Sung Bo Lee

Abnormal grain growth and grain boundary faceting in a model Ni-base superalloy

Abstract Normal or abnormal grain growth in a model Ni-base superalloy is observed to depend on the grain boundary structure when heat-treated in a solid solution temperature range above the solvus temperature (1150°C) of the γ′ phase. When heat-treated at 1200°C abnormal grain growth occurs and most of the grain boundaries are observed to be faceted by optical microscopy, transmission electron microscopy, and scanning electron microscopy at the intergranular fracture surface. Some of the grain boundary facet planes are expected to be singular corresponding to the cusps in the polar plot of the boundary energy against the inclination angle, and it is proposed that if these boundary segments move by a boundary step mechanism, the abnormal grain growth can occur. When heat-treated at 1300°C normal grain growth occurs, the grain boundaries are defaceted, and hence atomically rough. Normal growth is expected if the migration rate of the rough grain boundaries increases linearly with the driving force arising from the grain size difference. The correlation between the grain boundary structural transition and the growth behavior thus appears to be general in pure metals and solid solution alloys.

Investigation of grain boundaries in abnormal grain growth structure of TiO2-excess BaTiO3 by TEM and EELS analysis

Abstract BaTiO 3 with 0.2 mol% excess TiO 2 shows abnormal grain growth (AGG) behavior when sintered in air at 1250°C, below the eutectic temperature (1332°C). Abnormal grains have polyhedral shape and their grain boundaries are flat and parallel to the {111} planes of the abnormal grains. Many grain boundaries between matrix grains are faceted with small steps and long segments parallel to the {111} planes of one of the grains. It is shown through HRTEM that such a polyhedral shape is correlated with an ordered grain boundary phase between the abnormal grains and neighboring matrix grains. This is in contradiction to a generally accepted interpretation where some residual liquid phase induces the polyhedral abnormal grains. By EELS, the chemistry and the change in the atomic bonding of both the flat boundaries of the abnormal grains and the faceted grain boundaries of the matrix grains are studied. A correlation between faceting, AGG and the formation of the ordered layer at the grain boundaries of the abnormal grains is proposed.

Faceting behavior of an asymmetric SrTiO3 Σ5 [001] tilt grain boundary close to its defaceting transition

Abstract Using high-resolution transmission electron microscopy (HRTEM), an asymmetric Σ5 [001] grain boundary in SrTiO 3 with an inclination of 2.5° away from the symmetric (310) plane was studied. A defaceting transition is observed between 1500 and 1600 °C. From 1520 to 1570 °C, the grain boundary is faceted into the symmetric (310) and asymmetric (100)//(430) planes. At 1590 °C, the grain boundary is observed to be defaceted, which implies that the defaceting occurs between 1570 and 1590 °C. The faceted structure observed between 1500 and 1590 °C is interpreted in terms of the step free energy of the asymmetric (100)//(430) facet orientation. The faceting-defaceting transition is schematically represented as the temperature evolution of the equilibrium grain shape.

Temperature dependence of faceting in Σ5(310)[001] grain boundary of SrTiO3

Abstract Using high-resolution electron microscopy (HREM), temperature dependence of faceting of an asymmetric Σ5 grain boundary (GB) in SrTiO 3 is observed. The bicrystal samples have been fabricated by ultra-high vacuum diffusion bonding and heat-treated between 1100 and 1600 °C. Below 1300 °C, this GB facets into symmetric (310) and asymmetric (100)//(430) GB planes. At 1300 °C, in addition to the asymmetric facet, the symmetric {210} facet appears: three different facets are thus observed at this temperature. At 1400 and 1500 °C, the asymmetric facet disappears and the two kinds of symmetric facets remain. At 1600 °C, faceting disappears and the GB becomes defaceted. The faceting/defaceting transition behavior of the GB is interpreted in terms of the Wulff plot and its corresponding equilibrium crystal shape.

Improvement of Grain-Boundary Conduction in Gadolinia-Doped Ceria by the Addition of CaO

This study examined the effect of CaO addition on grain-boundary conductivity in 10 mol % Gd 2 O 3 -doped CeO 2 containing 500 ppm SiO 2 as an impurity. The addition of CaO increased the grain-boundary conductivity ∼ 50 times without affecting the grain-interior conductivity significantly. CaO was incorporated into the CeO 2 lattice completely, which means that a scavenging reaction between a large CaO-related second phase and siliceous impurity was not the reason for the increased conductivity. After adding CaO, grain boundary structure changed from random to faceted. Configuration change in the grain-boundary segregation in relation to the boundary-structure variation is suggested as a possible reason.

