Solidification characteristics and as-cast microstructures of a Ru-containing nickel-based single crystal superalloy

Abstract

The solidification characteristics and as-cast microstructures of a Ru-containing nickel-based single crystal superalloy were systematically investigated through thermal analysis, Thermo-Calc simulation, the planar interface solidification experiment, and the directional solidification quenching experiment. The main solidification transition temperature, segregation behavior, and solidification path were analyzed, and the microstructure evolution and phase formation mechanism were also discussed. The solidification began with the formation of primary γ dendrites (L → γ). Then Ni, Al, Ta, and Ru were enriched in the residual liquid, resulting in the precipitation of β-NiAl phase (L → β-NiAl). As the β-NiAl phase grew, the content of Ta gradually increased while the content of Al gradually decreased. Thus, the peritectic γ′ phases were precipitated on the surface of β-NiAl phase and coarsened by the incomplete peritectic reaction (L + β-NiAl → peritectic γ′ + β-NiAlResidual). The Al content further decreased and the Ta content further increased with the precipitation of the peritectic γ′ phase, leading to the formation of γ/γ′ eutectics on the surface of the γ dendrites or directly from liquid (L → γ/γ′ eutectic). Since the precipitation of β-NiAl phase and the subsequent incomplete peritectic reaction, Cr, Co, Mo, W, and Re were rejected into the residual liquid in the vicinity of β-NiAl phase, providing conditions for the nucleation of the TCP(R) phases (L → R). The wide freezing range of the alloy might be the cause of the severe micro-segregation and the precipitation of some secondary phases in the interdendritic regions.