Grain Boundary Diffusion of Hydrogen and the Embrittlement of Nickel PDF Download
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Author: Thomas Miller Harris
Publisher:
ISBN:
Category :
Languages : en
Pages : 140
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Author: Thomas Miller Harris
Publisher:
ISBN:
Category :
Languages : en
Pages : 140
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Author: Jiaqi Li
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
The mobility of hydrogen in metals is a key parameter for understanding the basic mechanisms of hydrogen embrittlement (HE). This problem is directly related to the mechanisms of diffusion and trapping of hydrogen within a crystal lattice. These mechanisms depend on the various microstructural heterogeneities and in particular the crystalline defects. In our work, we have focused on the diffusion and trapping of hydrogen in two elementary systems: nickel single crystals and bi-crystals. We developed a methodology combining experimental tools (electrochemical permeation / TDS, HRTEM, EBSD) and numerical methods (FEM-COMSOL / EAM-LAMMPS). The results obtained on the single crystals show a dependence of the diffusion coefficient of hydrogen with the crystallographic orientation and the hydrogen content. The thermodynamic analysis of the nickel-hydrogen-vacancy system shows a dependence of the chemical potential of hydrogen with the stress state induced by the formation of clusters of vacancies associated with the presence of hydrogen. The anisotropic character of the diffusion is then explained by the anisotropy of the elastic properties of the crystal lattice and the presence of these clusters. Moreover, we have characterized the processes of diffusion and trapping of hydrogen for nickel bi-crystals with different free volumes. The segregation energy of hydrogen depends on the nature of the site (the local free volume and the mechanical energy associated with the incorporation of solute). The diffusion of hydrogen is directly influenced by the nature of the grain boundary (the free volume and the distribution of the segregation sites). Our results, at the atomic scale, show a correlation between the solubility and the free volume of the grain boundary. The grain boundaries with a higher free volume have more favorable diffusion paths for hydrogen than in the crystal lattice and at the same time more segregation sites.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 14
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Author: Pavel Lejcek
Publisher: Springer Science & Business Media
ISBN: 3642125050
Category : Technology & Engineering
Languages : en
Pages : 249
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Book Description
Grain boundaries are important structural components of polycrystalline materials used in the vast majority of technical applications. Because grain boundaries form a continuous network throughout such materials, their properties may limit their practical use. One of the serious phenomena which evoke these limitations is the grain boundary segregation of impurities. It results in the loss of grain boundary cohesion and consequently, in brittle fracture of the materials. The current book deals with fundamentals of grain boundary segregation in metallic materials and its relationship to the grain boundary structure, classification and other materials properties.
Author: John Paul Hanson (Ph. D.)
Publisher:
ISBN:
Category :
Languages : en
Pages : 196
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Book Description
Hydrogen embrittlement of engineering alloys is characterized by a loss of ductility and unpredictable failure. These failures affect numerous industries, including nuclear power, oil and gas exploration, and hydrogen transportation and storage. In face-centered cubic alloys, the resultant fracture is intergranular and very sensitive to grain boundary character. We study this behavior in alloy 725, a popular nickel-iron superalloy with high strength and corrosion resistance. Using a suite of complementary experimental techniques we reveal the fracture behavior of individual grain boundaries in hydrogen embrittlement for the first time, providing critical understanding of the role of grain boundary character and informing improved microstructure design. We study crack propagation in hydrogen embrittled tensile test specimens using highenergy diffraction-microscopy, a non-destructive X-ray synchrotron technique capable of mapping grain boundaries in 3-D. We find that boundaries with low-index planes (BLIPs), defined as planes within 10° of [111], [110] or [100], resist crack propagation and improve toughness. We show that coherent twin boundaries (CTBs), a subset of BLIPs, also indirectly improve toughness by increasing the heterogeneity of the grain boundaries they intersect. In addition, we use electron backscatter diffraction and scanning electron microscopy to identify the grain boundaries along which cracks initiate and propagate on the sample surface. We unambiguously show that grain boundaries are the source of crack initiation, and we study a statistically significant number of cracking events, providing the ability to determine the role of grain boundary character. Surprisingly, we find that while CTBs resist crack propagation, they preferentially initiate cracks. These results inform a more nuanced approach to microstructure design. Typically grain boundary engineering techniques aim to maximize the fraction of low-S boundaries as designated by the coincident site lattice model. Our results suggest that these techniques should maximize the fraction of BLIPs instead. In addition, the dual nature of CTBs suggests the development of graded microstructures, with high concentrations of CTBs in the interior to resist crack propagation and reduced concentrations at the surface to limit crack initiation.
