On the origin of the high resistance to coarsening of Ω plates in Al-Cu-Mg-(Ag) alloys using Z-contrast microscopy

Conventional transmission electron microscopy (CTEM) has been used to measure the thickening kinetics of Omega plates in Al-4Cu-0.3Mg-0.4Ag (wt. %) at 200 degrees C, 250 degrees C and 300 degrees C. At 200 degrees C the plates thickened at a slow rate, remaining less than 10nm in thickness after 1000h exposure. At 250 degrees C the thickening kinetics exhibited two clearly discernible regimes. The first is characterized by a relatively slow rate of plate coarsening and the second by a much enhanced rate. The transition between the two regimes occurs after similar to 300h and at a plate thickness close to 13nm. At 300 degrees C, the Omega plates thicken rapidly and thicknesses in excess of 30nm are reached within 50h. Atomic resolution Z-contrast microscopy was used to examine the Omega plate/matrix interface as a function of time and temperature to attempt to explain the observed thickening kinetics. At all times and temperatures, two atomic layers of Ag (and possibly Mg) are present at the broad faces of the Omega plates. The ends of the plates and the risers of the growth ledges appear free of segregation. At plate thicknesses less than 13nm, the density of thickening ledges is very low and for plate thickness in excess of 13nm, the density increases dramatically. The change in ledge density is thought to be associated with the transition from full coherency to semi-coherency of the Omega plate. The critical plate thickness for this transition has been experimentally determined to be close to 13nm. The high resistance of Omega plates to thickening at temperatures up to 200 degrees C may be due to the high activation energy for ledge nucleation at plate thicknesses less than 13nm. The role of Ag (and Mg) seems to be in their contribution (structural and chemical) to the interfacial energy of the Omega phase and in determining the critical plate thickness for the loss of coherency. The necessary Ag (and Mg) redistribution accompanying a migrating growth ledge is probably of secondary concern in accounting for the observed thickening kinetics.