Papers by Author: Kozo Osamura

Abstract: The strain dependence of the critical current, Ic, of Bi2223/Ag/Ag-alloy composite superconducting tapes has been studied both experimentally and analytically under bending deformation for two types of tape used in the VAMAS bending round-robin program (classified as VAM1 and 3). Our former analysis showed that the experimentally obtained Ic values were between the calculated ones based on a damage-free initial state and a case where delamination occupied the full width of the tape mid-plane. The experimentally obtained Ic values were explained by the delamination occupying partial width of the tape mid-plane. However, the microscopic observation indicated that the delamination location in the thickness direction was not limited to the mid-plane. In the present study, the analysis was modified to incorporate the movement of the delamination location in the thickness direction. The calculated Ic values with delamination increased when the delamination location moved to compressive side of the tape, and decreased when that moved to tensile side of the tape. Finally, the experimental Ic values can be understood by the distribution of delamination in both width and thickness direction.

Abstract: Influence of applied tensile and bending strains on the local and overall transport critical current Ic and n-value at 77 K of multifilamentary Bi2223-composite superconductor was studied, where the n-value refers to the sharpness of the transition from super- to normal conducting state. Under both tensile and bending strains, the damage such as transverse and longitudinal cracking of the Bi2223 filaments and interfacial debonding between the filament and silver progressed. The extent of damage and accordingly the critical current was different among the local portions. The relation of the local current and n-value to overall ones was analyzed with a voltage summation model, with which the experimental result was described well. Further analysis revealed that the distribution of local critical current could be described by the Weibull distribution function and n-value could be expressed as a function of critical current. Based on these results, a Monte Carlo simulation was carried out to predict the overall critical current from the distribution of local critical current, with which the experimental results could be described.

Abstract: The Mesoalite alloy is formed using rapidly solidified powder metallurgy (RS-P/M) by hot extruding the RS powder produced by the atomization method. Meso20 is a Mesoalite alloy with a chemical composition of Al-9.5Zn3Mg-1.5Cu-4Mn-0.04Ag (mass%). Meso20 contains fine grains and precipitated intermetallic Mn compounds, and has a tensile strength of 910 MPa. During hot extrusion, dynamic recrystallization occurs and the fine grains develop. During heat treatment of Meso20, rod-like and granular Mn intermetallic compounds precipitate. The rod-like compounds are about 1 Ìm in length and the granular compounds are about 1 Ìm in diameter. X-ray diffraction measurement, transmission electron microscopy and energy dispersive X-ray (TEM/EDX) analysis and Rietveld analysis revealed the chemical composition of the granular and rod-like Mn intermetallic precipitates to be 86.5Al-10.9Mn-0.4Cu-0.9Zn-1.3Mg and 80.5Al - 10.3Mn-4.2Cu-2.5Zn-2.5Mg (mass%), respectively. The granular and rod-like compounds were identified as the Al6Mn and Q phases, respectively, with both belonging to the space group Cmcm. The lattice constants of Al6Mn were a=0.754 nm, b=0.648 nm c=0.855 nm and those of the Q phase were a=0.765 nm b=2.34 nm c=1.25 nm. Meso10, with a chemical composition of Al-9.5Zn-3Mg-1.5Cu-0.04Ag (mass%), contains no Mn and does not have fine grains, but rather coarse fibrous grains elongated along the extrusion direction. Thus the Mn intermetallic precipitates in Meso20 clearly affect the formation of fine grains. Microstructure development was studied during hot extrusion by observation using high resolution Electron Back Scattering Pattern method. Fine grains were found to develop in areas, which were relatively abundant in granular Mn intermetallic precipitates.

Abstract: The effect of extrusion rate and ratio on the Al3Zr induced dynamic recrystallization (DRX) that occurs during hot extrusion of RS-P/M Al-Zn-Mg-Cu-Zr alloys was investigated. An increase in the logarithm of extrusion rate promoted DRX and lead to a monotonic increase in the number of fine grains. Although DRX was also promoted and the grain size reduced by an increase in extrusion ratio from 10 to 20, the DRX behavior hardly changed, even when the extrusion ratio exceeded 20. However, with increasing extrusion ratio, the width of fibrous grain, i.e., the unrecrystallized region, decreased and the tensile strength increased to 879 MPa. When the extrusion rate and ratio exceeded 54 mm/min and 20, respectively, a marked grain coarsening occurred upon solution treatment, and the tensile strength tended to decrease, because of the high dislocation density induced by hot extrusion. By annealing at 563 K before solution treatment, it was possible to prevent grain coarsening, and thus prevent the strength decrease.

Abstract: In order to improve the high-temperature strength of an Al-Cu-Mg alloy, Mn was added at supersaturation to form a high-density dispersion of an intermetallic phase. In the P/M Al-3.6Mn- 6.4Cu-3.6Zn-1.7Mg alloy (mass%), rod-like Al-Mn-Cu-Zn quaternary intermetallic phases (Q phase) several hundred nanometers in length were dispersed in the matrix. The chemical composition of the Q phase was determined by TEM/EDX to be 78.8Al-12Mn-8Cu-1.2Zn (at%). The crystal system, space group, and lattice parameters of the unit cell were identified to be orthorhombic, Cmcm and a = 0.76, b = 2.11, c = 1.25 nm, respectively, by Rietveld analysis. Since the matrix of the alloy obtained was of the Al-Cu-Mg-(Zn) system, age-hardening occurred by formation of a GPB zone at room temperature and 448 K. At the peak level of age-hardening at room temperature, the tensile strength at room temperature was 704 MPa, and the elongations were 8.0%. The high temperature strengths at 523 and 573 K were 319 and 141 MPa, respectively, and the elongations were 17 and 34%, respectively.

Abstract: The XN-05C/2017, XN-05C/MESO-10 and BGC152/MESO-10 aluminum composites reinforced by low young’s modulus amorphous carbon fiber and particle, respectively, were fabricated by a PM route followed by hot extrusion and rolling in this study. The mechanical properties of the prepared composites were investigated subsequently. The XN-05C/2017 and XN-05C/MESO-10 were found to exhibit low elastic modulus of 50~60GPa within the fiber volume fraction of 0.10~0.15, while the BGC152/MESO-10 shows a data of 50GPa with a particle volume fraction of 0.10. The tensile strength of XN-05C/MESO-10 was evaluated to be 400~600MPa in the case of Vf=0.10~0.15, and become to decease with increasing fiber volume fraction. Damping application would be a potential consideration for the amorphous carbon fiber reinforced aluminum composites.

Abstract: Bending behavior of Bi2223/Ag superconducting composite tapes, and the influence of the bending damages on critical superconducting current Ic and n-value that is a measure for the sharpness of the transition from super- to normal conduction, were studied. Following damages, responsible to the reduction in critical current at high bending strains, were observed. (i) Transverse and (ii) longitudinal crackings of the filaments, (iii) interfacial debonding between filaments and silver, and (iv) buckling in compression side. The damage of the filaments arose first locally, resulting in a reduction of the Ic and n-value in the corresponding local portion. The overall Ic and n-values were determined by such a local portion. With increasing strain, the damage of the other portions arose successively, resulting in loss of superconductivity in all portions.