The formation and evolution of NiAl phase in a fourth generation nickel-based single crystal superalloy

The formation mechanism of the Ru-rich beta-NiAl phase in a nickel-based single crystal superalloy and its evolution course during the long-term aging were investigated. The beta-NiAl phase confirmed via the XRD and TEM analysis was observed in the solidification process for casting alloy. The intensive study on the formation mechanism of the beta-NiAl phase was conducted based on the analyses of the phase diagram, the incipient melting experiment, and the segregation behavior. In addition, a new phase identified as the 8 phase was also observed in the beta-NiAl phase, which was enriched in Re, Co, W, and Ni. The so- lidification path was determined as L -> beta, L -> beta + delta, L + beta -> gamma', L -> gamma', L -> (gamma + gamma'), which took into consideration the influence of the beta-NiAl phase and the 8 phase. Besides, the high melting point of the beta-NiAl phase induced impediment in homogenizing heat treatment, long-term aging at 1100 degrees C from 100 h to 1000 h was carried out to eliminate the beta-NiAl phase. Hence, the gamma'/beta-NiAl and gamma/beta-NiAl diffusion couples were put forward to interpret the evolution of the beta-NiAl phase during the long-term aging. The TCP phase which was determined as mu phase via the selected diffraction spot was examined in the full heat-treated and long-term aging specimens. The morphology of the TCP phase could be divided into three types: needle-like, rod-like, and bulk-like. The influence of the beta-NiAl phase on the formation of the TCP phase was researched in detail. (C) 2020 Elsevier B.V. All rights reserved.