Cyclic deformation behavior and micro-mechanism of crack initiation during thermomechanical fatigue in an intermediate temperature range (573 K ↔ 723 K) in Timetal 834 alloy

Abstract

In this work, thermomechanical fatigue (TMF) life of Timetal 834 alloy is studied under clockwise diamond (CD) and counterclockwise diamond (CCD) conditions in the intermediate temperature range (i.e., 573 K ↔ 723 K). TMF tests were performed at strain amplitude of Δ${\epsilon }_m$/2 = ±0.6 % and ± 1.0 % and post-failure, microstructure of each specimen was characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. TMF results show that life is shorter under CD condition than CCD one at a given strain amplitude due to accumulation of greater tensile damage during each cycle. The local average misorientation analyses confirm that strain localization is greater under CD condition than CCD one at a given strain amplitude. In this study, crack initiation and the associated failure mechanism are also investigated. EBSD analyses show band-like structure inside the primary alpha (α_p) grains where strain localization is substantially greater in both TMF conditions. TEM results corroborate with the EBSD findings and show dense dislocation density at the concavities around α_p grain boundaries. The fractography studies show faceted α_p grains near the crack initiation sites. EBSD analyses reveal that facets are interrelated with strain localization in α_p grains, oriented along basal or prismatic with high Schmid factor.