Influence of molybdenum contents on the microstructure, mechanical properties and oxidation behavior of multi-elemental Nb–Si based ultrahigh temperature alloys

The mechanical properties, including room-temperature fracture toughness, microhardness and compressive strength at 1250 °C, and oxidation resistance at 1250 °C of Nb–Si based alloys with different Mo contents (0, 2, 5 and 10 at.% Mo additions) were systematically evaluated. The roles of Mo content additions in the relationships between these properties and microstructures in a multi-elemental system of Nb–Ti–Si–Cr–Al–V-Hf-Zr-Mo were revealed. The results show that the four alloys are all composed of primary γ(Nb,X)₅Si₃ blocks, three types of Nbss/γ(Nb,X)₅Si₃ eutectic and a small amount of Nbss/Cr₂Nb eutectic. Alloying with Mo obviously decreases the amount and size of primary γ(Nb,X)₅Si₃ blocks, and correspondingly increases the concentration of Si in the remaining liquid during solidification, which results in preferable formation of regular eutectic. Both the microhardness of microstructural constituents at room temperature and the compressive strength at 1250 °C of the alloys are obviously improved due to the solid solution strengthening effect by Mo addition in the alloys. Adding 2 at.% Mo in the alloys simultaneously ameliorates the room-temperature fracture toughness as well as the oxidation resistance at 1250 °C of the alloys, while the additions of 5 and 10 at.% Mo in the alloys deteriorate them. Moreover, the adhesion of oxide scales is significantly improved by alloying with Mo.