Growth mechanisms of AuCu II plates

Abstract

The growth mechanisms of AuCu II plates are considered upon the basis of the measurements of their lengthening and thickening rates as reported by Pedraza and Kittl ( Acta Met. 24, 835, 1976). These rates are 3–4 orders of magnitude smaller than those estimated for incoherent AuCu II: disordered matrix (interphase) boundaries. During growth, the interphase boundaries are deduced to be coherent. At the edges of plates, coherency is maintained because the succession of ledges is too rapid to give the misfit dislocation structure time to develop. At the broad faces of plates, the closely spaced twins (initially, ca. 50 Å apart) in the AuCu II greatly diminish the driving force for introduction of misfit dislocations until substantial coarsening of the twinned structure has occurred. Ledge growth at the broad faces is inhibited by the requirement that such growth occur parallel to the twins; this effect appears responsible for the order of magnitude larger spacing between ledges at the broad faces than at the edges of the plates and for the growth of AuCu II crystals as plates rather than as idiomorphs. The principal features of AuCu II growth were shown to be consistent with a massive rather than a martensitic mode of transformation.