Thermodynamic modeling and experimental investigation of multicomponent nickel-based superalloys and magnesium alloys

In this thesis, two subjects have been explored: the study of Ni-based superalloys (Chapter 1 to 4) and magnesium alloys (Chapter 5-7).;Our effort in the former subject was focused on the thermodynamic description development of constituent key sub-systems of multi-component Ni-based superalloys including Al, Cr, Hf, Pt and Ir. The Calphad approach coupled with first-principles calculations as well as experimental investigations of phase equilibria was employed for the development of thermodynamic description. To deal with the order/disorder transition occurring in the Ni-base superalloys, a physical sound model, Cluster/Site Approximation (CSA) was used to describe the fcc phases. Thermodynamic descriptions of constituent binary systems, Cr-Pt, Cr-Ir and Ni-Ir, were first modeled based on the first-principles calculated energetics and literature phase-equilibrium results. Critical reassessment was performed on the Al-Ir system based on the experimental data both from us and those in the literature. Thermodynamic modeling and experimental investigation were also performed on the constituent ternaries, such as Ni-Al-Hf, Ni-Al-Pt and Ni-Al-Ir systems. The calculated phase diagrams using these thermodynamic descriptions agree well with the experimental data. A thermodynamic description of the Ni-Al-Cr-Ir quaternary system was also obtained via extrapolation. Some key Ni-Al-Cr-Ir alloys were identified and subjected to experimental investigation. The experimental results of these alloys under both as-cast and annealed states were in good agreement with our thermodynamic calculations, which validates the reliability of the current Ni-Al-Cr-Ir thermodynamic description.;In the later subject, the application of computational thermodynamics on the multi-component Mg alloys was discussed. Based on our previous studies, a description of Mg-Al-Ca-Sr and a preliminary description of Mg-Al-Ca-Sn were obtained by us. Directional solidification experiments were then performed on their constituent lower order systems as well as these two quaternary systems in order to validate the reliability of these thermodynamic descriptions. Microsegregation investigation on the directionally solidified Mg-4Al, Mg-5Al-3Ca and AXJ530 alloys was also carried out by microprobe using area scan method in this study. The measured Al concentration profiles within these alloys were presented and compared with the Scheil calculations.