Grain Boundary Plane Research Articles

A study of grain-boundary structure in non-stoichiometric NiAl by atomistic simulation and electron microscopy

Abstract The atomic structure of a Σ =5,(310)[001] grain boundary in NiAl has been determined by a synergistic approach using high-resolution electron microscopy (HREM) and atomistic structure calculations. The effect and distribution of point defects at the grain boundary were determined using molecular statics calculations employing N-body empirical potentials to calculate the relaxed grain-boundary structures and energies resulting from various initial structures and stoichiometries. Monte Carlo calculations confirm the stability of the lowest-energy structure, which contains Ni antisite defects adjacent to the grain-boundary plane. Multislice image simulations of this structure are in good agreement with the experimental HREM image. This is the first combined application of experimental and theoretical structure determinations to an intermetallic grain boundary.

High-resolution electron microscopy of grain boundaries in Ni3(Al0·8Ti0·2) bicrystals

Abstract The grain-boundary structure of [001] small-angle tilt boundaries in L12Ni3(Al0·8Ti0·2) bicrystals prepared by hot pressing was investigated by high-resolution transmission electron microscopy. Dislocations with a Burgers vector a[100] which are dissociated into two superpartials with Burgers vectors (a/2)[110] and (a/2)[110] connected by an antiphase boundary (APB) were found. The dissociated superpartials and the corresponding APB lie along the grain-boundary plane. The cores of the dissociated superpartials extend (or spread) over a few {011} atomic planes. Also, the preferred configuration of the dislocation array observed was discussed on energy considerations.

High-resolution electron microscopy of grain boundaries in Ni3(Al0·8Ti0·2) bicrystals II. Coincidence site lattice boundaries

Abstract The grain-boundary structure of Σ = 5, [001] (36.8°), Σ = 25, [001] (16·26°) and Σ = 3, [011] (109·.47°) coincidence site lattice (CSL) tilt boundaries in the Ll2Ni3(Al0·8Ti0·2) bicrystals prepared by hot pressing were investigated by high-resolution transmission electron microscopy with the help of image simulation. An atomic arrangement in the Σ = 5, [001] (36·8°) boundary is asymmetric with a small quantity of rigid-body translation; accommodation of atoms is small and is localized on a near-grain-boundary plane. An atomic arrangement in the Σ = 25, [001] (16·26°) boundary is asymmetric without appreciable rigid-body translation; some atoms are withdrawn and it is interpreted as an array of dissociated (a/2)⟨110⟩ superpartials. An atomic arrangement in the Σ = 3, [011] (109.47°) boundary is primarily symmetric, and local accommodation of atoms and rigid body translation vary with deviation from the grain-boundary plane. In the three CSL boundaries observed in this study, atomic-size cavities a...

The relationship between crystal misorientation and conductive mode contrast of grain boundaries in additive-free zinc oxide

Grain boundaries in additive-free ZnO show different types of conductive mode contrast, depending on the electrical properties. Electron beam scattering patterns have been used to determine the misorientation of grain boundaries showing certain types of conductive mode contrast in order to correlate electrical properties with grain-boundary structure. No correlations were apparent using the coincident-site lattice model, but other criteria suggest that grain-boundary electrical properties may be crystallographically controlled. More detailed analysis will require the full determination of the grain-boundary plane.

Transport properties of an engineered

Single grain boundaries (GB's) of an engineered [001] tilt series of bulk ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ (YBCO) bicrystals were electrically characterized to probe the intrinsic and extrinsic factors influencing GB properties of bulk melt-processed (MP) YBCO. The bicrystal series ranged from 1.5\ifmmode^\circ\else\textdegree\fi{} to 45\ifmmode^\circ\else\textdegree\fi{} misorientation angle and displayed tendencies similar to those of thin film GB's at 77 K in self-field. Subtle differences between thin film and bulk GB transport behavior that were observed may be attributed to the thin film substrate. The dependence of normal-state resistance (${\mathit{R}}_{\mathit{n}}$) at 77 K on the [001] tilt angle has been noted. An anticorrelation between ${\mathit{R}}_{\mathit{n}}$ and critical current (${\mathit{I}}_{\mathit{c}}$) exists such that the product, ${\mathit{I}}_{\mathit{cR}\mathit{n}}$, of each bicrystal from 10\ifmmode^\circ\else\textdegree\fi{} to 45\ifmmode^\circ\else\textdegree\fi{} falls within a narrow band of 10 to 20 \ensuremath{\mu}V. Such a narrow characteristic voltage range suggests highly reproducible growth conditions. It is also reasonable to suggest that naturally grown boundaries in bulk MP YBCO have more uniform GB character than their thin film counterparts. This advantageous growth technique allows us to further probe the role of the GB plane in influencing transport properties. \textcopyright{} 1996 The American Physical Society.

