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.

The effect of section thickness on the creep properties of a single crystal superalloy, PWA 1484 was investigated. For this purpose, a series of isothermal, constant load creep tests were conducted on uncoated sheet specimens in air at temperatures ranging from 760°C to 982°C and at stresses varying from 207 MPa to 758 MPa. The average time to reach 1% creep strain showed very weak dependence on specimen thickness at all test conditions. In contrast, the thickness debit associated with rupture life showed a complex dependence on test conditions. For example, creep rupture lives obtained at 760°C and 758 MPa decreased sharply (60%) with a reduction in specimen thickness from 3.18 mm to 0.38 mm. However, rupture lives of specimens tested at 871°C and 982°C exhibited a relatively modest dependence on specimen thickness. The results have been interpreted qualitatively in terms of the effect of plastic constraint on the deformation and fracture of ligaments between the interdendritic creep voids created during creep deformation.

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