• PDF / 2,956,830 Bytes
• 15 Pages / 612 x 792 pts (letter) Page_size
• 12 Downloads / 250 Views

DOWNLOAD

REPORT

---

mperature structural materials to replace nickel-based superalloys for future propulsion systems, transition-metal silicides have received considerable recent interest.[1–7] These alloys constitute a unique class of ultrahigh-temperature intermetallic materials, with high melting points (molybdenum and titanium silicides have melting points in excess of 2000 ⬚C) and the capability of forming protective silicon oxide films for enhanced oxidation resistance at elevated temperatures in hostile environments. In addition, specific silicides, such as Mo5Si3 (T1), have excellent creep resistance at temperatures as high as 1400 ⬚C.[2] Despite these advantages, most refractory silicides invariably display very poor fracture toughness at low temperatures[3,4] and can be susceptible to oxidation problems (“pest” reactions*) at temperatures below ⬃1000 ⬚C.[8,9,10] *The pest reaction, i.e., accelerated oxidation at intermediate temperatures, is generic to all forms of molybdenum silicides. For example, MoSi2 is prone to pest reaction in air between 300 ⬚C and 600 ⬚C,[8,9] whereas monolithic Mo5Si3 exhibits severe pest reaction at 800 ⬚C.[12,14]

H. CHOE, formerly Graduate Student with the Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Engineering, University of California, Berkeley, is Postdoctoral Researcher, Northwestern University, Evanston, IL 60208. J.H. SCHNEIBEL, Senior Scientist, is with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115. R.O. RITCHIE, Professor, is with the Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Engineering, University of California, Berkeley, CA 94720. Contact email: [email protected] Manuscript submitted April 23, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A

A potential solution to these problems may be provided by the boron-modified molybdenum silicide system.[11] These alloys generally consist of thermodynamically stable two-phase mixtures of Mo and Mo5SiB2 (T2) or threephase mixtures of Mo, Mo5SiB2, and Mo3Si, have high melting temperatures above ⬃2000 ⬚C, improved low-temperature fracture toughness properties (compared to MoSi2),[3,7] and excellent high-temperature oxidation resistance that increases with increasing boron content.[12–18] Indeed, in contrast to MoSi2 and Mo5Si3, which are very prone to pest reactions,[10,12,14] tertiary Mo-Si-B alloys have a reduced susceptibility to such intermediate temperature oxidation due to the formation of a protective borosilicate layer.[12–14,16–18] However, as the compositions that promote such oxidation resistance, i.e., higher B and Si, may not be the ones that promote toughness, i.e., higher Mo, optimization of these alloys requires a trade-off between crack growth and oxidation resistance. Although several recent studies have focused on the oxidation and pesting properties of Mo-Si-B alloys,[8–10,11–18] little research has been devoted to their fracture and fatigue properties, part