On the fracture toughness of fine-grained Mo-3Si-1B (wt.%) alloys at ambient to elevated (1300 °C) temperatures

New structural alloys based on borosilicides of molybdenum have been considered as potential replacements for current Ni-base superalloys, as they show promise as highly oxidation- and creep-resistant materials while still maintaining a moderate level of damage tolerance. Two alloys, each composed of Mo-3Si-1B (wt.%) with nominally similar fine-grained microstructures, have been developed utilizing markedly differing processing routes. Here, we study the influence of processing route on the fracture toughness of alloys containing ∼55 vol.% ductile α-Mo and ∼45 vol.% brittle intermetallics (Mo3Si (A15) and Mo5SiB2 (T2)). The room temperature toughness of these two alloys is significantly lower than that of previously evaluated coarser-grained Mo-Si-B alloys with similar composition; however at 1300 °C, the crack-initiation toughness of the fine- and coarse-grained alloys are nearly identical. At lower temperatures, the current finer-grained materials behave in a brittle manner as the smaller grains do not provide much impediment to crack extension; cracks can advance with minimal deflection thereby limiting any extrinsic toughening. Plastic constraint of ductile α-Mo grains by the hard intermetallic grains also serves to lower the toughness. Silicon impurity concentrations in the grain boundaries in the fine-grained alloys are much higher, leading to lower grain-boundary strengths and contributing to the much lower room temperature initiation toughnesses of these alloys (no stable crack growth was observed), as compared to the coarser-grained alloys. At 1300 °C, the increased ductility of α-Mo allows for significant plasticity; the correspondingly much larger contribution from intrinsic toughening results in significantly enhanced toughness, such that the finer grain morphology becomes less important in limiting crack growth resistance. Further optimization of these alloys, however, is still required to tailor their microstructures for the mutually exclusive requirements of oxidation resistance, creep resistance and damage tolerance.