Low-cycle fatigue-crack initiation study in René 95

Abstract

A microstructural study of fatigue deformation and cracking was conducted on René95, which is a thermomechanically processed superalloy developed for use as discs in advanced gas turbine engines. Optical, replica, scanning and transmission electron microscopy were used in order to study the deformation structures and mode of cracking during crack initiation under low-cycle fatigue. As in a previous tensile study, it was found that the deformation occurred very homogeneously throughout the material. This is believed to be due to the slip dispersive effect of the substructure in the warm worked grains and the very small size of the necklace grains. The study also showed that the number of load cycles to produce crack initiation can be strongly affected by brittle constituents of the microstructure, such as MC carbides. It was found that the specimens that had shorter lives were characterized by MC carbide cracking at the site of the crack initiation, whereas those which had longer lives under the same conditions of loading and temperature were characterized by only slip band cracking with no evidence of MC carbide cracking or decohesion in influencing the initiation.