Author: Bachir Osman Hoch
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
The diffusion of hydrogen in metals is a key factor for understanding the basic mechanisms of hydrogen embrittlement. However, the contribution of grain boundaries to the hydrogen diffusion is not well established. In this this work, we first investigated the effects of a heterogeneous grain boundary networks on the effective diffusivity in polycrystalline materials, using finite elements modeling. To do so, hydrogen diffusion through heterogeneous materials, modeled by a ternary continuum composite media, was simulated. We showed, by characterizing the grain-boundary connectivity, that there are strong correlations between the grain-boundary connectivity parameters and the effective diffusivity. It was found also that these correlations are more significant for nanocrystalline materials. Moreover, by using a homogenization method, it was evidenced that the percolation behavior of the effective diffusivity is controlled by the grain-boundary network evolution, without exhibiting the same percolation threshold than the latter. A second approach, using EBSD-based microstructures, was conducted to evaluate the effect of microstructural constraints on the grain boundary connectivity and to compare the effective diffusivity numerically obtained with experimental data on polycrystalline nickel from literature. In parallel, experimental analyses were performed to analyze the effects of the grain boundaries on the local hydrogen concentration. This highlighted the significant impact of grain-boundary character on the hydrogen distribution around grain boundaries, which can not be explained by the only diffusion process.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 11
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Author: R. M. Latanision
Publisher:
ISBN:
Category :
Languages : en
Pages : 11
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Book Description
Earlier work has shown an association between the intergranular embrittlement of nickel by hydrogen and the presence of hydrogen recombination poisons at the grain boundaries. The present effort demonstrates that the susceptibility of nickel to embrittlement may be controlled by heat treatment sequences which affect the partitioning of impurities. It is also suggested that the association between impurities and microchemistry may apply in part as well as to the temper embrittlement of steels. (Author).
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 17
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Author: David H. Lassila
Publisher:
ISBN:
Category :
Languages : en
Pages : 246
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Experiments were performed to determine the effects of hydrogen segregation at grain boundaries on intergranular fracture of charged nickel deformed in tension. Two modes of hydrogen segregation are investigated; lattice diffusion and dislocation transport of Cottrell atmospheres of hydrogen. The effect of cosegregation of hydrogen and sulfur is also explored. The results of this study show that the parameters affecting the diffusive segregation of hydrogen to grain boundaries play a fundamental role in conditions which result in embrittlement. If a sufficient quantity of hydrogen at grain boundaries is attained, the fracture mode of nickel will change from ductile rupture to intergranular. Experiments performed to evaluate the role of dislocation transport of hydrogen in the embrittlement of nickel suggest that dislocation transport has no effect on embrittlement and that the embrittlement mechanism is not dependent on hydrogen mobility. An analysis of the thermodynamics and kinetics of hydrogen segregation yielded values for the binding enthalpy of hydrogen to nickel grain boundaries and the breadth of hydrogen enhancement at boundaries. By considering these values and other test results a mechanism of hydrogen embrittlement of nickel is proposed. Three series of nickel tensile specimens were tested with various amounts of sulfur segregation present at grain boundaries. Segregation on the order of 0.1 monolayer has been shown to severely increase the embrittlement susceptibility of hydrogen charged nickel. (Author).
