Effect of C addition on microstructure and mechanical properties of Nb–Si–Ti based alloys

To improve the mechanical properties of Nb–Si alloy, the influence of C addition on microstructural evolution and mechanical properties of Nb–16Si–24Ti based alloys is studied. Results show that with the C content increase, the microstructure of Nb–16Si–24Ti-xC transforms from hypoeutectic structure to nearly eutectic structure, and the phase constitutions transform from Nbss + (Nb, Ti)₃Si to Nbss + γ-(Nb, Ti)₅Si₃. The microstructure transforms into fully eutectic structure and TiC phase is precipitated by addition of 4 at.% C. The element C is mainly dissolved in the Nbss phase, and the nanoindentation hardness of Nbss phase obviously increases with the increasing of C content, which is attributed to the solid solution strengthening by C addition. The fracture toughness increases from 6.3 MPa m^1/2 to 14.9 MPa m^1/2, as the C content increases from 0 at.% to 3 at.%, since with the C content increase, the c/a ratio decreases, the Nbss phase content increases, and the Nbss phase becomes more continuous. The addition of 4 at.% C induces the formation of TiC phase in the matrix, and thus decreases the fracture toughness.