Fracture and Fatigue-Crack Growth Behavior in Mo-12Si-8.5B Intermetallics at Ambient and Elevated Temperatures

perhaps the most progress has been made with alloys based on nickel and especially titanium aluminides. These alloys can Boron-containing molybdenum silicides have received some exhibit significant room temperature ductility, at least compared interest of late due to their superior low-temperature "pest" to other intermetallics, but have the major disadvantage that their resistance and comparable high-temperature oxidation resistance operating temperatures would be limited to less than lOOO"C, to MoSiz-based silicides; however, like many ordered which is too low for certain emerging applications [I]. One intermetallics, they are plagued by poor ductility and toughness candidate material which could be used at significantly higher properties. Of the various multiphase Mo-Si-B intermetallic temperatures is molybdenum disilicide, due to its excellent systems available, alloys with compositions of Mo-12Si-8.5B oxidation resistance, high melting temperature, and relatively easy (at.%), which contain Mo, Mo3Si, and T2 phases, are anticipated processibility. However, the structural use of molybdenum to have higher toughnesses because of the presence of the silicides is severely limited by their low ductility and poor fracture relatively ductile Mo phase. In this study, we examine the toughness at ambient temperatures. ambient to high (1300°C) temperature fracture toughness (RIn an attempt to enhance the ductility and fracture toughness of curve) and fatigue-crack growth characteristics of Mo-l2Si-8.5B, these alloys, one approach is through in situ ductile-phase with the objective of discerning the salient mechanisms governing toughening. The idea here is to process the silicide with excess crack growth. It is found that this alloy displays a relatively high Mo; then provided the crack intercepts the resulting primary aintrinsic (crack-initiation) toughness at 1200°C (-10 MPadm), but Mo phase, catastrophic fracture can be hindered through the only limited extrinsic R-curve (crack-growth) toughness. formation of unbroken ductile-particle ligaments in the crack Although the lack of extrinsic toughening mechanisms is not wake. The resulting crack bridging and plastic deformation of the beneficial to quasi-static properties, it does imply in a brittle particles, together with crack deflection and interfacial debonding, material that it should show only minimal susceptibility to provide the main contributions to toughness [2-71. Since premature failure by fatigue. This relative insensitivity to fatigue molybdenum silicides with Si concentrations less than those of was confirmed by fatigue-crack growth testing at temperatures MoSiz, i.e., MoSSi3 and Mo3Si, suffer from poor oxidation from ambient up to 1300°C. resistance, boron is added in the current alloy to promote oxidation resistance by forming protective borosilicate glass [8]. It has been reported that the addition of as little as 1 wt.% boron improved the oxidation resistance by as much as five orders of magnitude at moderate to high temperatures (80O0 to 1500°C) [l]. Consequently, the current alloy was processed with a composition of Mo-12Si-8.5B (at.%), such that it contained the a-Mo, Mo3Si, and Mo5SiBz (T2) phases. Indeed, the toughness of this material was expected to be higher than that of MoSiz due to the presence of the relatively ductile Mo phase. The resulting microstructure indicates that a-Mo forms as isolated particles in the Mo3S