Predicting the size- and temperature-dependent shapes of precipitates in Al-Zn alloys

We study theoretically the size versus shape versus temperature relation of precipitates in AI-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 depende nces of the precipitate shape. We find that aging experiments can be explai ned in terms of equilibrium shapes. The precipitates change from a nearly s pherical to a more ellipsoidal/hexagonal shape with increasing size and dec reasing temperature. They always flatten in the [111] direction, which can be interpreted as a consequence of a mechanical instability of face-centere d cubic Zn when rhombohedrally distorted along [111] and a strong anisotrop y of the chemical energy. The excellent agreement between experiment and th eory shows that our model can be used to quantitatively predict precipitate shapes and sizes. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Sc ience Ltd. All rights reserved.