Grain-Boundary Conduction in Gadolinia-Doped Ceria: The Effect of SrO Addition

This study examined the effect of adding SrO on the grain-boundary conduction of Ce0.9Gd0.1O1.95 gadolinia-doped ceria containing 500 ppm SiO2. The apparent grain-boundary resistivity at 300°C decreased drastically from 746.7 to 0.90–1.97 k cm upon doping with 1 mol % SrO, while the grain-interior resistivity increased gradually from 3.1 to 11.6 k cm as the SrO concentration was increased up to 5 mol %. Therefore, doping with 1 mol % SrO resulted in the minimum total resistivity. The electron probe X-ray microanalysis and the analysis of the lattice parameters suggest that the 140–500-fold enhancement in the grain-boundary conduction is attributed to the scavenging of the highly resistive siliceous phase by the SrO-containing phase.

Dielectric and magnetic properties in Ta-substituted BiFeO 3 ceramics

The dielectric and magnetic properties were investigated in Ta-substituted BiFeO 3 polycrystalline ceramics synthesized by a solid-state reaction. The Ta substitution decreased the grain size by two orders of magnitude compared with that of unsubstituted ceramics and increased the electrical resistivity by 6 orders of magnitude. The high resistivity and low dielectric loss allowed the dielectric constant to be determined at room temperature. The magnetic hysteresis loops were observed in the Ta-substituted BiFeO 3 , and the appearance of ferromagnetism was closely associated with the distortion of the oxygen octahedra by the Ta substitution. The coupling between the electric and magnetic dipoles was examined by determining the changes of the dielectric constant with the external magnetic field.

TEM observations of singular grain boundaries and their roughening transition in TiO2-excess BaTiO3

Abstract When BaTiO3 powder compacts with 0.2 mol% excess TiO2 are sintered in air at 1250°C, below the eutectic temperature, some grains grow abnormally to very large sizes with double twins in the 〈111〉 directions at their centers. The abnormal grains, elongated in the 〈111〉 directions, form mostly straight grain boundaries, lying on their {111} planes with the fine matrix grains. Some grain boundaries between the nearly (111) face of an abnormal grain and the matrix grains are faceted with some facet planes lying on either the (111) plane of the abnormal grain or (210) plane of the matrix grain. These flat grain boundaries, as observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), must be singular, corresponding to cusps in the polar plots of the grain boundary energy against the boundary normal. A schematic analysis with the capillarity vectors shows that the straight {111} grain boundaries across the triple junctions with the matrix grains are...

Threading-dislocation blocking by stacking faults formed in an undoped GaN layer on a patterned sapphire substrate

The microstructure of an undoped GaN grown on a hemisphere-patterned sapphire substrate is characterized by transmission electron microscopy. Interestingly, basal-plane stacking faults are formed in GaN at the height of hemispheres, leading to a substantial reduction in threading-dislocation density. It is believed that the formation of the stacking faults is characteristic of the lateral growth mode. This study looks at an unexplored feature of the lateral growth behavior that both so-called epitaxial lateral overgrowth mechanism and stacking faults function to reduce the dislocation density for the GaN growth on hemisphere-patterned sapphire substrates.

Transmission electron microscopy observations of Cu-induced directional crystallization of amorphous silicon

By annealing at 500°C for 1h with an electric field of 180V∕cm, a Cu-deposited a-Si/glass sample undergoes a partial crystallization of amorphous Si, whose microstructure is characterized by conventional and high-resolution transmission electron microscopy. The Si crystallites grow in the ⟨111⟩ and ⟨211⟩ directions with their {011} planes parallel to the amorphous Si film surface, assuming a needlelike shape. Copper silicides are not observed at the leading edges of the crystalline Si needles. The growth directions of ⟨111⟩ and the {011} surface plane orientations are accepted to arise from elastic anisotropy of c-Si and can be explained by considering the strain energy under the uniaxial stress state and the plane stress, equibiaxial strain state. Crystallization behavior of a-Si in the Ni∕a-Si layer is also briefly discussed.