Aging characteristics in three different Al-205wt%Li-1.5wt%Cu-1,2,3wt%Mg alloys, have been studied to clarify the effect of Mg content on the precipitation of δ' and S' phase. Hardness measurement was performed to examine aging behavior. The microstructural changes in both matrix and grain boundary were examined by transmission electron microscopy. Differential scanning calorimetry has been used to study the kinetics of GPB zone formation.
At aging for temperatures below 190℃, hardness curves exhibit a rapid increase region followed by a plateau. This tendency is marked with decreasing aging temperature and with decreasing content of Li. This is attributed to the difference in density of S' phase, formed in the matrix. For the three alloys, the principal strengthening agents were δ' and S' phase. For the alloys with 2 and 3wt% Mg, the $T_2$ phase nucleated in matrix and grain boundary. In the peak aging condition, the 30% of total hardening were found to be due to the S' phase.
Metastable, coherent spherical δ' phase nucleates homogeneously and show a $Ll_2$ structure, The coarsening of δ' particles well obey the LSW kinetics.
Precipitate free zone(PFZ) across a grain boundary is formed by a solute depletion mechanism. The width of PFZ increases proportionally to $t^{\frac{1}{2}$. The increase in Mg content tends to increase the width of PEZ.
At early stage of aging, the S' phase precipitates heterogeneously at subgrain boundaries and the supersaturation of Mg enhances the nucleation of S' phase, according as aging, the S' phase precipitates in matrix. The alloy containing lower Li exhibit more dense precipitation of S' phase. Lithium slow down the precipitation of GPB zone and S' phase.
