Crack nucleation mechanism of austempered ductile iron during tensile deformation

The relationship between the crack nucleation and stress-induced martensitic transformation in the retained massive austenites (RM-gamma's) of austempered ductile irons (ADIs) was examined in detail by carrying out tensile tests and scanning electron microscope (SEM) observations for an ADI material. The SEM observations revealed that cracks were not nucleated in the peripheral regions of graphite nodules but were nucleated in the RM-gamma's. Surface relief due to stress-induced martensitic transformation was observed near the cracks in the RM-gamma's, and it was also observed in the RM-gamma's in which cracks were not visible. For this reason, the cracks were concluded to be nucleated mainly in the RM-gamma's subjected to stress-induced martensitic transformation.