Austenite decomposition structures in the gibeon meteorite

摘要

A sample of the Gibeon meteorite has been examined, using several modern metallographic techniques, in particular, electron backscattering diffraction (EBSD). The original single crystalline austenite structure had transformed during slow cooling to a mixture of Widmanstätten ferrite and a two-phase structure known as plessite, which consists of ferrite grains and residual austenite. All the ferrite orientations bear relationships to the austenite structures that are spread between the Nishiyama-Wassermann (N-W) and Young-Kurdjmov-Sachs (Y-K-S) conditions, with close matching of their respective close-packed planes. The plessite consists of a structure of ferrite grains and subgrains, with particles of austenite at the boundaries. However, the scale of the structures in the plessite varies by some two orders of magnitude between different regions, despite these regions having the same chemical composition. It is concluded that this diversity of structures is due to the independent nucleation of ferrite at different temperatures in the different volumes of austenite, depending on the availability of nucleants. The transformation shows evidence of both diffusional and displacive mechanisms. Abnormal grain growth is evident in some plessite regions; this leads to small islands of austenite being trapped inside larger grains of ferrite. Such particles may adopt platelike morphologies, when their crystal lattices rotate to different variants of the Y-K-S relationship.