Microstructure and Hardness of Isothermally Aged Cu–20 at.% Ni–6.7 at.% Al Alloy

Abstract

The microstructural evolution and strengthening of Cu–20 at.％ Ni–6.7 at.％ Al alloy during isothermally aging at 450 to 650℃ were investigated to understand age–induced precipitation and hardening behavior for Cu–Ni–Al system. In an initial stage of aging, fine ordered Ni₃Al particles with a size of several tens nm was coherently formed in parent grains of Cu solid solution in a continuous precipitation manner, together with cellular components composed of Cu solid solution and fine fibrous Ni₃Al phases by grain boundary reaction. During furthermore aging, fine Ni₃Al particles in grains were continuously nucleated and grown, while cellular components at grain boundaries seemed not to be developed. The alloy exhibited a maximum hardness more than 320 Hv by aging at 550 to 650℃, which was greatest among conventional copper alloys. The hardening in the alloy was principally contributed to a high–density dispersion of fine ordered Ni₃Al particles in grains. According to the time–temperature transformation （TTT） diagram for the alloy, aging at 600 to 650℃ should be effective to obtain a microstructure with fine dispersion of ordered Ni₃Al particles and without cellular components, resulting in an excellent strength with the alloy.