The chemistry of precipitates in an aged Al-2.1Zn-1.7Mg at.% alloy

Abstract

Ageing processes in most aluminum alloys are complex and the decomposition of saturated solid solutions obtained by quenching takes place in several stages. Precipitation in Al-Zn-Mg alloys has also been widely considered to involve three stages, the nature of which depends on the alloy composition and in turn the Zn:Mg ratio. For higher ratios, G.P. zones (rich in Zn and Mg) are thought to be replaced gradually by the formation of the intermediate precipitate {eta}{prime} (commonly accepted to have the composition MgZn{sub 2}) and the equilibrium phase {eta} (MgZn{sub 2}), both of which are hexagonal although the lattice parameters are different. For alloys with lower Zn:Mg ratios, the intermediate precipitate may be the cubic T{prime} phase (probably Mg{sub 32}(Al,Zn){sub 49}) and the equilibrium phase T, of the same composition, which is also cubic. The present work uses a combination of transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) in an attempt to examine the precise compositions of precipitates in a high-purity, ternary Al-Zn-Mg alloy.