Chemomechanical Origin of Hydrogen Trapping at Grain Boundaries in fcc Metals (original) (raw)
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Acta Materialia, 2009
The feasibility of using ''grain-boundary engineering" techniques to reduce the susceptibility of a metallic material to intergranular embrittlement in the presence of hydrogen is examined. Using thermomechanical processing, the fraction of ''special" grain boundaries was increased from 46% to 75% (by length) in commercially pure nickel samples. In the presence of hydrogen concentrations between 1200 and 3400 appm, the high special fraction microstructure showed almost double the tensile ductility; also, the proportion of intergranular fracture was significantly lower and the J c fracture toughness values were some 20-30% higher in comparison with the low special fraction microstructure. We attribute the reduction in the severity of hydrogen-induced intergranular embrittlement to the higher fraction of special grain boundaries, where the degree of hydrogen segregation at these boundaries is reduced. Published by Elsevier Ltd on behalf of Acta Materialia Inc.
Metallurgical and Materials Transactions A, 2013
Material strengthening and embrittlement are controlled by complex intrinsic interactions between dislocations and hydrogen-induced defect structures that strongly alter the observed deformation mechanisms in materials. In this study, we reported molecular statics simulations at zero temperature for pure a-Fe with a single H atom at an interstitial and vacancy site, and two H atoms at an interstitial and vacancy site for each of the h100i, h110i, and h111i symmetric tilt grain boundary (STGB) systems. Simulation results show that the grain boundary (GB) system has a smaller effect than the type of H defect configuration (interstitial H, H-vacancy, interstitial 2H, and 2H-vacancy). For example, the segregation energy of hydrogen configurations as a function of distance is comparable between symmetric tilt GB systems. However, the segregation energy of the h100i STGB system with H at an interstitial site is 23 pct of the segregation energy of 2H at a similar interstitial site. This implies that there is a large binding energy associated with two interstitial H atoms in the GB. Thus, the energy gained by this H-H reaction is~54 pct of the segregation energy of 2H in an interstitial site, creating a large driving force for H atoms to bind to each other within the GB. Moreover, the cohesive energy values of 125 STGBs were calculated for various local H concentrations. We found that as the GB energy approaches zero, the energy gained by trapping more hydrogen atoms is negligible and the GB can fail via cleavage. These results also show that there is a strong correlation between the GB character and the trapping limit (saturation limit) for hydrogen. Finally, we developed an atomistic modeling framework to address the probabilistic nature of H segregation and the consequent embrittlement of the GB. These insights are useful for improving ductility by reengineering the GB character of polycrystalline materials to alter the segregation and embrittlement behavior in a-Fe.
Modeling Dislocation-Mediated Hydrogen Transport and Trapping in Face-Centered Cubic Metals
Journal of Engineering Materials and Technology, 2021
The diffusion of hydrogen in metals is of interest due to the deleterious influence of hydrogen on material ductility and fracture resistance. It is becoming increasingly clear that hydrogen transport couples significantly with dislocation activity. In this work, we use a coupled diffusion-crystal plasticity model to incorporate hydrogen transport associated with dislocation sweeping and pipe diffusion in addition to standard lattice diffusion. Moreover, we consider generation of vacancies via plastic deformation and stabilization of vacancies via trapping of hydrogen. The proposed hydrogen transport model is implemented in a physically based crystal viscoplasticity framework to model the interaction of dislocation substructure and hydrogen migration. In this study, focus is placed on hydrogen transport and trapping within the intense deformation field of a crack tip plastic zone. We discuss the implications of the model results in terms of constitutive relations that incorporate hy...
