The Role of Ledge Nucleation/Migration in Ω Plate Thickening Behaviour in Al‐Cu‐Mg‐Ag Alloys

Abstract

The thickening kinetics of Q plates in an Al-4Cu-0.3Mg-0.2Ag (wt. %) alloy have been measured at 200 OC, 250 "C and 300 "C using conventional transmission electron microscopy techniques. At all temperatures examined the thickening showed a linear dependence on time. At 200 "C the plates remained less than 6nm in thickness after lOOOh exposure. At temperatures above 200 "C the thickening kinetics are greatly increased. Atomic resolution Z-contrast microscopy has been used to examine the structure and chemistry of the (OOl), 11 ( 11 l)c( 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 growth ledges and the ends of the plates were free of segregation. No significant levels of Ag or Mg were detected inside the plate at any time. 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 S2 plates at temperatures up to 200 "C. Plates transformed at 200 'C 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 'C, concomitant with accelerated plate thickening kinetics. The high resistance to plate coarsening exhibited by $2 plates at temperatures up to 200 'C, is due to limited ledge nucleation under these conditions. The prohibitively high barrier to coherent ledge nucleation on the broad faces of plates aged at 200 'C arises from the contribution to the total free energy change attending nucleation from elastic interactions between the misfitting coherent ledges and the significant strain field that can exist normal to the broad face of the B plate.