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Title: Cyclic deformation, oxidation damage and fatigue crack growth in nickel-based superalloys
Author: Kashinga, Rudolph J.
ISNI:       0000 0004 7970 945X
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2018
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Cyclic deformation, oxidation damage and fatigue crack growth in nickel-based superalloys have been studied. Strain-controlled low cycle fatigue tests showed strong anisotropy and strain-rate dependency for a directionally solidified alloy. Essentially, the material showed different stiffness and fatigue life when the loading direction was changed from parallel to normal to the solidification direction. Imposition of dwells at peak strain level resulted in stress relaxation. Stabilised evolution of stress amplitude up to failure indicated limited cyclic softening/hardening. Crystal plasticity modelling was carried out to simulate cyclic deformation of the material, using finite element models developed from EBSD analysis of failed specimens. The model showed good predictive capability for the observed low cycle fatigue behaviour. To explain the reduced fatigue life in specimens loaded normal to the solidification direction, a study of stress distribution was conducted using the finite element crystal plasticity model. It was discovered that grain misorientations arising from a cluster of smaller grains caused severe localised stress concentrations, leading to earlier crack initiation and shortened fatigue life. Oxidation damage of single crystal and directionally solidified alloys was studied by testing thin disc specimens under isothermal conditions. Oxidation kinetics was assessed based on thermogravimetric analysis data. To analyse oxidised specimens, focused-ion beam (FIB) sectioning, complemented by SEM-EDS/EDAX analyses, was carried out. Oxidation was evident in the growth of both surface and internal oxides. This resulted in chemical composition changes near exposed surfaces and consequently led to microstructure change due to dissolution of γ'-precipitates. Load-controlled nano-indentation testing was used to rationalise microstructure change due to oxidation damage. At continuum level, deformation of a crack tip subjected to fatigue loading in vacuum and air was studied using finite element simulations for a polycrystalline alloy. In vacuum, material damage was purely mechanical and therefore, described by accumulated plastic strain. In air, a diffusion-based approach was developed and applied to investigate the interaction between fatigue loading and oxygen penetration at a crack tip. For selected loading conditions, progressive increase in oxygen concentration with fatigue cycles was evident. A local compressive stress due to dilatation effect was induced, which compensated part of the tensile stress caused by mechanical loading. A crack-growth criterion based on accumulated plastic strain and oxygen concentration at the crack tip was, therefore, developed to predict the crack growth rates under fatigue-oxidation conditions, in comparison against experimental results. Application of the diffusion-based approach was finally extended to a directionally solidified superalloy. To consider the effects of microstructure, grain-structures and textures were taken into account in the model, from which the influence of grain orientations on crack tip deformation and crack growth behaviour was studied. Again, the approach was able to quantify the different crack growth behaviour in vacuum and air conditions.
Supervisor: Not available Sponsor: EPSRC ; GE Power (UK) ; Copperbelt University (Kitwe ; Zambia)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Mechanical Engineering not elsewhere classified ; Low cycle fatigue ; Microstructure ; Finite element analysis ; Stress-distribution ; Crack growth ; Diffusion-based modelling ; Fatigue-oxidation conditions ; Superalloys ; Nickel