2022 Fiscal Year Research-status Report
空孔型欠陥のin situおよび二次元分布計測による金属水素脆化支配欠陥
| Project/Area Number |
21K04627
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| Research Institution | Chiba University |
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Principal Investigator |
Chiari Luca 千葉大学, 大学院工学研究院, 助教 (20794572)
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| Project Period (FY) |
2021-04-01 – 2024-03-31
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| Keywords | 水素脆化 / 空孔型欠陥 / 金属 / その場測定 |
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| Outline of Annual Research Achievements |
The main purpose of this research is to improve the positron annihilation spectroscopy technique, which is the only method for detecting atomic vacancies and to obtain confirmation of the defects thought to be responsible of the hydrogen embrittlement in common metals used in the hydrogen energy infrastructure, that is the vacancy-hydrogen complexes. An in-situ positron annihilation lifetime measurement method in a hydrogen environment, as well as in a hydrogen environment and under tensile stress, was developed and the existence of the vacancy-hydrogen complex, which becomes unstable by room temperature aging due to the hydrogen desorption after unloading, was proved in pure bcc-iron, which has a very large hydrogen diffusion coefficient.
A Teflon electrolytic cell was devised for in-situ positron annihilation measurements under electrolytic hydrogen addition and stress loading. Positron lifetime measurements were carried out while charging charging the samples that were strained at fast or slow strain rate up to 25% in a hydrogen environment. The results for the sample strained at fast strain rate showed the formation of vacancy clusters irrespective of the measurements during in-situ hydrogen charge or in air, in accordance with previous studies. On the other hand, in the sample strained at slow strain rate the formation of vacancy-hydrogen complexes was detected during the hydrogen charge. Subsequently, these defects grew into increasingly larger vacancy clusters as a function of aging time in air due to the hydrogen desorption from the sample.
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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
The current status of this research is overall evaluated to be progressing rather smoothly. The reason for this evaluation is that the experiments in the first two years of this project have been implemented on schedule and according to the original plan described in the research proposal. The conducted research followed in detail the program outlined at the beginning of the project and the results obtained so far are very satisfactory, because they confirm the outcomes anticipated in the research proposal.
The formation of vacancy-hydrogen complexes, that is the controlling defects of the hydrogen embrittlement, in pure iron and nickel was confirmed experimentally for the first time by in-situ positron lifetime measurements in a hydrogen environment and under tensile stress load and through high-speed positron lifetime measurements using a high-intensity positron beam. Moreover, the change over time of these defects into increasingly larger vacancy clusters by room temperature aging was investigated with high temporal resolution using a high-intensity positron beam, as well as by measurements during the in-situ hydrogen charge.
In this research, it was directly demonstrated for the first time that the dominant defects of the hydrogen embrittlement in pure iron are monovacancy defects bound with hydrogen, i.e. vacancy-hydrogen complexes, and that they are the precursors of the vacancy clusters, which are confirmed to exist in the fractured samples and are thought to originate the voids and cracks that develop into the brittle fracture.
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| Strategy for Future Research Activity |
The research in the remaining one year of this project is scheduled to progress on time and following the original research plan described in the research proposal. In particular, we anticipate to carry out the experiments planned for the third year of the project as originally outlined. The planning and preparation for such experiments is progressing smoothly, so that the experiments can be conducted on a timely manner and within the time limits of this project.
In the third year of this project, we aim to continue and complete the in-situ positron lifetime measurements in a hydrogen environment and under tensile stress loading, by carrying out the measurements at different strain values and check if there is any correlation with the defects formed under those conditions. Moreover, we plan to expand the observation of the vacancy-hydrogen complex and vacancy behavior by aging in air in practical steels, such as SUS 304, SUS 316 and SUS 316L, by implementing positron lifetime measurements using the in-situ hydrogen charging technique under constant tensile stress.
In addition, we would like to verify the accumulation of atomic vacancies in high-strain regions during straining by acquiring two-dimensional defect map of hydrogen-embrittled samples quenched at low temperatures to stabilize the vacancy-hydrogen complexes by positron lifetime measurements using a a high-intensity positron microbeam. By changing the chemical composition of the samples, we plan to clarify the causal relationship between the atomic defect behavior and the hydrogen embrittlement generation conditions.
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| Causes of Carryover |
The expenses incurred for the development and construction of the in-situ positron lifetime measurement technique were partly deducted from the present research project. Moreover, high-speed positron lifetime measurements were conducted without great expenses in terms of purchasing new equipment by using an existing infrastructure at an external research facility that does not charge any usage fee.
Travel expenses to attend domestic conferences were charged to the funds of this project. On the other hand, we attended international conferences that were basically held only online, so that no travel expenses were charged to the funds of this project.
As a consequence of the above reasons, not all the direct costs that were anticipated in the research proposal have been incurred so far. The incurring amount will be fully used in the last year of this project, by purchasing the new equipment needed to complete the in-situ measurements under hydrogen + stress loading environment and the two-dimensional mapping of defects, as described in the research proposal. Moreover, we plan to attend international and domestic conferences to present the results of this project.
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