Transmission electron microscopy observations of the lamellar structure inthe Arispe iron meteorite

The Arispe iron meteorite contains approximately 7 at.% Ni. The structure c onsists of coarse ferrite plate-like crystals with a Widmanstatten pattern. In between some of these plates is retained austenite, some of which has d ecomposed to a variety of structures (e.g., martensite, spheroidized struct ure, etc.). One of these structures is a lamellar arrangement of alternatin g plates of a low-Ni and a high-Ni phase. The structure has been identified using transmission electron microscopy. The low-Ni phase is ferrite and co ntains approximately 6 at.% Ni. The high-Ni phase contains approximately 48 at.% Ni and has the ordered NiFe (L1(o)) structure. The Fe-Ni phase diagra m indicates that the lamellar structure formed by a monotectoid reaction of austenite decomposing to ferrite and disordered austenite containing appro ximately 48 at.% Ni. Upon further cooling, the austenite ordered to form Ni Fe. (C) 2001 Elsevier Science Inc. All rights reserved.