Nanoscale structural evolution of Al3Sc precipitates in Al(Sc) alloys

Precipitation of the Al3Sc (L1(2)) phase in aluminum alloys, containing 0.1, 0.2 or 0.3 wt% Sc, is studied with conventional transmission and high-resolution (HREM) electron microscopies. The exact morphologies of the Al3Sc precipitates were determined for the first time by HREM, in Al-0.1 wt% Sc and Al-0.3 wt% Sc alloys. The experimentally determined equilibrium shape of the Al3Sc precipitates, at 300 degreesC and 0.3 wt% Sc, has 26 facets, which are the 6 {100} (cube), 12 {110} (rhombic dodecahedron), and 8 {111} (octahedron) planes, a Great Rhombicuboctahedron. This equilibrium morphology had been predicted by first principles calculations of the pertinent interfacial energies. The coarsening kinetics obey the (time)(1/3) kinetic law of Lifshitz-Slyozov-Wagner theory and they yield an activation energy for diffusion, 164 +/-9 kJ/mor, that is in agreement with the values obtained from tracer diffusion measurements of Sc in Al and first principles calculations, which implies diffusion-controlled coarsening. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.