Developments in superalloy castability and new applications for advanced superalloys

Abstract

Increasing emphasis is being paid to optimising castability of cast superalloys to absolutely minimise defects which impact casting yield and alloy performance and to improve structural weldability of weld repairable alloys. The objective is to maximise casting yield and quality and minimise any rectification costs. To accomplish these aims the controls on deleterious elements such as [N], [O], S, P and Si at the ppm level, including accurate measurement, are critical. Conversely, the addition of highly reactive elements, La and Y at the ppm level, in conjunction with very low (1 ppm or less) sulphur content has dramatically improved bare oxidation resistance (for unshrouded blade tip life) and thermal barrier coating (TBC) adherence and life including prime reliant applications. This technology has been applied to the following new or improved cast superalloys: CM 939 Weldable^®, CMSX-486^® and CMSX-4^® (SLS) [La+Y] (Cannon-Muskegon Corporation). In this paper, the role (both positive and negative) and control of critical elements at the ppm level as it applies to many advanced superalloys will be examined and discussed.