| Outline of Research at the Start |
This study aims to evaluates the effects of the small defects and microstructures on the fatigue strength of AM Alloy 718 under the hydrogen environment. The determining mechanism of the fatigue limit, the effects of defects and hydrogen, and the factors impacting fatigue resistance are explored.
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| Outline of Annual Research Achievements |
I have examined the fatigue limit of an additively-manufactured, Ni-based superalloy 718 (AM Alloy 718) in terms of the small, fatigue-crack growth-threshold. Based on the macro- and microscopic analyses, the determining mechanism of the fatigue limit, the effects of small defects and the factors impacting fatigue resistance were all thoroughly explored. The principal conclusions are summarized as follows:
1.The average value of twist angle along the paths of cracks generated during push-pull fatigue tests was substantially lower in the AM Alloy 718 specimens than in the wrought one. In addition, small cracks preferentially grew along favorable slip planes with low twist and/or tilt angles. The aforementioned results implies that the twist angle dominated fatigue-crack resistance, while the tilt angle played a secondary role under push-pull fatigue loading.
2.Similar to the GB misorientation, the role of ATBs was also interpreted based on the geometrical relationship between slip/crack planes in the twin and the matrix. Namely, in the presence of compatible relationships between the crack planes in both the twin and the matrix with a low twist angle, small cracks are expected to easily penetrate through ATBs. However, in order to enhance our understanding of the dominant role of this boundary, it is necessary to gather more experimental data on the interaction between arrested cracks and ATBs.
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