Effects of grain boundary and boundary inclination on hydrogen diffusion in a-iron
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Diffusion of interstitial hydrogen atoms in a-iron was investigated using molecular dynamic simulation. In particular, hydrogen diffusivities in bulk, on (001) surface and within a R5 [100]/(013) symmetric tilt grain boundary (STGB) were estimated in a temperature range of 400 and 700 K. Furthermore, hydrogen diffusivities in a series of R5 [100] tilt grain boundaries with different inclinations were also determined as a function of temperature. The inclination dependence of activation energy for diffusion exhibits two local maxima, which correspond to two STGBs. Additional calculation of inclination dependence of boundary energy and boundary specific excess volume shows two local minima at the same STGBs. This suggests hydrogen diffusion into and within a grain boundary might be assisted by grain boundary excess volume and stress. Simulation of effects of hydrostatic pressure on diffusion shows tensile stress can promote hydrogen diffusion in lattice into grain boundary or surface traps, while compressive stress leads to a decrease in diffusivity, and a slower rate of filling these traps.
I. INTRODUCTION
Hydrogen embrittlement, whereby hydrogen reduces ductility and fracture stress and thus degrades the mechanical properties of almost all metals and alloys, is believed to be one of most common causes leading to an increasing number of material failures in variety of engineering materials, e.g., oil/gas transmission pipelines carbon steels and nuclear reactor pressure vessel steels.1–9 Extensive experiments have been conducted to relate macroscopic and microscopic properties using different techniques such as permeation measurement,10 autoradiography,11,12 thermo analysis method,13,14 and nuclear resonance reaction analysis.15 It is commonly accepted that diffusion and segregation of interstitial hydrogen atoms to material defects (e.g., voids, dislocations, grain boundaries, and cracks) and the consequent interactions between hydrogen atoms and defects play an important role in hydrogen embrittlement. Based upon experimental observations, different mechanisms have been proposed, including hydrogen-induced hydride formation,16 hydrogen-induced lattice decohesion model,1,17 hydrogen-enhanced localized plasticity model,18,19 and a recent hydrogen-enhanced stressinduced vacancy mechanism.20 Unfortunately, a complete understanding of hydrogen effects on materials mechanical behavior is still lacking owing to the complexity of microstructure of metallic materials and the difficulty of direct observation of hydrogen interaction with defects mainly a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.262 J. Mater. Res., Vol. 26, No. 21, Nov 14, 2011
caused by small size, high diffusivity, and low solubility of hydrogen in metals.10 With recent increases in computer processor speed along with the development of massive parallel computing architectures, computer simulation is becoming an alternative approach to study hydrogen physics in iron. Ab initio calculation based on densi
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