There are considerable efforts in developing new materials for the increased demand on the high-temperature structural components.  Ceramic matrix composites and intermetallics are receiving particular attention for this purpose.  Many researchers have had interest in nickel aluminide and titanium aluminide because they exhibit significant room temperature ductility compared to other intermetallics.  However, their operating temperatures are limited to less than 1000°C, which is already too low for some emerging high temperature applications.  One of the candidates is molybdenum silicides, which possess a number of unique properties, including excellent oxidation resistance up to 1700°C and relatively easy processability.  However, they demonstrate relatively poor oxidation resistance at elevated temperatures and low fracture toughness at room temperature.
    Of various Mo-Si compounds, only MoSi₂ has been well characterized, because of its excellent oxidation resistance in the 800-1700°C.   Despite the good oxidation resistance of MoSi₂, this material has a high creep rate above 1200°C presumably due to its brittle-to-ductile transition around the temperature, making it unsuitable for a high-temperature structural material in its monolithic state.

     In 1954, Nowotny et al. reported that Mo-Si forms a stabilized hexagonal structure with the addition of carbon, which has led to research into other light element additions such as boron.  Boron-containing molybdenum silicides have recently received substantial interest due to their comparable oxidation resistance to MoSi₂-based silicides resulting from the formation of borosilicate glasses, and better mechanical properties.  It is interesting to note that without boron, molybdenum silicides only form an initial porous oxide scale, but small additions of boron to the system promote the growth of continuous, non-porous protective scale less than 10 micrometers thick.  It was reported that the addition of as little as 1 wt% boron improved the oxidation resistance by as much as five orders of magnitude from moderate to high temperatures (800 - 1500°C).  Depending on the exact composition, various Mo-Si-B systems (multiphase intermetallics) of Mo, Mo₃Si, T1 (Mo₅Si₃), and T2 (Mo₅SiB₂) phases can be produced.  The alloy system consisting of Mo, Mo₃Si, and T2 is expected to have higher fracture toughness than Mo₃Si-T1-T2 system, because of the presence of molybdenum, yet less oxidation resistance for the same reason.  In this study, mechanical properties of this Mo-Mo₃Si-T2 alloy system are being investigated both at ambient and high temperatures (1000-1300°C).  The Mo-Mo₃Si-T2 alloy system exhibits relatively high toughness (greater than 7 MPaÖm) and bend strength (>600 MPa) at room temperature after annealing at 1600°C.  Efforts are currently being made to examine R-curve behavior and cyclic fatigue-crack growth rate at ambient and elevated temperatures.

Full Text in PDF form: TMS Conference Presentation in Fall 2000

Ambient to high temperature fracture toughness and fatigue-crack propagation behavior in a Mo-12Si-8.5B (at.%) intermetallic by H. Choe, D. Chen, J.H. Schneibel, and R.O. Ritchie. Intermetallics, 2001

Fracture and Fatigue Properties of Mo-Mo3Si-Mo5SiB2 Refractory Intermetallic Alloys at Ambient to Elevated Temperatures (25 C- 1300 C) by H. Choe, J. H. Schneibel, and R. O. Ritchie, Metallurgical and Materials Transactions A, vol. 34A (2), Feb. 2003, pp. 225-239.

                      Ongoing Research

Current research is being conducted based on Materials-by-Design approach.  Two principal improvements will be made using the materials-by design concept.  First, oxidation resistance is expected to be enhanced due to the increase in Si contents.  Second, its restance to crack propagation (both in monotonic and cyclic loadings) will be improved by a continuous distribution of Mo phase to obtain more effective crack trapping mechanism, based on the results that crack trapping acts as a dominant toughening mechanism in this alloy.  The mechanical properties both at room and high temperatures and oxidation resistance of the newly processed alloy are being extensively investigated in conjunction with Oak Ridge national Lab.