A new, fourth generation, single crystal superalloy has been jointly developed by GE Aircraft Engines, Pratt & Whitney and NASA. The focus of the effort was to develop a turbine airfoil alloy with long-term durability for use in the High Speed Civil Transport. In order to achieve adequate long-time strength improvements at moderate temperatures and retain good microstructural stability, it was necessary to make significant composition changes from 2 nd and 3 rd generation single crystal superalloys. These included lower chromium levels, higher cobalt and rhenium levels and the inclusion of a new alloying element, ruthenium. It was found that higher Co levels were beneficial to reducing both TCP precipitation and SRZ formation. Ruthenium caused the refractory elements to partition more strongly to the ' phase, which resulted in better overall alloy stability. The final alloy, EPM-102, had significant creep rupture and fatigue improvements over the baseline production alloys and had acceptable microstructural stability. The alloy is currently being engine tested and evaluated for advanced engine applications.

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Single crystal superalloy turbine blades have recently entered production for JTSD commercial engine applications. This significant technical advance was made possible by the development of an alloy with improved properties and the development of a production casting process. The absence of grain boundary strengthening elements provided considerable alloying and heat treatment flexibility that resulted in single crystal Alloy 454 with an outstanding balance of properties. Major improvements in temperature gradients, the use of helical grain selectors, and the incorporation of these advances into existing vacuum furnaces have led to the rapid development of a production casting capability.

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