ON THE FORMATION OF TWINNED PRECIPITATES IN AL-GE ALLOYS

High-resolution microscopy has been used systematically to obtain a detailed understanding of the morphologies and substructures of Ge needle or lath precipitates in an Al-Ge alloy. We show that the many orientation relationships observed in twinned <100> and <110> needles can be related to only three basic lattice correspondences. The feature common to <100> and <110> needles is the arrangement of five cozonal twin segments whose relative degrees of development determines the cross-section morphology. Proposed growth mechanisms based on multiple twinning can account for the formation of all the different observed precipitate morphologies and defect substructures. The central role of twinning and the critical need for vacancies during nucleation are discussed in terms of possible models for nuclei that maximize the number of Ge-Ge bonds through twinning and thus minimize the need for vacancies.