Influence of Orientation on Properties of Grain Boundaries in NaCl

Simple (100) tilt, twist, and double-tilt bicrystals of NaCl, grown by the Kyropoulos technique from melts of high-purity NaCl, alone and with controlled impurity additions, were examined for mechanical strength and structure of the grain boundaries. Grain boundary fracture strengths, measured in three-point bending, showed that high-purity bicrystals with simple tilt orientations were stronger than those with simple twist at high mismatch angles (37° to 45°). The results did not show a functional dependence of strength on angle of mismatch in either tilt or twist bicrystals. Pips observed on parted grain boundaries of high-purity NaCl (100] twist and double-tilt bicrystals were believed to represent regions of continuity across the boundary. This feature was rare in similar-purity NaCl (100) tilt bicrystals. Separate additions of 100 ppm SiO₂, CaCl₂, FeCls, and KCl to the melt had no apparent effect on the character of the grain boundary. However, an addition of 1000 ppm CaCl₂ nearly doubled the strength of a (1001) 30° twist bicrystal, whereas the same addition weakened a (100) 45° tilt bicrystal. Sodium chloride (100) tilt grain boundaries, examined in situ under dark-field illumination, showed randomly distributed spots, believed to be impurity segregations, with their maximum density at the boundary. The spot densities increased with increasing tilt angle for angles of 15° and greater. The spots were not observed in the low-angle tilt boundaries (<15°) and were seen only in intermittent clusters in the few twist bicrystals examined.     

Stress Distributions in Al2O3 Bicrystal Tensile Specimens

Stress distributions in Al₂O₃ bicrystals were analyzed for certain misorientations across the grain boundary in straight-sided tensile specimens with the grain boundary normal to the tensile axis. Stresses were found using the numerical analysis technique of direct stiffness calculations or by finite element analysis. When the anisotropy in elastic constants of the bicrystals was considered, misorientation of principal axes yielded stress concentrations at the boundary as high as 1.50. The assumption of uniform stress states in bicrystal mechanical response studies, therefore, is not generally accurate.     

Current–Voltage Characteristics of Σ1 Boundaries with and without Cobalt Ions in Niobium-Doped SrTiO3 Bicrystals

Current–voltage ( I – V ) characteristics across Σ1 boundaries of Nb-doped SrTiO₃ bicrystals were investigated with a special interest in the effect of an impurity ion, cobalt. The Σ1 boundary with cobalt ions was fabricated by hot-joining technique after evaporating metallic cobalt onto the (001) plane used for a contact plane. High-resolution transmission electron microscopy (HREM) study associated with X-ray energy dispersive spectroscopy (EDS) revealed that cobalt solved and distributed around the Σ1 boundary which was perfectly joined without any second films. It was found that the Σ1 boundary with cobalt ions exhibited a nonlinear I – V relationship while the Σ1 boundary without cobalt ions showed a linear I – V relationship. This result indicates that the presence of cobalt itself at the boundary can form a potential barrier irrespective of grain boundary coherency.     

Dislocation Structures of Low-Angle and Near-Σ3 Grain Boundaries in Alumina Bicrystals

Alumina bicrystals with low-angle and near-Σ3 <0001> tilt grain boundaries were fabricated using diffusion bonding to study the dislocation structures in alumina grain boundaries. The resulting grain-boundary structures were investigated using high-resolution transmission electron microscopy, and the grain-boundary energies were analyzed using theoretical calculations. It was found that partial dislocations with Burgers vectors of the type return ⅓<10[Onemacr]0> were periodically located in the boundaries and that a stacking fault between pairs of partials was formed in such boundaries. The length of the stacking fault decreased with increased misorientation angles, which was reasonably predicted by the theoretical calculation. In the case of a near-Σ3 grain boundary, an array of displacement shift complete dislocations with the Burgers vector of return ⅓<1[Onemacr]00> was periodically formed along the boundaries. These boundaries did not have stacking faults. The spacing between the dislocations decreased with increased deviation angle from the exact-Σ3 boundary with the tilt angle of 60°.     

