SYNTHESIS AND FORMATION MECHANISMS OF MOLYBDENUM SILICIDES BY MECHANICAL ALLOYING

The synthesis and formation of MoSi2, Mo5Si3, and Mo3Si compounds by t he mechanical alloying of Mo-Si powder mixtures has been investigated. Ball-milling experiments were conducted for the composition range of 10-80 at.% Si. The formation of molybdenum silicides, especially MoSi2 , during mechanical alloying and the relevant reaction rates markedly depended on the powder composition. The spontaneous formation of alpha MoSi2 during mechanical alloying at 67 at.% Si (MoSi2 stoichiometry) proceeded by a mechanically-induced self-propagating reaction (MSR), t he mechanism of which is analogous to that of the self-propagating hig h-temperature synthesis (SHS). At the compositions of 54 and 80 at.% S i, however, the formation of MoSi, proceeded by the gradual formation of both the alpha and beta phases instead of the MSR mode. The formati on of Mo5Si3 during mechanical alloying was characterized by a slow re action rate as the reactants and product coexisted over a long period. The milling of Mo-rich powder mixtures up to 150 h did not lead to th e direct formation of Mo3Si. The Mo3Si phase appeared only after brief annealing at temperatures of 800 degrees C and above.