Mechanical alloying behavior in molybdenum-silicon system

The mechanical alloying of Mo-Si powder mixtures by ball milling has been studied by X-ray diffraction and differential thermal analysis. Experiments were conducted at the compositions of 10, 25, 38, 54, 67, and 80 at.% Si. The reaction rate and the end-product markedly depended on the powder composition. The formation of molybdenum disilicide (MoSi₂) by mechanical alloying occurred at the compositions of 54, 67, and 80% Si. At the composition of 67% Si (MoSi₂ stoichiometry), single phase α-MoSi₂ formed rather abruptly, which suggested that the reaction occurred by a mechanism similar to that of the self-propagating high-temperature synthesis (SHS). At the compositions of 54 and 80% Si, however, the reaction proceeded gradually and both the low-temperature α and the high-temperature β phases of MoSi₂ were formed. The formation of Mo₅Si₃ at 25, 38, and 54% Si compositions was characterized by a slow reaction rate as the reactants and product coexisted for a long period of time. The Mo₃Si compound was formed only after heating the milled Mo₇₅Si₂₅ powder sample to 1000°C. The formation of molybdenum suicides by the mechanical alloying of Mo and Si elemental powders and the relevant reaction rates seemed to depend on the exothermicity of the reactions as well as the thermodynamic properties of the products involved.