2019 Fiscal Year Annual Research Report
Ultra-high Cycle Fatigue Characterization and Ultra-slow Crack Growth of Titanium Alloys
| Project/Area Number |
19F19730
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| Research Institution | Kyushu University |
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Principal Investigator |
陳 強 九州大学, 工学研究院, 教授 (30264451)
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| Co-Investigator(Kenkyū-buntansha) |
YANG KUN 九州大学, 工学(系)研究科(研究院), 外国人特別研究員
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| Project Period (FY) |
2019-07-24 – 2021-03-31
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| Keywords | Fatigue / Titanium Alloys / Crack Initiation / Bimodal Microstructure / Ultrasonic Fatigue |
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| Outline of Annual Research Achievements |
We investigated high and very high cycle fatigue properties of titanium alloys with bimodal microstructure. Two types of bimodal microstructures, which respectively contained equiaxed alpha grains and lamellar beta grains with different volume fractions, were generated by tailoring different heat treatment route. Microstructure analysis showed that the hot-rolled microstructure contained about 15% beta grains and 85% alpha grains, and the double-annealed microstructure consisted of about 24% beta grains and 76% alpha grains.
Ultrasonic fatigue experiments (20 kHz) were carried out on the titanium alloys in the hot-rolled and double-annealed conditions. Higher fatigue strength was detected in the double-annealed condition, especially in the high cycle fatigue regime, which indicated that the fatigue resistance of the bimodal titanium alloy can be enhanced by increasing the volume fraction of the lamellar beta grain.
Fracture surface observation revealed that for both the heat treatment conditions, surface crack initiation occurs in the entire fatigue life range, while interior-induced failure appears only in the VHCF regime. On the other hand, fatigue micro-cracks initiated mainly along the maximum shear stress direction of the equiaxed alpha grains by dislocation slips. The length of initiated micro-cracks was just about 2-3 μm. This caused that the facet-like morphology, which prevails in the crack initiation region of titanium alloys, is difficult to observe in this study.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
As planned, two types of bimodal microstructures were tailored and ultra-high cycle fatigue tests were performed. Fracture surface examination and microstructure characterization were conducted to illuminate fatigue failure mechanism. The results obtained will be presented in the Eighth International Conference on Very High Cycle Fatigue, and will be submitted to the journal of Materials Science and Engineering: A. Moreover, we prepared the technologies for machining micron-sized notches, which were originally scheduled for the next fiscal year. On the whole, we have achieved the research goals set up for FY2019, and the progress of the project was partially advanced.
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| Strategy for Future Research Activity |
Previous research results showed that the grains surrounding the crack initiation grain may also influence crack initiation behavior. Thus, the microstructure characteristics of crack initiation grains and their adjacent grains will be studied together. Significant difference in fatigue life was found obviousely even though the same cyclic stress was applied, and the same kind of crack initiation mechanism happened, i.e. cracking either from surface or from interior. We inferred that the difference may result from the critical microstructure configurations for fatigue crack initiation. Based on the detected critical microstructure configurations, the relationship between the microstructure configurations and fatigue life will be determined. Meanwhile, we will continue to implement the project in accordance with the research plans for FY2020.
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