13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Dislocation dynamics analysis of hydrogen embrittlement in alpha iron based on atomistic investigations Shinya Taketomi1,*, Honami Imanishi2, Ryosuke Matsumoto2, Noriyuki Miyazaki2 1 Department of Mechanical Engineering, Saga University, 840-8502, Japan 2 Department of Mechanical Engineering and Science, Kyoto University, 606-8501, Japan * Corresponding author: taketomi@me.saga-u.ac.jp Abstract Hydrogen embrittlement is well known phenomenon in which hydrogen lowers the strength of materials. Hydrogen atoms show unique characteristics such as high diffusivity and low concentration in metals, so the direct observations of hydrogen effects on mechanical properties are still difficult. Nevertheless, experimental studies have been revealed many valuable results; HEDE (Hydrogen Enhanced Decohesion), HELP (Hydrogen Enhanced Localized Plasticity), HESIV (Hydrogen Enhanced Strain Induced Vacancy) and others have been proposed as a fracture mechanism of hydrogen embrittlement so far. However, the overview of the hydrogen embrittlement under various conditions is still unclear. In this study, we focus on the edge dislocation motion in the presence of hydrogen atom as one of the elementary process of hydrogen embrittlement in alpha iron. Our previous studies showed that the dislocation velocity increace (softening) occurs at lower hydrogen concentration and lower applied stress conditions. In contrast, dislocation velocity decrease (hardening) occurs at higher hydrogen concentration or higher applied stress conditions. Therefore, we performed the dislocation dynamics calculation around a crack tip based on the results obtained by our atomistic calculations. The results indicate that the hydrogen embrittlement mechanisms possibly change depending on boundary conditions (hydrogen concentration and applied stress intensity factor rate). Keywords Hydrogen embrittlement, Dislocation dynamics, Atomistic simulation, Alpha iron 1. Introduction Hydrogen embrittlement is well known phenomenon in which hydrogen lowers the strength of materials. Hydrogen atoms show unique characteristics such as high diffusivity and low concentration in metals, so the direct observations of hydrogen effects on mechanical properties are still difficult. Nevertheless, experimental studies have been revealed many valuable results; e.g. macroscopic hydrogen embrittlement fracture process, stress-strain relation and so on. HEDE (Hydrogen Enhanced Decohesion [1]), HELP (Hydrogen Enhanced Localized Plasticity [2]), HESIV (Hydrogen Enhanced Strain Induced Vacancy [3]) and others have been proposed as a fracture mechanism of hydrogen embrittlement so far. However, the overview of the hydrogen embrittlement under various conditions is still unclear. Here, we believe the evaluation of the elementary process can lead the understanding of whole image of hydrogen embrittlement, and we also believe the atomistic simulations can contribute the understanding of the hydrogen embrittlement, since the atomistic simulations show information in high resolution, and can easily exchange the boundary conditions (i.e. stress, temperature, hydrogen concentration). In this study, we focus on the edge dislocation motion in the presence of hydrogen atom as one of the elementary process of hydrogen embrittlement in alpha iron. Our previous studies [4-5] showed that the dislocation velocity increase (softening) occurs at lower hydrogen concentration and lower applied stress conditions. In contrast, dislocation velocity decrease (hardening) occurs at higher hydrogen concentration or higher applied stress conditions. These contradictive behaviors attribute the applied stress dependent competition between dislocation motion and hydrogen diffusion [4-5]. These atomistic studies, however, evaluated the individual dislocation motion due to the limitation of calculation cost. Therefore, in this study, we perform the dislocation dynamics calculations around a
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