Atomistic modeling of He embrittlement at grain boundaries of α -Fe: a common feature over different grain boundaries (original) (raw)
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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.
Journal of Nuclear Materials, 2019
Presented here are atomistic simulations of the interaction of a collision cascade in α-Fe with different types of grain boundaries (GBs). The collision cascade was generated by a primary knock-on Fe atom of 3 keV kinetic energy. Four different types of symmetric tilt GB configurations (low angle and high angle GBs, Σ3 and Σ11 GBs) with two tilt axes 1 1 2 and 1 1 0 were considered. Difference in potential energy, strength and average displacement of atoms between pristine and irradiated GB configurations were analysed to bring out the effect of irradiation. The vacancy and interstitial formation energies calculated using molecular statics simulations were found to be lower in the GB than in the grain. Collision cascade resulted in defect clusters in and around the GB. Low angle GBs were found to be better sinks for radiation induced defects. Tensile deformation of the irradiated configurations resulted in dislocation loops being nucleated from interstitial clusters. The loss in strength after irradiation was high for Σ3 GBs and negligible for other GBs. It was suggested that materials with low angle GBs and a high dislocation density would be preferred for use in radiation environment.
Properties of He clustering in α-Fe grain boundaries
Journal of Nuclear Materials, 2015
Classical molecular dynamics and density functional theory calculations are performed to study the impact of two distinct Fe grain boundaries (GBs) on the clustering properties of helium (He) and the possible He effect on GB decohesion. Several He concentrations are considered. Common properties of He clustering are found for the both GBs, which are visibly different from the bcc bulk. In particular, He clusters in the GBs are always elongated in the directions parallel to the interface and contracted in the direction normal to the GB plane, while they are isotropic in the bcc bulk. When the He number in the clusters is sufficiently large, the strong local pressure promotes the occurrence of loop punching, which is easier to trigger in the GBs than in the bulk, resulting in a lower He-to-vacancy ratio in the GB clusters. The emitted self-interstitial atoms (SIAs) can more easily dissociate from the clusters in the GBs than in the bulk, leading to relatively lower local pressures around the clusters in the GBs, and facilitating the clusters growth. He is found to decrease GB cohesion, and the embrittling effect of He increases with its concentration. But interestingly, this effect decreases with He clustering. The present findings are fully compatible with existing experimental evidence, for instance, for a stronger GB embrittlement due to He at rather low temperatures than at higher temperatures.
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.
Energetic landscape and diffusion of He in α-Fe grain boundaries from first principles
Physical Review B, 2013
Combined density functional theory and empirical-potential calculations are performed to investigate the lowest-energy sites and migration mechanisms of He in various α-Fe grain boundaries (GBs). Before the defect calculations, we show that structural optimizations, including simulated annealing and atom removal, are crucial for locating the stable GB structure in a given temperature regime. Then, the He formation energies for all the substitutional and interstitial sites in two different GBs are evaluated, showing a strong He segregation tendency. At variance with the bulk Fe case, the formation energy of an interstitial He is either lower than or similar to that of a substitutional He in the GBs. Finally, both static and dynamic barriers for interstitial He diffusion in the GBs are determined. Although the diffusion details and precise paths are GB dependent, some common features are identified: (1) The He atom always remains confined to the GB region while diffusing; (2) the He diffusion is highly anisotropic along the GBs; (3) the GB diffusion of an interstitial He atom is found to be always slower than its bulk diffusion, but it can still be faster than the bulk diffusion of a substitutional He.
Consistency Check of Segregation Energies of Elements at Ni and Fe Grain-Boundaries
arXiv: Materials Science, 2020
The need for advanced functional materials is expected to provide a boost in powder metallurgy, where the impurities on powder surfaces is incorporated as at grain boundary segregation. This paper has three aims. First, we analyze whether the reported data of Ni and Fe hosts can be correlated to basic thermodynamic data on chemical elements. The second aim is the consistency check, which is suggested to be applied for any data base. The third aim, is whether information of 50 most important elements can give additional information for prediction of unknown data. The data are analyzed whether a set of basic data is sufficient for characterizing the segregation. The data of the solvents were analyzed using the software R for principal component analysis (PCA). We grouped and correlated the data to Mendeleev number and thermodynamic data on pure elements. As a result, we found that the embrittlement depends strongly on the chemical bonding, but weakly on mechanical factors. Surprisingl...
MD modeling of defects in Fe and their interactions
Journal of Nuclear Materials, 2003
Ferritic/martensitic steels considered as candidate first-wall materials for fusion reactors experience significant radiation hardening at temperatures below $400°C. A number of experimental studies in ferritic alloys, performed at higher temperatures, have shown the existence of large interstitial loops with Burgers vector 1 2 h1 1 1i and h1 0 0i in the bulk, which may provide a significant contribution to the hardening caused during irradiation at lower temperatures. Hardening arises from a high number density of loops, voids and small precipitates, which pin system dislocations, impeding their free glide. In this work, we review the nature of the different interstitial dislocation loops observed in a-Fe and ferritic materials, assess the effect of substitutional impurities on migrating 1 2 h1 1 1i clusters, and apply atomistic modeling to investigate the mechanisms of formation and growth of h1 0 0i loops from smaller cascade-produced 1 2 h1 1 1i clusters. The proposed mechanism reconciles experimental observations with continuum elasticity theory and recent MD modeling of defect production in displacement cascades. In addition, the interaction of screw dislocations, known to control the low-temperature plastic response of b.c.c. materials to external stress, with h1 0 0i dislocation loops is investigated with MD, where the main physical mechanisms are identified, cutting angles estimated and a first-order estimation of the induced hardening is provided.
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.