MICROSTRUCTURAL ASPECTS OF FRACTURE IN AUSTEMPERED DUCTILE IRON.

The microstructural details of fracture in various austempered ductile irons have been investigated. Surface crack initiation and propagation were observed with the scanning electron microscope (SEM) upon incremental straining of a fracture test specimen at room temperature. Partially fractured specimens were fully fractured in a brittle manner so that the progress of existing partial cracks in the interior of specimens could also be observed. Upon initial straining, decohesion takes place between the primary graphite nodules and the austempered matrix. Localized plastic deformation is concentrated around the graphite nodules primarily in the ferrite laths or at ferrite/austenite interfaces. Microcracks initiate preferentially at many nodule sites and in intercellular delayed transformation regions where brittle microconstituents may be found. A large 'plastic zone' with many microcracks can be observed ahead of initiating or propagating crack fronts. The primary crack front grows or forms by successive link-up of favorably oriented microcracks.