Effect of impurities on structural, cohesive and magnetic properties of grain boundaries in α-Fe

The mechanical properties of polycrystalline metals are strongly dependent on the microscopic structure, stability, and elastic properties of grain boundaries (GBs). By using ab initio local energy, local stress, and local Young’s modulus, we attempt to provide a comprehensive view on the stability and structural properties of a series of 〈110〉 symmetrical tilt GBs (STGBs) in bcc Fe. We deal with four representative STGBs, the Σ3 (112), Σ3 (111), Σ11 (332), and Σ9 (221) GBs with the rotation angles ranging from 109.47° to 38.94°. The Σ3 (112) GB shows special stability due to stable structural units of four-membered rings with negligible bond-length changes except for substantial bond-direction changes, like stacking faults or twins. The other GBs are constructed by periodic arrangement of 5–3 and bulk structural units as usual coincidence GBs, while the 5–3 unit in the Σ9 (221) GB has an aspect of an edge-dislocation array in a small-angle tilt GB such as alternate compressive and tensile stresses at both the edges and relatively wide spread of local energy and local stress on both sides. In the GBs other than the Σ3 (112) GB, there exist commonly two kinds of interface atoms; atoms with larger atomic volumes reveal higher local energies, tensile stresses, and enhanced magnetic moments, while the other kind of atoms forming compressed bonds reveal lower local energies, compressed stresses, and reduced magnetic moments. For the four GBs, the local Young’s modulus averaged in the structural units ranges from 60 to 90 % of the bulk Young’s modulus in accordance with the degree of structural disorder or GB energies. There exists clear correlation among the local Young’s modulus, local energy, local stress, local magnetic moment, and local bonding nature at the structural units in the Fe GBs.

Energetics of point defect interacting with bi-crystal Σ3 copper grain boundaries

Lithium lanthanum titanate (LLTO) is one of the excellent candidates for an electrolyte in the all-solid-state Li-ion battery, owing to the high Li-ion conductivity in the bulk. However, the Li-ion conductivity at the grain boundary (GB) is largely reduced, and it is therefore important to reveal the origin of Li-ion conductivity reduction at the GB. Here, by using atomic-resolution scanning transmission electron microscopy combined with atomic force microscopy, we investigate the charge states, Li-ion conductivities, atomic and electronic structures at the LLTO Σ5 and Σ13 GBs. Although the Σ5 GB has no significant influence on Li-ion conductivity, the Σ13 GB shows the evident reduction of Li-ion conductivity. We further elucidate that the Σ13 GB is positively charged by the formation of oxygen vacancies at the GB. Such a positive charge would form the Li-ion depletion layers adjacent to the GB, which causes the significant reduction of Li-ion conductivity.

The effect of interstitial and substitutional impurities on grain boundary (GB) cohesion in the series of B2-TiMe alloys is studied from first principles using pseudopotential approach. It is shown that the TiMe Σ5(310) symmetrical tilt GB cohesion is reduced by the segregation of hydrogen while it is increased due to boron or carbon segregation. We analyze also the combined and accumulation effect of interstitial B (C) and H impurities on the change of the Griffith work.

Degradation in the weldability of a helium-containing vanadium alloy

In the present Letter, we report on a combined ab initio density functional theory calculation, multislice simulation, and electron holography study, performed on a Σ9 grain boundary (GB) in a CuGaSe2 bicrystal, which exhibits a lower symmetry compared with highly symmetric Σ3 GBs. We find an electrostatic potential well at the Σ9 GB of 0.8 V in depth and 1.3 nm in width, which in comparison with results from Σ3 and random GBs exhibits the trend of increasing potential-well depths with lower symmetry. The presence of this potential well at the Σ9 GB can be explained conclusively by a reduced density of atoms at the GB. Considering experimental limitations in resolution, we demonstrate quantitative agreement of experiment and theory.

Nonmetallic trace elements induced rhenium co-segregation in nickel Σ5 [001](210) symmetrical tilt grain boundary

A multiple-slip rate-dependent crystalline constitutive formulation that is coupled to the evolutionary equations of mobile and immobile dislocation densities is used with specialized finite-element methodologies to obtain a detailed microstructural prediction of the inelastic deformation and interrelated physical mechanisms that affect different failure modes in f.c.c. crystalline materials. The effects of grain and grain-boundary (GB) orientations, grain shape and size, GB strength, and mechanical properties on the inelastic finite deformation and failure mode mechanisms in polycrystalline aggregates with Σ3 and Σ17b GBs have been investigated. Results from this study are consistent with experimental observations that microstructures with desired material properties, such as resistivity to crack nucleation, can be potentially achieved by the control of grain aggregate size and CSL GB orientations. Furthermore, it is also shown that transgranular failure modes can occur in aggregates with a high frequency of Σ3 GBs, whereas intergranular fracture modes can occur with a high frequency of Σ17b GBs.

