Akihiko Nagata

Phase Compatibility and Superconductivity of Y-Ba-Cu-O Compounds

Superconducting properties of Y-Ba-Cu-O compounds were investigated in relation to precipitation of second phases. Contour lines of Jc, Tc and diamagnetic signal intensities were presented in the YO1.5-BaO-CuO system. These superconducting properties decreased steeply with BaCuO2 precipitation, and the maximum Jc was obtained in the yttrium-rich composition of Y1.2Ba2Cu3O7-y. It is considered, therefore, that the BaCuO2 precipitating around the YBa2Cu3O7-y grains interrupt the superconducting current pass at the grain boundary. Yttrium-rich composition, which inhibits BaCuO2 precipitation, is effective for the preparation of the high-Jc sample.

Superconductivity in dilute Cu-Nb-Sn alloys containing Nb3Sn precipitates

AbstractThe dilute Cu-Nb-Sn alloys containing small amounts of Nb and Sn less than 1 at % exhibited superconductivity after quenching from the liquid state and ageing. The best superconducting properties (

Microstructure of 2 mass% Si Dual-Phase Steels Carburized at Elevated Temperature.

T_{{\text{c}}_2 }

Strain dependence of critical current associated with constitution of practical Nb3Sn superconducting wires.

andJc=130 A cm−2) in a Cu-0.30 at % Nb 0.15 at % Sn alloy were obtained when the sample was aged at 550° C for 384 h. This sample exhibited a structure of fine Nb3Sn precipitates of 200 to 500Å diameter distributed homogeneously in the Cu matrix, and therefore it was concluded that superconductivity in these alloys resulted from the proximity effect of Nb3Sn particles. In spite of the similar structure obtained by ageing at 800° C, the Cu-Nb-Sn alloys showed inferior superconducting properties compared to the Cu-0.4 at % Nb alloy and this would be explained qualitatively by the difference in the mean free path in the two alloys.

Superconductivity and Microstructural Observation of Er0.5Y0.5Ba2Cu3O7-y

The superconducting properties (Tc and Jc) were investigated by the resistivity measurement, and the structure was observed by a transmission electron microscopy in the dilute copper alloys containing small amounts of V3Ga particles prepared by quenching from the liquid state and aging. No superconductivity could be observed at 4.2 K in as‐quenched and/or in overaged samples. The highest Tc2 (=12.5 K), Tc1 (=9.6 K), and Jc (=525 A/cm2) were obtained by aging at 700 °C for 96 h and corresponded with the structure of fine V3Ga precipitates dispersed homogeneously in the copper matrix. Therefore, it is concluded that superconductivity in these alloys results from the superconducting path due to the proximity effect of the superconducting V3Ga particles.

Transmission electron microscopic observations of a commercial Nb-Ti superconducting alloy

The carburizing time is expected to be short at high temperatures, where grains of the dual-phase steel are fine and hardly grow. If it can be used for the elevated temperature case hardening steel, the processing time might become short by reducing carburizing time and by skipping the first step quenching for grain refinement of the un-carburized core. In the present study, the applicability of the dual-phase steel for the elevated temperature case hardening steel has been investigated.The 2.0 mass% Si-(0.01-0.08) mass% C-(0.1 mass% V, 0.05 mass% Nb and 0.05 mass% Ti) steel were carburized at 1323 K for 10.8 ks, quenched from various temperatures and then tempered at 423 K. The microstructural observation, hardness measurement and tensile test of the core were carried out. When the microstructure was controlled to be dual-phase with α+γ at the carburizing temperature, the grains were fine and no grain growth was observed during the carburizing process. Therefore, the first step quenching for grain refinement might be skipped. The core showed the excellent strength-ductility combination compared with the conventional case hardening steel. The distribution of hardness in the carburized layer showed a gentle gradient due to the elevated temperature carburizing. The carburizing time was reduced to be 1/ 3 compared with the conventional carburizing. The grains in the carburized layer were also refined by additions of V, Nb and Ti.

Microstructure of rapidly and uni-directionally solidified aluminum alloys

In the previous paper, we reported the low alloyed dual-phase steel could be used for elevated temperature case hardening steels without the first step quenching from microstructure observations. In the present work, mechanical properties and optimum quenching temperature of the dual-phase steels were investigated for practical case hardening applications.The microstructural and mechanical properties of the 0.05 mass% C-2.0 mass % Si-(0.05 mass% Nb and 0.05 mass% Ti) steels carburized at 1323K for 10.8ks, quenched from 1123K-1223K and then tempered at 423K were studied. The maximum tensile and fatigue strength of the 2.0 mass% Si steel and the 2.0 mass% Si-(0.05 mass% Nb and 0.05 mass% Ti) steel were obtained after quenching from 1123K and 1223K, respectively. These properties were much higher than those of the conventional case hardening steel obtained by two step quenching. The wear resistance was not affected by quenching temperature and was much superior in the regions of higher and lower sliding speed compared with the carbon tool steel (SK 3). Skipping the first step quenching and carburizing at elevated temperatures make the processing time shorter and energy consumption to fab-ricate materials smaller. Thus, the dual-phase steel is considered to have a potential for practical use for the elevated temperature case hardening steel.

On the Decomposition of a Highly Supersaturated Solid Solution of Al-6 wt% Mn

An investigation was made on the strain dependence of critical cuurent Ic for three kinds of practical Nb3Sn superconducting wires which are in the course of development, with an emphasis on the correlation with wire constitution.In the internal tin diffusion processed wires with a single tin core at the center, the reversible strain limit, eirrev was small and the strain sensitivity of Ic was high. This is ascribed to the strain concentration induced by Kirkendall voids observed in the center of fracture surface.Internal reinforcement using SUS 405 steel increased both em, the strain corresponding to peak Ic, and the strain sensitivity, and decreased the eirrev in addition to a marked increase of flow stress. In the light of both the serration in the stress-strain relation and fracture surface morphology, the degradation of strain characteristics are attributable to the strain concentration in Nb3Sn layer associated with localized deformation of the steel at low temperatures.In the external tin diffusion in situ processed wires, both Nb3Sn layer formed in the periphery and the unreacted core in the center of the wire controlled the strain characteristics. The smaller the core size was, the larger the critical current density and the smaller the em or eirrev. Inhomogeneity of tin diffusion in the cross section as well as in the longitudinal direction resulted in deviation of the core and tin balling, respectively. They deteriorated the strain characteristics of the wire significantly.Improvements of the strain characteristics in these wires through proper constitution and heat treatment were briefly discussed.