Phase equilibria in the Nb–Ti side of the Nb–Si–Ti system at 1200 °C and its oxidation behavior

Abstract

Abstract Nb–Si based composites are one of the most promising superalloy for the next generation turbine airfoil materials. Alloying Ti can efficiently improve their poor oxidation resistance. In this work, five equilibrium alloys were used to determine the unclear but underlying phase equilibria in the Nb–Ti rich region of the Nb–Si–Ti system at 1200 °C. Convincing evidences on three-phase equilibria of bcc(Nb,Si,Ti) + αNb(Ti) 5 Si 3 + (Nb,Ti) 3 Si, αNb(Ti) 5 Si 3 + (Nb,Ti) 3 Si + Ti(Nb) 5 Si 3 and αNb(Ti) 5 Si 3 + (Nb,Ti) 3 Si + Ti(Nb) 5 Si 3 at 1200 °C were obtained. The important tie-triangle of bcc(Nb,Si,Ti) + αNb(Ti) 5 Si 3 + (Nb,Ti) 3 Si turns out to be much narrower than the previous calculation. An optimized thermodynamic description validated in the Nb–Ti rich (≤37.5 at.% Si) region between 1000 and 1500 °C of the Nb–Si–Ti system was subsequently acquired based on calculation of phase diagram (Calphad) method. Furthermore, the oxidation kinetics of three equilibrated alloys with increasing Ti content (5–30 at.%) were evaluated at 1050 °C. The one with bcc(Nb,Ti,Si) + (Nb,Ti) 3 Si equilibrium performed the lowest oxidation rate, which might due to the formation of compact TiO 2 film and the absent of fine eutectic structure bcc(Nb,Si,Ti) + αNb(Ti) 5 Si 3 . These knowledge is contributive to understanding the Ti effect on oxidation resistance of Nb–Si based alloys along with the variation of phase constitutions and microstructures.