Microstructure Evolution and Mechanical Properties of Nb–Si–Hf and Nb–Si–Zr Based High Temperature Alloys

Abstract

NbSi based alloys have been received worldwide attention for their potential application as next generation gas turbine blade materials due to their high melting point (>1750oC), low density (6.67.2g/cc), good high temperature strength and creep resistance. However, the inadequate fracture toughness at room temperature and poor oxidation resistance at high temperature limit their use for high temperature applications. These two properties need to be improved in order to make use of them for high temperature applications.
The desirable phases for obtaining better mechanical properties in Nb–Si based alloys are Nbss and αNb5Si3. Alloying is an effective way of achieving these phases. Among several alloying elements that have been investigated, the elements Zr and Hf are attractive for achieving balanced properties in NbSi based alloys. However, no systematic and comprehensive investigations have been carried out on NbSi alloyed with Hf and Zr. Hence, in the present research work, an attempt has been made to investigate the individual effect of Hf and Zr additions on the microstructure evolution and mechanical properties of NbSi alloy. The results show that Nb–16Si at. % alloy mainly exhibits only Nbss and Nb3Si phases. The addition of Hf does not have any significant influence on the modification of the phases and microstructure of Nb–16 at. % Si alloy. Both compressive strength and d of alloys increase with Hf addition.
The desirable microstructure consisting of Nbss and αNb5Si3 phases are obtained in Nb–16 at. %Si alloy with addition of 4 at. % Zr. The addition of Zr enhances the kinetics of eutectoid decomposition of Nb3Si (Nb3Si → Nbss + α–Nb5Si3) during cooling of alloy after solidification. The possible mechanism for obtaining Nbss + α–Nb5Si3 phases in the as cast condition has been proposed. The compressive strength, fracture strain and fracture toughness of the alloys increase with Zr addition. Nb–25at. %Si alloy exhibits Nbss, Nb3Si and β–Nb5Si3 phases. Whereas Nb–25 at. %Si–4 at. %Zr alloy shows only Nbss and α–Nb5Si3 phases. Both compressive strength and fracture toughness of Nb–25 at. % Si alloy increases appreciably with addition of 4 at. % Zr.
First time the desirable phases Nbss and αNb5Si3 are obtained in Nb–Si based alloys without any heat treatment.