| Summary: | <p>The property of polycrstalline nickel alloys are crucially dependent upon grain boundaries, as they are the strength limiting sites at high temperatures. Engineering of grain boundary structure and phases can be effective means of pursuing optimum properties of a component. Grain boundary morphologies are known to have critical impact on time dependent deformation, such as creep and fatigue. However, the reason for such an improvement is only understood qualitatively, the mechanisms of its formation or strengthening are not well understood. Roles of second phase particles on mechanical properties, such as carbides and borides, are still in an open debate. The present research tackled the questions from a fundamental perspective. The results accomplished have gained a deep understanding on the underlying science and satisfied industrial interests. This is achieved by carefully controlled experimentation and advanced characterisation. First, grain boundary morphologies are shown to be sensitive to the heat treatment that is given, and intergranular phases are proven to be responsible for obtaining serrated boundaries. Next, mechanical response between different grain boundary structures are distinctive, which is further elucidated by strain and tomography analysis. Last, minor element additions showed drastic improvement of creep life by redecoration of grain boundary phases. Overall, the thesis presents the mechanistic study of grain boundary structures, which is thoroughly investigated through the processing parameters on alteration of second phase particles and its influence on mechanical properties.</p>
|
|---|