Microstructures of rapidly-heated/quenched and transformed Nb/sub 3/Al multifilamentary superconducting wires

Abstract

Microstructures of rapidly-heated/quenched and transformed Nb& multifilamentary wires were studied by using transmission electron microscopy. Nb/AI composite wires are fabricated by a jelly-roll process. The Nb/AI composite filaments changed into Nb-AI supersaturated bcc solid solution with the rapidly-heating/quenching. The Nb-AI bcc phases consists of many crystal grains with diameters of 2-4 pm, surrounded with large-angle grain boundaries. Some spherical voids, about 0.1 micron in diameter, were also observed at the intra-and intergrains. All grain boundaries of the Nb-AI bcc phases are simple flat planes. Then, the Nb-AI bcc phases were transformed into A15 phases (grain size: 0.5-2.0 pm in diameter) with additional annealing. Secondary phases were not observed in the A15 filaments. Grain boundaries of the A15 phases show zigzag shape unlike those of the Nb-AI bcc phases, and every grain of A15 phases is an aggregation of sub-grains of 80-150 nm in diameter. Sub-grain boundaries are small-angle ones. Moreover, we found that many stacking faults formed in the A15 sub-grains in parallel with spaces of 10-20 nm. These numerous plane defects seem to be the main pinning centers in the rapidlyheated/quenched and transformed NbJAl wires. Index

Microchemical and microstructural comparison of high performance Nb/sub 3/Al composites

“…The as-quenched untransformed Nb(Al) in the 1.275 mm wire has a mean grain Feret diameter of 3 m (similar to the 2-4 m reported by Kikuchi et al [4] from TEM analysis. Processed to Cu-clad tape the grain size is reduced and the aspect ratio of the grains increases from 1.53 to 1.92.…”

“…Two successful modifications to the basic RHQT technique specifically address the stacking fault issue: In the double rapidly-heating/quenching, DRHQ, technique the standard RHQT process is followed but instead of a conventional low temperature ( 1000 C) transformation heat treatment a second RQH process produces the A15 phase and this is followed by a long range ordering heat treatment at 800 C for 12 hrs (7). The for these stacking fault free strands has been measured at 18.4 K, which suggests very good stoichiometry; furthermore a critical current density of 135 A/mm was obtained at 25 T [4]. A disadvantage of this technique is that the brittle A15 phase is formed during high temperature processing.…”

“…P. J. Lee [4] to stacking faults formed in the A15 phase [5], [6]. Two successful modifications to the basic RHQT technique specifically address the stacking fault issue: In the double rapidly-heating/quenching, DRHQ, technique the standard RHQT process is followed but instead of a conventional low temperature ( 1000 C) transformation heat treatment a second RQH process produces the A15 phase and this is followed by a long range ordering heat treatment at 800 C for 12 hrs (7).…”

“…A further anneal at 800 C then provides long range ordering to the A15 phase. In this paper we step back in magnification from the excellent high resolution TEM work of Kikuchi et al [4] and use a Field Emission Scanning Electron Microscope, FESEM to look at macroscopic variations in the microchemistry and microstructure of Nb Al wires and tapes in the RQ untransformed, RHQT and TRUQ conditions. We have previously shown that the FESEM can be used to probe sub-micron variations in Nb Sn chemistry by backscattered electron imaging, BEI, [9] and can be used to make accurate measurements of Nb Sn grain size using fractured cross-sections for strands with average grain sizes as small as 57 nm in diameter [10].…”

Observation of A15 phase transformation in RHQ-Nb3Al wire by neutron diffraction at high-temperature

“…It has been reported that when a RHQ treatment is applied to Nb and Al raw materials, a solid solution of NbAl y with a bcc structure is formed [6][7][8][9][10][11][12]. Upon further heat treatment, this undergoes an A15 phase transformation to form Nb 3 Al.…”