Deformation and Fracture of Polycrystalline Lithium Fluoride

Techniques for the fabrication of polycrystalline LiF test specimens were developed and evaluated using single-crystal LiF as a control. An etch was developed which revealed dislocations on all crystallographic faces of LiF. Large-grained polycrystalline specimens tested in four-point loading underwent 0.076 to 0.798% plastic strain before fracture. In most cases their yield stress was similar to that for single crystals favorably oriented for flow on {110}〈110〉 slip systems. Deformation was inhomogeneous among the grains because of differences in orientation with respect to the applied stress and within individual grains because of interactions at grain boundaries. Grain boundaries were barriers to slip, but stresses resulting from slip in one grain were transmitted to neighboring grains and often caused local deformation near the boundary. In one case, local boundary slip occurred on an (010) plane. Three-grain junctions were areas of high residual stresses, and fractures originated at boundaries at or near three-grain junctions. Fractures were mixed transgranular and intergranular.     

Direct Evidence of Dopant-Enhanced Grain-Boundary Sliding in Yttria-Stabilized Zirconia Bicrystals

Compression tests were conducted at 1400°C in air for undoped and Si-doped yttria-stabilized zirconia (YSZ) bicrystals with the same orientation relationship (Σ=5, [001]/{210} grain boundary). It was found that the macroscopic grain-boundary slidings are observed during the deformation in both undoped and doped bicrystals, and the sliding displacements increase almost linearly with increasing total displacements. It is distinctly demonstrated that the Si-doped bicrystal exhibits a sliding displacement that is a few times larger than the undoped bicrystal. In addition, the total strain of the Si-doped bicrystal at a failure is much larger than that of the undoped bicrystal. It can be stated that Si doping in YSZ not only enhances the grain-boundary sliding but also suppresses its failure.     

Structure of Sapphire Bicrystal Boundaries Produced by Liquid-Phase Sintering

The structure and composition of sapphire bicrystal boundaries produced by liquid-phase sintering depended on the crystallographic misorientation of the crystals across the boundary and on the orientation of the boundary. Basal twist boundaries of 15° or 30° were not wetted by glass, but contained significant amounts of Ca and Si at the boundary. For tilt boundaries of 7° or 12°, the glass wetted segments of boundaries that contained the basal plane of either crystal. Boundary segments with orientations of 40° or more from the basal plane, however, were dewetted (i.e., "dry"). Boundary segments oriented less than ∼40° from the basal orientation were partially wetted, consisting of segments of wetted and dry grain boundaries. For the 12° tilt boundary, Ca and Si could be detected on portions of the boundary that contained no glass. For bicrystal boundaries having tilts of ≤4°, dewetting occurred for all observed boundary orientations. Basal-oriented segments in these small angle tilt boundaries contained noticeable concentrations of adsorbed Ca and Si, while nonbasal segments were apparently free of Ca and Si. Most results could be explained based on a combined Wulff plot construction, which predicts partially wetted grain boundaries and "missing" angles for unwetted grain boundaries. Results that could not be explained by the construction included growth step ledges bounded by nonequilibrium facet planes.     

Current-Voltage Characteristics across (0001) Twist Boundaries in Zinc Oxide Bicrystals

Current-voltage ( I – V ) characteristics across (0001) twist boundaries with various misorientation angles were investigated in undoped ZnO bicrystals fabricated by a hot-joining technique. It was confirmed by high-resolution transmission electron microscopy that the boundaries were perfectly joined without intergranular phase. None of the bicrystals prepared in this study exhibited nonlinear I – V characteristics irrespective of coherency at the boundaries. Therefore, grain-boundary atomic configuration had no relation to the formation of double Schottky barriers at the (0001) twist boundaries in ZnO.     

Energy and Structure of (001) Coincident-Site Twist Boundaries and the Free (001) Surface in MgO: A Theoretical Study

A computer code is described which permits investigation of the energy and structure of coincident-site lattice (CSL) grain boundaries, stacking faults, and free surfaces in ionic crystals. The code uses computational techniques similar to those of Harwell's HADES code for the study of point defects in bulk ionic crystals. Two sets of short-range potentials are used to determine the energy and structure of the free (001) surface and of (001) CSL twist boundaries in MgO with values of Γ, the inverse density of CSL sites, ranging between Γ= 5 and Γ= 65. From comparison of the results obtained by means of the different potentials it is concluded that the Van der Waals attraction between oxygen ions on opposite sides of the interface is mainly responsible for the rather weak cohesion predicted for such bicrystals, whereas Coulombic interactions are found to play only a minor role. It then follows that (i) the (001) plane is not a favored plane for the formation of grain boundaries in pure oxides with NaCl structure and (ii) similarities should exist between (001) twist boundaries in ionic crystals with NaCl structure and in fee metals. These similarities are investigated by comparing calculated boundary structures and energy-vs-misfit angle curves for MgO with recent results for aluminum, copper, silver, and gold, in which a broad spectrum of different interatomic potentials was used. It is suggested that the rather strong cohesion of the pressure-sintered MgO bicrystals of Sun and Balluffi may be due to impurities which have migrated to the boundary during sintering.     

