Cu-diffusion-retardation mechanism and thermal stability of precipitate in Al-Mg-Si-Cu-X alloy

초록

This study extends the microalloying elements delaying precipitation kinetics of Al-Mg-Si-Cu alloy from Sc to other elements, and emphasizes the novel mechanism of enhancing thermal stability of Al-Mg-Si-Cu alloys. Extensive study on the Yb, Y, La and Er additions revealed that they have same delaying effects on the precipitation kinetics with Sc addition in the Al-Mg-Si-Cu alloy. Experimental studies have been performed representatively on only Yb addition instead of some elements (Sc, Y, La, and Er) which were expected to have same delaying effects. The results revealed that the Cu-diffusion-retardation mechanism found in Yb-modified alloy, showing same effects caused by Sc addition. The mechanism of Cu-diffusion-retardation phenomenon has been discussed using solubility at the aging temperature and thermodynamic calculation. Cu-X (X = Sc, Yb, Y, La, Er) pairs found by atom probe tomography is regarded to intermediate phase of Al8Cu4X intermetallics. Unlike highly-alloyed Al-Cu alloy, the Cu-X pair is assumed to be too low stability for permanently depleting Cu solutes in diluted Cu-contained Al-Mg-Si-Cu alloy system. Accordingly, Cu-X pair retards Cu diffusion in the matrix based on the assumption that the pair repeatedly forms and dissociates. The solubility at aging temperature has a crucial factor determining which X solutes form Cu-X pair. Combining atom probe tomography result and other experimental study, it is concluded that X solutes exceeding the maximum solubility at the aging temperature are contributed to form Cu-X pair and Cu-diffusion-retardation phenomenon.

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