Morphological evolution of primary β-Nb₅Si₃ phase in Nb-Mo-Si alloys

Nb-20Si–xMo (x = 2,4,6)-based ultrahigh temperature alloys were prepared by high-vacuum non-consumable arc melting in a water-cooled copper crucible. The morphological evolution of the primary β-Nb₅Si₃ phase in the Nb-Mo-Si-based alloys was discussed in detail based on the data of X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. The results indicate that the phase constituents in the studied alloys consist of Nb solid-solution (Nbss) phase and β-Nb₅Si₃ phase, with Nb₃Si phases also appearing in the Nb-20Si-2Mo alloy. The Mo addition does not change the morphology of the primary β-Nb₅Si₃ phase in the Nb-20Si alloy. The primary β-Nb₅Si₃ phase in the investigated alloys displays various shapes, such as dendritic, L-like, hollow square and square ones. The morphological evolution of the primary β-Nb₅Si₃ phase follows two main paths in the Nb-Mo-Si alloys. (I) At a lower cooling rate, the primary β-Nb₅Si₃ morphology undergoes a transformation from L-like shapes to hollow square shapes before finally forming square shapes, and (II) at a higher cooling rate, the primary β-Nb₅Si₃ morphology transforms from Y-like shapes to hollow square shapes and ends with forming square shapes.