Balancing the fracture toughness and tensile strength by multiple additions of Zr and Y in Nb–Si based alloys

To obtain the balanced mechanical properties of Nb–Si based alloys, the effects of multiple additions of Zr and Y on microstructure, fracture toughness and tensile strength were investigated. The compositions of the alloys were designed as Nb–16Si–23Ti–4Cr–2Al-2Hf-(3Zr-0.6Y), (5Zr-0.3Y) and (5Zr-0.6Y). The addition of Zr promotes silicide transition from α-Nb₅Si₃ to γ-Nb₅Si₃; in 5Zr alloys, the γ-Nb₅Si₃ is the sole silicide. Added Y accelerates the spheroidization rate of silicide during the heat treatment process. The toughness of 3Zr-0.6Y, 5Zr-0.3Y and 5Zr-0.6Y alloys are 23.1, 17.9 and 20.0 MPa m^1/2, respectively. Alloys containing more Y and less Zr have better toughness, accompanied by the greater degree of crack deflections, more microcracks and rougher fracture surfaces. The tensile strength of the three alloys are 919, 1209 and 1159 MPa, respectively. The higher strength of 5Zr alloys is attributed to greater orientation difference and immobile dislocations. The two mechanical properties are balanced successfully in the 5Zr-0.6Y alloy.