Effect of Sc-Addition on the Coarsening Behavior of Al₃Li Precipitates in Al–Li Alloys

Abstract

DSC and TEM analyses, as well as hardness test, were carried out on Sc containing and Sc-free Al–Li alloys in an attempt to investigate the effects of Scandium on the coarsening process of the δ′-Al₃Li precipitate, which is a main hardener in the Al–Li base alloys.
In the DSC measurement, both exothermic and endothermic peaks tend to move to the higher temperature side with increasing heating rate. The exothermic peak corresponding to the δ′-Al₃Li precipitation in the Sc-containing alloy shifts to the higher temperature side relative to the corresponding Sc-free alloys, suggesting that Sc atoms in solution retard the precipitation and growth of the δ′-Al₃Li phase. The apparent activation energy estimated from the analysis of the DSC data is 47 kJ/mol for the Sc free binary Al-2.6%Li alloy, 60 kJ/mol for the Al-2.4%Li-0.20%Sc alloy and 70 kJ/mol for Al-2.5%Li-0.1%Mg-0.5%Cu-0.1%Sc alloy.
The analysis of TEM micrographs revealed a much decreased coarsening rate of the δ′-Al₃Li particles for the Sc containing alloys.
No Sc-peaks were detected by the EDX analysis of the δ′-Al₃Li phase in the Sc containing alloy, indicating that there is no solutionization of Sc in the δ′-Al₃Li phase.
The present experimental results imply that the reason for the observed retardation by the Sc addition is closely related to the high affinity of Sc atoms to vacancies.