Effect of retrogression and reaging on the precipitates in an 8090 Al-Li alloy

The microstructure of the precipitates in an 8090 Al-Li alloy has been studied qualitatively and quantitatively by techniques of transmission electron microscopy through the stages of retrogression and reaging (RR). The retrogression was performed for various times at temperatures of 240 and 260 degrees C. The reaging treatment was carried out at 170 degrees C. The behavior of the precipitates was specific for each temperature of retrogression: while for retrogression at 240 degrees C there was only partial dissolution of delta' particles, total dissolution of delta' was observed after 30 min of treatment at 260 degrees C. Reaging following retrogression at 240 and 260 degrees C resulted respectively in the full and only partial restoration of the volume fraction of the delta' phase. Change in the delta' precipitate structure was the dominant factor which was responsible for the changes in hardness of the Al-Li 8090 alloy in the course of RR treatment for a retrogression temperature of (RR-240). On the other hand, changes in the hardness of the alloy subjected to RR-260 treatment could not be explained simply in terms of changes in the delta' particle structure. The precipitation of an additional phase, T-1, concomitant with the dissolution of delta' phase at 260 degrees C, also contributes to the hardness on this retrogression stage of the treatment. (C) 1998 Elsevier Science S.A.