Metastable phase transitions in Mo-Si and V-Si systems activated by high energy ball milling

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Metastable phase transitions in Mo-Si and V-Si systems activated by high energy ball milling

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, 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 leads to the formation of both α-MoSi2 (room temperature phase) and β-MoSi2 (high temperature phase), but lower energy milling favors the formation of β phase, while higher energy milling promotes the formation of α phase. In addition, if the milling energy is high enough, the Mo/Si reaction is 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 leads to amorphous transition, whereas intensive milling causes the formation of V3Si and V5Si3 intermetallic compounds. Finally, the thermodynamics and kinetics related to the different phase transitions in the two systems were discussed.