Properties of He clustering in α-Fe grain boundaries (original) (raw)
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Atomistic simulations of the effect of helium clusters on grain boundary mobility in iron
2015
A series of molecular dynamics simulations was performed in this work to investigate the kinetic interaction between helium clusters and grain boundaries in iron. Helium cluster formation and size distributions were found to be markedly different in the bulk compared to the region of a stationary boundary. Upon reaching a steady-state cluster distribution, the spatial fluctuation of cluster-enriched boundaries was analyzed to determine the grain boundary mobility using the random walk method. Segregated clusters reduced the boundary mobility, the drag effect of clusters increasing as the bulk solute concentration increases. The drag effect was further rationalized by employing Cahn's solute drag model using the effective binding energy of He clusters and the grain boundary diffusivity of a single He atom, their magnitudes having been determined from the segregation level and from monitoring the trajectory of a solute atom in the investigated grain boundaries, respectively. The model is found to provide a satisfactory explanation of the simulation results in the zero velocity limit.
Calculation of helium defect clustering properties in iron using a multi-scale approach
Journal of Nuclear Materials, 2006
Electronic structure calculations were used to study the relaxation, formation and binding energies of small helium clusters in iron. We considered three He defect configurations: two He atoms in an interstitial position and two and three He atoms located in one vacancy. To study He-vacancy clusters containing more He atoms, we used a multi-scale approach and constructed an empirical potential fitted to both formation and relaxation energies of a single He defect and small He clusters obtained from the first principles calculations. The potential consists of a repulsive pair-interaction part and a many-body attractive term describing the cohesion. The potential was used to study stability of He-vacancy clusters at zero temperature. The binding energy of a He atom to a He-cluster varies from 1.3 eV to 1.9 eV depending on the cluster size. When more than six He atoms are placed into a vacancy an Fe self-interstitial atom (SIA) is produced. The SIA binding energy to a He-di-vacancy cluster decreases from 5.0 eV to 0.7 eV as the number of He atoms increases. The results obtained are consistent with experimental observations of helium desorption reported in the literature.
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.
Modelling and Simulation in Materials Science and Engineering, 2013
He atoms introduced to materials may lead them to intergranular fracture, and understanding such an effect is a key issue for the design of future fusion reactors. In the present study, we investigated the decrease of grain boundary (GB) strength caused by He segregation at several kinds of α-Fe GBs by exploiting the first principles calculations and a set of empirical potentials. We found enough evidence to support the notion that the GB cohesive energy, a critical measure of GB strength, approximately scales with the He concentration at the GB surface, regardless of the GB type.
Interaction of He and He–V clusters with self-interstitials and dislocations defects in bcc Fe
The understanding of helium effects in synergy with radiation damage is crucial for the development of structural steels for fusion applications. Recent investigations in ultra-pure iron, taken as a basic model, have shown a drastic impact of dual beam (He-Fe) exposure on the accumulation of radiation-induced dislocation loops in terms of strong bias towards a0/2<111> family, while a0<100> loops are mostly observed upon Fe ion beam. In this work we perform a series of atomistic studies to rationalize possible mechanisms through which He could affect the evolution of microstructure and bias the population of a0/2<111> loops. Strong suppression of a0/2<111> loop migration, prohibiting their mutual interaction resulting in the formation of a0<111> and disappearance at sinks, is proposed to be caused by the He decoration occurring via helium-loop drag mechanism. A scenario for the microstructural evolution in the single-and dual-beam conditions is discussed.
Atomistic studies of formation and diffusion of helium clusters and bubbles in BCC iron
Journal of Nuclear Materials, 2011
In fusion applications, helium created by transmutation plays an important role in the response of reduced-activation ferritic/martensitic (RAFM) steels to neutron radiation damage. We have performed extensive atomistic simulations using the ORNL three-body Fe-He interatomic potential combined with three interatomic potentials for the iron matrix. Some of the results obtained are summarized in this review. Interstitial helium is very mobile and coalesces together to form interstitial clusters. We have investigated the mobility of these clusters. When an interstitial He cluster reaches sufficient size, it punches out an Fe interstitial, creating an immobile helium-vacancy cluster. If more helium atoms join it, more Fe interstitials can be created; the He-V defect is a nascent bubble. These mechanisms are investigated together in simulations that examine the nucleation of He defects. Mobile interstitial He clusters and helium bubbles 1-6 nm across are also simulated separately. Results are compared based on temperature and interatomic potentials used.
Molecular dynamics simulations of 50 Fe grain boundaries were used to understand their interaction with vacancies and self-interstitial atoms at all atomic positions within 20Å of the boundary, which is important for designing radiation-resistant polycrystalline materials. Site-to-site variation within the boundary of both vacancy and self-interstitial formation energies is substantial, with the majority of sites having lower formation energies than in the bulk. Comparing the vacancy and self-interstitial atom binding energies for each site shows that there is an energetic driving force for interstitials to preferentially bind to grain boundary sites over vacancies. Furthermore, these results provide a valuable dataset for quantifying uncertainty bounds for various grain boundary types at the nanoscale, which can be propagated to higher scale simulations of microstructure evolution.
Ab initio study of helium in α-Fe: Dissolution, migration, and clustering with vacancies
Physical Review B, 2005
Density functional theory calculations have been performed to study the dissolution and migration of helium in ␣-iron, and the stability of small helium-vacancy clusters He n V m ͑n,m = 0 to 4͒. Substitutional and interstitial configurations of helium are found to have similar stabilities. The tetrahedral configuration is more stable than the octahedral by 0.2 eV. Interstitial helium atoms are predicted to have attractive interactions and a very low migration energy ͑0.06 eV͒, suggesting that He bubbles can form at low temperatures in initially vacancy-free lattices. The migration of substitutional helium by the vacancy mechanism is governed by the migration of the HeV 2 complex, with an energy barrier of 1.1 eV. The activation energies for helium diffusion by the dissociation and vacancy mechanisms are estimated for the limiting cases of thermal-vacancy regime and of high supersaturation of vacancies. The trends of the binding energies of vacancy and helium to helium-vacancy clusters are discussed in terms of providing additional knowledge on the behavior of He in irradiated iron, necessary for the interpretation of complex experimental data such as thermal He desorption spectra.
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.
Interaction of helium atoms with edge dislocations in a-Fe
Formation energies, binding energies, and migration energies of interstitial He atoms in and near the core of an a/2h1 1 1i{1 1 0} edge dislocation in a-Fe are determined in atomistic simulations using conjugate gradient relaxation and the Dimer method for determining saddle point energies. Results are compared as a function of the proximity of the He to the dislocation core and the excess interstitial volume in regions around the dislocation. Interstitial He atoms have negative binding energy on the compression side of the dislocation and strong positive binding energy on the tension side. Even at low temperatures, interstitial He atoms in the vicinity of the dislocation easily migrate to the dislocation core, where they form crowdion interstitials oriented along the close-packed slip direction, with binding energies in excess of 2 eV. Crowdion interstitial He atoms diffuse along the dislocation core, transverse to the crowdion direction, with a migration energy of 0.4–0.5 eV.