Effect of the Cr 2 Nb Phase on the Tensile Behavior and Oxidation Resistance of a Two-Step Heat-Treated Nb-24Ti-12Si-10C
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INTRODUCTION
ALTHOUGH the operating temperatures of the hottest section of advanced turbine engines approach the stability limit of nickel-based superalloys, there is ongoing demand for further improvements in engine performance.[1] Nb-Ti-Si-based alloys have gained considerable attention due to their high melting points, relatively low densities, and excellent high-temperature mechanical properties.[2] In this alloy system, NbSS ensures good room-temperature fracture toughness and tensile strength, silicides offer high-temperature strength, and Cr2Nb provides oxidation resistance.[3] An excellent combination of high- and low-temperature mechanical properties has been achieved in the Nb-24.7Ti-8.2Hf-2.0Cr-1.9Al-16.0Si (MASC) alloy.[4] Nevertheless, the long-term oxidation resistance of the Nb-Ti-Si-based alloys is insufficient, which presents a major bottleneck that restricts their application.[5–8] Extensive efforts have shown that the addition of alloying elements such as Cr, Al, Ti, B, and Ge can improve oxidation resistance.[9–12] Among these elements, chromium is the most efficient element for conferring oxidation resistance; therefore, oxidation resistance is promoted by the formation of the Cr2Nb phase. Unfortunately, Cr2Nb is very brittle, with a fracture toughness of approximately 1.4 MPa m1/2 at ambient temperature, and this brittleness persists even above 1273 K (1000 C).[13,14] The results presented by
Zhang indicated that the macrohardness of the Nb-Si-based alloys increases with the addition of Cr.[15] A variety of processing schemes have been used to alleviate the detrimental effects of incorporating Cr2Nb. However, modifying the morphology of Cr2Nb seems difficult. The size of Cr2Nb phases in Nb-Cr alloys obtained via rapid solidification techniques is much smaller than that in alloys obtained via conventional casting, but this phase exhibits a continuous network morphology that enables easy crack propagation. Davidson has demonstrated that slower cooling rates would result in larger scale Cr2Nb particles that would break at a lower stress than smaller particles.[16] This finding indicates that smaller Cr2Nb particles reduce stress. After heat treatments are applied, the contiguity of Cr2Nb blocks, which prefer direct formation from liquid melting, cannot be altered easily. Fortunately, this phase can be dissolved during high-temperature heat treatments. In this study, two-step heat treatments were employed to obtain fine Cr2Nb particles in a Nb-24Ti-12Si-10Cr-2Al-2Hf alloy. The microstructure of the heat-treated alloys was studied in detail, especially that of the twice-heat-treated alloys. The purpose of the first heat treatment was to ensure the decomposition of Nb3Si and dissolve the solidified Cr2Nb blocks, while the aim of the second heat treatment was to obtain fine Cr2Nb particles.
II.
L.N. JIA and H. ZHANG are with the School of Materials Science and Engineering, Beihang University, No. 37, Xueyuan Road, Beijing 100191, China. Contact e-mail: [email protected] Manuscript submitted September 9, 2016
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