Effect of Sulfur on Microstructures and Solidification Characteristics of a Nickel-Base Single Crystal Superalloy

The effects of sulfur on the microstructures and solidification characteristics of a second-generation single crystal superalloy were clarified through optical microscope (OM), scanning electron microscope (SEM), differential scanning calorimetry (DSC) and electron probe microanalysis (EPMA). When sulfur was added to the alloy, the area fraction of the eutectic and microporosity decreased with the area fraction of MC carbides increasing. The γ / γ’ eutectic morphology changed from mosaic + fan + coarse γ’ to mosaic + fan as the sulfur content increased, and very few γ / γ’ eutectic was found in the alloy with the highest sulfur content. Moreover, sulfur tended to segregate at the interdendritic regions and form Hf / Cr- rich sulfides, which were adjacent to the carbides or eutectic. This work not only uncovers some sulfides wrapped by eutectic or carbides, but also presents the solidification sequence of the interdendritic regions in high sulfur content alloy was discussed.