Strain partitioning between first-to-solidify (FTS) and last-to-solidify (LTS) regions upon tensile loading of ductile iron was investigated by combining in-situ X-ray tomography with digital volume correlation and postmortem metallographic examinations. The results indicate that the plastic shear bands form mainly by linking graphite particles contained in the same FTS region. A special distance function is introduced to show that this is due to the lower strength of the FTS regions compared to the LTS regions, but also to the higher stress concentration associated with the particles. The methodology is general and therefore extendable to material systems containing similar microstructural heterogeneities.