THE MECHANISM OF NUCLEATION AND PRECIPITATION IN 7075-0.7 LI ALLOY

The effect of 0.7 wt% Li on the nucleation mechanism and precipitation behaviour of 7075 has been studied. A nucleation mechanism has been s uggested to explain the observed precipitation scheme: vacancy-rich G. P. zone-->T'-->T (Al,Zn)(49)Mg-32 in 7075-0.7 Li which replaces the pr ecipitation of solute-rich G.P. zone-->eta'-->eta MgZn2 in 7075. Accor ding to this mechanism, the lithium-vacancies (Li-v) aggregates act as nuclei for subsequent clustering of Zn and Mg atoms, and lead to limi ted formation of vacancy-rich G.P. zones. The vacancy concentration, t he ratio of Mg/Zn and the crystallographic structure are all modified inside zones because trapping of vacancies by Li inhibits the diffusio n of Zn and Mg into zones. Therefore, the morphology, size, distributi on, ageing kinetics and age-hardening are all changed. Moreover, narro w precipitation free zones (PFZs) are also produced. In contrast to th e early inhibition of nucleation, promotion of T phase precipitation o ccurs on prolonged ageing at 120 degrees C with the aid of crystal def ects such as dislocations and voids. The inhibited nucleation and redu ced hardening can be improved either by increasing the solution heat t reatment (SHT) temperature or by applying two-stage ageing.