Correlation of Grain Boundary Character with Wetting Behavior

Wetting of grain boundaries (GB's) by liquid Cu in an Fe-30wt%Mn-10wt%Cu alloy has been studied as a function of the five macroscopic degrees of freedom (DoF's) of grain boundary character. These were chosen to consist of two grain boundary normals (or bounding planes) and a twist angle. The five DoF's of 975 GB's were determined by electron backscattering patterns and serial sectioning, after annealing at 1120°C, and each GB was categorized as being either wet, dry, or mixed (i.e. partly wet and partly dry). Interpretation of the wetting behavior by means of a model of GB energy, which includes consideration of the five macroscopic DoF's, led to correct predictions of wet and dry behavior in 80% of the GB's studied.     

Grain Boundary Wetting Statistic in Zn/Ga System and its Application to Grain Boundary Energy Spectrum Estimation

The grain boundary statistic in zinc polycrystals in contact with saturated Ga(Zn) melt has been studied. The misorientation angle distributions for zinc thin foil and zinc plates were obtained. The influence of the misorientation angle value on the wetting probability p of grain boundaries was observed. The grain boundary energy distribution parameters were obtained by using the p() relationship. The dihedral angles in triple lines of non-wetted zinc samples were also measured and their distribution was used to obtain the grain boundary energy distribution function. The parameters obtained by two different methods correspond to one other.     

Misorientation Dependence of the Grain Boundary Energy in Magnesia

Geometric and crystallographic data obtained from a well annealed magnesia polycrystal have been used to specify the five macroscopic degrees of freedom for 4665 grain boundaries. The results indicate, that for this sample, the five parameter grain boundary character space is fully occupied. A finite series of symmetrized spherical harmonics has been used to approximate the misorientation dependence of the relative grain boundary energy. Best fit coefficients for this series were determined by assuming that the interfacial tensions at each triple junction are balanced. The grain boundary energy function shows Read-Shockley behavior at small misorientations and a broad minimum near the 3 misorientation. Furthermore, misorientations about the ‹100› axis create boundaries with relative energies that are less than those created by misorientations about the ‹110› or ‹111› axes.     

Thermodynamics of Vacancies and Impurities at Grain Boundaries

The thermodynamics of vacancy and impurity adsorption at interfaces and grain boundaries (GBs) in solids is considered. Theoretical expressions are derived for the GB/interface free energy change caused by various levels of vacancy or impurity adsorption. This information is used to predict the behavior of vacancies at interfaces and GBs in a stress gradient and to forecast the effect of impurities on GB fracture strength. The latter predictions provide an interpretation of intergranular fracture behavior in terms of impurity adsorption and GB structural parameters such as GB width and value.     

Atomic Structure and Properties of the (310) Symmetrical Tilt Grain Boundary (STGB) in SrTiO3. Part I: Atomistic Simulations

The relaxed structure and energy of the (310) symmetrical tilt grain boundary (STGB) in SrTiO₃ have been calculated using static lattice energy minimization methods. In principle, the (310) GB plane can either be a cation-rich, positively charged SrTiO plane or a negatively charged oxygen plane, and both scenarios have been considered in this report. The effect of point-defect reconstruction at the GB core region, manifested either as completely missing columns or as half-filled columns of ions as suggested by experiments, has been analyzed. The results indicate that while Schottky defects are very strongly preferred energetically at the GB core, there is not significant gain in energy by having half-filled columns, as opposed to fully-dense and fully-empty columns, at the GB core. The simulation results have been analyzed in the context of Pauling's rules of crystal chemistry and bicrystallography. The results form the basis for an objective comparison with experimental studies in Part II of the paper.     

Excess Volume of the Solid/Liquid Interface in Fe-6 at.%Si Bicrystals Wetted by Liquid Zinc

The pressure effect on grain boundary wetting in Fe-6 at.%Si bicrystals of different misorientation angles but constant misorientation axis has been studied. The wetting agent was liquid zinc. It was found that the pressure for the dewetting transition is higher for the near 5 boundary than for the other general boundaries, where is the inverse density of the coincidence sites in the two misoriented crystal lattices. This result was explained assuming a thinner liquid film wetting the near 5 boundary than in the case of nonperiodic grain boundaries. Furthermore, the wetting angle increased with increasing pressure. The wetting angle dependence on pressure could be understood assuming a excess surface volume of the solid/liquid (S/L) interface higher than 0.2 nm. This is considerably higher than the estimated excess volumes of grain boundaries based on computer simulations. To explain this result, it was postulated that in the system studied, where diffusion of Zn, Fe and Si perpendicular to the S/L interface takes place, the S/L interface is relatively thick and the interaction between the two crystals separated by the melt extends over more than 2 nm distance. This long-range interaction was rationalized in terms of clusters of several atoms, detaching from the solid and dissolving in the melt at some distance from the bulk.     

