Thermomechanical fatigue mechanism in a modern single crystal nickel base superalloy TMS-82

Thermomechanical fatigue (TMF) in a < 0 0 1 > oriented nickel base single crystal TMS-82 superalloy was studied in an effort to clarify the mechanisms of stress relaxation and failure. Detailed observations of the microstructural evolution from the interior and outer surfaces of the specimens after TMF tests were made using transmission electron microscopy, scanning electron microscopy and optical microscopy. The stress relaxation took place during a hold time in compression at 900 degrees C, and the associated mechanisms varied with the following cycles. During TMF cycling, three stages of stress relaxations were identified: (1) primary stress relaxation; (2) steady stress relaxation; and (3) tertiary stress relaxation; each stage exhibits a distinct microstructural evolution. The first stage is related to the filling of dislocations in the gamma channels; the second stage involves dislocation annihilation; and the final stage is associated with the de-twinning of deformation twins. The main crack was found to originate from the intersection of deformation twin plates with the specimen surface, and oxidation then assists the growth of the crack. The stress concentration at the crack tip results in a high density of deformation twins, and the propagation of the crack along the twin boundaries can lead to TMF failure of the specimen. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.