We study theoretically the size versus shape versus temperature relation of precipitates in Al-Zn via quantum-mechanical first-principles simulations. Our parameter-free model, based on a mixed-space cluster expansion, allows the prediction of the experimentally observed size and temperature dependences of the precipitate shape. We find that aging experiments can be explained in terms of equilibrium shapes. The precipitates change from a nearly spherical to a more ellipsoidal/hexagonal shape with increasing size and decreasing temperature. They always flatten in the  direction, which can be interpreted as a consequence of a mechanical instability of face-centered cubic Zn when rhombohedrally distorted along  and a strong anisotropy of the chemical energy. The excellent agreement between experiment and theory shows that our model can be used to quantitatively predict precipitate shapes and sizes.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys