Atomistic study of the competitive relationship between edge dislocation motion and hydrogen diffusion in alpha iron
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Ryosuke Matsumoto and Noriyuki Miyazaki Department of Mechanical Engineering and Science, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan (Received 9 December 2010; accepted 21 March 2011)
The interaction between a dislocation and hydrogen is considered to play an important role in hydrogen-related fractures for metals; it has been experimentally reported that hydrogen affects the dislocation mobility. These studies, however, show different macroscopic softening and/or hardening effects in iron, and the interaction between the dislocation and hydrogen remains unclear. In this study, we investigated the occurrence of interactions between a {112},111. edge dislocation and a hydrogen atom via the estimation of the stress-dependent energy barriers for the dislocation motion and hydrogen diffusion in alpha iron using atomistic calculations. Our results show the existence of boundary stress conditions: dislocation mobility increment (softening) occurs at a lower applied stress, dislocation mobility decrement (hardening) occurs at an intermediate stress, and no effects occur for the steady motion of a dislocation at a higher stress in this analysis condition. I. INTRODUCTION
The phenomenon of hydrogen affecting the mechanical property of metals, known as hydrogen embrittlement, is one of the fracture mechanisms that should be clarified for the safe use of structural materials. Many studies have been performed to clarify the fundamental effect of hydrogen on material strength to prevent hydrogen embrittlement.1–13 One of the typical effect of dissolved hydrogen appears on the plastic deformation behavior, which can be observed in the stress–strain relation and/or the fracture surface.13 Therefore, the dislocation motion in the presence of hydrogen is considered to play a fundamental role on hydrogen-related fractures for metals.5,6,9–11 For example, different effects of hydrogen on plastic deformation (macroscopic softening/hardening) have been reported in iron.14 These results show that the fundamental effect of hydrogen on plastic deformation is still unclear. Recently, much attention has been paid to the increment of the dislocation mobility (softening) as an elementary process for hydrogen embrittlement; this increment is known as the hydrogen-enhanced localized plasticity (HELP) mechanism.5,6 The HELP mechanism is considered to be responsible for the reduction of the energy barrier for dislocation motion5 or the hydrogen shielding effect due to the reduction of the interaction between dislocations.6 Sofronis et al. have reported that a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.106 J. Mater. Res., Vol. 26, No. 10, May 28, 2011
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the hydrogen atmosphere shields the effective shear stress between dislocations under extremely high hydrogen concentrations.6 On the other hand, we reported that the energy barrier for {112},111. edge dislocation motion decreases in the presence of hydrogen
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