Papers by Author: Sadahiro Tsurekawa

Abstract: An AC magnetic field (0.5Tesla) is applied with the field direction perpendicular to the rolling direction during annealing of a 76% cold-rolled IF steel sheet. Microstructure and texture evolution in the as-annealed specimens were determined using SEM based OIM technique. It is found that the recrystallization is noticeably retarded by AC magnetic field annealing. At the early stage of recrystallization (annealing at 650°C for 30min), the development of (111) <123> orientations was favored by the AC magnetic field. With progress of recrystallization (annealing at 700°C and 750°C for 30min), the applied AC magnetic field suppressed the development of γ-fiber recrystallization textures to some extent.

Abstract: Crystallization kinetics and texture evolution in iron-based amorphous alloys (Fe78Si9B13, Fe73.5Si13.5B9Nb3Cu1) under a magnetic field have been studied. We have found that the application of a magnetic field during crystallization at a temperature ranging the Curie point of the amorphous phase and that of the crystalline bcc-Fe(Si) phase can produce a {110} texture in the alloys. X-ray diffraction with the Schulz method revealed that {110} oriented grains were preferentially nucleated due to the magnetic field. The studies of crystallization kinetics found that the nucleation rate of the α-Fe(Si) phase in amorphous Fe73.5Si13.5B9Nb3Cu1 was enhanced approximately three times as high with the 6T magnetic field than without field. The volume fraction of crystalline α-Fe(Si) phase produced was increased by the magnetic field.

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: In last two decades it has been extensively studied whether the grain boundary engineering can be effectively applied to controlling intergranular fracture and brittleness of different kinds of brittle materials. Grain boundary engineering has been well established. A new processing method based on magnetic field application has reached a new stage of grain boundary engineering.

Abstract: Grain boundary engineering through the control of grain boundary character distribution (GBCD) has been extensively employed as a powerful tool for achieving enhanced properties and for development of high performance both structural and functional polycrystalline materials. Many efforts were made firstly to increase the frequency of low-energy CSL boundaries of polycrystalline materials in grain boundary engineering. However, the connectivity of grain boundaries can be an important microstructural parameter governing bulk properties of polycrystalline materials as well as the GBCD. In the present work, the connectivity of random grain boundaries was quantitatively evaluated using both the triple junction distribution and random boundary cluster length on the basis of SEM-EBSD/OIM observations, and then these evaluated parameters were linked to intergranular corrosion of SUS304 stainless steel. We have found that the length of the maximum random boundary cluster drastically decrease with increasing CSL boundaries in the fraction ranging 60 – 80% CSL boundaries, which leads to percolation threshold occurring at approximately 70±5% CSL boundary fraction (at 30±5% random boundary fraction). The experimentally observed percolation threshold is much higher than theoretically obtained one based on randomly assembled network (at 35% resistant bonds for a 2D hexagonal lattice). In addition, the fraction of resistant triple junctions is found to increase with increasing the the CSL boundary fraction. An increase in the frequency of resistant triple junctions can enhance intergranular corrosion resistance of polycrystalline austenitic stainless steel even if the GBCD is the same.

Abstract: This paper gives an overview of our recent works on the effect of magnetic annealing, i.e. annealing in a magnetic field, on the evolution of texture and grain boundary microstructure in ultra-fine grained and nanocrystalline magnetic materials differently produced; rapidly solidified Fe-6.5mass%Si ribbons, electrodeposited nanocrystalline nickel, and nanocrystalline Fe78Si9B13 alloy ribbon crystallized from the amorphous state. It was found that the effect of magnetic annealing was powerful and useful for controlling grain growth resulting in the evolution of different types of texture and grain boundary microstructure, depending on the condition of magnetic annealing. In particular, the magnetic crystallization of amorphous Fe78Si9B13 alloy was found to be powerful for producing a nanocrystalline material with a sharp texture and a special grain boundary microstructure.

Abstract: This paper discusses micropstructural aspects of brittleness fracture of polycrystalline materials caused by intergranular fracture. Structure-dependent intergranular brittle fracture in bicrystals and polycrystals are discussed and predicted theoretically. Experimental evidence for the structure-dependent intergranular fracture is shown and some general features are discussed to demonstrate the relationship between grain boundary structure/character, grain boundary energy and intergranular fracture strength. Theoretical prediction of the fracture toughness based on the strongest-link theory is introduced for polycrystals with different grain boundary microstructures, primarily defined by the grain boundary character distribution, grain boundary connectivity. Finally recent achievements of successful control of intergranular brittleness by grain boundary engineering based on the strongest-link theory are introduced for different materials.

Abstract: The stability of the plane matching boundary is discussed in connection with the deviation from the exact plane matching orientation and the atomic density per unit area of the lattice plane. The statistic calculation of the plane matching boundary distribution in the different types and sharpness of texture revealed that the coverage by plane matching boundaries (>80%) is much higher than the coverage by CSL boundaries (~60%). The sharpness of texture strongly affects the distribution of plane matching boundaries.

Abstract: The effect of grain boundary character distribution (GBCD) on intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels in high temperature water was verified experimentally. GBCD control using the strain annealing method increased the fraction of low- S coincidence site lattice (CSL) boundaries and the segmentalized network of random grain boundaries in austenitic stainless steels. The fractions of low- S CSL boundaries of GBCD controlled steels were 75–85%, while those of uncontrolled steels were 60–70%. Creviced bent beam tests were conducted at 561 K in pure water containing 8 ppm dissolved oxygen for stress corrosion cracking (SCC) evaluation. The tests revealed that GBCD control suppressed IGSCC initiation or propagation and that cracks were predominantly propagated along random grain boundaries. It is considered that induced lower- S CSL boundaries result in high resistance to IGSCC.