Dislocation structures produced by dissolution of discontinuous precipitate products

Abstract

Discontinuous precipitation lamellar products in Al-22at.%Zn have been subjected to dissolution treatments. The main purpose of this study was to see whether the dissolution process at temperatures slightly above the solvus was continuous or discontinuous. Detailed transmission electron microscopy examination of the resulting microstructures has shown that the dissolution process involved volume diffusion and that the grain boundaries remained essentially static during dissolution. The most striking features of the post-dissolution microstructures are walls of screw dislocations marking the planes of impingement of pairs of α′-β dissolution interfaces. Occassionally, edge dislocation walls were formed perpendicular to the original screw walls. The dynamics of the recovery process accompanying the dissolution rapidly organize the dislocations into subcess. Dark field imaging combined with convergent beam and microdiffraction techniques in the as-aged-and-quenched steady state lamellar products suggested that a small but reproducible twist distortion is accomplished during growth and is preserved during dissolution. The origin of this deformation is discussed.