Coarsening kinetics of nanoscale Al3Sc precipitates in an Al-Mg-Sc alloy

The effects of Mg alloying on the temporal evolution of Al3Sc (L12 structure) nanoscale precipitates are investigated, focusing on the morphology and coarsening kinetics of Al3Sc precipitates in an Al-2.2 Mg-0.12 Sc at.% alloy aged between 300 and 400 °C. Approximately spheroidal precipitates are obtained after aging at 300 °C and irregular morphologies are observed at 400 °C. The coarsening behavior is studied using conventional and high-resolution transmission electron microscopies to obtain the temporal evolution of the precipitate radius, and atom-probe tomography is employed to measure the Sc concentration in the (alpha)-matrix. The coarsening kinetics are analyzed using a coarsening model developed by Kuehmann and Voorhees for ternary systems [Kuehmann CJ, Voorhees PW. Metall Mater Trans A 1996;27:937]. Values of the interfacial free energy and diffusion coefficient for Sc diffusion in this Al-Mg-Sc alloy at 300 °C are independently calculated, and are in good agreement with the calculated value of interfacial free energy [Asta M, Ozolins V, Woodward C. JOM 2001;53:16] and the experimental diffusivity obtained for the Al-Sc system [Marquis EA, Seidman DN. Acta Mater 2001;49:1909; Marquis EA. Ph.D. Thesis. Materials Science and Engineering Department, Northwestern University, 2002; Fujikawa SI. Defect Diffusion Forum 1997;143–147:115].