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

Microalloying additions of Ag (∼0.1 at.%) increase the hardening response of Al-Zn-Mg alloys to elevated temperature ageing in the range 100-200°C due to the formation of a high density of very fine η′ precipitate plates. The present study employed transmission electron microscopy (TEM) and three-dimension atom probe (3DAP) to study the early stages of ageing in the alloy Al- 1.8Zn-3.4Mg-0.1 Ag (at.%) in an attempt to identify the role of Ag in stimulating precipitation hardening. During isothermal ageing at 90°C, the hardening response is attributed to a high density of Zn-Mg-Ag rich solute clusters and GP zones, During ageing at 150°C, η′ precipitates nucleate at Zn-Mg-Ag rich solute clusters, the former growing as (111)_α platelets with an average composition of approximately 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 formation of the Zn-Mg-Ag rich solute clusters, The GP zones and η′ precipitates were observed to possess a Zn:Mg ratio close to 1:1, whereas the equilibrium η precipitates possessed compositions consistent with MgZn₂. Furthermore, partitioning of Ag was observed inside all precipitate phases, viz. G.P. zones, η′ and η.