EFFECT OF STRETCHING ON THE PRECIPITATION KINETICS OF AN AL-2.0LI-2.8CU-0.5MG(-0.13ZR) ALLOY

Abstract

The influence of stretching on the precipitation kinetics of an Al-2.0Li-2.8Cu-0.5Mg(-0.13Zr) alloy has been quantitatively investigated using transmission electron microscopy (TEM) and hardness testing. The microstructure of the alloy consists primarily of delta', S', and T1 phases. Stretching exerts little influence on the coarsening kinetics of delta' phase. The activation energy for coarsening is estimated to be approximately 100 kJ/mole, regardless of whether the alloy is unstretched or stretched. The volume fraction of delta' phase, however, is significantly reduced in the stretched condition. The stretching treatment greatly accelerates the nucleation kinetics of T1 phase at the expense of S' phase; however, the growth rate of T1 phase (and also of S' phase) is significantly reduced in the stretched condition due to overlapping of the diffusion field. A kinetic model is presented that addresses this problem. The model successfully predicts the effect of stretching on the lengthening rate of T1 plates. The activation energies for the lengthening of T1 plates and S' rods are approximately 100 to 110 kJ/mole and approximately 110 to 120 kJ/mole, respectively, regardless of the condition of stretch. It is concluded that, for both unstretched and stretched material, the growth of the T1 and S' phases is controlled by the diffusion of copper along dislocations. The nucleation of S' phase tends to occur at dislocations with a Burgers vector that is nearly parallel to the [010]s, direction, which is the direction that exhibits maximum misfit.