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The mechanics of crack propagation in dual-phase austempered ductile iron is studied in this paper. Finite element modelling and experimental tests were performed in order to assess the effects of the microstructure topology on the crack propagation behaviour. To this end, specimens with matrix microstructures ranging from fully ferrite to fully ausferrite were studied and the preferential zones and phases for crack propagation were identified in every case. The effectiveness of the ausferrite phase as a reinforcement of the ferritic matrix via the encapsulation of the brittle and weak last-to-freeze regions was confirmed. It is found that the encapsulations induce the so-called crack deflection mechanism which conducts to an increment of the material fracture toughness. On the other hand the presence of small pools of allotriomorphic ferrite increased the crack propagation resistance, when compared with completely ausferritic matrices. Keywords: Ductile iron, Dual-phase ADI, Matrix microstructure, Fracture toughness, Finite element modelling.