Microstructure and mechanical properties of 4% cobalt and nickel alloyed ductile irons

In the study, 4% cobalt and nickel elements were added to ductile iron (DI) to investigate the effects of the alloying elements on both its microstructure and its mechanical properties. The results show that 4% cobalt alloyed ductile iron (4% Co-DI) not only increases the nodule count and ferrite content in the microstructure, but also improves the mechanical properties such as yield strength, tensile strength, impact toughness and fracture toughness (KIC) as compared to unalloyed DI. In contrast, with 4% nickel addition, there is a decreased nodule count and increased pearlite content in the matrix. However, this material (4% Ni-DI) has the highest hardness and strength but the lowest ductility and toughness among all the ductile irons. In the case of microstructure, all kinds of iron have a sufficient nodularity up to 90% and the sequence of the nodule count is 4% Co-DI (150 nodules/mm2) > DI (106 nodules/mm2) > 4% Ni-DI (85 nodules/mm2). Moreover, it appears that cobalt plays an important role in stabilizing the ferrite phase, whereas 4% nickel accelerates the formation of pearlite in ductile iron.