Effects of Co addition on the microstructure and properties of multi-elemental NbSi based ultrahigh temperature alloys

Five multi-elemental NbSi based ultrahigh temperature alloys with nominal compositions of Nb-22Ti-14.8Si-4Cr-2.5Al-xCo (x = 0, 1, 2, 4, and 8 at.%) were prepared via vacuum non-consumable arc-melting. The effects of Co addition on the microstructure, nanoindentation hardness, compressive strength and isothermal oxidation resistance at 1250 °C of the alloys were investigated. The results reveal that the content of both the Nbss/γ(Nb,X)₅Si₃ eutectic and the Nbss phase decreases with increasing Co content in the alloys and Co addition promotes the morphological transformation of Nbss toward a dendritic structure. Meanwhile, the addition of Co promotes the formation of new polymorphs of silicide phases; the increase in Co content changes the phase fractions of the alloys. In alloys with 0, 1, and 2 at.% Co, the only silicide phase is γ(Nb,X)₅Si₃, but in alloys with Co content above 2 at.%, two other types of silicide phases (α(Nb,X)₅Si₃ and β(Nb,X)₅Si₃) also exist except for γ(Nb,X)₅Si₃. Furthermore, Co addition promotes the formation of low-melting-point phases such as Cr₂(Nb,X), Tiss and Ti₂Co between Nbss/γ(Nb,X)₅Si₃ eutectic cells and along the Nbss boundaries, which degrades the compressive strength at 1250 °C of the alloys. However, Co addition increases the nanoindentation hardness of both Nbss and γ(Nb,X)₅Si₃ due to the solid solution strengthening effect of Co. The addition of Co has no significant effect on the oxidation resistance at 1250 °C of the alloys.