Structure of Alumina Grain Boundaries Prepared with and without a Thin Amorphous Intergranular Film

The presence of a thin amorphous intergranular film along grain boundaries in alumina is expected to affect the properties of the interface and hence those of the material. In the present study, two types of grain boundaries have been formed in hot-pressed alumina bicrystals. In one case, the surfaces of the sintered crystals were kept as clean as possible, while in the other a thin layer of SiO₂ was intentionally deposited onto the surface of one crystal. The distribution of SiO₂ along the resulting grain boundary was then monitored by transmission electron microscopy and compared with the morphological features of the interface. In the special cases chosen here, the glass receded into large pores which grew into the alumina itself. However, the presence of the glassy phase during the early stages of sintering clearly did influence the characteristics of the resulting grain boundaries.     

Energies and Grooving Kinetics of [001] Tilt Boundaries in Nickel Oxide

Relative energies of symmetrical [001] tilt boundaries in nickel oxide bicrystals were determined from the dihedral angle at the groove root with a surface profilometer. At 1535°C in air, the relative grain boundary energy increases with tilt angle up to about 20 degrees and then remains relatively constant with increasing angle. A strong temperature dependence of boundary energy is explained by segregation of point defects to the boundary. Grooving kinetics are controlled by volume diffusion of nickel, and a theoretical discussion of mass transport in nonstoichio-metric solids is given, predicting this behavior under certain conditions.     

High-Temperature Deformation of Alumina Double Bicrystals

Deformation and strength of alumina double bicrystals fabricated from verneuil-grown stock were studied as functions of temperature (1210° T 1820°C) and strain rate (0.0005 ε 0.05 min^-1). The oriented specimens had c axes parallel or perpendicular to the compressive stress axes; + a axes and c axes were matched and/or mismatched across the (1123) interfaces of the double bicrystals. Undoped ("pure") double bicrystals in which the c axes and stress axis were coincident deformed by rhombohedral twinning and, at T 1550°C, by slip. Double bicrystals containing crystal segments having c axes perpendicular to the stress axis deformed in part by basal kinking. Interfacial impurity additions of mullite or spinel thin films promoted grain-boundary fracture and sliding, whereas "pure" double bicrystals, even at stresses comparable to the ultimate strengths of single crystals in similar orientations, did not exhibit macrosliding of the grain boundary.     

Characterization of Grain Boundaries in Superplastically Deformed Y-TZP Ceramics

The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (1150°C), the grain boundaries are covered by an ultrathin (1nm) yttrium-rich amorphous film. After deformation at 1200°–1300°C under low stress, some grain boundaries are no longer covered by the amorphous film. Yttrium segregation seems to occur only at wetted grain boundaries. Evidence has been found that the extent of dewetting increases with increasing applied stress.     

Grain boundary energies in yttria-stabilized zirconia

The three-dimensional microstructure of 8% yttria-stabilized zirconia (YSZ) was measured by electron backscatter diffraction and focused ion beam serial sectioning. The relative grain boundary energies as a function of all five crystallographic grain boundary parameters were determined based on the assumption of thermodynamic equilibrium at the internal triple junctions. Grain boundaries with (100) orientations have low energies compared to boundaries of other orientations, and all [100] twist boundaries have relatively low energies. Other classes of boundaries with lower than average energies include [100] symmetric tilt boundaries with disorientations less than 40° and [111] twist boundaries with disorientations greater than 20°. At fixed misorientations, the relative areas of boundaries are inversely correlated to the relative grain boundary energy. The results suggest that texturing microstructures to increase the relative areas of [100] twist boundaries might increase the oxygen ion conductivity of YSZ ceramics.     

Technique for Measuring Grain Boundary Mobility and Its Application to Potassium Chloride

A simple technique for investigating boundary mobility is outlined and applied to the study of boundary migration in melt-grown bicrystals of KCl containing pure twist boundaries. An unexpectedly high activation energy, 4 to 6 ev, is found for movement of the twist boundaries.     

