Abstract
Initiation and growth of short cracks in a nickel-based single crystal were studied by carrying out 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. 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 at both room and high temperature (650 °C). These slip traces were caused by accumulated shear deformation of activated octahedral slip systems, which were specifically identified by analysing the surface slip traces and crack-propagation planes. The crack-growth rates were evaluated against stress intensity factor range, revealing the anomaly of slip-controlled short-crack growth. The effects of crystallographic orientations and temperature on fatigue crack growth were subsequently analysed and discussed, including the influence of microstructural features such as carbides and pores.
Highlights
Nickel-based superalloys are predominantly used to produce turbine blades and discs in aircraft engines and land-based gas turbines [1]
The in-situ scanning electron microscope (SEM) observations of processes of fatigue crack initiation and propagation are selectively shown in Fig. 2 for [001] orientated specimen, loaded up to failure
Deformation by crystallographic slip was responsible for crack initiation, while at 650 °C, the mode-I type fracture indicated the onset of cracking at the notch root
Summary
Nickel-based superalloys are predominantly used to produce turbine blades and discs in aircraft engines and land-based gas turbines [1]. Such components are generally subjected to low-cycle fatigue during start-up, normal operation and shut-down of the system. Dislocations multiply and accumulate within the material, resulting in significantly increased levels of their density. These dislocations arrange themselves in configurations with the lowest energy by forming persistent slip bands (PSBs), which are precursors for crack initiation. In precipitation-hardened materials, such as nickelbased superalloys, PSBs form as dislocations cut through the γ matrix and γ′ precipitates in a planar slip manner
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