Abstract
A high stress bending fatigue test has been conducted on a cold-rolled and annealed Fe-8.2wt.%Al-30.1wt.%Mn steel which exhibits a mixed dual-phase structure containing 45 vol.% austenite and 55 vol.% ferrite. The axis of the applied stress is either perpendicular (T) to or parallel (L) to the rolling direction of specimens. It is found that the tensile property of the L specimens is much superior to that of the T specimens, but their fatigue properties are comparable. Transmission electron microscopy (TEM) examinations indicate that a heterogeneous planar dislocation substructure was developed by the cyclic bending stress within the austenite region, and the tangled dislocation cell substructure was observed within the ferrite region. The former is associated with the early persistent slip bands and the easy initiation and propagation of crack. However, the latter is accompanied by the cross-slip of dislocations. From the in situ surface crack propagation study, it is found that the crack is always initiated from and moved along the slip bands within the austenite region. The twin and grain boundaries, and austenite-ferrite interfaces are less favourable. The fatigue crack path cannot durably stay within the ferrite region (arrays) of the bending fatigue specimen. This result implies that the ferrite phase serves as a crack arrester. One reason is that the annealed ferrite phase is softer than austenite and dislocation easily cross-slips in the ferrite structure, so that the blunting effect during fatigue testing may retard the crack propagation. The other important reason for the better fatigue properties of the ferrite phase than the austenite is the shortage of carbon in the present material. © 1989.