Mechanical Dissolution of Cu₅Zr Phase and Formation of Supersaturated Solid-Solution Nanocrystalline Structure by High-Pressure Torsion in a Hypoeutectic Cu–2.7 at%Zr Alloy

Abstract

Microstructural evolution and changes in hardness and electrical conductivities of a cast hypoeutectic Cu–2.7 at%Zr alloy processed by high-pressure torsion (HPT) were investigated. The cast alloy had a net-like microstructure composed of a primary Cu phase and a eutectic consisting of layered Cu and Cu₅Zr phases. The Cu and Cu₅Zr phases in the eutectic had a cube-on-cube orientation relationship. The cast alloy with the hardness of 137 HV exhibited a value of electrical conductivity of 32%IACS. With increasing the number of HPT-revolutions, the eutectic was severely sheared and elongated along the rotational direction. In addition, mechanical dissolution of the Cu₅Zr phase into the Cu phase by the HPT was confirmed after 5 HPT-revolutions through XRD measurements and TEM observations. After 20 HPT-revolutions, the Cu phase was significantly refined and formed the lamellar structure having an average grain size of 15 nm. The electrical conductivity decreased and saturated at a value of 8%IACS after 50 HPT-revolutions. The significant decrease in the electrical conductivity was primarily attributable to the mechanical dissolution of the Cu₅Zr phase into the Cu phase by the HPT, followed by the formation of a nanocrystalline Cu–Zr supersaturated solid-solution alloy with the hardness of 430 HV.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Copper 60 (2021) 98–103.

Fig. 9 Lattice constant and concentration of Zr solutes in the Cu phase vs. the number of HPT-revolutions.