Isothermal fatigue-induced precipitation in high-chromium ferritic steel: A microstructural analysis

HiperFer-steels exhibit thermo-mechanically triggered Laves phase precipitation, leading to cyclic hardening during high-temperature fatigue. Thereby, particle-free zones (PFZ) at grain boundaries are formed, which act as crack initiation sites. At higher load amplitudes, the widths of these PFZ become smaller. To understand the underlying mechanism, high-resolution microstructural analyses were performed in this work, showing that in the PFZ a very low dislocation density is present, while the adjacent regions show a high dislocation density. It is assumed that the Laves phase-forming elements diffuse from interior grain regions to the PFZ and use the dislocations at the boundary between precipitation front and PFZ to form Laves phase particles. Thus, the PFZ become smaller with increasing plastic deformation, while no increase in dislocation density occurs.