Influence of grain boundary misorientation on hydrogen embrittlement in bi-crystal nickel
International Journal of Hydrogen Energy, 2014
Computational techniques and tools have been developed to understand hydrogen embrittlement and hydrogen induced intergranular cracking based on grain boundary (GB) engineering with the help of computational materials engineering. This study can help to optimize GB misorientation configurations by identifying the cases that would improve the material properties increasing resistance to hydrogen embrittlement. In order to understand and optimize, it is important to understand the influence of misorientation angle on the atomic clustered hydrogen distribution under the impact of dilatational stress distributions. In this study, a number of bi-crystal models with tilt grain boundary (TGB) misorientation angles (θ) ranging between 0°≤ θ ≤ 90° were developed, with rotation performed about the [001] axis, using numerical microstructural finite element analysis. Subsequently, local stress and strain concentrations generated along the TGB (due to the difference in individual neighbouring crystals elastic anisotropy response as functions of misorientation angles) were evaluated when bi-crystals were subjected to overall uniform applied traction. Finally, the hydrogen distribution and segregations as a function of misorientation angles were studied. In real nickel, as opposed to the numerical model, geometrically necessary dislocations are generated due to GB misorientation. The generated dislocation motion along TGBs in response to dilatational mismatch varies depending on the misorientation angles. These generated dislocation motions affect the stress, strain and hydrogen distribution. Hydrogen segregates along these dislocations acting as traps and since the dislocation distribution varies depending on misorientation angles the hydrogen traps are also influenced by misorientation angles. From the results of numerical modelling it has been observed that the local stress, strain and hydrogen distributions are inhomogeneous, affected by the misorientation angles, orientations of neighbouring crystal and boundary conditions. In real material, as opposed to the numerical model, the clustered atomic hydrogens are segregated in traps near to the TGB due to the influence of dislocations developed under the effects of applied mechanical stress. The numerical model predicts maximum hydrogen concentrations are accumulated on the TGB with misorientation angles ranging between 15°<θ<45°. This investigation reinforces the importance of GB engineering for designing and optimizing these materials to decrease hydrogen segregation arising from TGB misorientation angles.
Submitted for the MAR10 Meeting of The American Physical Society Strain-induced metal-hydrogen interactions across the first transition series-An ab initio study of hydrogen embrittlement JOHANN VON PEZOLD, UGUR AYDIN, JÖRG NEUGEBAUER, Max-Planck-Institut fuer Eisenforschung GmbH-The attractive interaction between hydrogen and distorted regions of the host matrix underlies all the currently discussed mechanisms of hydrogen-induced embrittlement of metals, such as hydrogen enhanced local plasticity (HELP), hydrogen enhanced decohesion (HEDE) and stress-induced hydride formation. In this study we investigate these interactions systematically by determining heat of solutions, H-H binding energies within the metal matrix, as well as phase diagrams as a function of the lattice strain and the H chemical potential across the first transition series (3d elements) using Density Functional Theory (DFT) calculations. The results will be interpreted in terms of the likely embrittlement mechanisms of these metals.
Atomistic simulations of hydrogen and carbon segregation in α-iron grain boundaries
IOP Conference Series: Materials Science and Engineering
During material deformation, the coincidence site lattice (CSL) grain boundaries (GBs) are exhibiting deviations from their ideal lattice structure. Hence, this will change the atomic structural integrity by generating full and partial dislocation joints on the ideal CSL boundaries. In this analysis, the ideal ∑5 (310) GB structures and its angular deviations iniron within the limit of Brandon criterion, in order to conserve the dislocation core structure, will be studied in depth using molecular statics simulations. Firstly, the hydrogen and carbon atoms energetics within the GBs core structure and their free surfaces are calculated. Then Rice-Wang cohesive structure model is applied to compute the embrittlement/strengthening effect of the solute atoms on the ideal and deviated GB structures. Hydrogen showed significant embrittlement and degradation in the mechanical properties of-iron, while carbon showed a desirable atomic strengthening effect.
