CMSX-4® Plus (SLS): An Improved 3rd Generation Single Crystal Alloy

Ongoing demand for advanced aero gas turbine engines with lower fuel burn and commensurate reduced CO₂ emissions require single crystal (SX) superalloys capable of operating at higher gas and metal temperatures beyond the capability of 2^nd generation, 3% rhenium (Re)-containing SX alloys, currently used extensively in commercial and military flight engines. These complex cooled turbine blades and vane castings must have an excellent balance of high temperature mechanical properties, producibility, oxidation/hot corrosion resistance, coating compatibility including TBC performance and phase stability. 

The highest strength nickel-base SX superalloys, without ruthenium, currently in production (3^rd generation CMSX-10K^® and CMSX-10N^® alloys) contain 6-7% Re. These highly alloyed, specialty alloys have some application drawbacks including some secondary reaction zone (SRZ) phase instability in the base alloy adjacent to the coatings, low temperature internal oxidation/hot corrosion attack requiring sophisticated dual role internal and external coating and difficulty in production solution heat treatment. In addition, current 3^rd generation SX alloys have relatively higher density which is a disadvantage in terms of weight and inertia in rotating part applications, and high cost due to the elevated Re content.

Cannon-Muskegon (CM) has developed an improved, lower Re content (4.8%), 3^rd generation SX superalloy, CMSX-4^® Plus (SLS), suitable to replace both current 2^nd and 3^rd generation SX alloys.  CMSX-4 Plus (SLS) has improved properties and performance over current 3^rd generation SX alloys, while lacking the drawbacks. Coatings have been successfully developed which are compatible with the base alloy and suitable bond coats for TBC application. The development and characterization of CMSX-4 Plus (SLS) alloy, including composition, mechanical and physical properties, oxidation properties and phase stability, along with production status and applications are presented in this paper.