Diffusion Induced Grain-Boundary Migration and Enhanced Grain Growth in BaTiO3

The effect of diffusion induced grain-boundary migration (DIGM) on grain growth has been studied in a model system of BaTiO₃-PbTiO₃. When sintered BaTiO₃ samples of two different grain sizes were heat-treated in contact with PbTiO₃, DIGM occurred in the coarse-grained samples (200 m in average size) while fast grain growth was observed in the fine-grained samples (4 m). Energy dispersive spectroscopy (EDS) analysis confirmed that the fast growth of BaTiO₃ grains was accompanied by the alloying of Pb and thus related to DIGM. A calculation of coherency strain energy for the BaTiO₃-PbTiO₃ system showed that the coherency strain energy of a coherent (Ba_0.8Pb_0.2)TiO₃ layer on BaTiO₃ was between 2 and 3 MJ/m³ depending on the surface orientation. The calculated coherency strain energy values are much higher than the capillary energy due to the grain boundary curvature of 4 m grains in the fine-grained sample. The observed enhancement of grain growth appears therefore to be a result of DIGM. Such grain growth enhancement by DIGM is thought to occur in materials processing under chemical inhomogeneity or inequilibrium, for example, in the sintering of powder mixtures and in the annealing of chemically inhomogeneous polycrystals.     

Interdiffusion in two-layer Pd/Ag films. IV. Diffusion-induced Σ13 boundary migration

Fast growth of grains with homogeneous composition and 13 orientation was observed in Pd/Ag single-crystal thin films during annealing at 400°C. It was found that nucleuses of these orientation are contained in an initial structure. The grain growth obeys the mechanism of diffusion-induced grain boundary migration and the migration velocity was >-10^-7 m/s. The assumption was made that an elementary act of fast grain boundary migration is a transition of an atomic group determined by a Coinsidence Site Lattice, and the reconstruction occurs as correlated displacements of n atoms resulting in the transition of m atoms from one grain to another. A free activation energy of such a process and velocity of the special grain boundary were estimated in the frames of the model.     

On grain boundary character distribution in materials with cubic structure

From analysis of numerous experimental data on grain boundary (GB) statistics in polycrystals it has been established that certain groups of materials with cubic structure reveal similar GB character distributions (GBCD) (distribution of GBs by reciprocal density of coincidence sites ). It has been shown that GBCD can be described with an empirical low with different parameters for various groups. Several criteria for classification of materials by these groups (the stacking fault energy value, hierarchy of GB energies and mechanism of replacement of high-energy GBs with low-energy ones) have been considered. It has been found that peculiarities of electronic structure of materials are correlated with the classification proposed.     

On the accommodation of extrinsic dislocations in grain boundaries

The relaxation of the extrinsic grain boundary dislocation (EGBD) stresses, created by interaction of lattice dislocations with grain boundaries (GBs), is a phenomenon which plays an important role in recrystallization and high temperature deformation of materials. The kinetics of this phenomenon, controlled by GB diffusion, are relatively well established. On the contrary, the processes which operate in order the GBs return to equilibrium are still controversed in vicinal and general GBs.The decomposition in discrete products and the rapid motion of the glissile components observed by High Resolution Electron Microscopy (HREM) in symmetrical tilt GBs support the recent incorporation model of EGBD accommodation. But, until now, these observations and this model are restricted to GBs described by the Structural Unit/Grain Boundary Dislocation (SU/GBD) model. Otherwise, the Spreading phenomenon generally observed by Transmission Electron Microscopy (TEM) in vicinal and general GBs is not clearly understood.This paper is an attempt to review the different EGBD accommodation models and to raise up the question of their relevance to account for the stress relaxation in any grain boundary.     

Intergranular Stress Corrosion Cracking of Pure Copper 〈111〉 Tilt Bicrystals

Behaviour of stress corrosion cracking (SCC) in a series of pure copper bicrystals with a symmetrical 111-tilt boundary has been investigated. Tests were performed by the slow strain rate technique (SSRT) in 1M NaNO₂ solutions. The small-angle tilt bicrystals fractured in both intergranular and transgranular manners accompanied by a large amount of plastic strain to fracture while the large-angle bicrystals fractured in almost intergranular manner with a smaller plastic strain. Susceptibility of SCC increases with increasing misorientation and becomes relatively constant in large-angle grain boundaries. The local minima appeared at the 7(321) and 3(211) boundaries, suggesting that the susceptibility was partially affected by grain boundary energy. Stress concentration generated by the pile-up of trapped dislocations at the grain boundary could account for the high susceptibility of the intergranular SCC in large-angle grain boundaries.     

