Effects of Si variation and Zr addition on the microstructure, oxidation resistance and mechanical property at 1523 K of Nb-Si-Ti/Zr/Hf-Cr/Al high-entropy superalloys

Abstract

The present work investigated the influences of the variation of Si content and the minor amount of Zr addition on the microstructure, oxidation resistance and mechanical property at 1523 K of Nb-Si-Ti/Zr/Hf-Cr/Al high-entropy superalloys systematically. Alloy compositions with Nb-21.6(Ti/Zr)-3.1Hf-(9.4–15.6)Si-6.3Cr-3.1Al-1.0B (at. %) were designed using a cluster formula approach to realize the substitution of similar elements, in which these alloys were solid-solutionized at 1573 K for 24 h and then aged at 1273 K for 24 h. It was found that all these aged alloys are mainly composed of BCC-Nb solid solution (Nbss), Nb5Si3 and Cr2Nb phases, where the increase of Si content and the addition of Zr could significantly increase the volume fraction of Nb5Si3 in a form of coarse and primary particles. Consequently, high-Si alloys possess outstanding mechanical property at high temperature, as evidenced by the fact that the yield strength (σYS = 373 MPa) at 1523 K of high-Si/Zr alloy containing 15.6 at. % Si and 1.6 at. % Zr is much higher than that (σYS = 266 MPa) of the low-Si alloy with 9.4 at. % Si and 0 Zr. It is mainly ascribed to the existence of primary Nb5Si3 particles, besides the eutectic microstructure composed of Nbss and Nb5Si3, as well as fine Cr2Nb precipitates in Nbss. Moreover, a minor amount of Zr substitution for the Ti is conducive to the phase transition of α-Nb5Si3 to γ-Nb5Si3. Thus, this high-Si/Zr alloy exhibits an improved oxidation resistance after 50 h-exposure at 1523 K, due to the lowest mass gain of about 80.9 mg/cm2. The constitutions of oxide scales in these alloys were characterized and compared, from which it was found that the better oxidation resistance is closely related to the formation of dense oxide scales. This work will provide a new approach to obtain an optimum combination of mechanical property and oxidation resistance of superalloys through multi-component alloying.