Shape refinement of Ge precipitates in an Al-Ge alloy by thermal cycling

It is shown that the shape of Ge precipitates in quenched and aged AI-Ge al loys can be refined by a thermal cycling heat treatment which causes partia l dissolution and regrowth of precipitates and leads to a shift in the dist ribution of precipitate morphologies. Using thermal cycling of material age d to contain precipitates in the cube-cube orientation relationship, it is shown that after several cycles of shrinkage and regrowth, the predominant precipitate shape is that of an octahedron. From direct in-situ observation in an electron microscope, the mechanism of shape refinement is found to b e the elimination of internal twins. This conclusion is in agreement with e arlier work, which showed that Ge precipitate shapes are controlled by twin ning. For material aged to contain predominantly lath-shaped precipitates, thermal cycling does not lead to twin-free precipitates. This behavior can be understood from symmetry considerations.