Single crystal superalloys and casting technology offer a combination of attractive properties for advanced gas turbine engine components. The alloys are designed to produce superior properties for a challenging combination of requirements: high temperature creep-strength, fatigue resistance, oxidation resistance, coating performance and retention of performance in thin-walled configurations. Application of this technology to new components has only been restricted by the competing criteria of affordability for first time introduction.The development of improved single crystal superalloys, CMSX-4 (SLS) [La+Y] and CMSX-486 provide an appealing combination of enhanced performance and production affordability for demanding, high temperature applications in advanced gas turbine engines.

Single crystal (SX) superalloys have wide application in the high pressure turbine section of aero and industrial gas turbine engines due to the unique combination of properties and performance. Since introduction of single crystal casting technology, SX alloy development has generally focused on increased temperature capability, and major improvements in alloy performance have been associated with the introduction of new alloying elements, including rhenium (Re) and ruthenium (Ru). 3% Re-containing second generation alloys, such as CMSX-4 ® , PWA 1484 and Rene' N5 have seen the greatest market utilization and have become the benchmark alloys for comparing new alloy developments. However, Re and Ru are rare elements and have limited production/availability and commensurate high costs, which has resulted in significant escalation of SX alloy costs. Consequently, there has been much interest in the development of improved SX superalloys with no Re or lower Re content compared to second generation alloys.Cannon-Muskegon ® has developed two new SX superalloys: CMSX ® -7 alloy, which contains no Re, and low Re CMSX ® -8 alloy, as alternatives to first and second generation alloys for applications which require slightly lower temperature capability compared to CMSX-4 alloy. This paper details development and characterization of these new, proprietary SX alloys.

Turbine inlet temperatures over the next few years will approach 1650°C (3000°F) at maximum power for the latest large commercial turbofan engines, resulting in high fuel efficiency and thrust levels approaching 445 KN (100,000 lbs.). High reliability and durability must be intrinsically designed into these turbine engines to meet operating economic targets and ETOPS certification requirements. This level of performance has been brought about by a combination of advances in air cooling for turbine blades and vanes, design technology for stresses and airflow, single crystal and directionally solidified casting process improvements, and the development and use of rhenium (Re) containing high γ′ volume fraction nickel-base superalloys with advanced coatings, including full-airfoil ceramic thermal barrier coatings. Re additions to cast airfoil superalloys not only improves creep and thermo-mechanical fatigue strength, but also environmental properties including coating performance. Re dramatically slows down diffusion in these alloys at high operating temperatures. A team approach has been used to develop a family of two nickel-base single crystal alloys (CMSX-4® containing 3 percent Re and CMSX®-10 containing 6 percent Re) and a directionally solidified, columnar grain nickel-base alloy (CM 186 LC® containing 3 percent Re) for a variety of turbine engine applications. A range of critical properties of these alloys is reviewed in relation to turbine component engineering performance through engine certification testing and service experience. Industrial turbines are now commencing to use this aero developed turbine technology in both small and large frame units in addition to aero-derivative industrial engines. These applications are demanding, with high reliability required for turbine airfoils out to 25,000 hours, with perhaps greater than 50 percent of the time spent at maximum power. Combined cycle efficiencies of large frame industrial engines are scheduled to reach 60 percent in the U. S. ATS programme. Application experience to a total 1.3 million engine hours and 28,000 hours individual blade set service for CMSX-4 first stage turbine blades is reviewed for a small frame industrial engine.

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.