Early Stage Oxidation Initiation at Different Grain Boundaries of fcc Fe–Cr Binary Alloy: A Computational Chemistry Study

Tight-binding quantum chemical molecular dynamics method has been applied in order to study the Σ3 (111), Σ5 (100) and random grain boundaries oxidation initiation mechanism of fcc Fe–Cr binary alloy in a boiling water reactor environment. The metal–water interaction at high temperatures causes diffusion of environmental species and segregation of metallic atoms. Water molecules favorably permeate through the random grain boundary (GB) to find the space generated by atomic rearrangement, although it is difficult to diffuse in the Σ3 (111) and Σ5 (100) grain boundaries. Moreover, applied strain creates extra spaces in the lattice that can facilitate the absorption of environmental species. The highly positively charged chromium and the negatively charged oxygen atoms or OH remain along the GB by forming bonds. The GB atoms selectively lose their valence electrons when dissociated atoms adsorb, indicating that the oxidation process is a possible mechanism of intergranular cracking initiation.     

Solute Segregation to Grain Boundaries in Al: A First-Principles Evaluation

Solute-induced grain boundary(GB) strengthening is eff ective in improving the toughness and tensile strength of polycrystalline alloys. In this work, GB segregation behaviors of solute elements in Al alloys and their potential eff ects on GB binding have been systematically investigated from fi rst-principles energetics. The low-energy Σ3(111) and Σ11(113) are immune to vacancy segregation, while high-energy Al GBs, such as Σ13(320), Σ9(221), Σ5(210), and Σ5(310), can attract both vacancies and...     

Effect of grain boundary fraction on castability of a directionally solidified nickel alloy

The effect of grain boundary (GB) fraction on hot tearing during directional solidification was explored. The increase of GB fraction was found to reduce the hot tearing tendency. The eutectic melt is finely dispersed and increasingly discontinuous at the GBs when the GB fraction is increased. The change of eutectic melt at the GBs is related to the reduced concentration of GB elements or impurities. The better castability is attributed to the uniform distribution of strain due to the presence of more GBs and the stronger GB cohesion because of the larger bridging areas.     

Effect of grain boundary character on sink efficiency

The dependence of the width of void-denuded zones (VDZs) on grain boundary (GB) characters was investigated in Cu irradiated with He ions at elevated temperature. Dislocation loops and voids formed near GBs during irradiation were characterized by transmission electron microscopy, and GB misorientations and normal planes were determined by electron back-scatter diffraction. The VDZ widths at Σ3〈1 1 0〉 tilt GBs ranged from 0 to 24 nm and increased with the GB plane inclination angle. For non-Σ3 GBs, VDZ widths ranged from 40 to 70 nm and generally increased with misorientation angle. Nevertheless, there is considerable scatter about this general trend, indicating that the remaining crystallographic parameters also play a role in determining the sink efficiencies of these GBs. In addition, the VDZ widths at two sides of a GB show different values for certain asymmetrical GBs. Voids were also observed within GB planes and their density and radius also appeared to depend on GB character. We conclude that GB sink efficiencies depend on the overall GB character, including both misorientation and GB plane orientation.     

Wetting of molybdenum grain boundaries by nickel: effect of the boundary structure and energy

The structural effect of the penetration of nickel along symmetrical [101] tilt grain boundaries (GBs) in two different molybdenum bicrystals is investigated. The selection of GBs (Σ=3{121} and Σ=11{323}) is governed by their different energy so that a different penetration behaviour is expected. The temperature of treatment is 1350°C, i.e. above the eutectic temperature. The analysis of the Mo–Ni phase formed on the surface of the bicrystal, the concentration profile along the GB and the identification of the nanophases present at the GB is performed by using several experimental techniques from microscopic to nanoscopic scales. Important differences in the penetration of nickel are found for the two investigated GBs.     

Atomistic simulations of grain boundary segregation in nanocrystalline yttria-stabilized zirconia and gadolinia-doped ceria solid oxide electrolytes

Hybrid Monte Carlo–molecular dynamics simulations are carried out to study defect distributions near Σ5(3 1 0)/[0 0 1] pure tilt grain boundaries (GBs) in nanocrystalline yttria-stabilized zirconia and gadolinia-doped ceria. The simulations predict equilibrium distributions of dopant cations and oxygen vacancies in the vicinity of the GBs where both materials display considerable amounts of dopant segregation. The predictions are in qualitative agreement with various experimental observations. Further analyses show that the degree of dopant segregation increases with the doping level and applied pressure in both materials. The equilibrium segregation profiles are also strongly influenced by the microscopic structure of the GBs. The high concentration of oxygen vacancies at the GB interface due to lower vacancy formation energies triggers the dopant segregation, and the final segregation profiles are largely determined by the dopant–vacancy interaction.     

