Microstructures and Nb-Rich Precipitation Behaviors of Inconel 718 Superalloy Under Sub-rapid Solidification Process
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UCTION
AS a most widely used precipitation strengthening nickel-based superalloy, Inconel 718 (IN718) has been widely used in gas turbine disks, rocket motors, aircraft engines, nuclear reactors, pumps, and blisks for its excellent mechanical properties, e.g., adequate strength, high ductility, and fatigue resistance up to 650 C[1–3]. The product properties of a casting alloy mainly depend on the microstructure which might be impaired by precipitates that develop during solidification. Laves phase is the most common brittle precipitates, which contain 10 to 30 wt pct Nb for the formation of strengthening phase in the interdendritic regions as a result of serious Nb and Mo elements segregation at the final solidification of IN718.[4] The presence of this deleterious phase drastically degrades mechanical properties. Thus, the control of Laves phase in solidification stage is of great importance due to induced fracture in dendrite leads to the crack initiation and propagation.[5] The interdendritic Laves phases dissolve in the austenitic matrix with homogenization heat treatment, which is the WANLIN WANG, CHENYANG ZHU, JIE ZENG, CHENG LU, HAIRUI QIAN, HUI XU, and PEISHENG LYU are with the School of Metallurgy and Environment, Central South University, Changsha 410083, China; National Center for International Research of Clean Metallurgy, Central South University, Changsha 410083, China. Contact email: [email protected] Manuscript submitted June 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
main way to eliminate it in the microstructure.[6,7] But the long-time and high-temperature heat treatment needs to consume a large amount of energy. It is well known that the formation, morphology, and composition of Laves phase depended strongly on cooling rate.[8,9] Thus, it is urgent and of great importance to develop a new production method to control the microstructure and precipitates during the solidification process. As the only industrialized sub-rapid solidification process, strip casting is one of the most important technological revolutions in recent steel industry, which can produce thin strip directly from the liquid metal and show a great potential for the reduction of operating and investment costs through eliminating several rolling steps.[10,11] Strip casting process is an important method to deal with the most difficult problems such as segregation, inclusion, uniformity, and energy consumption in the processing of metal materials.[12] In previous studies, two kinds of experimental apparatus were used to simulate the process of strip casting, i.e., dip tester[13,14] and droplet solidification test. Droplet solidification test is one of the most important and widely used techniques for modeling the sub-rapid solidification and its related heat transfer behaviors in strip casting. The droplet solidification test is important with respect to the fundamental study of strip casting technology given the common conditions of direct contact between cooling substrate (mold) and solidifying metal. This kind of experiment can
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