Grain growth prediction with inclination dependence of 〈1 1 0〉 tilt grain boundary using multi-phase-field model with penalty for multiple junctions

We present first-principle calculations on symmetric tilt grain boundaries (GBs) in bcc Fe. Using density functional theory (DFT), we studied the structural, electronic and magnetic properties of Σ3(111) and Σ11(332) GBs formed by rotation around the [110] axis. The optimized structures, GB energies and GB excess free volumes are consistent with previous DFT and classical simulation studies. The GB configurations can be interpreted by the structural unit model as given by Nakashima and Takeuchi (2000 ISIJ 86 357). Both the GBs are composed of similar structural units of three- and five-membered rings with different densities at the interface according to the rotation angle. The interface atoms with larger atomic volumes reveal higher magnetic moments than the bulk value, while the interface atoms with shorter bond lengths have reduced magnetic moments in each GB. The charge density and local density of states reveal that the interface bonds with short bond lengths have more covalent nature, where minority-spin electrons play a dominant role as the typical nature of ferromagnetic Fe. In order to understand the structural stability of these GBs, we calculated the local energy and local stress for each atomic region using the scheme of Shiihara et al (2010 Phys. Rev. B 81 075441). In each GB, the interface atoms with larger atomic volumes and enhanced magnetic moments reveal larger local energy increase and tensile stress. The interface atoms constituting more covalent-like bonds with reduced magnetic moments have lower local energy increase, contributing to the stabilization, while compressive stress is generated at these atoms. The relative stability between the two GBs can be understood by the local energies at the structural units. The local energy and local stress analysis is a powerful tool to investigate the structural properties of GBs based on the behavior of valence electrons.

Hydrogen diffusion behaviour at the coherent and incoherent Σ3 grain boundaries (GBs) of pure polycrystalline Ni was observed in situ using a polyaniline-based hydrogen video imaging system. The hydrogen flux at the coherent Σ3 GBs, also referred to as twin boundaries, was found to be the same as that in the grain interior. In contrast, enhanced hydrogen diffusion was observed at the incoherent portions of the Σ3 GBs. Hydrogen diffusion was notably enhanced at the micro/nanoscale incoherent portion, and the deviation angle of Σ3 GBs was identified as a critical factor influencing the hydrogen flux therein.

Effect of Σ3 grain boundaries on microstructure and properties of oriented Fe-6.5 wt%Si in twin-roll strip casting

A model of discontinuous precipitation of γ′ phase at grain boundaries (GBs) in polycrystalline Ni-based superalloys during continuous cooling from supersolvus temperature was developed. The model calculates the size and γ′ phase fraction in GB precipitates as a function of cooling rate and GB mobility. The model is based on the classical mechanism of diffusion-controlled precipitation. It considers fast diffusion of γ′-forming elements along the GB and the motion of the GBs under driving force induced by alloy decomposition. The model predicts either discrete GB gamma-prime particles and GB serration or the growth of fan-type structures depending on the cooling rate and GB mobility. We conclude from the model predictions that the GB mobility is the key factor controlling the type and morphology of GB precipitates. High GB mobility results in formation of fan-type structures, while the lower GB mobility leads to GB serration. The predicted dependence of the size of GB precipitates on the cooling rate is in good agreement with the experimental observations.

Effects of Cr, Mn on the cohesion of the γ-iron grain boundary

The incomplete to complete grain boundary (GB) wetting transition is controlled by GB energy and, therefore, by GB character. To find the GB character dependence of the wetting transition, experiments were carried out on polycrystalline Cu–In alloys, which are known to exhibit wetting behaviour between 715 and 986 °C. Electron backscatter diffraction was applied to determine the two-dimensional GB character, using the coincidence site lattice (CSL) model and applying the Brandon criterion. Special wetting behaviour was found in those GBs characterised as low-angle, Σ3 and Σ11. The low-angle and Σ3 GBs were not wetted until the sample melted, while the temperature for the transition to complete wetting of Σ11 GBs was over 100 °C higher than that for complete wetting of all other types of GB, including other CSL GB as well as random GB. It might be that a different Brandon type criterion is needed in the case of GB wetting to show the “special” wetting behaviour of the CSL GBs.