Phase ordering has a strong influence on many properties of intermetallic alloys. In particular mechanical properties, such as creep resistance, which decreases drastically when phase transition occurs. Since the transition takes place at elevated temperatures and is influenced by materials purity, it is experimental investigations are time consuming. On the other hand, new modeling techniques, such as ab initio pseudo potential calculations including phonon contributions make it possible to study ordering-disordering process in ideally pure systems. In particular, the temperature of such phase transition can be predicted. The results of the investigations are discussed with respect to the date obtained experimentally.