Role of particle size and nature of the reactants on the phase evolution, microstructure and densification of MoSi(2)-TiB(2) composites produced by in-situ reactions

The roles of particle size and nature (elemental vs. compound) of the reactants on the phase evolution, microstructure and densification of MoSi 2-TiB2 composites during in-situ reactions have been investigated. It is shown that the elemental reaction proceeds through a larger number of intermediate reaction steps and that the distribution of the MoSi2 and TiB2 phases in the final microstructures varied considerably depending on these factors.;In general, the small elemental reactant powders led to greater densification although the amount of oxides introduced into the system from the surface of the powders was higher. The dissolution and further segregation of the oxygen introduced into the system led to the formation of silica pockets, as the most distinctive characteristic of the resulting microstructures.;It was also found that the shape of the TiB2 grains is strongly influenced by the size of the initial reactant particles. Smaller powders led to highly compacted pellets and elongated TiB2 grains due to the simultaneous growth of a large number of fine grains. Larger powders produced more regular equiaxed TiB2 grains.;The densification obtained through the elemental reaction is significantly higher compared with the displacement reaction. This is related to the larger driving force for the initial reactions and earlier completion of the overall reaction allowing longer time for densification of the final phases.;The restrictions imposed by the stoichiometry of the reactions were surmounted by adding the appropriate compound powders to the charge prior to consolidation. It was found that no significant changes in the microstructure or densification were observed by adjusting the matrix-reinforcement ratio proving that this is a viable processing route for overcoming the stoichiometric constraints imposed by the reactants selected.;The effect of the initial powder size of the metallic components on the microstructure is shown specifically, the initial size of the molybdenum powders led to MoSi2 regions of similar size. Likewise, the size of the initial titanium powder particles resulted in partially-reacted final microstructures, this is related to the fact that the formation of TiB2 is the slowest reaction of all the sub-reactions that occur to produce the final MoSi 2-TiB2 composites.