Intergranular Fatigue Cracking Enhanced by Impurity Segregation

Abstract

It was previously reported that fatigue life of some alloys can be dramatically reduced if the grain boundaries contain a high level of impurity segregation before fatigue tests. In this paper the susceptibility of single phase brass samples (90Cu10Zn) to this form of damage is studied. After cold drawing of as cast brass bars, fatigue samples were heat treated at 800°C during 30min to promote recrystallization and impurity segregation at grain boundary. The samples were then tested under high frequency bending fatigue test at 200°C. After cracking, fracture surfaces were studied using both scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The SEM micrographs showed that the fractures were mostly intergranular. Chemical composition of intergranular cracks surface were analyzed using EPMA at low accelerating voltage. A high concentration of sulfur was found on most of grain boundary facets. The internal stress in alloys after fatigue was qualitatively estimated using electron backscattering diffraction in scanning electron microscopy. A high level of local misorientation was found near most grain boundaries. The mechanism of intergranular cracks formation during fatigue is discussed taking into account both the segregation of sulfur at grain boundaries and accumulation of plastic strain at grain boundaries