Mechanical alloying of refractory metal–silicon systems

Mechanical alloying of Mo–Si (Mo33Si67) and V–Si (V75Si25) powder mixtures was activated by high-energy ball milling at ambient temperature. The metastable phase transitions in both systems during milling were investigated by x-ray diffraction, and scanning and transmission electron microscopy. It is found that the alloying processes are closely related to the milling conditions. As far as the Mo–Si system is concerned, ball milling led to the formation of both αMoSi2 (room temperature phase) and βMoSi2 (high temperature phase), but lower energy milling favored the formation of β phase, while higher energy milling promoted the formation of α phase. In addition, if the milling energy is high enough, the Mo–Si reaction would be governed by a self-propagating high temperature process. On the other hand, two different pathways of phase transition in the V–Si system were also identified depending on the milling intensity, i.e. weak milling led to an amorphous transition, whereas intensive milling caused the formation of V3Si and V5Si3 intermetallic compounds. Finally, the thermodynamics and kinetics related to the different phase transitions in the two systems are discussed.