Damaging micromechanisms characterization in pearlitic ductile cast irons

Ductile cast irons (DCIs) are characterized by a wide range of mechanical properties, mainly depending on microstructural factors, as matrix microstructure (characterized by phases volume fraction, grains size and grains distribution), graphite nodules (characterized by size, shape, density and distribution) and defects presence (e.g., porosity, inclusions, etc.). Considering ferritic DCIs, ferritic matrix – graphite nodule debonding is only seldom observed and the main ferritic DCI damaging micromechanism consists in cracks nucleation and propagation in the graphite external shell coming from the reduced carbon solubility in phase during cooling process (“onion-like” mechanism). A second damaging micromechanism is sometimes observed together with the “onion-like” mechanism and consists in cracks nucleation and propagation corresponding to the graphite nodule center (“disgregation” mechanism). The importance of this damaging mechanism is dependent on the loading conditions and is probably influenced by the graphite nodule nucleation mechanism during solidification. In this work, the damaging micromechanisms in a pearlitic DCI have been investigated by means of the Scanning Electron Microscope (SEM) observation of the specimen lateral surface during the tensile tests (“in situ” tests). Damaging micromechanisms development has been analyzed, focusing both the role of the graphite nodules and the cracks initiation and propagation in the pearlitic matrix.