Grain-boundary plane crystallography and energy in austenitic steel

The presence of grain boundaries in polycrystalline materials affects the materials properties and performance. Recently it has been realized that boundaries can be manipulated to give better properties, and the design and control of grain boundaries is now an area of strong research interest in the search for high performance engineering materials. Grain boundaries can be classified using the Coincident Site Lattice Model (CSL), which defines the periodicity, i.e., the degree of fit between the two lattices which constitute the boundary. Using this model it is possible to divide boundaries into categories: low angle (up to 15{degree} misorientation), CSL and random i.e., high angle non-CSL. Some CSL boundaries have been shown to have special properties: an example from recent research in the same program as that currently reported has shown that twin boundaries ({Sigma} = 3 in CSL notation) in High Nitrogen Austenitic Stainless Steels do not favor the formation of Cr{sub 2}N precipitates. The research presented here examines grain boundary inclinations of surface grains in austenitic steel specimens which have been isothermally aged at higher 700 C or 800 C. Grain boundary plane crystallography has also been obtained for the 800 C aged sample.

Electronic bonding characteristics of hydrogen in bcc iron: Part II. Grain boundaries

Electronic structure calculations were carried out for bcc iron grain boundaries (GB) with or without hydrogen, using the self-consistent Discrete Variational embedded cluster method within the first-principles local density formalism. Bonding characteristics were mainly investigated. Simple rigid body translations perpendicular to the GB plane were used for estimation of relaxed GB geometry. Analysis of bond order summation over the GB shows considerable volume expansion normal to the GB plane of a dense 23(111) twist/tilt GB and some compression for the rather open 23(110) twist configuration. These results are discussed in the context of atomistic simulations which suggest that higher energy GB's generally have larger volume expansion normal to the GB plane. H in a 23(111) GB reduces Fe-Fe bonding strength by —3% within a 0.25 nm spherical volume around the H site, associated with reduction of the 4s and 4p occupancy of the nearest neighbor Fe. Since these orbitals contribute mainly to metallic bonding, the action of H atoms as an embrittlement inducer can be understood.

Grain boundary design and control for electrical and magnetic properties of polycrystalline materials

The concept of “grain boundary (GB) design and control” originally proposed for the control of mechanical properties by manipulating the proportion of low energy GBs in polycrystals has been developed to process materials with desirable electrical and magnetic properties. For this purpose various parameters of the GB network have been analyzed and it has been shown that the GB plane, density of GB defects, GB spatial distribution, i.e. mutual location of various kinds of GB, GB inclination (grain shape) and GB density (grain size) or size of clusters of low-energy GBs, can be used for GB engineering. Several examples illustrating the possibility of application of various ways of GB engineering to the control of electrical properties of polycrystalline silicon and aluminum thin films and magnetic properties of cobalt-chromium thin films have been given.

An investigation of grain-boundary plane crystallography in polycrystalline nickel

An investigation has been carried out to measure and categorize the grain-boundary plane indices of boundaries in pure nickel. Coincidence site lattices (excluding Σ = 3s) were found to be either asymmetrical tilt boundaries with high indices, or have irrational boundary planes. For the Σ=3s, almost half were asymmetrical tilt boundaries displaced from the 111/111 symmetrical tilt boundary on the 110 zone. Such boundaries have low energies compared to other Σ=3s. The 211/211 incoherent twin was not observed, which was explained on the basis of its higher energy compared to other boundaries on the 110 zone. The results are compared and contrasted with previous data, where boundaries abutted the specimen surface during annealing, which is not the case for the present data. Comments are made with respect to the relationship between macroscopic and atomic-level boundary geometry and implications of the results for grain-boundary properties.

Environment Sensitive Embedding Energies of Impurities, and Grain Boundary Stability in Tantalum

