Effect of Cooling Rate on Microsegregation During Solidification of Superalloy INCONEL 718 Under Slow-Cooled Conditions
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CKEL-BASED superalloy INCONEL 718 (IN718) is extensively used as raw materials to manufacture components for aerospace, marine, nuclear, and power-generation industries because of its combination of good corrosion resistance, high rupture strength, good mechanical properties, outstanding weldability, and excellent high-temperature structural stability in the range from – 253 C to 650 C.[1–3] The strengthening mechanism is mainly contributed by nanoscale particles that precipitate from solid-solution matrix (c phase)
XIAO SHI, SHENG-CHAO DUAN, WEN-SHENG YANG, HAN-JIE GUO, and JING GUO are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China and also with the Beijing Key Laboratory of Special Melting and Preparation of High-End Metal Materials, Beijing 100083, China. Contact e-mail: guohanjie@ustb. edu.cn Manuscript submitted November 27, 2017.
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during heat treatment and aging processes. One type of the principal strengthening phases is face-centered cubic intermetallic c¢-Ni3(Al,Ti), and the other is metastable intermetallic c¢¢-Ni3Nb with a body-centered tetragonal crystal structure. Mo is frequently added to IN718 to increase the mechanical resistance through solid-solution hardening.[2,4,5] However, in the solidification process, before heat treatment or forging, if the actual cooling rate is larger than the equilibrium rate, which is extremely slow, the high content of strengthening elements such as Nb, Mo, Al, and Ti in the IN718 alloy will be redistributed at the solidification front to various degrees. Therefore, the solid phases that crystallize at each temperature interval have different chemical compositions and do not have sufficient time to diffuse evenly, leading to the formation of a non-uniform matrix and detrimental brittle intermetallic compounds. Inevitably, this solute redistribution during non-equilibrium solidification will not only cause severe element segregation but also greatly affect the microstructure and morphology, resulting in considerable difficulties in the subsequent thermal processing
and in significant deterioration of the final casting properties. On the basis of the aforementioned phenomenon, several experimental methods have been employed to investigate the solidification behaviors of IN718 superalloy.[6–11] In general, the as-cast IN718 alloy exhibits Lfi(c + NbC) and Lfi(c + Laves) eutectic-type reactions during solidification. Two minor microstructural segregation constituents, NbC and Laves, are known to precipitate in the solute-rich interdendritic liquid at different stages of solidification. Fe and Si additions increase the amount of the c/Laves constituent, whereas C additions promote the formation of c/NbC. Furthermore, various segregative mechanisms and experimental techniques have been developed to explain the elemental partitioning and change in liquid density during solidification.[12–18] These studies confirms that Nb, Si, and C exhibit a strong propensity to form
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