Thermal stability of electrical and mechanical properties of cryo-drawn Cu and CuZr wires

Abstract

Low temperature deformation results in a considerable refinement of the microstructure of polycrystalline copper by extensive deformation twinning in the case of cryogenic wire drawing. In order to improve the thermal stability of the obtained microstructure, we investigate the deformation of CuZr alloys with up to 0.21 at% (0.3wt%) Zr by drawing in liquid nitrogen up to a true strain of 2.4. An ultimate tensile strength of 625MPa in combination with an electrical conductivity of about 60%IACS was found for an initially solution annealed CuZr0.21 alloy. For an initially precipitation treated CuZr0.21, a slightly lower ultimate tensile strength of 600MPa at about 86%IACS was observed in the as-deformed state. Deformation twinning could be identified in all investigated materials, leading to a refinement of the microstructure. Thermal stability of microstructure as well as electrical and mechanical properties can be significantly improved by the addition of Zr. For cryo-drawn copper the drop of hardness due to recovery and recrystallization occurs between 200°C and 225°C for an annealing duration of 30min. Small recrystallized grains are already identified at 175°C. In CuZr0.21, a stable microstructure is found up to 350°C for the solution annealed material while small recrystallized grains are found already at 300°C in the precipitation treated material. By annealing a homogenized and subsequently cryo-drawn CuZr0.21 at 350°C for 30min, ultimate tensile strength can be further increased up to 635MPa while electrical conductivity is enhanced significantly to 70%IACS by partial precipitation of Cu5Zr. Dynamic resistivity measurements indicate that this heat treatment subsequent to cryo-drawing does not affect the thermal stability of the prepared material.