Materials Science Forum Vols. 558-559

Abstract: When a polycrystal is in chemical equilibrium, the microstructure evolves as a result of grain growth under the capillary driving force arising from the interface curvature. As the growth rate of an individual grain is the product of the interface mobility and the driving force, the growth of the grain can be controlled by changing these two parameters. According to crystal growth theories, the growth of a crystal with a rough interface is governed by diffusion and its interface mobility is constant. In-contrast, the growth of a crystal with faceted interfaces is governed by the interface reaction and diffusion for driving forces below and above a critical value, respectively. As the growth rate is nonlinear for the regime of interface reaction control, the grain growth is nonstationary with annealing time. Calculations reveal that the types of nonstationary growth behavior including pseudo-normal, abnormal, and stationary are governed by the relative value of the maximum driving force, gmax, to the critical driving force for appreciable growth, gc. Recent experimental observations showing the effects of critical processing parameters on microstructural development also support the theoretical prediction. The principles of microstructural design are deduced in terms of the coupling effects of gmax and gc.

Abstract: The present paper presents an overview of present and future tools which can be used by the steel manufacturer in order to control the texture of the finished sheet product. The major solidstate- transformation processes (phase transformation, plastic deformation and recrystallisation) playing a role during thermo-mechanical processing will be addressed. The physical mechanisms that give rise to the appearance of specific texture components will be discussed in detail. In addition to current state-of-the-art process technology the potential of innovative processes will be described such as accumulative roll bonding (ARB). The present paper will also pay attention to the particular role of surface textures as an additional degree of freedom allowing to control the sheet texture with the potential to enhance the {001} or {110} fibre textures for magnetic applications.

Abstract: A new approach to grain boundary engineering for photovoltaic polysilicon has been attempted using a new processing method of unidirectional and rotational solidification from the melt, in order to control the grain boundary microstructure and to produce desirable bulk electrical properties. The effect of grain boundary microstructure on bulk electrical properties of polysilicon can be more precisely evaluated by introducing a new parameter “directional grain boundary density (DGBD)” in connection with basic knowledge of structure-dependent grain boundary electrical properties, the grain boundary character distribution (GBCD) and grain boundary geometrical configuration which can be experimentally determined by Orientation Imaging Microscopy (OIM). We report the usefulness of this approach to development of high performance polysilicon.

Abstract: SrTiO3 bicrystals with various types of grain boundaries were prepared by joining two single crystals at high temperature. By using the bicrystals, we examined their current-voltage characteristics across single grain boundaries from a viewpoint of point defect segregation in the vicinity of the grain boundaries. Current-voltage property in SrTiO3 bicrystals was confirmed to show a cooling rate dependency from annealing temperature, indicating that cation vacancies accumulate due to grain boundary oxidation. The theoretical results obtained by ab-initio calculation clearly showed that the formation energy of Sr vacancies is the lowest comparing with Ti and O vacancies in oxidized atomosphere. The formation of a double Schottky barrier (DSB) in n-type SrTiO3 is considered to be closely related to the accumulation of the charged Sr vacancies. Meanwhile, by using three types of low angle boundaries, the excess charges related to one grain boundary dislocation par unit length was estimated. In this study, we summarized our results obtained in our group.

Abstract: Grain growth in ZnO ceramics doped with 0.01 and 0.02 mol.% Bi2O3 and Sb2O3 in amounts appropriate for Sb2O3/Bi2O3 ratios of 0.8, 1.0 and 1.2, sintered at 1200oC for 2 and 10 hours, was investigated. Grain growth is promoted by a sufficient amount of the Bi2O3 liquid phase at the grain boundaries and also by the presence of IBs in the ZnO grains. While the doping of ZnO with such small amounts of Bi2O3 caused the exaggerated growth of some grains, the addition of Sb2O3 resulted, via the IBs-induced grain-growth mechanism, in uniform grain growth and the presence of IBs in most of the ZnO grains. The formation of the pyrochlore phase bounds the Bi2O3 and Sb2O3, which affects, depending on the Sb2O3/Bi2O3 ratio, the occurrence of the Bi2O3 liquid phase and also the amount of available Sb2O3 for the nucleation of IBs in the ZnO grains during the early stages of sintering. As a result, it influences the grain growth.

Abstract: Grain boundary character in samples of Zr701 annealed at two different temperatures has been investigated in terms of lattice misorientation. The main difference between the two samples was the extent of grain growth post-recrystallization. The textures were typical for the material. Differences between the texture-based misorientation distribution function (T-MDF) and the microstructure-based MDF (M-MDF) revealed significant preferences for certain grain boundary types, notably those with <11-20> rotation axes.

Abstract: Bicrystals of Nb-doped SrTiO3, having tilt angles of 4o~18 o with respect to [001], were prepared by joining two single crystals at 1873 K and then investigated to identify the effect of tilt angle on the grain boundary structure. The boundaries consisted of a regular array of dislocations but the positioning of cores along the boundary was found to be changed from a line to a zigzag as a tilt angle was increased up to 10o. The 14° - tilted boundary exhibited two kinds of boundary region exist at the same grain boundary; (1) the discrete cores region as observed in 4° ~ 10° - tilted boundaries and (2) the randomly oriented region as found in the 18° boundary. Thus it was observed that the structure of low-angle tilt boundary changed from the discrete dislocation structure to the randomly oriented structure as a tilt angle increases. These structural changes at the grain boundaries are considered to be related to a minimization of strain due to the high density of dislocations.

Abstract: Compression tests were performed on Fe-3%Si specimens with few grains. The deformation microstructure and microtexture were investigated by electron backscatter diffraction (EBSD) and related to the initial crystal orientation and grain boundary characteristics. Groups of microbands were found that are characterised by a periodic change in crystal orientation, shear at the grain boundary, and the formation of new grains. It is supposed that these microband groups represent an early stage of microshear band development.

Abstract: The grain boundary energy anisotropy in BCC Fe-based polycrystals is considered. The correlation between the energy in BCC random grain boundaries and the distribution of grain boundary planes in the bulk was examined with a special attention on the presence of low index (low surface energy) planes in the internal surfaces. For a BCC structure, {100} and {110} planes are known to be the lowest energy planes dominating the equilibrium crystal shapes. Experimental evidences demonstrated that these planes were predominant in the texture of surfaces controlled by surface energy [2]. Moreover, the relation between the grain boundary character distribution and the crystallographic dependence on the grain boundary energy in the bulk after annealing treatment was studied. The grain character boundary distribution (GCBD) was calculated using the crystallographic information obtained from OIM-EBSD maps from samples showing columnar grains. Preliminary results showed no particular distribution trend within the standard stereographic triangle (001-101-111).

Abstract: Cold-rolled micro-texture of polycrystalline 3%Si-Fe was investigated using high-resolution Electron BackScattering Patterns (EBSP) method. There were deformation bands near grain boundaries. The orientation relationship between the deformation bands and the surrounding deformed grains is explained by the orientation rotation around a <211> axis. The activated slip to generate these deformation bands is estimated from the <211> rotation. The S-value, which is a geometrical index of slip operation against applied stress, of this slip system was not maximum value of all, but it had a common slip plane with an adjacent grain. A hypothesis that the slip system having a common slip plane with an adjacent grain is activated was proposed from the present results.