Internal stresses in cold-deformed Cu–Ag and Cu–Nb wires

Abstract

The co-deformation of Cu–Ag or Cu–Nb composite wires used for high-field magnets has a number of important microstructural consequences, including the production of very-fine-scale structures, the development of very high internal surface-area-to-volume ratios during the drawing, and the storage of defects at interphase interfaces. In addition, the fabrication and co-deformation of the Cu and Ag or Nb, which differ in crystal structure, thermal expansion, elastic modulus and lattice parameter, lead to the development of short-wavelength internal stresses in both composites. In this paper, these internal stresses are characterized by neutron diffraction and transmission electron microscopy as a function of the imposed drawing strain. The internal stresses lead to important changes in the elastic–plastic response, which is related to both magnet design and service life. The second derivative ∂² σ/∂² ε of the stresses with respect to strain is used to describe the low-strain anelasticity of the composites. The internal stresses in Cu–Nb are higher than in Cu–Ag and, consequently, the absolute values of (∂² σ/∂² ε)_Cu–Nb are higher than those of (∂² σ/∂² ε)_Cu–Ag at low strains.

Acknowledgements

The US Department of Energy and National Science Foundation sponsored this research. The work has benefited from the use of the IPNS at Argonne National Laboratory and of the LANSCE at Los Alamos National Laboratory. IPNS and LANSCE are funded by the US Department of Energy, BES-Materials Science, under contract W-31-109-ENG-38, and by the US Department of Energy under contract W-7405-ENG-36 respectively. The authors wish to thank G. Saada, J. P. Hirth and J. L. Smith for discussions and contributions.