Out-of-phase thermomechanical fatigue of a single crystal Ni-base superalloy

The present work studies the out-of-phase thermomechanical fatigue (OP-TMF) behavior of the precisely oriented [001] single crystal Ni-base superalloy ERBO/1 (CMSX-4 type). The OP-TMF tests are performed between minimum and maximum temperatures of 1023 and 1223 K and a cyclic mechanical strain amplitude of ±0.5 %. In accordance with previous findings, the present study confirmed that isothermal low cycle fatigue (LCF) tests at 1023 and 1223 K show significantly longer fatigue lives than the OP-TMF tests, specimens with rougher surfaces fail earlier than polished specimens and deformation bands and cracks form in the specimen surface. Two new results were obtained: First, when comparing OP-TMF cycles with fast cooling (tensile part of cycle) and fast vs. slow heating (compressive part of cycle) one finds that the slower heating/compressive cycle is more damaging. Second, analytical scanning electron microscopy of the flanks of an OP-TMF crack allows to study the diffusion-controlled growth kinetics of the oxide and the underlying zone, which is depleted by the oxide forming elements. These results are discussed in the light of the microstructural, mechanical and chemical results of the present study considering previous work on OP-TMF of superalloy single crystals (SXs).