HIGH-RESOLUTION ELECTRON-MICROSCOPY OF THE EARLY DECOMPOSITION STAGE OF AL-LI ALLOYS

The early decomposition stages of Al-7.8at.%Li, Al-10.4at.%Li and Al-11.8at.%Li alloys were studied by high resolution electron microscopy. In all the above alloys, it was confirmed that in the as-quenched stage small ordered domains of L12 structure were present surrounded by the disordered matrix. From this microstructure, it was concluded that the alloy was already decomposed as in the as-quenched stage. No clear evidence was found for congruent ordering which was proposed to occur prior to spinodal decomposition. Examination of the sublattices of the individual ordered domains also failed to give convincing evidence for congruent ordering. The present observations suggest that the kinetics of spinodal decomposition of this system is too fast to be detected by TEM. In order to slow down the decomposition kinetics, solution treated samples were quenched to just above the solvus fine for delta' and then to room temperature. By this heat treatment, a solution treated sample with much weaker order spots could be obtained. However, this microstructure could not be interpreted as congruently ordered either based on the weak L1(2) spots. As there is a limitation to the quenching speed, it is suggested that ordering and spinodal decomposition progress concomitantly during the observable range of time.