Theory of material toughness with grain boundary oxide inclusions
File(s)
Author(s)
Cimbaro, Luca
Type
Thesis
Abstract
At high temperatures oxygen damages the nickel-based superalloy RR1000, which is used in rotor discs of jet engines, by forming a relatively small oxide inclusion at the tip of an intergranular crack. The inclusion is much more brittle than the surrounding material. Also, it is thickest at the tip of the crack, tapers along the grain boundary over a distance of a few microns, and has a positive misfit. A theory shows that the inclusion enhances the oxidation of the grain boundary ahead of the crack by deriving the hydrostatic stress using functional relationships. The inclusion increases the crack growth by reducing the material toughness of the superalloy. The theory predicts a critical size of the inclusion for the failure of the superalloy. The material toughness of ductile materials such as RR1000 can be orders of magnitudes higher than that of brittle solids. A theory describes the material toughness in crystalline solids from purely brittle to purely ductile behaviour. For shear mode fracture the theory shows that the material toughness is proportional to a critical value of the external stress squared and the length of the crack, whether there is a plastic zone or not. The material toughness is highest in ductile solids because the plastic zone provides additional resistance to crack growth. At the point of crack growth the Peach-Koehler force overcomes the resistance opposing dislocation motion in the plastic zone. An elastic region of variable size, being largest for purely brittle solids and smallest for purely ductile solids, separates the crack tip from the plastic zone. The theory derives a dimensionless variable, which comprises the macroscopic and the microscopic parameters governing the fracture. The dimensionless variable controls the elastoplastic nature of the fracture and determines the size of the elastic region at the crack tip.
Version
Open Access
Date Issued
2019-08
Date Awarded
2020-02
Copyright Statement
Creative Commons Attribution NonCommercial NoDerivatives Licence
Advisor
Sutton, Adrian Peter
Paxton, Tony
Balint, Daniel Stephen
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)