Significant decrease in interfacial energy of grain boundary through serrated grain boundary transition

The mechanism of serrated grain boundary formation and its effect on liquation behaviour have been studied in a wrought nickel-based superalloy – Alloy 263. It was newly discovered that grain boundaries are considerably serrated in the absence of γ?′-phase or M₂₃C₆ at the grain boundaries. An electron energy-loss spectroscopy study suggests that serration is triggered by the discontinuous segregation of C and Cr atoms at grain boundaries for the purpose of relieving the excessive elastic strain energy. The grain boundaries serrate to have specific segments approaching one {111} low-index plane at a boundary so that the interfacial free energy of the grain boundary can be decreased, which may be responsible for the driving force of the serration. The serrated grain boundaries effectively suppress grain coarsening and are highly resistant to liquation due to their lower wettability resulting from a lower interfacial energy of the grain boundary.     

Grain boundary chemistry and grain boundary dislocations in bulk YBa2Cu3O7-δ

We report the results of our microchemical analyses of low- large-angle grain boundaries in bulk YBa₂Cu₃O_7- using nanoprobe energy-dispersive-X-ray spectroscopy (EDX) and electron-energy-loss spectroscopy (EELS). We observed periodic variation in the concentration of Cu along the boundaries, and oxygen depletion at the boundaries. We found that the chemistry of the grain boundary is very sensitive to grain boundary dislocations (GBDs), while, in turn, the configuration of the GBDs is very sensitive to the boundary misorientation and the boundary plane normal. The strain field associated with closely spaced GBDs reduced the density of mobile holes at the boundary, which is expected to be detrimental to the superconducting properties of the boundary. The possible structural transition of the grain boundaries from an oxygen-deficient state to a fully oxygenated state near a coincidence orientation is discussed, based on the reduction of the elastic strain energy of the boundaries.     

The study of grain orientation distributions and grain boundary misorientation distributions in 304 and 316L stainless steels

Orientation distribution functions in two recrystallized austenitic stainless steels (AISI types 304 and 316L) with known grain boundary misorientation distributions have been studied. Previously obtained data on grain boundary spectra in these steels have been re-examined and analyzed from the point of view of texture analysis.The results obtained have shown that there is no unambiguous relatonship between grain boundary misorientation distribution and grain orientation distribution (ODF) determined by the X-ray analysis in the materials under study. This ambiguity is due to the following reason. In the grain boundary misorientation statistics only nearest-neighbor grains are taken into account, but in the orientation distribution function orientations are averaged over the entire volume of the specimen independent as to whether the grains are adjacent or not. Two main results were established for the steels under study: (i) Textures of the two steels differ, though their grain boundary misorientation distributions are similar; and (ii) misorientations of the majority of grain boundaries can be described as rotations about the axes close to 110.     

Evaluating Anisotropic Surface Energies Using the Capillarity Vector Reconstruction Method

By measuring the geometry and crystallography of the three interfaces that meet at grain boundary thermal grooves, it is possible to determine the anisotropy of the surface free energy. Previously, the surface energy of MgO at 1400°C in air was approximated by a truncated double Fourier series with coefficients that were determined by fitting the observations to Herring's condition for local equilibrium at a triple junction. The purpose of this paper is to describe an alternative analysis of the same data set that is not limited by an assumed functional form of the surface energy. In this case, the space of surface characters is discretized and each orientation is associated with a capillarity vector (according to the Cahn–Hoffmann definition). The set of capillarity vectors that most closely satisfies the condition for local equilibrium at each triple junction is then determined by an iterative method. The relative surface free energies derived from this analysis are more anisotropic than those derived from the series fit and more consistent with the observed faceting of MgO in air at 1400°C. The relative surface energies of the low index planes are ₁₁₀/₁₀₀ = 1.07±0.04 and ₁₁₁/₁₀₀ = 1.17±0.04.     

A two-channel model for transport across high T c bicrystal grain boundary junctions

This paper describes an investigation of the microscopic configuration of the barrier which exists at the boundary in high T _c grain boundary junctions. A model for describing the properties of bicrystal and biepitaxial grain boundary junctions is proposed. Two channels for transport currents are assumed, the supercurrent being carrier by one channel only. The measurable macroscopic parameters of a junction, (critical current density and normal state resistance) are computed in terms of the microscopic parameters that appear in this model. The model well describes major features of the characteristics observed for high T _c bicrystal junctions.     

Diffusion Along Migrating Grain Boundaries

The paper summarizes recent experiments on diffusion at migrating grain boundaries (GBs) occurring during discontinuous reactions, like discontinuous precipitation (DP) and diffusion induced grain boundary migration. Analytical electron microscopy was used for measurements of the solute concentration across individual solute-depleted lamellae. These data combined with information on the growth velocity and the thickness of an individual lamella allowed the determination of the local values of the diffusivities of the moving reaction front of the DP cell in Al–Zn, Ni–Sn, Cu–In and Co–Al alloys. The obtained diffusivities and activation energies are very similar to the relevant parameters of stationary GBs. This allows us to conclude that there is no significant difference in the rates of diffusion along migrating and stationary GBs in the systems investigated. It is therefore believed that the diffusivity values of the moving reaction front of the DP reaction can be a source of reliable information on interfacial diffusion characteristics, especially in systems and/or at temperatures where radiotracer data are not readily available.