THE EARLY STAGES OF PRECIPITATION IN AL-LI ALLOYS

The initial stages of precipitation in supersaturated binary Al - Li solid solution alloys are discussed. Results of experimental investigations referring mostly to ageing at or near room temperature are reviewed and compared with predictions of theoretical treatments. In alloys containing more than about 7 at.% Li a fast continuous ordering (spinodal ordering) takes place before the somewhat slower decomposition. There are indications that the decomposition starts with a homogeneous nucleation in the ordered matrix. In artificially aged samples further precipitation takes place during the subsequent room temperature storage by nucleation within the disordered matrix phase. The ordered nature of the precipitates determines their interaction with dislocations and thereby their effect on the mechanical properties of the precipitation-hardened alloy.