A transmission electron microscopy examination of plessite structures in the Arispe iron meteorite

The Arispe iron meteorite contains thin stingers of retained austenite (taenite) between plates of ferrite (kamacite). These stringers contain microstructures of various Fe-Ni phases called plessite. Plessite has been observed in the Arispe iron meteorite as fine and coarsened pearlitic (lamellar), spheroiditic, and microwidmanstatten a structure. Each of these structures consists of kamacite and a high-Ni unidentified taenite phase, identification of the high-Ni taenite phase can aid in the determination of the Fe-Ni phase diagram. Through TEM observations and EDS analyses of the pearlitic plessite, this high-Ni phase was determined to be tetrataenite, an ordered face-centered cubic phase with ~50% Ni. Furthermore, it was concluded that the pearlitic plessite formed by the monotectoid reaction which is prior to the transformation of the taenite to tetrataenite. The taenite phase of the microwidmanstatten a structure was found to contain -30% Ni, which is significantly less than the Ni content of tetrataenite. it is speculated that the microwidmanstatten a structure formed by a different mechanism. Based upon the observation of martensite in the microwidmanstatten a structure, the plessite formed from a stringer that was martensitic prior to transforming to microwidmanstatten a structure, it was not established why the taenite plates did not transform to a taenite phase as predicted from the Fe-Ni phase diagram.