Processing - structure - property relationships in Molybdenum Disilicide with ternary additions

Abstract

The following thesis focuses on the effect of ternary additions and process methods on the mechanical and structural properties of molybdenum disilicide (M0SI2). The principal lines of investigation included the investigation of phase formation during the mechanical alloying of Mo and Si with Al, Mg, Ti, Zr and Fe ternary additions, the investigation of silica content and morphology in M0SI2 consolidated with Al additions, and the investigation of room temperature mechanical properties using modified Vickers indentation techniques. The phase formation during the mechanical alloying of Mo and Si elemental powders with a ternary addition of Al, Mg, Ti, Zr or Fe was monitored using X-ray diffraction with Rietveld analysis used to quantify the phase proportions. When Mo and Si are mechanically alloyed in the absence of any additions, the tetragonal (I4/mmm) or equilibrium form of M0SI2 (/-M0SI2) forms by self-propagating high temperature synthesis (SHS). With additional milling, the tetragonal phase transforms to a hexagonal (P6222) structure (/1-M0SI2). The mechanical alloying of Al, Mg and Ti additions with Mo and Si tend to promote a more rapid transformation of /-M0SI2 to /1-M0SI2. In high concentrations, the addition of these ternary elements inhibits the initial SHS reaction of Mo and Si to I-M0SI2, instead promoting the formation of h-M0SI2. The addition of Zr and Fe tend to stabilise the tetragonal phase. A computer model of the mechanical alloying process has been developed to simulate the effect of the various alloying additions on the phase evolution observed during milling. Using the Arhennius equation, the model balances the formation rates of the competing reactions that are observed during milling. These reactions include the combustion of Mo and Si to form I-M0SI2, the diffusion of Mo and Si to form h-M0SI2, the transformation due to deformation of tetragonal to the hexagonal polymorph, and the recovery of I-M0SI2 from h-M0SI2 and deformed I-M0SI2. The addition of the various ternary additions change the free energy of formation of the associated M0SI2 alloys ie. Mo(Si,Al)2, Mo(Mg,Al)2, (Mo,Ti)SI2, (Mo,Zr)SI2 and (Mo,Fe)SI2, respectively. Variation of the energy of formation alone is sufficient for the simulation to accurately model the observed phase formations. ………………….