| Higher-temperature structural materials are in critical need for the enhancement of thermal efficiency and advanced engine systems to replace currently available alloys, such as Ni-based superalloys and NiAl-based aluminum alloys. Among potential candidates are multiphase Mo-Si-B intermetallics due to their high melting temperature (above ∼2000°C) and relatively good oxidation resistance. In this study, efforts are made to investigate the fracture toughness (R-curve) and fatigue-crack growth behavior of the multiphase Mo-Si-B intermetallics with varying compositions (Mo-12Si-8.5B, Mo-16.8Si-8.4B, and Mo-10Nb-12Si-8.5B (at.%)) at temperatures from ambient to 1300°C, with the objective of discerning salient toughening mechanisms.; The boron-doped molybdenum silicide alloys, containing α-Mo, Mo 3Si, and Mo5SiB2 phases, with varying compositions were fabricated both by ingot and powder metallurgy to generate different microstructures for comparison. It is found that Mo-12Si-8.5B (at.%) alloy, in particular, displays quite high intrinsic (crack-initiation) toughness both at ambient and elevated temperatures (∼7–10 MPa√m). Improvements in the initiation fracture toughness are attributed to a microstructure containing coarse α-Mo particles, where the crack arrests. More surprisingly, its fracture toughness increases up to ∼11.8 MPa√m at 1300°C.; Mechanistic studies indicate that crack trapping by the primary α-Mo phase is the principal mechanism of toughening. Conversely, some degree of ductile-phase bridging is evident at 1300°C, due to the improved ductility of α-Mo phase. In addition, extensive microcracking can also be seen in the form of a network of arrested cracks parallel to the main crack. The relative insensitivity to fatigue degradation is demonstrated by cyclic fatigue-crack growth testing both at ambient and elevated temperatures up to 1300°C.; Finally, careful studies are conducted to control the microstructures and develop a correlation with material processing and mechanical properties. By precisely controlling composition and processing, the microstructure of the Mo-Si-B alloy system will be manipulated to enhance the toughening effects. “Materials-by-design” concept is utilized to design a new microstructure based on the dominant toughening mechanism in the alloy system. |
