The Investigation of Synthesis and Phase Transformation in Molybdenum Disilicide by Mechanical Alloying

The molybdenum dicilicide is an intermetallic compound with two allotropies tetragonal ?-MoSi2 and hexagonal ?-MoSi2.The ?-MoSi2 phase is established to 1900 ?C and ?-MoSi2 phase from 1900 to 2050 ?C. This material has considerable properties that provide different applications in different industries such as air-space industry and gas turbines. The molybdenum synthesis can be down in various ways such as melting and casting, SHS (Self propagating High temperature Synthesis). Recently, the mechanical alloying method because of its considerable advantages such as powder particle compound homogenizing, fragmentation and contact area increasing and also crystal defect increasing that affects ductility increasing of brittle compound such as intermetallic compounds, is used for progressive materials synthesizing. In this research, the synthesis and formation of molybdenum diciliside was investigated by mechanical alloying method from stoichiometric mixing molybdenum and silicon (Mo/2Si) and also its phase transformations. A mixture of elemental molybdenum and silicon powders at the stoichiometric composition of MoSi2 were ball milled to 100 hours using a planetary ball mill. The milling was performed at rotational speeds (vial speeds) of 300 and 400 rpm and stainless steel balls (5 and 10 mm in diameter). Other synthesis conditions such as ball to powder weight ratio and mass of the charged powder were chosen similarly. The results demonstrate that mechanical milling affects the formation of nanocrystalline MoSi2 from elemental powders through solid state reaction. In other words, formation of molybdenum disilicide from its primary elements during mechanical alloying depends on effective parameters in mechanical alloying (collision frequency of ball to powder particles, contact area creation and heat transferred to powder particles view point) according to XRD results. The results indicate that increasing in milling intensity and decreasing in ball diameter cause to fast formation of MoSi2, which is derived of increasing in milling energy. The thermal analysis investigation confirmed affection of mechanical activation on the acceleration of MoSi2 synthesis with reduction of formation temperature of molybdenum disilicide during mechanical alloying and electron microscopic investigation showed that MoSi2 forms on the surface of molybdenum grains during mechanical alloying. It was also demonstrated that formation of MoSi2 (? and/or ?) depends on mechanical activation conditions and selection of parameters during mechanical alloying. So that if intensity of mechanical activation be chosen high and/or selection of parameters contribute with high heat production in milling vials, ?- MoSi2 phase alone or with ?- MoSi2 phase (that has low activation energy with respect to ?- MoSi2) will be produced. In lower milling intensity, usually at first ?- MoSi2 is produced. Phase transformation of ? ? ? occurs during mechanical alloying when the crystallite size decreases to approximately 12 nm. The final product of mechanical alloying is ?- MoSi2.