The effect of interstitial N on grain boundary cohesive strength in Fe

The effect of interstitial N on the cohesion of an Fe Σ3[1 1 0](1 1 1) grain boundary (GB) was investigated by ab-initio electronic structure calculations to reveal that free interstitial N produces a large strengthening energy, reduces the magnetic moments of the GB Fe atoms and is embrittling at the GB's.     

A new model of grain-boundary segregation with the formation of atomic complexes in a grain boundary

The process of grain-boundary segregation (GBS) has been considered under an assumption that the formation of associates (atomic complexes with a composition that corresponds to the nearest chemical composition in the phase diagram) is possible in a grain boundary (GB). The grain boundary is considered as a two-component mixture of A and B atoms, which can exist both in free and bound states (bound in a complex). The formation of complexes with an arbitrary composition and complexes of an AB type have been considered. It has been shown that, even in the absence of segregation (b = 1), the interactions of atoms in a GB that induce the formation of complexes leads to an enrichment of GBs in impurity atoms. It has been demonstrated that under certain conditions, a GBS isotherm can exhibit saturation corresponding to the chemical composition of the complex.     

Monte Carlo simulation of grain boundary segregation and decohesion in NiAl

Grand canonical Monte Carlo simulations are performed to study grain boundary segregation in the ordered intermetallic compound NiAl. The embedded atom method is applied to model atomic interactions in NiAl. The structure and chemical composition of Σ=5 (210) [001] and Σ=5 (310) [001] symmetrical tilt grain boundaries are studied as functions of the bulk composition at 1200 K. The grain boundaries tend to be enriched in Ni. Deviations of the bulk composition from the stoichiometry towards Ni-rich compositions increase local disorder and enhance Ni segregation at the grain boundaries. In one of the boundaries, the Ni segregation induces a structural transformation to a new metastable grain boundary structure. The effect of grain boundary disorder and segregation on grain boundary decohesion is evaluated by simulated tensile tests.     

Comparing EAM Potentials to Model Slip Transfer of Sequential Mixed Character Dislocations Across Two Symmetric Tilt Grain Boundaries in Ni

Slip transfer via sequential pile-up dislocations across grain boundaries (GBs) plays an important role in plastic deformation in polycrystalline face-centered cubic (FCC) metals. In this work, large scale concurrent atomistic-continuum (CAC) method simulations are performed to address the slip transfer of mixed character dislocations across GBs in FCC Ni. Two symmetric tilt GBs, a Σ3{111} coherent twin boundary (CTB) and a Σ11{113} symmetric tilt GB (STGB), are investigated using five different fits to the embedded-atom method (EAM) interatomic potential to assess the variability of predicted dislocation-interface reaction. It is shown that for the Σ3 CTB, two of these potentials predict dislocation transmission while the other three predict dislocation absorption. In contrast, all five fits to the EAM potential predict that dislocations are absorbed by the Σ11 STGB. Simulation results are examined in terms of several slip transfer criteria in the literature, highlighting the complexity of dislocation/GB interactions and the significance of multiscale modeling of the slip transfer process.     

Insight into gallium behavior in aluminum grain boundaries from calculation on Σ=11 (113) boundary

Gallium impurities affect the atomic processes and material properties of aluminum metal to a high degree. Various ab initio calculations have been performed on a Σ=11 (113) symmetric tilt boundary in aluminum with and without some gallium substitutions. A simple interpretation of the results emerges, which can be applied to grain boundaries in general. The calculations relate to the energetics of gallium substitution on various sites, local relaxation effects, vibrational frequencies and a barrier to grain boundary migration.     

Quasicontinuum study of incipient plasticity under nanoscale contact in nanocrystalline aluminum

Atomistic simulations using the quasicontinuum method are performed to examine the mechanical behavior and underlying mechanisms of surface plasticity in nanocrystalline aluminum with a grain diameter of 7 nm deformed under wedge-like cylindrical contact. Two embedded-atom method potentials for Al, which mostly differ in their prediction of the generalized stacking and planar fault energies, and grain boundary (GB) energies, are used and characterized. The simulations are conducted on a randomly oriented microstructure with 〈1 1 0〉-tilt GBs. The contact pressure–displacement curves are found to display significant flow serration. We show that this effect is associated with highly localized shear deformation resulting from one of three possible mechanisms: (1) the emission of partial dislocations and twins emanating from the contact interface and GBs, along with their propagation and intersection through intragranular slip, (2) GB sliding and grain rotation and (3) stress-driven GB migration coupled to shear deformation. Marked differences in mechanical behavior are observed, however, as a function of the interatomic potential. We find that the propensity to localize the plastic deformation at GBs via interface sliding and coupled GB migration is greater in the Al material presenting the lowest predicted stacking fault energy and GB energy. This finding is qualitatively interpreted on the basis of impurity effects on plastic flow and GB-mediated deformation processes in Al.     

