~ Pergamon Acta metall, mater. Vol.43, No. 4, pp. 1467-1476,1995 Copyright © 1995ElsevierScience Ltd 0956-7151(94)00378-5 Printed in Great Britain. All rights reserved 0956-7151/95$9.50+ 0.00 ON THE CORRELATION OF MICROSTRUCTURE AND ELECTROMAGNETIC PROPERTIES OF HEAVILY COLD WORKED Cu-20 wt% Nb WIRES F. HERINGHAUSt, D. RAABE~ and G. GOTTSTEIN Institut ffir Metallkunde und Metallphysik, Kopernikusstrasse 14, Rheinisch-Westf'alische Technische Hochschule Aachen, Germany (Received 15 July 1994) A~tract--Fibre or ribbon reinforced in situ metal matrix composites (MMCs) consisting of Cu and 20 wt% Nb were produced by large strain wire drawing. The microstructure of the composites was investigated by means of optical and electron microscopy. The normal and superconducting properties of the MMC wires in the presence of external magnetic fields were examined and compared to the electromagnetic properties of pure Cu wires. The findings are discussed on the basis of the microstructural changes during deformation. The current results substantiate that the amount of internal boundaries and the filament spacing have considerable influence on the normal and superconducting properties of Cu-20%Nb. Zus,~amenfassung--Faserverst/irkte in situ Verbundwerkstoffe mit metallischer Matrix (MMCs) aus Cu und 20 gew.% Nb wurden durch hohe Drahtverformung hergestellt. Die Mikrostruktur der Verbund- werkstoffproben wurde mit Hilfe licht- und elektronenoptischer Mikroskopie untersucht. Die normal- und supraleitenden Eigenschaften der MMC-Dr/ihte wurden unter dem Einflufi/iuBerer magnetischer Felder gemessen und mit den elektromagnetischen Eigenschaften reiner Cu-Dr/ihte verglichen. Die ermittelten Daten werden auf der Basis der mikrostrukturellen Entwicklung im Verlauf der Verformung diskutiert. Die vorliegenden Ergebnissebelegen, dab der Anteil an inneren Grenzfl/ichen und der Faserabstand einen betr~chtlichen EinfluB auf die normal- und supraleitenden Eigenschaften yon Cu-20%Nb haben. 1. INTRODUCTION Copper and Niobium have negligible mutual solubil- ity in the solid state [1, 2]. Ribbon or fibre reinforced in situ processed metal matrix composites (MMCs) can hence be manufactured by large degrees of deformation, e.g. by wire drawing or rolling of a cast ingot. Competitive binary systems of Cu with Ta, Cr, Mo or V show a similar thermodynamical behaviour, but exhibit considerable shortcomings as compared to Cu-Nb. The density of Ta, Cr, Mo and V strongly deviate from that of Cu so that gravitational segre- gation during solidification and the increase of the specific density of the final alloy deteriorate its poten- tial for technological applications. Moreover, the melting temperatures of Ta and Mo are much higher than that of Nb which complicates the melting process. Cu and Nb reveal almost equal densities (Pcu = 8890kg/m 3, PNb = 8580kg/m3). Therefore gravitational segregation does not occur, practically. Increased Nb content leads to a decrease of ductility, tPresent address: National High Magnetic Field Labora- tory, Tallahassee, Florida, U.S.A. ~To whom all correspondence should be addressed. i.e. of elongation to fracture [3], and to a degradation of electrical conductivity [4]. Thus, an alloy contain- ing 20 wt% Nb appears to combine optimum electri- cal and mechanical properties. Cu-Nb composites have been under intensive investigation for the past 15 yr mainly for the following two reasons. (1) The tensile strength of the deformed material is very high, in particular much greater than expected from the rule of mixtures [5-7]. Several models have been proposed to explain the observed strength anomaly on the basis of microstructural mechanisms. The barrier model by Spitzig and coworkers [6] attributes the strength to the difficulty of propagating plastic flow through the f.c.c.-b.c.c, interfaces (f.c.c=face centred cubic, b.c.c. =body centred cubic). Funkenbusch and Courtney interpret the strength in terms of geometrically necessary dislo- cations owing to the incompatibility of plastic defor- mation of the b.c.c, and f.c.c, phase [7]. In fact, both models succeed in describing the observed increase of strength assuming reasonable fitting parameters. In a recent approach, Raabe and Hangen [8] have suggested a physical model which accounts for the observed dislocation structures and for the crystallo- graphic textures of both phases. In such an approach the high tensile strength can be described nearly without using fitting parameters. 1467