the amorphous phase in order to predict the formation of amorphous alloys and supersaturated solid solutions by rapid quenching of the melt as well as during co-deposition and solid-state reaction of the elemental components.