Microstrain measurements and damage analysis during tensile loading of intercritical austempered ductile iron

The study focuses on the measurement of microstrain and the assessment of damage during tensile loading of intercritical austempered ductile iron by using digital image correlation technique and in situ observations. The results show that the first stages of damage involve the initiation and propagation of cracks along the matrix-nodule interface. By comparing the strain maps retrieved from digital image correlation analysis with the microstructure, plastic deformations were observed to be located preferentially in the ferritic internodular areas. However, for a more advanced stage of damage, cracks are seen to initiate in the metallic matrix at the boundary between ausferrite sheaves and propagate preferentially following the direction of the ausferrite plates. The final fracture shows a mix of quasi-cleavage and ductile morphology. The quasi-cleavage is produced by the nucleation and growth of multiple quasi-cleavage cracks formed on the ausferrite sheaves, while the overloaded ferritic zones are fractured by ductile tearing.