Metallurgy
Ductile iron is not a single material but is part of a group of materials which can be produced to have a wide range of properties through control of the microstructure. The common defining characteristic of this group of materials is the morphological structure of the graphite. In ductile irons, the graphite is in the form of spherical nodules rather than flakes (as in grey iron), thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its name. The formation of nodules is achieved by addition of nodulizing elements, most commonly Magnesium (note Magnesium boils at 1100C and Iron melts at 1500C) and, less often now, Cerium (usually in the form of Misch metal), into the melt. Tellurium has also been used. Yttrium, often a component of Misch metal, has also been studied as a possible nodulizer.
"Austempered Ductile Iron" (ADI) was invented in the 1950s but was commercialized and achieved success only some years later. In ADI, the metallurgical structure is manipulated through a sophisticated heat treating process. The "aus" portion of the name refers to austenite.
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