| 研究実績の概要 |
In the research last year, it has been found that the initial damage of nickel-based alloys under creep-fatigue loading at elevated temperature is manifested by the accumulation of dislocations and voids at the interface. This form of degradation can not only be embodied as intergranular cracks (Alloy 617) but also can be observed in the form of holes (Alloy 625). Although the forms of degradation are not the same, it has been confirmed that the main reason for the degradation is the diffusion of atoms. According to previous studies on the diffusion equation by Shinozaki, the required activation energy of atoms for diffusion could be significantly reduced under the influence of stress. The damage that occurs frequently around the interface indicates that in addition to the external load, there is also another force that drives the diffusion of atoms. It was considered that this force was caused by the lattice mismatch at the interface. Moreover, the magnitude of the stress field should be also related to the degree of the local lattice mismatch. The example of degradation of Alloy 625 has been a good proof of this theory. Defects preferentially accumulated around precipitates, because there is a higher degree of lattice mismatch at the interface between the precipitates and Ni-matrix. Based on the stress-dependent Arrhenius formula Shinozaki modified, the internal stress term was introduced to the diffusion formula. A mathematical model for quantitatively evaluation of the damage of materials under creep-fatigue loading at evaluated temperature was proposed.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
3: やや遅れている
理由
Due to the pandemic of COVID-19 last year, my research progress was delayed to a certain extent. Due to the school’s blockade, people outside the school were not allowed to enter the lab, which led to the failure of timely repair and maintenance of problematic experimental equipment. Around May last year, the progress of the experiment stagnated. This led to a shortage of experimental data, which made me encounter great difficulties in the final quantitative analysis of internal stress in my research.
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| 今後の研究の推進方策 |
In the research last year, a mathematical model for quantitatively evaluation of the damage of materials under creep-fatigue loading at evaluated temperature based on the diffusion formula was proposed. However, we failed to determine the magnitude of the internal and external stress, separately. This is because the crystal orientations of each grain inside the real material are different, the magnitude of the external stress is not exactly the same as the external load. In my opinion, it is almost impossible to quantitatively analyze internal stress based on the experimental data I had. Moreover, due to the complexity and instability of the experiment, even if the intermittent test is repeated many times, it is very difficult and time-consuming to obtain the experimental results that can quantitatively separate the internal stress and the external stress. Thus, in this year, I will try to perfect this mathematical model and quantitatively evaluate the magnitude of internal and external stress based on the simulation results. Compared with the experiments, the simulation method can let us see the influence of the stress field around the interface on the diffusion of atoms more intuitively. In addition, the effect of fatigue load on the total creep-fatigue damage will also be completed this year. The effect of changing the loading and unloading rate of the fatigue load and the frequency of the fatigue load on the initial damage rate of the material will be also determined by applying multiple sets of controlled intermittent experiments.
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