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

Abstract

Microstructural evolution and changes in hardness and electrical conductivity of a cast hypoeutectic Cu–2.7at%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 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 HPT, followed by the formation of nanocrystalline Cu–Zr supersaturated solid–solution alloy with the hardness of 430 HV.