AbstractMetalloid impurities have a very low solubility in Tantalum, and therefore prefer to segregate at the grain boundaries (GBs). In order to analyze the energetics of the impurities on the Tantalum GB, the LMTO calculations were performed on a simple 8-atom supercell emulating a typical (capped trigonal prism) GB environment. The so-called “environment-sensitive embedding energies” were calculated for Hydrogen, Boron, Carbon, Nitrogen, Oxygen, Phosphorus, and Sulphur, as a function of the electron charge density due to the host atoms at the impurity site. The calculations showed that, at the electron density typical of a GB, Carbon has the lowest energy (followed by Nitrogen and Boron) and thus would compete with the other impurities for the site on the GB, tending to displace them from the GB. The above energies were then used in a modified Finnis-Sinclair embedded atom approach for calculating the cohesive energies and the equilibrium interplanar distances in the vicinity of a (111) Σ3tilt GB plane, both for the clean GB and that with an impurity. These distances were found to oscillate, returning to the value corresponding to the equilibrium spacing between (111) planes in bulk BCC Tantalum by the 10th-12th plane off the GB. Carbon, Nitrogen and Boron somewhat dampen the deformation wave (making the oscillations less than in the clean GB), while Oxygen, Phosphorus and Sulphur result in an increase of the oscillations. The cohesive energies follow the same trend, the GB with Carbon being the most stable. Thus, Carbon, Nitrogen and Boron may be thought of as being cohesion enhancers, while Oxygen, Phosphorus and Sulphur result in decohesion effects.

Electrical and structural studies of copper and nickel precipitates in a Σ=25 silicon bicrystal

Deep-level transient spectroscopy measurements of electronic defect states in a coincidence Σ=25 grain boundary (GB) in silicon have been performed after quenching of heat-treated samples (900 °C, 2 h) containing copper and/or nickel. These elements are usually suspected to be nonintentionally contaminating impurities. The special care supplied to get processed samples free from transition-metal impurities (particularly copper) has led to the measurement of a continuous distribution of boundary levels between Ec−0.20 eV and Ec−0.38 eV which shift toward deeper energies (between Ec−0.25 eV and Ec−0.42 eV) with a gradual increase of the copper content. The heavily contaminated samples with Cu or with both Cu and Ni exhibit a single interface trap at Ec−0.54 eV. According to transmission electron microscopy (TEM) and energy-dispersive x-ray (EDX) analyses, as well as comparable data reported earlier, this interface state seems characteristic of copper precipitates obtained by quenching from 900 °C; however, the incorporation of nickel in a ‘‘copper-free’’ bicrystal has been found to result in an almost discrete boundary level at Ec−0.50 eV. From TEM, EDX, and electron-diffraction investigations, the corresponding precipitates were geometrically shaped, intersecting the GB plane, and identified as two types (A and B) of NiSi2 silicide.

Grain-boundary structures in hexagonal materials: Coincident and near coincident grain boundaries

Embedded atom method (EAM) simulations of the structure of grain boundaries in hexagonal metals are presented. The simulations use recently developed interatomic potentials for Ti and Co. Structures were calculated for various symmetrical tilt boundaries with the [1¯100] tilt axis. The structures obtained for both metals are very similar. The energies for the Co boundaries are higher than those for Ti by a factor of 2. The structural unit model was applied to the computed grain-boundary structures in these hexagonal materials. As in cubic materials, the structural unit model can describe a series of symmetrical tilt coincident boundaries. In addition, when the coincidence ratio in the grain-boundary plane varies with thec/a ratio, a structural unit-type model can describe the variation of grain-boundary structure withc/a ratio. This model is adequate for describing series of symmetrical tilt boundaries with the grain-boundary plane oriented perpendicular to a fixed crystallographic direction and varyingc/a ratios. For the structures of the so-called near coincident boundaries that appear in these materials, it was concluded that near coincident boundaries behave similarly to exact coincidence boundaries if there is a coincident periodic structure in the grain-boundary plane. This may occur even without a three-dimensional (3-D) coincident site lattice.

Monte Carlo simulation of the minority-carrier recombination at and around grain boundaries surrounded by a denuded zone revealed by light-beam-induced current mapping

Abstract The purpose of this paper is the simulation of the minority-carrier recombination in the case of a recombining grain boundary (GB) surrounded by a denuded zone. This denuded zone could be due to the segregation of impurities (metals and oxygen) to the GB plane. A direct Monte Carlo simulation is used to trace minority-carrier trajectories in a heterogeneous distribution of recombination centres and to evaluate the collection efficiency of a built-in field by averaging over a large number of runs. The collection efficiencies obtained by the simulation are checked by comparison with the results of analytical solutions proposed previously for a recombining GB plane embedded in a homogeneous bulk. Then the configuration of a GB surrounded by a zone of enhanced lifetime is discussed and compared with the experimental data.

Electron trapping and impurity segregation without defects: Ab initio study of perfectly rebonded grain boundaries.

