Effects of Mg on aging characteristics of Al-2.5wt%Li alloys with three different Mg compositions of 0, 1, 2 wt% have been investigated.
Aging behavior was studied by means of hardness and lattice parameter measurement. Transmission electron microscopy has been used to study the micro-structural changes in both the matrix and at grain boundary. Differential scanning calorimetry was used to study the kinetics of precipition of equilibrium $\delta$ phase.
Meastable, coherent spherical $\delta'$ phase nucleats homogeneously and show a $Ll_2$ ordered structure. Increasing the Mg content tends to accelerate the aging kinetics of the alloys. The addition of Mg to the Al-Li alloys appeared to have little effect on the solvus temperature for the $\delta'$ phase. The coarsening of $\delta'$ particles well obey the LSW kinetics, $\overline{\Gamma}^3$αt in all cases and the rate constant appeared not to be much influenced by the Mg addition.
Addition of Mg to the Al-Li alloys appears to induce more strengthening than that took into account by simple solid solution strengthening effect. The measurement of lattice parameter indicated that Mg may be incorporating in the $\delta'$ particles and producing a stronger $\delta'$ particles than in the case of binary counterpart.
Precipitate free zone (PFZ) across a grain boundary is formed by a solute depletion mechanism and the width of PFZ increases proportional to $t^{\frac{1}{2}}$. The increases in Mg content tends to increases the width of PFZ.
The rate of precipitation of equilibrium $\delta$ phase can be best described by an Avrami expression with n=1.5. Increasing the contents of Mg, decreases the activation energy for $\delta$ formation. The activation energies for $\delta$ formation in binary alloy was determined to be 141.3 KJ/mole, but this decreases to be 137.3 and 130.7 KJ/mole respectively with increasing the Mg content from 1.0 wt% to 2.0 wt% Mg.
