材料专业 优秀英文文献

Abstract
The impact of grain size on hydrogen diffusion and trapping mechanisms has been investigated for a wide range of grain size of nontextured pure nickel. Both aspects depend mainly on the nature of grain boundaries (GBs). In particular, we illustrate the effects of random and special boundaries on the different defects and trapping sites stored in the GBs, and their consequences on hydrogen transport and segregation. The high-angle random boundaries are considered as disordered phase where the hydrogen diffusion is accelerated, while the special boundaries constitute a potential zone for hydrogen trapping due to the high density of trapping sites as dislocations and vacancies. The predominance of one phenomenon over the other depends on several parameters, such as the grain size, the probability of grain boundary connectivity, the grain boundary energy and the excess of free volume. In addition, our experiments confirm that hydrogen promotes vacancy formation probably in GBs. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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conclusion that the TJs may also accelerate hydrogen diffusion and that diffusivity along these interfaces is higher than along GBs. Despite experimental and modeling approaches being proposed to explain short-circuit diffusion along the GBs and TJs in fcc materials, controversy still remains. Louthan et al. [19] and Tseng et al. [28] maintain that the acceleration of hydrogen diffusion along GBs is caused by the geometrically necessary dislocations (GNDs) stored in these interfaces. However, it was also showed that these defects represent trapping sites for hydrogen [29], inducing a slowdown of its diffusivity [30]. The use of new experimental and numerical techniques has shown that the configuration and energy of GBs can explain the short-circuit diffusion. Indeed, Ladna and Birnbaum [22,31] found using secondary ion mass spectrometry that the hydrogen diffusion is accelerated along the high-energy GBs, and by numerical simulations (density field theory), Pederson et al. [32] showed that the twist configuration of GBs favors hydrogen diffusion.
⇑ Corresponding author. E-mail address: abdelali.oudriss@univ-lr.fr (A. Oudriss).
exhaustive statistical analysis of a large data set, the values of hydrogen solubility and diffusivity in polycrystalline metals and alloys exhibit some incoherence [14,15]. Such problems appear to be caused primarily by the setting up of complex experiments as well as by insufficient microstructural characterization of the samples [16].
Recently, we have suggested that hydrogen diffusion along the GBs depends on the GND density, which is directly associated with the GB misorientation, h [33]. Nevertheless, this approach is only valid when the boundary misorientation is less than 15°. Indeed, when h becomes higher than 15°, the GB structure becomes complex and it is no longer accommodated by only one GND [34]. In this case, the GBs are defined by the coincidence site lattice (CSL) [34], whereby each boundary is assigned a number ‘R’ corresponding to the reciprocal number density of lattice sites that are common to both crystals. In fcc polycrystals, two categories are distinguished: (i) boundaries with R < 29 called “special” grain boundaries that have a superstructure, and whose misorientations are accommodated by a second set of dislocations network and vacancies [34]; and (ii) grain boundaries with high coincidence index (R > 29) qualified as “general” or “random” which are considered as an disordered phase [35,36]. Moreover, special and random boundaries and their degree of percolation play a significant role in several phenomena, including intergranular corrosion [37], creep [38], etc. However, their impact on hydrogen diffusion in fcc materials remains little studied. The objective of the present work is to identify the role played by the nature of GBs on the acceleration and/or slowdown of hydrogen diffusion in nickel.
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Acta Materialia 60 (2012) 6814–6828
www.elsevier.com/locate/actamat
Grain size and grain-boundary effects on diffusion and trapping of hydrogen in pure nickel
Hydrogen embrittlement (HE) requires a good knowledge of the hydrogen transport and segregation mechanisms in face-centered cubic (fcc) polycrystalline materials for the different reasons discussed previously. GBs and triple junctions (TJs) represent structural defects that may affect these mechanisms; their large variety and complex structure do not simplify the comprehension of HE. Ultimately, despite numerous studies [17–21], the influence of GBs and TJs on HE in fcc materials remains a matter of controversy. Indeed, several studies [17,22–25] have confirmed that hydrogen diffusion is accelerated along the GBs by a mechanism of short-circuit diffusion. This phenomenon is followed by an increase in the hydrogen “solubility”. Moreover, studies [21,26,27] on nanocrystalline materials have led to the
Keywords: Hydrogen; Diffusion; Trapping; Nickel; Grain boundary
1. Introduction
In various technological applications, hydrogen-induced embrittlement causes premature structural failure due to physical and/or chemical processes occurring on the material’s surface or in the bulk of the material (aggressive media, mechanical state, solute effect, etc.). Despite the fact that several studies have explored the nature, causes and control of metal degradation due to hydrogen, the diversity of the situations investigated (metallurgical state, surface reactivity, etc.) has led to erroneous interpretations and/or controversial discussions [1–13]. Therefore, studying the kinetics of hydrogen ingress, diffusion and trapping is decisive to a better understanding of hydrogen embrittlement. In annealed polycrystalline materials, grain boundaries (GBs) have a crucial impact on transport and segregation of hydrogen and on fracture mechanisms. According to
1359-6454/$36.00 Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2012.09.004
A. Oudriss et al. / Acta Materialia 60 (2012) 6814–6828
A. Oudriss ⇑, J. Creus, J. Bouhattate, E. Conforto, C. Berziou, C. Savall, X. Feaugas
LaSIE FRE CNRS 3474, Universite´ de la Rochelle, Av. Michel Cre´peau, 17042 La Rochelle, France Received 10 May 2012; received in revised form 30 August 2012; accepted 2 September 2012 Available online 1 October 2012