Nanostructural evolution of Al3Sc precipitates in an Al-Sc-Mg alloy by three-dimensional atom probe microscopy

The effects of Mg alloying on the precipitation of Al3Sc precipitates were investigated, focusing on nanostructural. evolution during isothermal aging at 300degreesC. Three-dimensional atom probe microscopy was performed on samples both in the as-quenched state and after aging for various times. Magnesium tends to segregate at the coherent Al/Al3Sc interface, with a measured value of 1.9 +/- 0.5 atom nm(-2) for the relative Gibbsian excess of Mg with respect to Al and Sc. This value is constant for all heat treatments, thereby demonstrating that the system is in global thermodynamic equilibrium. This study provides direct experimental evidence for first-principles calculations, which explain morphological changes of Al3Sc precipitates in the presence of Mg observed by high-resolution electron microscopy. Evidence for the presence of Mg atoms in the center of the precipitates is also found, and is discussed in terms of heterogeneous nucleation of the precipitates on Mg-Sc atomic clusters. Copyright (C) 2004 John Wiley Sons, Ltd.