Optimization of Mo-Si-B intermetallic alloys

Mo-Si-B intermetallics consisting of the phases Mo₃Si and Mo₅SiB₂, and a molybdenum solid solution (“α-Mo”), have melting points on the order of 2000 °C. These alloys have potential as oxidation-resistant ultra-high-temperature structural materials. They can be designed with microstructures containing either individual α-Mo particles or a continuous α-Mo phase. A compilation of existing data shows that an increase in the volume fraction of the α-Mo phase increases the room-temperature fracture toughness at the expense of the oxidation resistance and the creep strength. If the α-Mo phase could be further ductilized, less α-Mo would be needed to achieve an adequate value of the fracture toughness, and the oxidation resistance would be improved. It is shown that microalloying of Mo-Si-B intermetallics with Zr and the addition of MgAl₂O₄ spinel particles to Mo both hold promise in this regard. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 15–17, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.