Fine-grain mixture consisting of ferrite and stabilized austenite, which guarantees high strength and ductility of ADI (Austempered Ductile Iron).

Below a temperature between 400 and 500 °C, transformation is not possible; instead, short range diffusion takes place, which initially only produces ferrite. The carbon displaced from the ferrite accumulates in the austenite, whose carbon content may increase up to 2 % due to this process. This stabilizes austenite down to low temperatures and the resulting structure comprises a mixture of finely acicular ferrite and austenite.

Additional references:
Metal matrix of cast iron
Structure formation of cast iron