The effects of Mg addition on the aging characteristics of Al-2.5wt%Li-2.5wt$Cu alloys with three different Mg compositions of 0.5, 1.0, 1.5 wt% have been investigated. Aging behavior was studied by means of hardness and lattice parameter measurement. The microstructural changes in both the matrix and grain boundary have been investigated using transmission electron microscope. δ' solvus temperature was determined using hardness test.
Most of δ' $(Al_3Li)$ phase precipitates in normal spherical particles coherent with the matrix. Addition of Mg to the Al-Li-Cu alloys appeared to have little effect on the solvus temperature for the δ' phase. Similarly to the case of an Al-Li binary alloy, the coarsening of δ' particles well obey the LSW kinetics. $r^3\infty$ in all cases. Half width of δ' PFZ(h) increases linearly with the square root of the aging time. The increases in Mg content tends to increase the width of δ'PFZ.
In the case of alloy containing 0.5wt% Mg, homogeneous nucleation of S' $(Al_2CuMg)$ phase plays an important role in the precipitation os S' phase. On the other hands, in the alloys containing magnesuim more than 1.0wt% S' phase precipitates on the G.P. zones. The volume fraction of the S' phase increases with magnesium concentration. The growth rate of S' lath markedly decreases with increasing magnesuim content. It is related to the copper supersaturation in the matrix.
The principal strengthening agent were δ', S' and T1 phase in the case of alloy containing 0.5wt% Mg. In the alloys containing 1.0wt% Mg or more, the main contributor to coprecipitation hardening are δ' and S' phase. In the case of alloy containing 1.5wt% Mg, $T2(Al_6Cu(Li,Mg)_3)$ phase precipitates with very low volume fraction at the grain boundary at the low aging temperature(190℃).
