Effect of Co on microstructures and mechanical properties for Nb-Si based in-situ composites

Highlights: • The effect of Co on Nb-Si based in-situ composites was discussed for the first time. • Fracture toughness was increased to 13.14 MPa·m^1/2 by addition of 1% Co. • Excessive Co promotes the precipitation of β-(Nb,X)₅Si₃ and deteriorates fracture toughness. In this paper, six alloys with different Co contents (Nb-16Si-20Ti-4Cr-1.5Al-4Hf-4Zr-1Ta-xCo (x = 0, 1, 2, 3, 4 and 5)) were prepared by copper water-cooled crucible arc-melting. The effects of Co on phase selection, microstructure, room-temperature fracture toughness, and compressibility were investigated. The six alloys are composed of Nbss, α-(Nb,X)₅Si₃, γ-(Nb,X)₅Si₃, and Cr₂(Nb,X) phases. The addition of Co element promotes the precipitation of Tiss phase. Primary β-(Nb,X)₅Si₃ phase was stabilized at room temperature by adding more than 3% Co. Alloy changes from a near-eutectic to a hypereutectic by the addition of Co. The bulk of primary silicide phase deteriorates the room-temperature fracture toughness. At the same time, Co element solid solution in Nbss phase reduces the Si content in Nbss phase and increases the K_Q value of alloy. Therefore, the K_Q value of 1Co alloy reaches 13.14 MPa·m^1/2. The maximum compressive strength of 5Co alloy is 2155 MPa, which is mainly due to the solid solution strengthening of Co element and the highest silicide phase content.