Effect of Zr and Hf additions on microstructure and mechanical properties of Nb–Si based ultrahigh temperature alloys

To improve the fracture toughness, and reduce density and cost of Nb–Si alloy, the influence of the substitution of Zr for Hf in Nb–16Si–16Ti–xZr–yHf (x + y = 4) alloys is investigated in this study, and the microstructural evolution and mechanical properties, including fracture toughness and compressive properties are analyzed. The results show that the substitution of Zr for Hf in Nb–16Si–16Ti–xZr–yHf (x + y = 4) alloys can refine the microstructure, and promote the formation of (Nb, X)5Si3/Nbss eutectic. The fracture toughness KQ value increases with the increase of Zr content, since the high Zr content alloys exhibits the low c/a ratio and fine microstructure. Moreover, the addition of 1~4 at.% Hf in Nb–16Si–16Ti–4Zr alloy has less influence on the mechanical properties. The substitution of Zr for Hf in Nb–16Si–16Ti–xZr–yHf (x + y = 4) alloys can improve the fracture toughness of alloys, and the Zr element exhibits the low density and cost compared with Hf element, which indicates that Zr is a promising alloying element.