CRACK INITIATION I N CORROSION FATIGUE AT DEFINED GRAIN BOUNDARIES I N BI- AND POLYCRYSTALS

Abstract
The nucleation and growth of fatigue and corrosion fatigue cracks was examined for special grain boundaries in FeSi bicrystals and compared with the behavior of polycrystals. The bicrystals were oriented with one slip plane parallel to the boundary plane in one grain ; for the other grain the orientation was chosen to enhance the elastic anisotropy near the surface. These bicrystals as well as polycrystals were tested in corrosion fatigue in an ammonium carbonate solution held at fixed potential. Cracks nucleated at and, depending on frequency, propagated along the boundary. In the environment the fatigue life was reduced by orders of magnitude compared to the fatigue life in air. Corrosion fatigue cracks nucleated preferentially at boundaries which were favorably oriented for slip. In polycrystals these boundaries were responsible for the drastic reduction of the fatigue limit.