Bismuth segregation at copper grain boundaries

The dependence of Bi segregation on the grain-boundary (GB) geometry for various Bi concentrations in Cu at different temperatures is investigated systematically. The GB segregation of Bi in Cu(Bi) bicrystals was determined quantitatively using energy dispersive X-ray spectroscopy (EDS) in a dedicated scanning transmission electron microscope (STEM). The Miller indices of the corresponding GB planes were determined by high resolution transmission electron microscopy (HRTEM). The energies of all experimentally investigated GBs were calculated by atomistic computer simulations. Gibbs segregation free energies were determined from the experimentally measured amount of Bi segregation using the classical McLean segregation theory. The free energies decrease monotonically with increasing calculated GB energies. The Fowler–Guggenheim segregation theory yields an attractive interaction between the segregated Bi atoms. No Bi-induced changes in the bonding at the GBs could be detected by electron energy loss spectroscopy (EELS).     

Mechanical behavior of Σ tilt grain boundaries in nanoscale Cu and Al: A quasicontinuum study

Molecular simulations using the quasicontinuum method are performed to understand the mechanical response at the nanoscale of grain boundaries (GBs) under simple shear. The energetics and mechanical strength of 18 Σ 〈1 1 0〉 symmetric tilt GBs and two Σ 〈1 1 0〉 asymmetric tilt GBs are investigated in Cu and Al. Special emphasis is placed on the evolution of far-field shear stresses under applied strain and related deformation mechanisms at zero temperature. The deformation of the boundaries is found to operate by three modes depending on the GB equilibrium configuration: GB sliding by uncorrelated atomic shuffling, nucleation of partial dislocations from the interface to the grains, and GB migration. This investigation shows that (1) the GB energy alone cannot be used as a relevant parameter to predict the sliding of nanoscale high-angle boundaries when no thermally activated mechanisms are involved; (2) the E structural unit present in the period of Σ tilt GBs is found to be responsible for the onset of sliding by atomic shuffling; (3) GB sliding strength in the athermal limit shows slight variations between the different interface configurations, but has no apparent correlation with the GB structure; (4) the metal potential plays a determinant role in the relaxation of stress after sliding, but does not influence the GB sliding strength; here it is suggested that the metal potential has a stronger impact on crystal slip than on the intrinsic interface behavior. These findings provide additional insights on the role of GB structure in the deformation processes of nanocrystalline metals.     

Atomistic simulation of hydrogen diffusion at tilt grain boundaries in vanadium

Molecular dynamics simulations of hydrogen diffusion at Σ3 and Σ5 tilt grain boundaries in bcc vanadium (V) have been performed based on modified embedded-atom method interatomic potentials. The calculated diffusivity at the grain boundaries is lower than the calculated bulk diffusivity in a temperature range between 473 and 1473 K, although the difference between the grain boundary and bulk diffusivities decreases with increasing temperature. Compared with that of the other directions, the mean square displacement of an interstitial hydrogen atom at the Σ3 boundary is relatively small in the direction normal to the boundary, leading to two dimensional motion. Molecular statics simulations show that there is strong attraction between the hydrogen atom and these grain boundaries in V, which implies that the role of grain boundaries is to act as trap sites rather than to provide fast diffusion paths of hydrogen atoms in V.     

Relation between precipitate-free zone width and grain boundary type in 7075-T7 Al alloy

The effect of grain boundary (GB) type on precipitate-free zone (PFZ) width in friction stir-processed 7075-T7 Al alloy is investigated by transmission electron microscopy (TEM) and stereology. The average half width of PFZs at random GBs is 70.4 ± 0.7 nm. For low-angle GBs, an apparent transition of PFZ half width is observed at a misorientation of 11°. For coincidence site lattice (Σ) GBs, only Σ1, Σ3 and Σ5 have smaller PFZ width than that of random GBs. Crystal-frame stereology is used to recover the GB plane distribution. It is found that the GB plane distribution is relatively isotropic for most Σ GBs. Low/high index plane combinations are observed for most Σ GBs; furthermore, most Σ GBs have both tilt and twist components. The combined results of TEM and stereology suggest that smaller PFZ width is associated only with low Σ GBs since the formation and growth of PFZs at GBs depends closely on the core structure, in addition to the geometric structure of GBs.     

Fundamental studies on stress-corrosion cracking in iron and underlying mechanisms

Stress-corrosion cracking (SCC) in Fe and Fe-based alloys is critical to material failure for many applications of these materials. In this paper, the behavior of the Fe Σ3 (1 1 1) grain boundary (GB) is studied under precipitations of S, P, N, C and B to elucidate the effects caused by stress-corrosion, as well as its mechanisms, using first-principles calculations. There are certain locations where impurities tend to segregate more. Some elements, such as C and B, serve as strengtheners, while S, P and N weaken the Fe–Fe interactions at GBs. However, the stress-corrosion failures arise in a variety of ways. The accumulation of S and P results in separation of grains, which in turn gives rise to the intergranular cracking under sufficient stress. Cavities may grow at GBs due to the segregation of N and formation of nitrides; consequently, SCC could propagate through the connection of voids.