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Evidence for highly localized damage in internal tin and powder-in-tube Nb3Sn strands rolled before reaction obtained from coupled magneto-optical imaging and confocal laser scanning microscopy

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A A Polyanskii1, P J Lee1, M C Jewell1, E Barzi2, D Turrioni2, A V Zlobin2 and D C Larbalestier1

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Nb3Sn strands for high-current, high-field magnets must be cabled before reaction while the conductor is still composed of ductile components. Even though still in the ductile, deformable state, significant damage can occur in this step, which expresses itself by inhomogeneous A15 formation, Sn leakage or even worse effects during later reaction. In this study, we simulate cabling damage by rolling recent high performance powder-in-tube (PIT) and internal tin (IT) strands in controlled increments, applying standard Nb3Sn reaction heat treatments, and then examining the local changes using magneto-optical imaging (MOI), scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). These combined characterizations allow any local damage to the filament architecture to be made clear. MOI directly reveals the local variation of superconductivity while CLSM is extremely sensitive in revealing Sn leakage beyond the diffusion barrier into the stabilizing Cu. These techniques reveal a markedly different response to deformation by the PIT and IT strands. The study demonstrates that these tools can provide a local, thorough, and detailed view of how strands degrade and thus complement more complex extracted strand studies.


 
Corrections to figures 5, 7, 8, 12 and 13 were made to this article on 10 August 2009. The corrected electronic version is identical to the print version.
PACS

84.71.Fk Superconducting cables

74.25.Sv Critical currents

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

62.20.F- Deformation and plasticity

81.40.Lm Deformation, plasticity, and creep

74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)

Subjects

Superconductivity

Condensed matter: structural, mechanical & thermal

Dates

Issue 9 (September 2009)

Received 1 June 2009, in final form 14 July 2009

Published 5 August 2009



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