Effect of atomic scale plasticity on hydrogen diffusion in iron: Quantum mechanically informed and on-the-fly kinetic Mo
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M. Itakura Center for Computational Science and E-systems, Japan Atomic Energy Agency, Taito-ku, Tokyo 110-0015, Japan
M. Ortiz Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, California 91125
E.A. Cartera) Department of Mechanical and Aerospace Engineering and Program in Applied and Computational Mathematics, Princeton University, Princeton, New Jersey 08544 (Received 25 February 2008; accepted 2 July 2008)
We present an off-lattice, on-the-fly kinetic Monte Carlo (KMC) model for simulating stress-assisted diffusion and trapping of hydrogen by crystalline defects in iron. Given an embedded atom (EAM) potential as input, energy barriers for diffusion are ascertained on the fly from the local environments of H atoms. To reduce computational cost, on-the-fly calculations are supplemented with precomputed strain-dependent energy barriers in defect-free parts of the crystal. These precomputed barriers, obtained with high-accuracy density functional theory calculations, are used to ascertain the veracity of the EAM barriers and correct them when necessary. Examples of bulk diffusion in crystals containing a screw dipole and vacancies are presented. Effective diffusivities obtained from KMC simulations are found to be in good agreement with theory. Our model provides an avenue for simulating the interaction of hydrogen with cracks, dislocations, grain boundaries, and other lattice defects, over extended time scales, albeit at atomistic length scales.
I. INTRODUCTION
The interaction of hydrogen with metals is a longstanding problem of interest across several disciplines such as surface chemistry and catalysis, applied physics, metallurgy, and mechanical engineering. The deleterious effect of hydrogen on the mechanical properties of metals and alloys remains a vexing problem for structural applications and is still incompletely understood from a mechanistic standpoint due to the multiplicity of mechanisms by which hydrogen interacts with metals. Furthermore, it is now widely accepted that several of these phenomena are inherently coupled1–3 and the search for a single dominant mechanism, which has fueled much research and (inevitably) opposing viewpoints, is futile. Among several mechanisms proposed for hydrogen
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0340 J. Mater. Res., Vol. 23, No. 10, Oct 2008
embrittlement (HE) of metals, hydrogen-enhanced decohesion (HEDE)4–6 and hydrogen-enhanced local plasticity (HELP)2,7,8 have gained acceptance as the two most viable for stable phases. Material degradation via the formation of brittle hydride phases,9 possibly stabilized by local stresses,10 is also a viable mechanism but has not been found to be relevant for iron,1 which is the material of interest in this work. The HEDE mechanism postulates embrittlement due to localized reduction in cohesive strength induced by the segregation of hydrogen to defects such as grain boundaries, microcracks, notches, and second phase particles, among ot
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