OS05W0460 Investigation of fatigue crack propagation behavior in ultrafine-grained steel by AFM and EBSP

Abstract

Ultrafine-grained specimens with the average grain size of less than 2μm and medium-grained specimens were prepared from steel plates produced by an advanced thermo-mechanical control process. The smooth specimens were fatigued under cyclic axial tension compression at room temperature in air. The fatigue crack initiation process was investigated by atomic force microscopy and crystallographic orientation image microscope by EBSP. The results show that fatigue cracks initiated from the simple slip lines in the medium-grained specimens. On the other hand, in the ultrafine-grained specimens, complex slip deformation was formed in the vicinity of the grain boundaries prior to the initiation of fatigue cracks. Fatigue cracks were nucleated at the boundary between the grains with the concentrated complex slip deformation. The crystallographic analysis by EBSP revealed that the complex slip deformation was formed by the cross slip of active slip systems with the largest Schmid factors.