Influence of Microstructure on Crack Paths in a Ferritic- Martensitic Steel

The effect of microstructural morphology of ferritic martensitic dualphase steel on small crack initiation and propagation under cyclic loading wasinvestigated. Damage accumulation during the experiment was directly observed with along-distance microscope. Slip bands formed in ferrite grains after several thousandscycles and cracks initiated along some of them due to dislocation pile up. Most of thesecracks were oriented at around 45° with respect to the loading direction. Subsurfaceobservation by means of a focused ion beam (FIB) and additional crystallographicanalysis with electron backscatter diffraction (EBSD) measurement showed that thesecracks initiated as a result of activity of slip system having high Schmid Factor. A fewcracks initiated at phase boundaries of ferrite/martensite, which lay in the directionperpendicular to the loading direction, but they propagated in the ferrite grains underthe surface. The FIB tomography technique and the EBSD crystallographic techniqueshowed how crack growth in depth direction is affected by the microstructure.INTRODUCTION