Papers by Author: Shinji Kohara

Abstract: Liquid metal structure of Sn-Cu alloys studied by synchrotron high energy X-ray diffraction have been performed to investigate the effect of the trace elements Ni and Al. It has been demonstrated that trace Ni and Al additions are able to alter the liquid Sn-4wt%Cu structure. The refinement of the primary Cu₆Sn₅ phase after trace Al additions is proposed to be caused by the consumption of Cu atoms involved in a AlCu chemical ordering and an associated change in the undercooling for Cu₆Sn₅ nucleation.

Abstract: Solidification processes of Fe-B and Fe-C eutectic alloys have been investigated by a time-resolved synchrotron x-ray diffraction under containerless cooling conditions using a conical nozzle levitation technique. To observe relative variations of structure from the undercooled liquid to crystalline phase, we have conducted millisecond order time-resolved x-ray diffraction experiments with a two-dimensional detector. The structural variations observed during the solidification of the Fe₈₃C₁₇ alloy were identified as the phase transformation process expected from the Fe-C phase diagram. As for the Fe₈₃B₁₇ alloy, it was revealed that a metastable phase composed of Fe₂₃B₆ compound was precipitated as a primary crystalline phase from the undercooled liquid. In addition, decomposition of the metastable Fe₂₃B₆ phase showed dependence on the cooling rate of the sample. At the cooling rate of 30 K/s, the Fe₂₃B₆ phase decomposed to bcc-Fe and Fe₂B phases with decreasing temperature. On the contrary, at the cooling rate of 180 K/s, the metastable Fe₂₃B₆ phase remained in spite of an appearance of the bcc-Fe phase. By comparing the primary crystalline phase between the Fe₈₃C₁₇ and the Fe₈₃B₁₇ alloys, we suggest that the formability of the metastable Cr₂₃C₆-type compound is closely related with the glass-forming ability of Fe-metalloid binary alloys.

Abstract: With the arrival of the third generation of synchrotron sources and/or the introduction of advanced insertion devices (wigglers and undulators), the high energy (E > 50 keV) x-ray diffraction technique has become feasible, leading to new approaches in the quantitative study of the structure of disordered materials than was hither to available. Since we built the SPring-8 bending magnet beamline BL04B2 and two-axis diffractometer for disordered materials in 1999, we have studied on disordered materials from ambient to extreme condition. In this article, the high-energy x-ray diffraction beamline BL04B2 of SPring-8 and recent developments of ancillary equipment (automatic sample changer, conventional high-temperature furnace, aerodynamic levitation furnace) are introduced. Furthermore the structural analysis on the basis of diffraction data with the aid of computer simulations, which we performed in the last 10 years is reviewed.

Abstract: It is well known that multi-component alloys form bulk metallic glasses (BMGs) from the supercooled liquid state without rapid quenching. However, the mechanism of phase selection between crystal and glass states has not been fully clarified. To obtain an insight into the glass-forming processes, we carried out in-situ observation on the solidification of Zr-based BMG-forming alloys from its supercooled liquids by time-resolved X-ray diffraction combined with the conical nozzle levitation (CNL) technique to achieve a containerless melting. For Zr-based alloys, we succeeded in detecting the X-ray diffraction patterns during glass formation from the supercooled liquid state as well as the crystallization from the liquid state. Furthermore we performed the precise structure analysis of supercooled state of Zr-based binary liquids. Based on the liquid structure and in-situ observation results, we discussed about the phase selection mechanism between crystal and glass states.

Abstract: In order to obtain an insight into the high glass-forming ability of bulk metallic glasses, we have analyzed liquid structures of the Zr-Cu and the Zr-Ni binary alloys with different compositions. High-energy (E = 113 keV) x-ray diffraction experiments were carried out for the liquid alloys levitated by a conical nozzle levitation (CNL) technique. While a peculiar shoulder on the second peak was observed in the structure factors of the Zr-Cu liquid alloys, those of the Zr70Ni30 and the Zr50Ni50 liquids exhibit an asymmetric shape of the second peak. In addition, it was found that the effect of concentration variation in the liquid Zr-Ni alloys was significantly different from that of the liquid Zr-Cu alloys. The liquid structure analyses using the reverse Monte Carlo (RMC) simulation have clarified that a degree of the short-range correlation between the constituents in the liquids affects substantially the glass-forming ability of the binary Zr alloys.

Abstract: The atomic scale structure of amorphous Si-C-O ceramics fibers produced from the pyrolysis of a polycarbosilane precursor has been investigated by X-ray diffraction using high-energy synchrotron radiation at SPring-8. First peak in the total correlation function T(r) of the amorphous and the heat-treated fibers is analyzed to consist of two contributions: Si-C (1.89 Å) and Si-O (1.61 Å) bonds. The coordination number of C and/or O around Si is about four. This suggests that the Si-C-O fibers basically have a network structure that consists of two tetrahedral units: SiC4 and SiO4. The local chemical and structural orders vary continuously in the materials from the disordered network structure of SiC4 and SiO4 tetrahedra (mixture of amorphous SiC and SiO2) to nanocrystals of SiC and SiO2, through the ternary Si-C-O solid solution which is believed to have an intermediate structure between the amorphous and crystalline states.

Abstract: Two topics are described for structure analyses of glass and liquid using a combination of conical nozzle levitation (CNL) technique and diffraction experiments. The structure of high-purity bulk forsterite (Mg2SiO4) glass synthesized by a CNL technique has been determined by a combination of high-energy x-ray, neutron diffraction, and reverse Monte Carlo (RMC) modeling technique. The 3-dimensional atomic configuration derived from RMC modeling revealed that unusual network structure. In order to study structures of high-temperature and undercooled liquids, a CNL system has been developed and integrated with the two-axis diffractometer for glass, liquid, and amorphous materials at SPring-8, which is one of the third-generation synchrotron source. High-energy x-ray diffraction experiments were performed to obtain reliable diffraction data for the liquid phase of metallic glass-forming Zr-Cu binary alloys.