Abstract
We have elucidated the physical mechanisms governing the observed size- and temperature dependence of precipitate shapes in Al-Zn alloys via quantum-mechanical first-principles simulations. In remarkable quantitative agreement with alloy aging experiments, we find that with decreasing temperature and increasing average size, the precipitates change from spherical to plate-like shape. Although the precipitates are created by an inherently kinetic heat treatment process, the entire series of their size vs. shape relation can be explained in terms of thermodynamic arguments and understood in terms of strain and chemical energies.