Collaborative optimization of microstructure, phase composition and room-temperature fracture toughness of Nb−Si based alloys using Ti, Zr and Hf elements

The Nb−16Si−xTi−yZr−zHf (x=18, 22; y=0, 4; z=0, 4; at.%) alloys were prepared by arc melting to investigate the influence of elementsTi, Zr and Hf additions on the phase constitution, microstructure, fracture toughness and crack propagation behavior. The results show that the single addition of 4 at.% Zr promotes the eutectoid reaction of (Nb,X)3Si phase to Nb solid solution(Nbss)/γ-(Nb,X)5Si3 eutectic, and simultaneous addition of Ti, Zr and Hf further promotes the eutectoid reaction. The crack trends to propagate in (Nb,X)3Si phase, and the crack deflects when the crack meets Nbss. The fine Nbss/γ-(Nb,X)5Si3 eutectic and Nbss/γ-(Nb,X)5Si3 lamellar structures can induce the crack bridging and branching, and hinder the crack growth. Nb−16Si−22Ti−4Zr−4Hf alloy with the highest content of alloying elements exhibits the highest room-temperature fracture toughness (11.62 MPa·m1/2), which is 87.7% higher than that of the Nb−16Si−18Ti alloy. The performance improvement is mainly attributed to the presence of lamellar eutectic structure.