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 similar to 55 vol.% ductile alpha-Mo and similar to 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 degrees 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 alpha-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 degrees C, the increased ductility of alpha-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 growl:h 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. Published by Elsevier Ltd.