Processing of MoSi2-based intermetallics

The influence of processing on the microstructure and properties of MoSi2 and the adjacent hexagonal ternary compounds Mo(Si,Al)2 and Mo5Si3C has been investigated. Since powder processing is clearly the preferred processing method for these compounds, it is important to realize that the oxides present on the starting powder surfaces (i.e. silica on the silicon and silicide powders) will be incorporated into the compacted samples unless special care is taken to remove them prior to and/or during consolidation. For binary MoSi2, the reduction of silica has been accomplished using carbon additions, to produce SiC and gaseous CO/CO2 mixtures, or aluminum additions to produce alumina particles. For the Mo5Si3C studies, it is clear that the relatively large ternary phase field on previously published phase diagrams is in error and that this phase is closer to a line compound, making the production of single phase material difficult. In the case of the aluminum-modified MoSi2, aluminum additions beyond the solubility limit leads to the formation of the hexagonal C40 structure, which undergoes an unusual environmental degradation at near-ambient temperatures. In attempting to determine the mechanism of this degradation, it was noted that there are also significant inconsistencies in the published ternary Mo–Si–Al diagram. Finally, the relative importance of such variables as crystal structure, silica content, nature, size and volume fraction of second phase and matrix grain size on the high temperature properties of these silicides is described and compared with previous studies of MoSi2 in order to clarify some of the confusion that has existed in the literature dealing with the mechanical behavior of MoSi2 and its composites.