Acta Materialia, 2004
We propose that the ideal fracture energy of a material with mobile bulk impurities can be obtained within the framework of a Born-Haber thermodynamic cycle. We show that such a definition has the advantage of initial and final states at equilibrium, connected by well-defined and measurable energetic quantities, which can also be calculated from first principles. Using this approach, we calculate the ideal fracture energy of metals (Fe and Al) in the presence of varying amounts of hydrogen, using periodic density functional theory. We find that the metal ideal fracture energy decreases almost linearly with increasing hydrogen coverage, dropping by $45% at one-half monolayer of hydrogen, indicating a substantial reduction of metal crystal cohesion in the presence of hydrogen atoms and providing some insight into the cohesion-reduction mechanism of hydrogen embrittlement in metals.
We investigated the hydrogen distribution and desorption behavior in an electrochemically hydrogen-charged binary NieNb model alloy to study the role of d phase in hydrogen embrittlement of alloy 718. We focus on two aspects, namely, (1) mapping the hydrogen distribution with spatial resolution enabling the observation of the relations between desorption profiles and desorption sites; and (2) correlating these observations with mechanical testing results to reveal the degradation mechanisms. The trapping states of hydrogen in the alloy were globally analyzed by Thermal Desorption Spectroscopy (TDS). Additionally, spatially resolved hydrogen mapping was conducted using silver decoration, Scanning Kelvin Probe Force Microscopy (SKPFM) and Secondary Ion Mass Spectrometry (SIMS): The Ag decoration method revealed rapid effusion of hydrogen at room temperature from the g-matrix. The corresponding kinetics was resolved in both, space and time by the SKPFM measurements. At room temperature the hydrogen release from the g-matrix steadily decreased until about 100 h and then was taken over by the d phase from which the hydrogen was released much slower. For avoiding misinterpretation of hydrogen signals stemming from environmental effects we also charged specimens with deuterium. The deuterium distribution in the microstructure was studied by SIMS. The combined results reveal that hydrogen dissolves more preferably inside the g-matrix and is diffusible at room temperature while the d phase acts as a deeper trapping site for hydrogen. With this joint and spatially resolving approach we observed the microstructure-and time-dependent distribution and release rate of hydrogen with high spatial and temporal resolution. Correlating the obtained results with mechanical testing of the hydrogen-charged samples shows that hydrogen enhanced decohesion (HEDE) occurring at the d/matrix interfaces promotes the embrittlement.
Grain Boundary Segregations and Hydrogen Embrittlement
Le Journal de Physique Colloques, 1982
-Les relations entre ségrégation intergranulaire et fragilisation par I'hydrogène des métaux peuvent s'envisager d'un double point de vue : 1°) L'hydrogène a une forte tendance à ségrêger dans les défauts de structure et en particulier les joints de grains. Cette ségrégation peut conditionner les propriétés des matériaux hydrogênés (diffusion de l'hydrogène, fissuration induite par l'hydrogène, propriétés électriques dans les semi-conducteurs, etc) et particulièrement leur comportement mécanique (fragilisation proprement dite). 2°) L'existence de ségrégations intergranulaires d'impuretés ou d'éléments d'alliage peut modifier le comportement du matériau vis-à-vis de la fragilisation par l'hydrogène : effets synergétiques des ségrégations (S,P,Sb,...) et de l'hydrogène dans la fragilisation, ou au contraire, effets d'inhibition de certaines ségrégations (carbone par exemple) sur la fragilisation intergranulaire par l'hydrogène. Ces deux aspects sont bien entendu liés puisque la ségrégation de l'hydrogène dans les joints est fonction des interactions possibles de cet élément avec les autres éléments éventuel lèsent ségrégés. C'est en particulier sur ce sujet des interactions multiples au niveau du joint et de leurs conséquences sur la tenue mécanique des alliages que les recherches ont subi un développement ces dernières années. L'exposé tente une synthèse des résultats et des idées récemment publiés sur ce sujet en considérant à la fois l'aspect statique (interaction, ségrégation d'équilibre) et dynamique (transport par les dislocations, accumulation d'hydrogène).