English: Microstructure of Austempered Ductile Iron (ADI), selective colour etching, taken by light microscopy using polarized light, mag.1000x with interface digital camera. It is characterized by its spheroidal graphite nodules spaced within the matrix. These nodules reduce microsegregation of solutes within the material. For ADI, the material has been austempered such that the matrix is transformed into ausferrite, or a mixture of acicular ferrite and austenite. The microstructure is used to classify ADI into grades, which depend on the heat treatment process and not the composition of the material.