A Nanoscale Study of Dislocation Nucleation at the Crack Tip in the Nickel-Hydrogen System
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A simulation framework, to model hydrogen (H)–dislocation interaction and understand the associated H diffusion mechanism, using modified Monte Carlo (MC) dynamics is presented. It is well known that H influences the physical and chemical properties of metals and alloys.[1–6] Hydrogen can be absorbed at the surface causing surface embrittlement or can penetrate into the matrix causing hydride formation.[1,7–9] Fundamental issues in understanding H-enhanced dislocation mobility and dislocation nucleation are still a work in progress.[10–38] In the past few decades, these fundamental issues have been studied theoretically using various modeling techniques such as quantum mechanics/first principal,[35] atomistic,[36–39] continuum phenomenological material models.[34] Different experimental techniques have also been employed to describe underlying mechanisms related to H solute interaction with material microstructure under various stresses and their subsequent effect on material engineering properties.[1–6] The effects of H on the bulk deformation and fracture properties have been studied through experiments[17,18] K.N. SOLANKI, Assistant Professor (Research), and D.K. WARD, Postdoctoral Fellow, are with the Center for Advanced Vehicular Systems, Starkville, MS 39759. Contact e-mail: [email protected] D.J. BAMMANN, Professor, is with the Mechanical Engineering Department, Mississippi State University, Mississippi State University, Mississippi State, MS 39762. Manuscript submitted February 19, 2010. Article published online October 23, 2010 340—VOLUME 42A, FEBRUARY 2011
and simulations.[8,9,19,20] It has been shown that the H can cause a material to either harden or soften by interacting with dislocations or with impurities.[10,17] Hydrogen, on the one hand, can increase dislocation velocities[11,13,20,21,24–26] or, on the other hand, can shield the elastic interactions between dislocations and shortrange stress fields.[14] Hydrogen can also induce shear localization and associated plastic instabilities that lead to premature material failure.[8] Furthermore, H diffusion along the grain boundaries and interfaces encourages intergranular failure due to the H-induced decohesion mechanism.[27] Researchers also suggest that H promotes void nucleation based on experimental data for steels,[12,28–31] nickel (Ni) alloy,[32] and aluminum alloy.[33] Hydrogen can affect plastic flow by lowering the energy barrier for dislocation slip, thereby enhancing strain localization (H-enhanced localized plasticity (HELP)), and also by reducing the dislocation spacing that can affect observed strain hardening.[5] Hydrogen-assisted cracking, stress-corrosion cracking, or environmentally assisted cracking still remains incompletely understood from the mechanistic point of view. We attempt to understand a fundamental case of H diffusion and its interaction with the stress field. The approach investigates mechanisms operating at the atomic scale. Solanki et al.[40] and Horstemeyer et al.[41] have shown that the kinematic variables, such as displa
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