Effects of Sulfur Contamination on the Oxidation Resistance of Recycled Ni-Base Single-Crystal Superalloy PWA1484™

Cyclic oxidation behaviors of recycled Ni-base single-crystal superalloy PWA1484™ at 800 and 1100 °C were investigated to understand the effects of impurity S from scrapped turbine blades on the oxidation resistance of recycled blades. Two series of PWA1484™ alloy were prepared; alloys directly recycled from scrapped blades using either a conventional Al2O3 crucible or a CaO crucible for melting, and model alloys cast with intentionally added S varying between 6 and 88 ppm. PWA1484™ recycled through CaO crucibles showed improved cyclic and isothermal oxidation resistance at 1100 °C, compared with that recycled through an Al2O3 crucible, since calcium–aluminate slags can capture impurity S as CaS during melting. However, PWA1484™ recycled by Al2O3 crucible melting showed significant decrease in oxidation resistance at 800 °C. Microstructural and alloy chemistry analyses on two series of alloys suggested that increase in impurity S contents had accelerated the oxidation spallation rates at 1100 °C, while negative impacts such as degradation of oxide layer adhesiveness were limited for cyclic oxidation at 800 °C. These were due to differences in S distribution after oxidation; clear S segregation at the metal/oxide interface was detected at 1100 °C, while no segregation at the interface was observed at 800 °C. Furthermore, the slight changes in main alloying compositions through recycling process also affected the oxidation rate at both temperatures. Our findings show that not only the control of impurity S, but minor compositions adjustment of Al and Cr, which forms the protective oxide layers, are also important for recycling of scrapped blades.