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

Abstract

The formation mechanism of the Ru-rich β-NiAl phase in a nickel-based single crystal superalloy and its evolution course during the long-term aging were investigated. The β-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 β-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 δ phase was also observed in the β-NiAl phase, which was enriched in Re, Co, W, and Ni. The solidification path was determined as L → β, L → β + δ, L + β → γ′, L → γ′, L → (γ + γ′), which took into consideration the influence of the β-NiAl phase and the δ phase. Besides, the high melting point of the β-NiAl phase induced impediment in homogenizing heat treatment, long-term aging at 1100 °C from 100 h to 1000 h was carried out to eliminate the β-NiAl phase. Hence, the γ’/β-NiAl and γ/β-NiAl diffusion couples were put forward to interpret the evolution of the β-NiAl phase during the long-term aging. The TCP phase which was determined as μ 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 β-NiAl phase on the formation of the TCP phase was researched in detail.