Precipitation behavior and strengthening mechanisms in two Al-Li-Cu(-Zr) and one Al-Li-Cu-Mg(-Zr) alloys have been investigated by means of Vickers hardness, differential scanning calorimetry, tensile test and transmission electron microscopy. Retrogression technique has been employed in order to estimate the contribution of each type of precipitates to the total strengthening effect. The strength of the alloys was greatly increased on applying a stretch treatment and its magnitude was dependent on the Cu and Mg content. The sizes and volume fraction of δ' phase are slightly decreased by the stretching treatment. The coarsening data of δ' particles were well fitted to LSW coarsening kinetics and its activation energy was slightly lowered in the stretched condition. The coarsening rate of the δ' particles, which was shown to be controlled by the diffusion of Li atoms, was rarely affected by the presence of Cu or Mg atoms. The addition of Mg lowered the volume fraction of the δ' phase and it was attributed to its effect (i.e., an enhancement) on the precipitation of equilibrium δ phase. The stretching treatment greatly enhanced the nucleation rate of the $T_1$ plates, resulting in a more uniform distribution and higher volume fraction. A less pronounced effect was observed in the case of Mg containing alloy. The growth rate of the $T_1$ plates was significantly decreased in the stretch condition as compared to the no-stretch condition. The lengthening kinetics of $T_1$ plates was found to be controlled by the diffusion of Cu along the dislocations. The $T_1$ plate tended to be sheared by dislocation even in the peak aging condition. The nucleation of S' phase was accelerated by the stretching treatment but the effect was somewhat smaller as compared to that of $T_1$ plates. The strengthening mechanisms of $T_1$ phase in ternary Al-Li-Cu alloy have been analyzed in terms of modified chemical strengthening mechanism considering the effect of the orientation relationship. The result yielded the surface energy 3.264 J/㎡. The order strengthening mechanism was found to be responsible for the δ' strengthening, in which the trailing dislocation pulls completely off from the encountering particles. The superposition parameter in the addition rule of the δ' and $T_1$ strengthenings was determined to be 1.34. In the case of Mg containing alloy, the order strengthening mechanisms were also responsible for the δ' particles, but the antiphase boundary energy was found to slightly increase as compared to the Mg-free alloy.
