Calorimetric studies of 8090 and 1441 Al–Li–Cu–Mg–Zr alloys of conventional and retrogressed and reaged tempers

Abstract

The differential scanning calorimetry (DSC) technique has been used to examine the solid state reactions of GPB zones formation, precipitation of δ′, T₁, T₂, S′, δ phases and their dissolution occurring in the 8090 and 1441 Al–Li–Cu–Mg–Zr alloys of the water-quenched (WQ), peak aged T8 and T6, over aged T7, retrogressed (R), retrogressed and reaged (RRA) T77 tempers. All the exothermic and endothermic peaks in the DSC thermograms have been identified and discussed. The noticeable differences observed in the thermograms of the 8090 and 1441 alloys have been explained and this is attributed to the variation in the concentrations of the solute elements Li, Cu and Mg in the alloys. The peak temperatures as well as the heat evolved and absorbed during the precipitation and dissolution reactions have been determined with the help of the built-in software of the simultaneous thermal analyzer (STA) used for DSC studies. X-ray diffractograms and a few TEM micrographs have been illustrated to correlate the reactions in these alloys. Further, all the thermograms of the WQ state of the 1441 alloy, taken at four different heating rates, have exhibited overlapping peaks of GPB zones formation and the δ′ phase precipitation. The overlapping peaks cause restrictions in determining the kinetic parameters of activation energy and growth parameter, but, interestingly, the thermograms of retrogressed tempers have showed separate peaks of GPB zones formation and δ′ phase precipitation which will easily enable to find out kinetic parameters, by varying heating rate method.