Slip Band Formation and Crack Initiation during Very High Cycle Fatigue of Duplex Stainless Steel – Part 1: Mechanical Testing and Microstructural InvestigationsTuesday (27.09.2016) 15:00 - 15:15 Part of:
Duplex Stainless Steel (DSS) is increasingly used in applications with high requirements on mechanical strength and corrosion resistance over a long period, e.g., in off-shore technologies or chemical industry. In this study, detailed mechanical experiments were carried out to investigate slip band formation and crack initiation during very high cycle fatigue (VHCF). Therefore, electrolytically polished shallow-notched hourglass-type specimens of a 22wt% Cr, 5wt%Ni, 3% Mo duplex steel (German designation: 1.4462) were fatigued in an ultrasonic fatigue testing machine at a stress ratio of R = -1. The formation of slip bands was simultaneously observed in-situ by an optical far-field microscope and scanning electron microscope (SEM). It was found that early fatigue damage manifests itself in the formation of slip bands exclusively in the softer austenite phase and with a gradually increasing density. Crack initiation is observed to occur at phase boundaries where a high slip band density impinges on an austenite/ferrite phase boundary. Once initiated, phase and grain boundaries appear as microstructural barriers for crack propagation, depending on their crystallographic misorientation relationship (measured by automated electron back-scatter diffraction EBSD). Correspondingly, a decrease of the crack propagation rate has been observed, while after passing the barriers, the crack propagation rate increases exponentially. A second part of the study is focused on the influence of the atmosphere on crack propagation and VHCF life. The fracture surfaces are generally smoother for experiments under vacuum conditions and the fatigue life by a factor of 50 longer as compared to VHCF experiments in air and corrosive environment (5% NaCl spray atmosphere). The strong environmental effect is attributed to the pronounced slip irreversibility during cycling which is observed in form of preferential single slip deformation and fracture of slip bands, which produce a rough fatigue surface and a strong decrease of fatigue life.
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