DISCONTINUOUS PRECIPITATION IN HIGH LI CONTENT AL-LI-ZR ALLOYS

The morphology of discontinuous precipitation of the delta' phase is i nvestigated in very high Li concentration Al-Li-Zr alloys using transm ission electron microscopy. Contrary to previous observations, the rea ction was found to initiate during quenching following solution heat t reatment. Furthermore, the reaction was found to be very sensitive to the cooling rate from the solution treatment temperature since this co uld change the solute distribution in the vicinity of a grain boundary . It is demonstrated that the initiation of discontinuous precipitatio n in AI-Li alloys can not be explained in terms of recrystallisation p ressures and that the grain boundaries are repelled by developing disc ontinuous delta' colonies leading to gross distortion of the grain bou ndaries. The reaction mechanisms involved, and existing and new models for the discontinuous precipitation of delta', are discussed in the l ight of these observations.