Effects of chromium and copper additions on precipitation in Al?Zn?Mg alloys

The effects of chromium and copper additions on precipitation in several Al?Zn?Mg alloys have been investigated. Results show that chromium additions heterogenize precipitation in aged Al?Zn?Mg alloys by creating special nucleation sites. Multirowed bands of incoherent precipitates appeared in the grain boundaries and subboundaries in an Al-5 pct Zn-2 pct Mg-0.1 pct Cr alloy. It is believed that fine nuclei associated with the existence of chromium-rich regions are formed during solidification and are retained after solution heat treatment. These nuclei would lead to the formation of incoherent precipitates during quenching and aging. Chromium is, therefore, considered to causehigh temperature nucleation. Copper additions to Al?Zn?Mg alloys accelerate precipitation at lower aging temperatures and increase the density of G. P. zones nucleated at relatively lower temperatures (20 to 90°C). In this way copper considerably strengthens Al?Zn?Mg alloys. Copper, in contrast to chromium, causes increased low-temperature nucleation of G. P. zones.