Characterization of deformation mechanisms during low cycle fatigue of a single crystal nickel-based superalloy

Abstract

The deformation and fracture mechanisms of a single crystal nickel-based superalloy CMSX-4 have been investigated during low cycle fatigue (LCF) tests at tem- peratures of 750, 850, and 950 C under strain-controlled R = 0. It was found that LCF lives at 750 and 850 C were similar and longer than those at 950 C. The specimens tested at 750 and 850 C showed fatigue crack initiation at internal pores, and their failure occurred by cracking at persistent {111} slip bands. On the other hand, at 950 C the crack initiated at the oxide-layered surface and propa- gated along \100( c channel until fracture. At the two lower temperatures, a/2\110( dislocations with low den- sity was rarely present within c channels, and a/3\211( partial dislocations were occasionally seen to shear c 0 leaving superlattice stacking faults behind. At 950 C, homogeneous deformation was produced by perfect dislo- cation movements of cross-slip and climb in the c channel and a limited c 0 shearing by superdislocation was observed. At total strain range lower than 0.6%, well-developed polygonal dislocation network formed at rafted c 0 interface. Comparison of dislocation structures revealed that load- controlled LCF tests lead to more severe deformation to specimens than strain-controlled tests.