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The influence of Nb3Sn strand geometry on filament breakage under bend strain as revealed by metallography

Matthew C Jewell, Peter J Lee and David C Larbalestier

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The non-Cu critical current density of Nb3Sn strands has been pushed towards 3000 A mm−2 (12 T, 4.2 K) by increasing the Sn content and reducing the inter-filamentary Cu. We compare the susceptibility to A15 filament breakage (under 0.5% bend strain) of the new high-Jc internal Sn conductor geometries with both high-Jc powder-in-tube (PIT) and low hysteresis loss distributed filament ITER designs. In all but the PIT designs, there was significant filament breakage on the tensile side of the strand cross-section with little if any evidence for cracking on the compressive side. Where there is significant inter-filamentary Cu remaining after reaction the highest frequency of A15 filament breakage is observed at the edges of the filament packs. This suggests that the breakage is most likely to occur where filaments receive less mechanical support from the filament-Cu matrix. In very high Jc strands, where individual Nb filaments coalesce into large A15 tubes during reaction, breakage can occur across the entire sub-element. In the PIT design composite, filament breakage did not occur at 0.5% bend strain. At 0.6% bend strain the PIT filaments cracked in both the tensile and compressive regions.


PACS

74.25.Sv Critical currents

81.70.Bt Mechanical testing, impact tests, static and dynamic loads

74.25.Ld Mechanical and acoustical properties, elasticity, and ultrasonic attenuation

Subjects

Superconductivity

Condensed matter: structural, mechanical & thermal

Dates

Issue 9 (September 2003)

Received 8 April 2003, in final form 24 June 2003

Published 4 August 2003



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