J. Myers, S. Vega, S. Woodard, Pratt and Whitney Aircraft Engines, East Hartford, CT

Summary:

The use of Mo-Si-B alloys as a structural material at elevated temperatures is dependent on both environmental resistance and structural capability. A high volume fraction of intermetallic in these alloys is desired for oxidation resistance, while a low volume fraction of intermetallic within a continuous metal matrix is optimum for meeting demanding mechanical property requirements. Alloys near the Mo-3Si-1B / Mo-8.9Si-7.7B (wt% / at%) composition are attractive because they approach a balance between these competing material requirements. These alloys consist of Mo₅SiB₂ and Mo₃Si intermetallics within an a-molybdenum matrix.

Powder processing improvements have resulted in the production of powder with a solidification microstructure consisting of fine primary Mo(_ss) dendrites and a eutectic mixture of Mo(_ss), Mo₅SiB₂ and Mo₃Si. Consolidation methods such as hot isostatic pressing (HIP) and extrusion change both the microstructure size and morphology. Understanding these changes is a vital part of the effort aimed at the production of an ideal microstructure during subsequent thermo-mechanical processing. The effect of different heat treatment conditions on the microstructures of these alloys will also be discussed.