The microstructure of austempered ductile iron was investigated by electron backscatter diffraction technique. The results show that the orientation relationship between acicular bainitic ferrite and austenite is Greninger–Troiano relationship. A single austenite grain is divided into four packets and each packet contains six variants that share a {011} α (i.e., {111} γ ) plane. When two γ grains are twinned, the twins share a {111} γ plane and have seven packets. The adjacent acicular bainitic ferrite plates (or laths) sharing a ? 001 ? γ axis have small misorientation of about 5.7°. The adjacent acicular bainitic ferrite plates (or laths) not sharing a ? 001 ? γ axis have two high misorientation angles of ~54.3° and ~60.0°. Further, the low angle boundary to high angle boundary ratio is far less than the ratio of the variant pairs with small misorientation to the ones with large misorientation. This work is available for structures obtained as a consequence of the heat treatment of austempering.
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