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 alloys can be refined by a thermal cycling heat treatment which causes partial dissolution and regrowth of precipitates and leads to a shift in the distribution of precipitate morphologies. Using thermal cycling of material aged 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 be the elimination of internal twins. This conclusion is in agreement with earlier work, which showed that Ge precipitate shapes are controlled by twinning. 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.