MICROSTRUCTURAL EVOLUTION OF SINGLE CRYSTAL AND DIRECTIONALLY SOLIDIFIED REJUVENATED NICKEL SUPERALLOYS

摘要

The possibility of using rejuvenation heat treatments to restore theoriginal cuboidal γ′ microstructure from the rafted structuredeveloped in four different nickel based superalloys during creeptesting at elevated temperatures has been explored in this research.Creep data are presented for each of four different nickel-basedalloys considered, and the rafting of the γ′ structure linked tocalculations of the lattice mismatch. An image analysis techniquehas been developed for images taken using a scanning electronmicroscope in order to rapidly quantify the γ′ particle sizedistributions before and after high temperature heat treatment.Thermodynamic calculations have been used to determine aprocessing window in which high temperature heat treatments canbe carried out in order to re-solution the γ′ without risk ofincipient melting. These have demonstrated that it is possible torestore the original γ′ structure in the case of some of the alloys.Examination of the interactions of an oxidation resistant coatingwith each of the alloys both experimentally and using a combinedthermodynamic and kinetic model has shown slightly differentbehaviour depending on the chemical composition of the alloy,which may impact upon how coated materials are processed toenable γ′ rejuvenation. Uncoated regions have also beenconsidered for one alloy with the aim of minimising surfacerecrystallisation and development of detrimental phases duringhigh temperature processing. The research has shown that it ispossible to determine suitable heat treatment windows to alter γ′morphology, quantify these changes and strategies have beendeveloped to take into account both coated and uncoated surfaceinteraction effects for potential rejuvenation of components.