Dislocation structure of an austenitic stainless steel in different stages of fatigue

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Abstract
The dislocation structure of an 18–13 type austenitic stainless steel in different stages of fatigue has been studied by means of transmission electron microscopy. For stress amplitudes well below the endurance limit (defined as the stress amplitude giving fracture after 107 cycles) a structure of randomly distributed dislocations of a low density was built up and remained unchanged from 104 cycles. At the end of the lifetime of specimens fatigued above the endurance limit, the dislocations occurred in dense tangles, often arranged in cell walls. The tangles were clustered with dislocation loops elongated in a direction. Also long parallel rows of such elongated loops were observed. Irregularly distributed stacking-fault tetrahedra were found. The stacking-fault energy of the material was estimated from the edge length of the largest observed tetrahedra. It is proposed that the stacking-fault tetrahedra as well as the observed elongated loops may be formed by a dislocation mechanism. This mechanism includes the process of screw dislocation cross slip, and it is suggested that cross slip in stainless steel during cyclic loading takes place also at moderate stresses, in spite of its comparatively low stacking-fault energy.