Fatigue microcrack initiation in polycrystalline alpha-iron with polished and oxidized surfaces

Abstract

Plastic-strain-controlled fatigue crack initiation experiments were conducted on unoxidized and oxidized, vacuum-melted iron. In the unoxidized, as-polished condition at low plastic strain amplitudes, e.g., 1 or 5 X 10−4, microcracks initiated along well defined slip bands and in the troughs of surface rumples. Such microcracks tended to stop short of grain boundaries. On increasing the plastic strain amplitude, initiation of fatigue cracks along grain boundaries became important. When the specimens were surface oxidized, intergranular microcrack initiation was the dominant mode even at the plastic strain amplitude of 5 x 10−4, where transgranular microcracks formed only very infrequently and then only considerably later in the fatigue lifetime. At 1 x 10−3 amplitude, transgranular microcracks initiated very early in the cycling compared to the polished condition, but such cracks did not grow or increase in number. Intergranular cracks formed later and led to failure. Surface oxidation led to approximately a 20 pct reduction in lifetime to final failure.