Mechanisms of short crack propagation in austenitic-ferritic duplex steel

For applications requiring both high strength and high corrosion resistance, austenitic-ferritic duplex steels are often the material of choice. In this study, cyclic deformation experiments were performed on the austenitic-ferritic duplex stainless steel 1.4462. By measuring the crack opening and crack sliding displacement in situ in a scanning electron microscope, the characteristics of the different crack propagation mechanisms in the two phases are determined. In the ferritic phase, two different appearances of short cracks can be observed, one exhibiting a very smooth and the other one a rather rough surface crack path. Electron backscatter diffraction measurements on the crack-containing grains in addition with high resolution imaging of the topography of the crack flanks reveal that contrary to common assumptions in the literature, short cracks in ferrite do not propagate via single slip. Instead, two different slip systems with an identical slip direction, but different slip planes, are activated. In this context, the specific appearance of different crack paths can be explained with the orientation of the respective grains. Furthermore, a model for discontinuous crack propagation especially of rough cracks in ferrite is developed. Finally, a correlation between the crack propagation rate and the plastic deformation of the crack tip is revealed and the possibility of determining the barrier effect of grain and phase boundaries via the measurement of the plastic deformation of the crack tip is investigated.