Microstructural mechanisms of cyclic plasticity and fatigue damage in Ni-based superalloys
File(s)
Author(s)
Piglione, Alessandro
Type
Thesis
Abstract
Ni-based superalloys combine exceptional mechanical properties at elevated temperatures with an excellent resistance to aggressive environments. They are employed in numerous applications that require them to withstand cyclic stresses; thus, to ensure their structural integrity, their fatigue properties must be well understood. Metal fatigue is intimately related to the gradual accumulation of microstructural plastic strains during cyclic loading. Hence, this thesis investigates the relationship between microstructural mechanisms of cyclic plasticity and the fatigue properties of two Ni-based superalloys. The alloys of study are DD6, selected as a representative example of single-crystal superalloys, and additively manufactured IN718, representative of polycrystalline superalloys. In the former case, the development of plasticity is shown to be governed by the interactions between dislocations and the cuboidal precipitates characteristic of single-crystal superalloys; the dependence of such interactions on the temperature and the cyclic strain amplitude is thoroughly investigated and discussed, and further related to the alloy’s cyclic response and fracture behaviour in the different testing conditions. Moreover, a novel study combining experimental investigations and microstructure-based crystal plasticity modelling is dedicated to the investigation of local plasticity developing near a notch in a fatigued DD6 specimen. The differences between local and bulk deformation are thoroughly studied and the key factors determining such differences are discussed, providing new insights into local plasticity in the fatigue crack initiation region in single-crystal superalloys. In turn, the development of cyclic plasticity in IN718 is shown to be governed by the interactions between mobile dislocations and the solidification cells characteristic of as-built additively manufactured alloys. The cyclic response of the alloy is correlated with the presence of high dislocation densities in the as-built condition, to their existing arrangement in cells and to the retainment of such cells during cyclic loading. In addition, the differences in dislocation densities, cell sizes and dislocation arrangements at cell walls induced by two distinct scanning patterns are linked to the different cyclic responses, thus linking processing parameters, resulting microstructures and associated fatigue properties.
Version
Open Access
Date Issued
2021-08
Date Awarded
2021-12
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Pham, Minh-Son
Dunne, Fionn
Sponsor
AECC Beijing Institute of Aeronautical Materials (BIAM)
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)