On the Fracture and Fatigue Properties of Mo-Mo3Si-Mo5SiB2 Refractory Intermetallic Alloys at Ambient to Elevated Temperatures (25 °C to 1300 °C)

The need for structural materials with high-temperature strength and oxidation resistance coupled with adequate lower-temperature toughness for potential use at temperatures above ,1000 8C has remained a persistent challenge in materials science. In this work, one promising class of intermetallic alloys is examined, namely, boron-containing molybdenum silicides, with compositions in the range Mo (bal), 12 to 17 at. pct Si, 8.5 at. pct B, processed using both ingot (I/M) and powder (P/M) metallurgy methods. Specifically, the oxidation (“pesting”), fracture toughness, and fatigue-crack propagation resistance of four such alloys, which consisted of ,21 to 38 vol. pct a -Mo phase in an intermetallic matrix of Mo3Si and Mo5SiB2 (T2), were characterized at temperatures between 25 8C and 1300 8C. The boron additions were found to confer improved “pest” resistance (at 400 8C to 900 8C) as compared to unmodified molybdenum silicides, such as Mo5Si3. Moreover, although the fracture and fatigue properties of the finer-scale P/M alloys were only marginally better than those of MoSi2, for the I/M processed microstructures with coarse distributions of the a -Mo phase, fracture toughness properties were far superior, rising from values above 7 MPa!m at ambient temperatures to almost 12 MPa!m at 1300 8C. Similarly, the fatigue-crack propagation resistance was significantly better than that of MoSi2, with fatigue threshold values roughly 70 pct of the toughness, i.e., rising from over 5 MPa!m at 25 8C to ,8 MPa!m at 1300 8C. These results, in particular, that the toughness and cyclic crack-growth resistance actually increased with increasing temperature, are discussed in terms of the salient mechanisms of toughening in Mo-Si-B alloys and the specific role of microstructure.