Effect of processing and microstructure on Rene 95

Rene 95 is a high-strength, wrought nickel-base superalloy used primarily for compressor and turbine disks in advanced aircraft engines. A special thermomechanical processing sequence is described which imparts in Rene 95 a duplex microstructure, consisting of large warm-worked grains surrounded by a fine-grain necklace. Compared with the 100 percent fine-grain material usually produced by conventional processing, the duplex material is shown to have superior high-temperature properties, especially in rupture life/ductility, notched sustained-peak LCF, and residual cyclic life. This study demonstrates the inferior 538 to 6490C mechanical properties associated with the fine-grain material are attributable to oxidation of surface carbides, which allows crack initiation, followed by rapid crack propagation. This environmental effect nucleated failure mechanism was identified in 6490C stress rupture tests and is verified by improved rupture life and ductility of the fine-grain material tested in vacuum or by periodic surface removal during testing in air. In contrast to the fine-grain structure, the duplex structure offers greater resistance to crack propagation by virtue of its large warm worked grains and hence exhibits superior rupture ductility