Initiation of corrosion fatigue cracks at defined grain boundaries in bicrystals

Abstract

The fatigue tests in air show that persistent slip bands (PSB's) and cracks nucleate very early at special grain boundaries. At stress amplitudes for which no persistent slip band nucleation was observed in single crystals and which were oriented for single slip, cracks still nucleate at grain boundaries. The endurance limit for special bicrystals lay 50% below the endurance limit of polycrystals. In air, the cracks nucleate at the boundary but propagate within the PSB. From the stress amplitudes at which PSB's nucleate in single crystals and in bicrystals, which have one grain with the same orientation as the single crystals, the additional shear stress due to elastic anisotropy was measured to be 55 MPa. These tests yield an understanding of the behavior of short cracks, which nucleate at special boundaries but cannot propagate further, if they hit an unfavorably oriented boundary for which higher local stresses for propagation were needed. With the same bicrystals, corrosion fatigue tests were carried out in ammonium carbonate solutions. In the solution, crack nucleation was found to depend on frequency and amplitude. Contrary to the behavior in air, the cracks nucleate at and propagate along the grain boundary. Specimens which last for 10 5 cycles in air only survive 6·10 3 cycles at the lowest frequencies tested. In addition, if the stress amplitude is reduced by 27%, it was observed that, for a given frequency, the fatigue life is reduced by more than 90% relative to the fatigue life in air at the same stress level. The susceptibility of special boundaries against corrosion fatigue combined with the observed dependencies on stress amplitude and frequency could be understood on the basis of the slip step dissolution model for SCC.