We present the results of an extensive ab initio study of the \ensuremath{\Sigma}=5 tilt [310] grain boundary in germanium. We find that the boundary reliably reconstructs to the tetrahedrally bonded network observed in high-resolution electron microscopy experiments without the proliferation of false local minima observed in similar twist boundaries. The reduced density of bonds crossing the grain-boundary plane leads us to conjecture that the boundary may be a preferred fracture interface. Though there are no dangling bonds or miscoordinated sites in the reconstruction, the boundary presents electron-trap states just below the conduction band. Further, we show that lattice relaxation effects are irrelevant to the segregation of impurities to tetrahedrally reconstructed defects and that the interfacial electron-trap states give rise to an electronic frustration mechanism that selectively drives the segregation of only n-type dopants to the boundary.

Cobalt intergranular segregation in WC-Co composites

Interfaces between carbide grains in the tungsten carbide-cobalt composite have been considered. Different techniques, such as transmission electron microscopy and energy dispersive X-ray analysis have been used to characterize the orientation relationship, the nature of the planes and the chemical composition of the grain boundaries.The cobalt concentration at WC-WC grain boundaries was determined by X-ray energy selective analysis in the TEM. Cobalt profiles were performed across low-angle grain boundaries, coincidence and general grain boundaries. Cobalt segregation was found whenever dislocations were imaged in the grain-boundary plane of a low-energy grain boundary. The segregation value was compared with the segregation ratio measured in special grain boundaries characterized by a coincidence site lattice.

Second-harmonic generation in a spin-glass microstructure

We studied magnetoconductance of microregions containing 2d electron-gas adjacent to a grain-boundary plane in bicrystals of Hg 0.79Cd 0.19Mn 0.02Te. In addition to weak localization magnetoresistance and universal conductance fluctuations persisting up to ∼ 1 K, strongly nonlinear-current voltage characteristics were detected in the vicinity of spin-glass freezing occurring at 100 mK.

Stress Distribution and Mass Transport Along Grain Boundaries During Steady-State Electromigration

ABSTRACTStress distribution and mass flux in the plane of each grain boundary within a polycrystalline thin-film conductor have been calculated during electromigration for zero flux divergence (steady state) and various boundary conditions. Steady state, representing a balance between the (applied) electric and (induced) stress driving forces, is assumed to develop after a short transient time. Boundary conditions at the intersection of grain boundaries with the top and bottom conductor surfaces (surface junctions) and with the conductor edges (edge junctions) are assumed to be of two types: open (flux passes freely) and closed (zero flux). Flux is assumed to pass freely at the intersection of grain boundaries with each other (triple Junctions). Several grain boundary configurations are considered, including individual boundaries, single triple junctions, and combinations thereof, assuming that bottom surface junctions (conductor/ substrate interface) are closed and that top surface junctions are either open (bare conductor) or closed (passivation layer). Results clearly show the formation of incipient holes and hillocks near the intersection of triple junctions and/or closed (blocked) edge junctions with open surface junctions.

The effect of stress on grain boundary grooving

A theory of the effect of stress on grain boundary (GB) grooving is developed under the assumptions of matter transport by surface and GB diffusion, and steady-state diffusion in the GB. A uniaxial stress normal to a planar GB induces a flux at the GB root, out of (in to) the boundary for compression (tension), proportional to the stress. The corresponding solution is joined to that for thermal grooving at the surface using the continuity of chemical potential and of flux at the groove root. For planar geometry and small slopes, grooving under stress is described by a superposition of the solution for stress-free grooving and that generated by the stress-induced flux with no groove. Limits of the approximations are discussed. The analysis predicts an acceleration of grooving by a tensile stress and offers the possibility of an absolute measurement of the GB diffusion coefficient for mass transport.

Quasiperiodic features in the atomic structure of long-period grain boundaries

Atomic relaxations in grain boundaries (GBs) can assume quasiperiodic character even in periodic GBs, as we show for a Σ = 41 GB in gold, which is analyzed by transmission-, high-resolution electron microscopy (HREM) and atomistic computer simulations. We find that misfit localization within a large planar GB unit cell can generate densely spaced regions of structural disorder, akin to misfit dislocations. The observations illustrate that atomic relaxations are typically dominated by short-range interactions and the tendency of the solid to assume a local atomic environment similar to the bulk.

Inhomogeneity of charge carrier concentration along the grain boundary plane in oxide superconductors

Spatially resolved transmission high energy electron loss spectrometry (EELS) has been utilized to probe the spatial variation of the hole concentration along the grain boundary (GB) plane in YBa 2Cu 3O 7−δ (Y123). The O K pre-edge peak in the EELS spectra, which offers a quantitative measure of the hole concentration in p-type doped cuprate superconductors including Y123, has been observed to vary along the GB plane. These observations conform to the Dayem-Bridge-type model for GBs in Y123. A modified Dayem-Bridge model is presented based on the experimental observations of varying oxygen stoichiometry along the GB plane.