Study of fatigue crack initiation and propagation mechanisms in a directionally solidified superalloy: Effects of microstructure, anisotropy and oxidation

Abstract

The Directionally solidified (DS) superalloy CM247LC was first developed by Cannon Muskegon Corporation and is extensively used in turbine blades and vane applications, due to its high strength at elevated temperatures, and excellent resistance to creep comparable with some single crystal (SX) superalloys. Studies on the fatigue behaviour, particularly the interplay between microstructure, oxidation, creep and fatigue at more moderate elevated temperatures (i.e. typical of those seen at stress concentration features like the blade root or at cooling holes) are however relatively few. The thesis provides a deeper understanding of the contribution of these factors to the alloy’s failure mechanisms by systematic test design and careful in-situ and post characterization. Fatigue anisotropy is correlated with the columnar grain elongation direction with the respect to the loading direction and general crack propagation direction. The former relationship induces the anisotropy in the Young’s modulus and yield stress, which affects the entire fatigue life, while the later affects the specific fatigue crack initiation and propagation directions. Much shorter fatigue life is found in the specimen (termed as L or LR) with the columnar grains aligned along the loading direction when the applied maximum stress is close to the yield stress. This is attributed to the much lower Young’s modulus inducing a higher local plastic strain and hence faster crack initiation. The fatigue short crack propagation behaviours are mainly affected by the relationship between the columnar grain elongation direction and crack propagation direction. The effects of microstructural features on the fatigue short crack initiation and propagation have been assessed by data rich imaging approaches. Short cracks preferentially initiate from carbides or pores, instead of at slip bands, even though the intensity of the localized strain is at similar scale to these features in the early stages of crack initiation. The observation of crack evolution in three dimensions (3D) via X-ray CT further illustrates the fatigue cracks propagate along several slip systems simultaneously instead of a single favoured slip system. Oxidation preferentially occurs at the stress/strain localisation features such as slip bands and carbides. The detailed oxidation mechanism of the carbides is revealed by 3D reconstruction of the oxidised carbides at different testing times: (Co, Ni) oxides are formed firstly at the interface between carbides and matrix, then protrude beneath these carbides, finally intruding into the bulk, leading to crack formation inside the carbides. Crack initiation from oxidised carbides is widely observed, but their propagation is blocked by the surrounding oxides in fatigue testing with 1-1-1-1 trapezoidal waveform. Thus, the main crack initiation sites transfer from the surface to the subsurface pores. The phenomenon of oxides prohibiting crack propagation is also shown in the long fatigue crack propagation tests. The degree of crack arrest is linked to testing temperatures and frequencies, as more significant oxidation occurs in the lower frequency and higher temperature tests. Internal (Al, Cr)-rich oxides intrude into the materials, especially ahead of the crack tip, while the external (Co, Ni) oxides fill the crack. Oxidation induced crack closure is thought to be the main factor contributing to observed crack arrests, caused by the thick external oxides. A modified model based on the thickness of the external oxidation layers is proposed to quantitatively evaluate the effective stress intensity, which shows the thicker oxidation layers formed at lower frequencies, causes a dramatic reduction in the crack tip stress intensity and leads to crack arrest. Further detailed analysis on cracks was performed to assess the steps in the internal (Al, Cr)-rich oxide formation. Al-rich oxides form inside the γ՛ in the form of nanoparticles, while Cr-rich oxides are formed at the γ channel in the form of stripes. The potential stress assisted oxidation mechanism is inferred to be: (1) the formation of external oxides depletes γ՛ of Ni, causing the formation of Al-rich oxides inside the γ՛ (2) after the γ՛ is fully oxidized, O diffuses into the γ channel, forming stripe-like Cr-rich oxides; (3) As dense internal oxidation layers are not formed, allowing continuous diffusion of Ni and Co elements, this forms thick external oxidation layers, resulting in oxidation-induced crack closure.

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Identifiers

ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

ORCID for Nong Gao: orcid.org/0000-0002-7430-0319

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