On the origin of the high coarsening resistance of Omega 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 transmi ssion electron microscopy techniques. At all temperatures examined the thic kening showed a linear dependence on time. At 200 degreesC the plates remai ned less than 6 nm in thickness after 1000 h exposure. At temperatures abov e 200 degreesC the thickening kinetics are greatly increased. Atomic resolu tion Z-contrast microscopy has been used to examine the structure and chemi stry of the (001)(Omega)parallel to (111)(alpha) interphase boundary in sam ples 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 th e thickening ledges and the ends of the plates were free of Ag segregation. The necessary redistribution of Ag and Mg accompanying a migrating thicken ing ledge occurs at all temperatures and is not considered to play a decisi ve role in the excellent coarsening resistance exhibited by the Omega plate s at temperatures up to 200 degreesC. Plates transformed at 200 degreesC ra rely contained ledges and usually exhibited a strong vacancy misfit normal to the plate. A large increase in ledge density was observed on plates tran sformed at 300 degreesC, concomitant with accelerated plate thickening kine tics. 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 th e plate. Results also suggest that accommodation of the large misfit that e xists normal to the broad face of the plate is unlikely to provide the driv ing force for Ag and Mg segregation. (C) 2001 Acta Materialia Inc. Publishe d by Elsevier Science Ltd. All rights reserved.