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 suggested 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 precipitation of solute-rich G.P. zone-->eta'-->eta MgZn2 in 7075. According to this mechanism, the lithium-vacancies (Li-v) aggregates act as nuclei for subsequent clustering of Zn and Mg atoms, and lead to limited formation of vacancy-rich G.P. zones. The vacancy concentration, the ratio of Mg/Zn and the crystallographic structure are all modified inside zones because trapping of vacancies by Li inhibits the diffusion of Zn and Mg into zones. Therefore, the morphology, size, distribution, ageing kinetics and age-hardening are all changed. Moreover, narrow precipitation free zones (PFZs) are also produced. In contrast to the early inhibition of nucleation, promotion of T phase precipitation occurs on prolonged ageing at 120 degrees C with the aid of crystal defects such as dislocations and voids. The inhibited nucleation and reduced hardening can be improved either by increasing the solution heat treatment (SHT) temperature or by applying two-stage ageing.