Effects of withdrawal rate on microstructure and stress rupture properties of a Ni₃Al-based single crystal superalloy IC6SX

As high-temperature structural materials, Ni₃Al possesses excellent comprehensive properties, such as good strength at high temperature, low density, excellent ductility, superior oxidation resistance and so on. A high performance Ni₃Al-based alloys IC6 has been developed by authors and successfully applied for aero-engine vanes. It has been found that the high temperature mechanical properties of the single crystal IC6 alloy are superior to both equiaxial crystal and directional columnar IC6 alloys. A lot of research work for IC6 alloys has been carried out on the equiaxial crystal and directional columnar IC6 alloys during the last two decades. In order to further improve the mechanical properties and meet the requirement of turbine blade materials, the single crystal Ni₃Al-based alloy IC6SX has been developed and the research work on single crystal processing has been carried out recently. In the present investigation, the influence of withdrawal rate on the microstructure and stress rupture properties of a Ni₃Al-based single crystal superalloy IC6SX were studied. The single crystal specimens were prepared by screw selection crystal method. The microstructure analysis by SEM and OM reveals that the microstructure of specimens is dendrite structure, and the primary dendrite arm spacing decreases from 480 μm to 390 μm with withdrawal rate increasing from 1.5 mm/min to 6.0 mm/min. Different with the common Ni-based superalloys, the size of the γ′ phase in the dendritic core is larger than that in the interdendritic area. As withdrawal rate increases the size of the γ′ phase decreases. The average size of the γ′ phase in the dendritic core decreases from 3 μm to 1.3 μm and the average size of the γ′ phase in the interdendritic area decreases from 1.6 μm to 0.8 μm with withdrawal rate increasing. Meanwhile, the shape of γ′ phase tends to regular, and more primary eutectic γ′ phase precipitates form in the interdendrite region. The volume fractions of primary eutectic γ′ phase precipitates are 0.07%, 0.18%, 0.36% and 0.77%, respectively at the withdrawal rates 1.5, 3.0, 4.5 and 6.0 mm/min. It has been found that the stress rupture properties are affected by primary dendrite arm spacing and size of γ′ phase significantly, i.e., the rupture lies of as-cast specimens increase with increasing withdrawal rate, which may e attributed to the refining microstructure. The above results can be used to optimize the process design for both single crystal specimen preparation and component fabrication of the present alloy IC6SX and may be used as a reference for other Ni-based single crystal alloys.