| Effects of Sc on the coarsening process of δ`-Al₃Li precipitate in Al-Li allays, which have been regarded as a main hardener, were investigated by means of DSC, TEM and hardness tester. From the result of DSC analysis, both exothermic and endothermic peaks of the Sc-containing alloy, which correspond to the precipitation and dissolution of δ` particles, were observed to shift to the higher temperature sides, compared to those of the Sc free alloys. This suggests that Sc solutes could act as a prohibiter of precipitation and growth of the δ` particles. The estimated activation energy for δ` precipitation and growth in the Sc-containing alloy is 60 kJ/㏖ larger than Sc-free alloy 47 kJ/㏖. From the result of microstructure observation by means of TEM, both alloys revealed that the growth of mean diameter of δ` particles is proportional to the cube root of time and, especially in the Sc containing alloy, a much decreased coarsening rate of the δ` particles is observed. According to the result of EDX analysis, no peak of Sc solute was detected in the δ` particle of the Sc containing Al-Li alloy, which indicates that there is no Sc enrichment in the δ` particle. Therefore, based on the present experimental results, the reason for the retardation of δ` precipitate by the Sc addition is closely related to the strong interaction between Sc solutes and vacancies in the matrix. |
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