Atomic Distribution in Aluminum-Silver Alloys During Aging

Abstract

Laue photographs of aluminum-rich aluminum-silver crystals during aging at 20° and 150°C show more or less clearly defined streaks along certain zonal ellipses, in addition to the streaks caused by thermal agitation. These have been analyzed by stereographic and reciprocal lattice projections and are found to result from two-dimensional gratings parallel to planes of the form {111}. The streaks with 20° aging are more diffuse than with 150° aging; no streaks were found with 200° aging. When the precipitate lattice is fully developed, it yields sharp spots along the streaks at positions predicted from earlier studies of the Widmanstätten structure; preceding this state there exist very thin plate-like nuclei on randomly spaced {111} planes, which govern the orientation and shape of the fully developed precipitate. These may consist of (a) clusters of silver atoms or (b) imperfect lattices caused by (111) layers of atoms shifting parallel to themselves as required for the transformation from the face-centered cubic to the hexagonal close-packed lattice but with the shifting occurring on random planes, thus destroying the lattice periodicity normal to these planes.