Investigation On Phase Diagrams, Phase Equilibria And Phase Transformations Of Ni Alloys

The key to a successful materials design is to obtain reliable thermodynamic and kinetic information of the investigated system. The present work is a fundamental investigation on the industrial Ni alloys. The studied systems are Ce-Ni, B-Ni, Cu-Mg-Ni, Fe-Ni-Zn, Al-Cr-Ni and Al-Ni-Si. On the aspect of industrial application, one purpose of this work is to expand the thermodynamic database of the Ni-based alloys, and to provide some useful information of phase transformations for materials design. On the aspect of computational thermodynamics of materials, this work is dedicated to solving some remaining issues in thermodynamics by employing the designed key experiments, CALPHAD (CALculation of PHAse Diagram), and the first principles calculations so as to explore an effective method for the construction of alloy phase diagrams.The major contents of the present work can be summarized as follows:1. Using a combined method of experiments and the first principles calculations, the phase diagrams of Ce-Ni, Cu-Mg-Ni, Fe-Ni-Zn, Al-Cr-Ni and Al-Ni-Si have been well constructed. The assessed thermodynamic parameters in this work can be applied to predict phase equilibria, phase transformations and thermodynamic properties accurately, such as, the enthalpy of formation of intermetallic phases, activities, etc. The present thermodynamic studies provide a fundamental basis to design Ni-based materials, and assist to reveal some complex phase transformations in the related alloys.2. The importance of evaluation of the boundary systems has been emphasized in the thermodynamic investigation on the Cu-Mg-Ni system. Furthermore, some suggestions on the related topics have been made. In addition, the phase stabilities of the Laves phases have been predicted successfully in this work.3. In the study of the Fe-Ni-Zn system, the thermodynamic issues in galvanizing have been elucidated. The binary Fe-Zn and Ni-Zn systems have been reassessed due to some defects of thermodynamic models reported in literature. The present work shows the advantages of using a combined method of the first principles calculations and CALPHAD approach to study the Zn-contained systems which are difficult to handle in experiments. The present work not only provides thermodynamic descriptions of the Fe-Zn and Ni-Zn systems, but also demonstrates a reliable method to study thermodynamics in galvanizing.4. The Ce-Ni system has been studied by using a hybrid approach of the designed key experiments, first principles calculations, and CALPHAD method. In addition, a considerable solubility limit of the CeNi5 phase has been determined by using electron probe microanalysis. An accurate thermodynamic description of the Ce-Ni system has been performed.5. In the study of the B-Ni system, a low-cost and efficient method to measure the specific heat of alloys has been developed. The present method reduces the errors resulting from the mass difference between crucibles of sample and reference. Therefore, it increases the accuracy and flexibility of measurement. The specific heat of intermetallic NiB has been determined between 40 and 950℃.6. The order-disorder transition model has been successfully applied to describe the fcc_L12 and bcc_B2 ordered phases in the Al-Ni-Si and Ni-Zn systems, respectively. The present modeling can be used to predict phase stability of the coherent precipitation by using a single model to describe ordered and disordered structures. The present study on the Al-Ni-Si and Ni-Zn systems shows the significance of the order-disorder model.7. The present investigation on the Al-Ni-Si system shows the necessity of the designed experiments for phase diagram determination. Compared with the conventional methods, specific experiments with logical and careful design can improve the precision and minimize the workload evidently.This work performed some case studies on the Ni alloy systems, and reviewed the tendency of future researches. Some remaining issues in computational thermodynamics have been discussed extensively. This work demonstrated an efficient way to construct the thermodynamic database of Ni alloys. By incorporating the other systems, this work will be used to construct a high precision thermodynamic database of Ni alloys for industrial application.