On the origin of the high coarsening resistance of Ω plates in Al-Cu-Mg-Ag alloys

The thickening kinetics of fl plates in an Al-4Cu-0.3Mg-0.2Ag (wt. %) alloy have been measured at 200, 250 and 300 degreesC using conventional transmission electron microscopy techniques. At all temperatures examined the thickening showed a linear dependence on time. At 200 degreesC the plates remained less than 6 nm in thickness after 1000 h exposure. At temperatures above 200 degreesC the thickening kinetics are greatly increased. Atomic resolution Z-contrast microscopy has been used to examine the structure and chemistry of the (001)(Omega)parallel to (111)(alpha) interphase boundary in samples treated at each temperature. In all cases, two atomic layers of Ag and Mg segregation were found at the broad face of the plate. The risers of the thickening ledges and the ends of the plates were free of Ag segregation. The necessary redistribution of Ag and Mg accompanying a migrating thickening ledge occurs at all temperatures and is not considered to play a decisive role in the excellent coarsening resistance exhibited by the Omega plates at temperatures up to 200 degreesC. Plates transformed at 200 degreesC rarely contained ledges and usually exhibited a strong vacancy misfit normal to the plate. A large increase in ledge density was observed on plates transformed at 300 degreesC, concomitant with accelerated plate thickening kinetics. The high resistance to plate coarsening exhibited by Omega plates at temperatures up to 200 degreesC, is due to a prohibitively high barrier to ledge nucleation in the strong vacancy field normal to the broad face of the plate. Results also suggest that accommodation of the large misfit that exists normal to the broad face of the plate is unlikely to provide the driving force for Ag and Mg segregation. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.