Stage I fatigue cracking in MAR-M 247 superalloy at elevated temperatures

Abstract

Nickel base superalloys exhibit fatigue fracture behavior with features of brittle-like cleavage cracking under high cycle fatigue loading at temperatures up to approximately 800 °C. This specific fracture behavior was already documented in several studies, but a possible mechanism of fatigue crack propagation under this mode has not been made completely clear yet. The aim of this paper is to put more light on the phenomenon by using advanced electron microscopy techniques like electron back-scattered diffraction (EBSD) and focused ion beam (FIB) sectioning. Fractured specimens after high cycle fatigue tests were thoroughly examined with the aim to localize the fatigue crack initiation sites and accompanying features of the fatigue crack propagation. Several specimens were cross-sectioned in order to characterize active slip systems, cyclic plastic deformation localization and fatigue crack propagation. Dislocation structures were studied by transmission electron microscopy (TEM).Nickel base superalloys exhibit fatigue fracture behavior with features of brittle-like cleavage cracking under high cycle fatigue loading at temperatures up to approximately 800 °C. This specific fracture behavior was already documented in several studies, but a possible mechanism of fatigue crack propagation under this mode has not been made completely clear yet. The aim of this paper is to put more light on the phenomenon by using advanced electron microscopy techniques like electron back-scattered diffraction (EBSD) and focused ion beam (FIB) sectioning. Fractured specimens after high cycle fatigue tests were thoroughly examined with the aim to localize the fatigue crack initiation sites and accompanying features of the fatigue crack propagation. Several specimens were cross-sectioned in order to characterize active slip systems, cyclic plastic deformation localization and fatigue crack propagation. Dislocation structures were studied by transmission electron microscopy (TEM).