Oxidation Resistance and Critical Sulfur Content of Single Crystal Superalloys

Abstract

Single crystal superalloys had greatly improved cyclic oxidation resistance when their sulfur content was reduced from impurity levels, typically 5–10 ppmw in past years, down to 1 ppmw or below currently. Excellent alumina scale adhesion has been documented for PWA 1480, PWA 1484, Rene’N5, Rene’N6, and CMSX 4, all without reactive element (Y) additions. Hydrogen annealing was used for effective desulfurization to below 0.1 ppmw, as well as for achieving controlled intermediate levels. This paper summarizes the direct relationship between cyclic oxidation behavior and sulfur content. An adhesion criterion has been proposed based on the concentration of sulfur needed to initiate spallation due to a monolayer of interfacial segregation. This suggests that a level down to ∼0.2 ppmw would be needed to maximize adhesion for a 1 mm thick sample. It is in reasonable agreement with the experimental results.

Ultra-Low Sulfur Determination in High Purity Base Metals and High Temperature Nickel Base Alloys

“…The determination of sulfur in many metals and alloys has become very important, especially at concentration levels below 5 ppm. Recent research has indicated that the desirable sulfur content of high-temperature single-crystal superalloys is less than one ppm [1]. A technique that is routinely used to determine sulfur in these materials is High-Temperature-Combustion Infrared-Detection [2].…”

Coating and Surface Technologies for Turbine Airfoils

“…Sulfur levels below 1 ppm have been shown to give significant benefits to the oxidation resistance of the alloy [4,5]. Utilizing a low sulfur superalloy with a bond coat system can result in TBC life improvements [6].…”