2021 Fiscal Year Annual Research Report
Characterization of Crack Nucleation in Titanium Alloys with Metastable Microstructures
| Project/Area Number |
20F40737
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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) |
LIU HANQING 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) |
2020-11-13 – 2023-03-31
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| Keywords | Titanium alloy / Phase transformation / Slip deformation / Crack nucleation |
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| Outline of Annual Research Achievements |
Metastable microstructures in titanium alloy joints could be formed during the solidification process because of the high cooling rate, resulting in a heterogeneous mechanical response to the tensile and fatigue loading. The topmost layer exhibits the best plasticity properties due to the martensite transformation behavior, on contrary, the metastable thermal martensite formed during solidification could damage the ductility of the weld joint. Such microstructural heterogeneities could therefore facilitate the discrepancy in fatigue crack nucleation behavior. The formation of thermal martensite could facilitate the crack nucleating from the matrix grain boundary, while slip deformation could be actived within the thermal martensite free grains, resulting in fatigue crack nucleation. Such microstructural deficiencies could be reduced by the following direct aging treatment with optimized parameters, and therefore enhance the mechanical properties of the materials.
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
Generally, we have accomplished the planned research. Further research on the deformation behavior of precipitates during fatigue crack nucleation and the microstructural mechanisms on the crack extension activities will be carried out. The relatated mechanical tests are under conducted and the obtained results meet our expectations.
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| Strategy for Future Research Activity |
1. Crack extension mechanism will be carried out by site specifically introducing micro defects into the regions with target microstructures to study the influence of twin boundary and Schmidt factor (SF) on cyclic deformation precess using EBSD. A quantitative analysis will be carried out to investigate the crack propagation rate before the Paris domain.
2. The influences of precipitates on the will be carried out by controlling the holding time of aging treatment and therefore modifying the size and volume fraction of the material. The interaction of dislocation with the interface of precipitates and matrix, as well as the evolution of solution atoms neighboring to the dislocation will furtherly be characterized using AC-STEM.
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