  Zr and Sc precipitate in aluminum alloys to form the Al\_3Zr\_xSc\_{1-x}
compound which, for low supersaturations of the solid solution, exhibits the
L1\_2 structure. The aim of the present study is to model at an atomic scale
the kinetics of precipitation and to build mesoscopic models so as to extend
the range of supersaturations and annealing times that can be simulated up to
values of practical interest. In this purpose, we use some ab initio
calculations and experimental data to fit an Ising type model describing
thermodynamics of the Al-Zr-Sc system. Kinetics of precipitation are studied
with a kinetic Monte Carlo algorithm based on an atom-vacancy exchange
mechanism. Cluster dynamics is then used to model at a mesoscopic scale all the
different stages of homogeneous precipitation in the two binary Al-Zr and Al-Sc
alloys. This technique correctly manages to reproduce both the kinetics of
precipitation simulated with kinetic Monte Carlo as well as experimental
observations. Focusing on the nucleation stage, it is shown that classical
theory well applies as long as the short range order tendency of the system is
considered. This allows us to propose an extension of classical nucleation
theory for the ternary Al-Zr-Sc alloy.

Materials Science