LOCAL ATOMIC ARRANGEMENTS IN AGE HARDENABLE ALLOYS

The traditional model for the aging of Al-Cu alloys has recently been challenged. This note compares and contrasts these two approaches. The traditional model assumes that there is a sequence of different regions that form with aging time. In particular, single Cu layers left brace 100 right brace planes of the Al-rich matrix (GP 1 zones) form first, followed by a thick coherent ordered Al-Cu platelet called GP 2 and than a partially incoherent intermetallic phase theta prime and the fully incoherent equilibrium phase Al//2 Cu. The GP 2 phase is usually thought to be two Cu layers surrounding some Al layers. In the new model the first two stages do not involve separate coherent phases but instead a coarsening of the Cu-rich platelets so that the thick zones are essentially all copper atoms. Whether or not the initial state is simply all single Cu layers or a mixture of single and multiple layered zones may depend on the quench rate, but the most recent studies always find a mixture. This new approach is based on examination of the diffuse intensities in an extended volume in reciprocal space. 22 Refs.