The effects of a trace addition of silver upon elevated temperature ageingof an Al-Zn-Mg alloy

Microalloying additions of Ag (similar to0.1 at.%) increase the hardening r esponse of AI-Zn-Mg alloys to elevated temperature ageing in the range 100- 200 degreesC due to the formation of a high density of very fine eta' preci pitate plates. The present study employed transmission electron microscopy (TEM) and three-dimension atom probe (3DAP) to study the early stages of ag eing in the alloy Al-1.8Zn-3.4Mg-0.1Ag (at.%) in an attempt to identify the role of Ag in stimulating precipitation hardening. During isothermal agein g at 90 degreesC, the hardening response is attributed to a high density of Zn-Mg-Ag rich solute clusters and GP zones. During ageing at 150 degreesC, eta' precipitates nucleate at Zn-Mg-Ag rich solute clusters, the former gr owing as {111}(alpha) platelets with an average composition of approximatel y 20 at.% Zn, 20 at.% Mg and 1.4 at.% Ag. The 3DAP data indicates that the co-segregation of Zn and Ag and subsequently Zn and Mg atoms precedes the f ormation of the Zn-Mg-Ag rich solute clusters. The GP zones and eta' precip itates were observed to possess a Zn:Mg ratio close to 1:1, whereas the equ ilibrium eta precipitates possessed compositions consistent with MgZn2. Fur thermore, partitioning of Ag was observed inside all precipitate phases, vi z. G.P. zones, eta' and eta. (C) 2001 Elsevier Science Ltd. All rights rese rved.