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

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.