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Title: Cyclic deformation, fatigue crack initiation and crack growth in a nickel-based single-crystal superalloy
Author: Zhang, Lu
ISNI:       0000 0004 7970 7761
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2019
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Single-crystal nickel-based superalloys are dominantly used for turbine blades due to their superior mechanical properties, fatigue and creep resistance at elevated temperature. Typically, gas turbines operate at high alternating load during engine start-up and shut-down, which subjects nickel-based superalloys to severe cyclic loads in harsh environments. For gas turbine blades, a large proportion of service life is spent in the initiation and growth of short fatigue cracks. Therefore, it is important to understand the mechanism of cyclic deformation, crack initiation and the early stage of crack propagation for reliable service-life prediction and optimal design of gas turbine systems. Cyclic deformation, fatigue crack initiation and short crack growth in nickel-based singlecrystal superalloys have been studied experimentally and numerically in this thesis. Straincontrolled low-cycle fatigue (LCF) tests were carried out, with a focus on the effects of crystal orientation and temperature. From transmission electron microscope (TEM) analyses, a connection between microstructure and mechanical behaviour of the alloy was discussed. Initiation and growth of short fatigue cracks were studied by in-situ fatigue experiments within a scanning electron microscope (SEM). Specimens with two different crystallographic orientations (i.e. [001] and [111]) were tested under load-controlled tension fatigue in vacuum at both room and elevated temperatures. In addition, crystal-plasticity finite-element method (CPFEM) were applied to simulate the mechanical behaviour of the alloy under cyclic loading condition at room temperature and 825°C, considering both [001] and [111] orientations. The model was further extended to investigate slip band formation, crack initiation and short fatigue crack growth, with validation of in-situ SEM experiments. Cyclic hardening/softening was observed during the LCF process, depending on the strain amplitude, crystallographic orientation and temperature. Using TEM, it was found that the processes of γ'-precipitate dissolution and dislocation recovery were responsible for cyclic softening. Alignments and pile-ups of dislocations in the γ matrix contributed to cyclic hardening. In in-situ SEM experiments, slip-caused crack initiation was identified at room temperature while initiation of a Mode-I crack was observed at 650°C. Slip traces continuously developed ahead of the crack tip once initiated and acted as nuclei for early-stage crack growth. The crack-growth rates were evaluated against stress intensity factor range ΔK, revealing the anomaly of slip-controlled short-crack growth. CPFE simulation results, in terms of stressstrain loops and stress evolution, were in good agreement with the experimental data. The crystal plasticity model was able to capture the slip localisation and direction of slip-trace development. The simulated results were also correlated well with the behaviours of crack initiation and propagation observed with in-situ SEM. In summary, the alloy shows strong anisotropic behaviour during cyclic deformation, related to crystallographic orientations. Cyclic plasticity was depicted by dislocation-precipitate interactions, explaining the different deformation behaviour observed for different orientations and temperatures. Crystallographic slip was responsible for initiation and growth of short cracks at both room and high temperatures, causing a large scatter of crack growth rate when correlated with the stress intensity factor range. The CPFE model was shown capable of simulating the mechanical behaviour under LCF and predicting the locations of fatigue crack initiation and early-stage growth behaviour.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Mechanical Engineering not elsewhere classified ; Nickel superalloy ; Single crystal ; Fatigue ; Crack ; Crystal plasticity ; Finite element model