In-situ observations of the effects of orientation and carbide on low cycle fatigue crack propagation in a single crystal superalloy

Abstract

Based on in situ observations with scanning electron microscopy (SEM), we investigated the low cycle fatigue (LCF) propagation behaviors of a Ta-rich single crystal nickel-based superalloy at 550oC in vacuum. Specimens in three different orientations, i.e. [010], [011] and [111], were tested in stress-controlled tension-tension LCF tests with R=0.1. Slip traces can be observed ahead of the crack and distinct crystallographic fracture was dominated. By analysis on the surface slip traces and the crack propagation planes, the operated slip systems were examined to be octahedral slip and the active octahedral plane was identified in this context. The whole cracking process of small fatigue cracks were meticulously tracked and recorded. The evaluation of short crack growth rate for specimens in three orientations was carried out by fracture mechanics approach. The influence of crystallographic orientations on fatigue crack growth are analyzed and discussed. Especially, the interaction between the short crack and local microstructure such as the carbides ahead of the crack were analyzed, altering the propagation path as well as influencing the crack growth rate. The parameter of crack opening displacement was examined to correlate well the crack growth data in different orientations.