Abstract
We obtained thermal desorption spectra of hydrogen for a small-size iron specimen to which strain was applied during charging with hydrogen atoms. In the spectra, a shoulder-shaped peak in the high-temperature side was enhanced compared with the spectra of the specimen to which only strain was applied. We also observed that the peak almost disappeared by aging processes at ≥ 373 K. Then, assuming that the shoulder-shaped peak results from hydrogen atoms released by vacancies, we simulated the thermal desorption spectra using a model incorporating the behavior of vacancies and vacancy clusters. The model considered up to vacancy cluster {{V_9}}, which is composed of nine vacancies, and employed the parameters based on atomistic calculations, including the H trapping energy of vacancies and vacancy clusters that we estimated using the molecular static calculation. As a result, we revealed that the model could, on the whole, reproduce the experimental spectra, except two characteristic differences, and also the dependence of the spectra on the aging temperature. By examining the cause of the differences, the possibilities that the diffusion of clusters of {V_2} and {V_3} is slower than the model and that vacancy clusters are generated by applying strain and H charging concurrently were indicated.
Highlights
HYDROGEN embrittlement (HE) is known as one cause of delayed fracture of high-strength steels and cold cracking of weld alloys.[1,2,3,4] As is widely known, in HE, H atoms gather at the location of stress concentration resulting from tensile stress or residual stress and lead to toughness degradation in materials by promoting the production of defects and/or by weakening the strength of atomic bonds, and the toughness degradation causes cracks or fractures.[5,6]Manuscript submitted July 6, 2020; accepted October 17, 2020
Because ductility loss is observed in a tensile test of the steels and of the alloys even after H atoms charged for the vacancy formation are removed, as shown in Figure 1, it is implied that the vacancies themselves bring about HE, and it is mentioned that the phenomenon may support the H-enhanced strain-induced vacancy (HESIV) model that is proposed as a mechanism of HE[12,13,14]
We simulated the experimental spectra using the model based on the McNabb-Foster theory, which considered the migration of vacancies and vacancy clusters as well as the growth and dissociation of vacancy clusters
Summary
HYDROGEN embrittlement (HE) is known as one cause of delayed fracture of high-strength steels and cold cracking of weld alloys.[1,2,3,4] As is widely known, in HE, H atoms gather at the location of stress concentration resulting from tensile stress or residual stress and lead to toughness degradation in materials by promoting the production of defects and/or by weakening the strength of atomic bonds, and the toughness degradation causes cracks or fractures.[5,6]Manuscript submitted July 6, 2020; accepted October 17, 2020. The report[8] mentions the effect of H atoms on the vacancy formation in deformed steel based on PAL measurement. Similar vacancy formation is observed in tempered martensitic steels prestressed cyclically during H charging[9], in Inconel 625, iron[10], and nickel alloys[11], which are deformed and H charged concurrently. Because ductility loss is observed in a tensile test of the steels and of the alloys even after H atoms charged for the vacancy formation are removed, as shown, it is implied that the vacancies themselves bring about HE, and it is mentioned that the phenomenon may support the H-enhanced strain-induced vacancy (HESIV) model that is proposed as a mechanism of HE[12,13,14]. How many vacancies bring about HE is still unclear, and the amount and size of vacancy clusters that can be generated by vacancy agglomeration relating to HE are unknown
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