Microstructural Instability and Precipitation Behaviors of Intermetallic Phases in a Nb-Containing CoNi-Based Superalloy
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TRODUCTION
THE development of aviation industry has undoubtedly brought considerable convenience for humans, but the increasing air traffic causes the emission of greenhouse gas to be uncontrollable. Accordingly, global warming has gradually been exacerbated. One of the main solutions is to maximize the operating temperature of turbine engines so as to increase the thermal efficiency, thus decreasing the toxic emission. To this end, the pivotal requirements are enhancements and innovations of turbine materials, which have troubled material scientists for decades. Recently, the re-discovery of superlattice ordering in Co-Al-W alloys by Sato et al.,[1] which was firstly reported in 1971,[2] has attracted considerable research interest. Similar to commercial Ni-based superalloys, the Co-Al-W alloy consists of coherent L12-c¢ precipitates, c¢-Co3(Al,W), and a A1-c matrix, such two-phase microstructure leads ZHONGDING FAN, HAO CHEN, ZHIGANG YANG, and CHI ZHANG are with the Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China Contact e-mail: [email protected]. XINGUANG WANG and YANHONG YANG are with the Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Manuscript submitted on October 21, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
to an anomalous yield stress behavior. The combination of yield stress anomaly and the higher solidus temperature, approximately 1450 C, renders the ternary Co-Al-W superalloy possessing great potential to become an improved turbine material compared to Ni-based superalloys.[1,3] However, major drawbacks of the Co-Al-W superalloy including high density and relatively low c¢-Co3(Al,W) solvus temperature considerably limit its applications. The low c¢ solvus temperature leads to an insufficient stability of the microstructure at elevated temperatures and further undermines the high-temperature mechanical properties. For the purpose of developing a Co-Al-W alloy with enhanced and well-balanced properties, the investigation on alloying effects of various elements is necessary. It has been reported that Ni and Ti are prone to partition to the c¢-Co3(Al,W) phase[4,5] and thus increase its solvus temperature,[6–8] while improving the high-temperature mechanical properties of Co-Al-W superalloys including 0.2 pct flow stress[9–11] and creep resistance.[12–14] Moreover, exceptional enhancements on high-temperature mechanical properties could be achieved by moderate additions of refractory metals that are known to be strong solid solution strengtheners in superalloys. Previous studies have shown that refractory elements such as Ta and Nb are effective for increasing the c¢ solvus temperature of Co-Al-W superalloys, substituting 1 at. pct of Ta for Co
would raise the c¢ solvus temperature by 45 C.[3,7,15,16] Besides, addition of Ta and Nb was also proved to stabilize the c¢ phase of the newly developed Co-Al-Mo alloys.[17–19] It should, ho
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