Current–Voltage Characteristics of Cobalt-Doped Inversion Boundaries in Zinc Oxide Bicrystals

Undoped and cobalt-doped basal inversion boundaries were fabricated in ZnO bicrystals to investigate their current–voltage characteristics. High-resolution transmission electron microscopy observations and energy-dispersive X-ray spectroscopy analyses for a cobalt-doped bicrystal revealed that the boundary was highly coherent and free from intergranular phases and precipitates, but a certain amount of cobalt was present near the boundary. The cobalt-doped bicrystals exhibited nonlinear characteristics that depended on cooling rates from annealing temperature, in contrast to linear characteristics of the undoped bicrystals. It is suggested that the presence of cobalt impurities enhances the formation of acceptor-like native defects near the boundaries to generate electrostatic potential barriers.     

High-Temperature Tensile Deformation of Glass-Doped 3Y-TZP

Amorphous grain boundary phases in 3-mol%-yttria-stabilized zirconia ceramics (3Y-TZP) were studied to determine the influence of intergranular amorphous silicate phases on tensile superplasticity at temperatures of 1300–1500°C. Controlled additions (1 wt%) of compositionally distinct barium silicate and borosilicate phases were used. The initial grain sizes of the pure, barium silicate added, and borosilicate-added samples were 0.45, 0.55, and 0.55 μm, respectively. Systems with added barium silicate and borosilicate glass both exhibited a 60% reduction in flow stress as compared with pure 3Y-TZP, with the lower-viscosity barium silicate system exhibiting a slightly greater reduction in flow stress. The higher-viscosity borosilicate glass/3Y-TZP materials exhibited the greatest elongation to failure, while the barium silicate/3Y-TZP materials had the least elongation. Yttrium was found to segregate to grain boundaries in the pure and borosilicate-containing samples, and both yttrium and barium were found to segregate to grain boundaries in the barium silicate containing samples. No silicon was observed along two-grain boundaries in any of the samples, even those containing pockets of glass. The difference in deformation behavior may be due to a combination of viscosity of the glass addition, grain boundary segregation, and grain boundary bond character.     

Grain boundary curvatures in polycrystalline SrTiO3: Dependence on grain size,topology, and crystallography

By mapping grain orientations on parallel serial sections of a SrTiO₃ ceramic, it was possible to reconstruct three-dimensional orientation maps containing more than 3000 grains. The grain boundaries were approximated by a continuous mesh of triangles and mean curvatures were determined for each triangle. The integral mean curvatures of grain faces were determined for all grains. Small grains with fewer than 16 neighbors mostly have positive mean curvatures while larger grains with more than 16 neighbors mostly have negative mean curvatures. It is also possible to correlate the mean curvature of individual triangles with the crystallographic characteristics of the grain boundary. The mean curvature is lowest for grain boundaries with (100) orientations and highest for grain boundaries with (111) orientations. This trend is inversely correlated to the relative areas of grain boundaries and directly correlated to the relative grain boundary energy. The direct correlation between the energy and curvature is consistent with the expected behavior of grain boundaries made up of singular orientations. Furthermore, because both the relative energy and curvature of grain boundaries with (100) orientations are minima in the distributions, these boundaries also have the lowest driving force for migration.     

Molecular Dynamics Simulation of Oxygen Ion Diffusion in Yttria Stabilized Zirconia Single Crystals and Bicrystals

Molecular dynamics simulation studies have been performed to study the oxygen ion diffusion in yttria stabilized zirconia single crystals and bicrystals separated by tilt grain boundaries (GBs). Two types of GBs which are Σ 5 (3 1 0) and Σ 13 (5 1 0) are studied at temperatures between 1,000 K and 2,000 K. The effect of grain size, which is the distance between two GBs, and the effect of GB orientations are systematically investigated in this study. The oxygen diffusion in the bicrystals is found to be blocked by the GB, and the blocking effect increases with decreasing grain size and is affected by different grain orientations. In addition, the oxygen diffusion along the GB plane is most hindered.     

Origin of Grain-Boundary Diffusion in MgO

Preferential diffusion of Ni²⁺ and Co²⁺ along grain boundaries was observed in certain bicrystals of MgO. This enhancement is attributed to impurity segregation at the boundary. The identified impurities responsible for the effect are the principal impurities in the single-crystal MgO: Ca, Si, and Fe. No enhancement was observed in any bicrystal prepared above 1300°C, a temperature similar to that at which studies of the mechanical properties of MgO have implied a reabsorption of impurity precipitates into solid solution. It is concluded that enhanced grain-boundary diffusion of cations in MgO is an extrinsic, rather than an intrinsic, property of the boundary.