Microstructural evolution of single crystal and directionally solidified rejuvenated nickel superalloys

The possibility of using rejuvenation heat treatments to restore the original cuboidal ' microstructure from the rafted structure developed in four different nickel based superalloys during creep testing at elevated temperatures has been explored in this research. Creep data are presented for each of four different nickel-based alloys considered, and the rafting of the ' structure linked to calculations of the lattice mismatch. An image analysis technique has been developed for images taken using a scanning electron microscope in order to rapidly quantify the ' particle size distributions before and after high temperature heat treatment. Thermodynamic calculations have been used to determine a processing window in which high temperature heat treatments can be carried out in order to re-solution the ' without risk of incipient melting. These have demonstrated that it is possible to restore the original ' structure in the case of some of the alloys. Examination of the interactions of an oxidation resistant coating with each of the alloys both experimentally and using a combined thermodynamic and kinetic model has shown slightly different behaviour depending on the chemical composition of the alloy, which may impact upon how coated materials are processed to enable ' rejuvenation. Uncoated regions have also been considered for one alloy with the aim of minimising surface recrystallisation and development of detrimental phases during high temperature processing. The research has shown that it is possible to determine suitable heat treatment windows to alter ' morphology, quantify these changes and strategies have been developed to take into account both coated and uncoated surface interaction effects for potential rejuvenation of components.