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Damage mechanisms in nickel-based superalloy René 41 subjected to fatigue
Babinský, Tomáš ; Pippan, Reinhard (oponent) ; Růžička,, Milan (oponent) ; Polák, Jaroslav (vedoucí práce)
The present study deals with the aspects of fatigue damage in a wrought Ni-based superalloy René 41. The material of interest was investigated in two thermodynamical states: solutionised and aged. Uniaxial, fully-reversed low-cycle fatigue tests were conducted at room and elevated (800 °C) temperatures under total strain amplitude control. To study the relevant damage mechanisms and reveal their connection to plastic deformation, a detailed analysis of the cyclic response using a statistical approach was performed, and advanced microscopy techniques were employed, namely scanning and transmission electron microscopy. Interrupted fatigue tests were performed to study the evolution of surface relief and the underlying dislocation structure. After each interruption, the evolution of the microstructure was analysed using electron microscopy techniques. At room temperature, the damage was mostly transgranular, and the cracks initiated predominantly within grains from the roughened surface. Comparing the two investigated material states, the aged material attained higher stresses and exhibited higher lifetimes in service-relevant stress-life relation. Cyclic deformation was found to localize to persistent slip bands since the first cycle, and the majority of slip bands formed very early in the fatigue life. Hence, shearing of the strengthening phase, coherent ’ precipitates, occurred largely only in the first few cycles. Due to the cyclic slip localization, surface roughening emanated in the form of slip steps, extrusions and intrusions. The formation of surface relief was thoroughly discussed with existing models. The exposure to high temperatures led to an environmental attack, due to which an oxide layer formed on the surface. The cracks initiated and propagated along weakened grain boundaries, and the resulting damage was thus intergranular.
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