Two cast irons differing in graphite shape — flakes versus nodules.
In gray iron the graphite forms flakes, which act as built-in chip breakers and give excellent damping and machinability, but also act as internal stress raisers so the material is brittle in tension.
In ductile (nodular) iron the graphite forms spheres, which removes the stress concentration. The result is real ductility — 12 to 18% elongation — at some cost in damping and machinability. The two are chosen for different jobs and are not interchangeable.
Assuming all cast iron is brittle. Ductile iron has elongation comparable to steel, and the two grades machine quite differently.
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