Effect of Sc-addition on the coarsening behavior of Al3Li precipitates in Al-Li alloys

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 delta'-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 delta'-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 delta'-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 delta'-Al3Li particles for the Sc containing alloys. No Sc-peaks were detected by the EDX analysis of the delta'-Al3Li phase in the Sc containing alloy, indicating that there is no solutionization of Sc in the delta'-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.