Direct observation of carbon-decorated defects in fatigued type 304 stainless steel using positron annihilation spectroscopy

We have directly observed carbon decoration of defects in fatigued 304 stainless steel using positron annihilation spectroscopy. The formation and evolution of defects during fatigue was determined by positron annihilation lifetimes and electron momentum distributions in a series of samples. We find an initial rapid change in the defect concentrations that saturates around 10% of the cycles to failure into two distinct open-volume defect populations that both trap the positrons. Analysis of the momentum distributions of atomically bound electrons demonstrates that one of the defects has high levels of carbon decoration. Electron momentum distributions also show evolution in the carbon decoration of the defects with increasing fatigue all the way to failure.