Solidification and Evolution of β-NiAl Phase in a Re-Containing Single Crystal Superalloy

The β-NiAl phase, which may exacerbate the high-temperature mechanical properties of Ni-based superalloy, was observed in a designed Re-containing single crystal superalloy. The solidification mechanism of β-NiAl phase and its evolution after solution treatment as well as long-term aging at 1120 °C were systematically investigated. The β-NiAl phase was confirmed by the analyses of XRD and TEM in as-cast specimen. Combined the analyses of TEM, DSC, phase diagrams and chemical segregation behaviors obtained via EPMA, the solidification path in interdendritic region was determined as L → (β-NiAl + Cr) eutectic, L → β + δ, L + β → γ′, L → γ′. The β-NiAl phase could be diminished by solution treatment and/or long-term aging treatment. After the solution treatment at 1330 °C for 30 min, the β-NiAl phase split into small bulks and the δ phase dissolved, contributing to the formation of the γ + γ′ structure and large blocky δ phase. During the long-term aging at the 1120 °C, the β-NiAl phase transformed into γ and γ′ phase. With aging time prolonging, the Cr-rich phase dissolved, which promoted the precipitation of TCP confirmed as μ phase with three morphologies.