Precipitation Process in a Cu-Ni-Be Alloy

Abstract

The precipitation process in an aged Cu-1.9wt%Ni-0.3wt%Be alloy has been examined by high-resolution transmission electron microscopy. The precipitation sequence found is: Guinier- Preston (G.P.) zones → γ'' → γ' → stable γ. The disk-shaped G.P. zones and the disk-shaped γ'', γ' and γ precipitated phases are composed of monolayers of Be atoms on {100}α of the Cu matrix and alternative Be and Ni matrix layers parallel to {100}α. The γ'' phases consisting of two to eight Be-layers has a body-centered tetragonal (bct) lattice with a=b=0.24 nm and c=0.28 nm. The γ' or γ phase is bct with a=b=0.24 nm and c=0.26 nm or a=b=0.26 nm and c=0.27 nm. The γ'', γ' or γ phase aligns with the matrix according to the Bain orientation relationship. The growth kinetics of disk-shaped γ precipitates on aging at 500°C has been also investigated. The {001}α habit planes of the γ precipitates migrate by a ledge mechanism. The average thickness of the γ disks increases with aging time t as t1/2. An analysis of experimental data using a kinetic model yields the diffusivity of solute in the Cu matrix, which is in agreement with the reported diffusivity of Ni in Cu.