Ambient to high temperature fracture toughness and fatigue-crack propagation behavior in a Mo-12Si-8.5B (at.%) intermetallic

Boron-containing molybdenum silicides have been the focus of significant research of late due to their potentially superior low-temperature "pest" resistance and high-temperature oxidation resistance comparable to that of MoSi₂-based silicides; however, like many ordered intermetallics, they are plagued by poor ductility and toughness properties. Of the various multiphase Mo-Si-B intermetallic systems available, alloys with compositions of Mo-12Si-8.5B (at.%), which contain Mo, Mo₃Si, and T2 phases, are anticipated to have higher toughnesses because of the presence of the relatively ductile Mo phase. In this study, we examine the ambient to high (1300°C) temperature fracture toughness (R-curve) and fatigue-crack growth characteristics of Mo-12Si-8.5B, with the objective of discerning the salient mechanisms governing crack growth. It is found that this alloy displays a relatively high intrinsic (crack-initiation) toughness at 800 up to 1200°C (∼10 MPa√m), but only limited extrinsic R-curve (crack-growth) toughness. Although the lack of extrinsic toughening mechanisms is not necessarily beneficial to quasi-static properties, it does imply in a brittle material that it should show only minimal susceptibility to premature failure by fatigue, as is indeed observed at temperatures from ambient to 1300°C. Of particular significance is that both the fracture toughness and the threshold stress intensity for fatigue are increased with increasing temperature over this range. This remarkable property is related to a variety of toughening mechanisms that become active at elevated temperatures, specifically involving crack trapping by the α-Mo phase and extensive microcracking primarily in the Mo₅SiB₂ phase.