Microstructural Characterization of Ni-Base Superalloy As-Cast Single Crystal (CMSX-4)

Abstract

The as-cast microstructure of a single-crystal CMSX-4 Ni-base superalloy produced by directional solidification was studied. The 〈100〉 misorientation of the angled and spiral grain selectors was evaluated for 3.33 and 5.0 mm/min withdrawal rates. The dendritic microstructure presented primary dendrite arm spacing around 370 µm (for a withdrawal rate of 3.33 mm/min) and 320 µm (for 5.0 mm/min); and interdendritic γ–γ′ islands, whose volume fraction was around 5.4% (for 3.33 mm/min) and 6.1% (for 5.0 mm/min). The microsegregation profiles of one withdrawal condition (for 5.0 mm/min) were used to extrapolate the solidus and γ′ solvus temperatures using the Thermo-Calc software. These microsegregation profiles of the as-cast microstructure also affected the solid-state γ′ precipitation in terms of shape, size and volume fraction. In the center of the dendrite, the γ′ precipitates were regularly cubic-shaped with a face of approximately 0.5 µm and a volume fraction of 55%. In the periphery of the dendrite, the morphology of the γ′ precipitates was more asymmetric, and the volume fraction of the γ′ phase increased to 65%. Two morphologies of the interdendritic γ–γ′ islands, fine and coarse, were observed, and a eutectic solidification sequence was proposed. The results indicated that the chemical composition of the interdendritic liquid progressed from hypoeutectic (fine interdendritic structure) to hypereutectic (coarse interdendritic structure) during the solidification.