Ultra high strength nanofilamentary conductors: the way to reach extreme properties

To enhance the intensity of non-destructive magnetic fields with long pulse duration, reinforced conductors are needed with extremely high mechanical strength and good electrical conductivity. The ideal conductors for this ap plication should have an action integral close to that of pure copper. An e laboration process based on cold drawing and restacking has been developed at LNCMP for this purpose. The best results have been obtained with Cu/Nb n anocomposite wires with a section of 3 x 10(-2) mm(2) composed of a copper matrix embedding 9 x 10(6) continuous parallel niobium whiskers with a diam eter of 40 nm. The ultimate tensile strength is 1950 MPa at 77 K. The funda mental properties linked to the effect of nanometer size have been investig ated. Nevertheless, because of their small section these conductors cannot be practically used in the winding of our magnets. Therefore, we are elabor ating a new generation of optimized Cu/Nb nanostructured wires exhibiting u ltra high strength in a section of 2mm(2). The latest developments are pres ented. Concurrently, we are developing Cu/Ta multifilamentary conductors. S ince the shear modulus of tantalum is greater than that of Nb (mu (Ta) appr oximate to 2 mu (Nb)), the Cu/Ta UTS should be enhanced. However, drawing o f Cu/Ta billets leads to the formation of a macroscopic roughness at the Cu /Ta interface and to the fracture of Ta. This phenomenon is interpreted in terms of stress-driven rearrangement (Grinfeld instabilities). We have inve stigated some solutions to prevent its formation. (C) 2001 Elsevier Science B.V. All rights reserved.