In-situ SEM observation on the fracture of austempered ductile iron

The microprocesses of crack initiation and propagation in an austempered ductile iron have been investigated by using an scanning electron microscope with a microtensile holder. It was revealed that microcracks always initiated at, and propagated along, the graphite–matrix interface. Microcracks in the matrix initiated after the interface cracking. The graphite nodules in this cast iron could not be regarded as voids with no strength because internal fracture of the graphite was observed. When the cracks propagated in the matrix, they often grew along the bainitic ferrite–austenite interface. Crack deflection, shear ligament formation, and crack branching occurred during the connection of main cracks and microcracks, which contributed to the strength and toughness of the ADI.