The formation of plessite in meteoritic metal

available online at http://meteoritics.org 553 © The Meteoritical Society, 2006. Printed in USA. The formation of plessite in meteoritic metal J. I. GOLDSTEIN1* and J. R. MICHAEL2 1Department of Mechanical and Industrial Engineering, 313 Engineering Lab, University of Massachusetts, 160 Governors Drive, Amherst, Massachusetts 01003, USA 2Materials Characterization Department, Sandia National Laboratories, P.O. Box 5800, MS 0886, Albuquerque, New Mexico 87185, USA *Corresponding author. E-mail: [email protected] (Received 14 February 2005; revision accepted 04 November 2005) Abstract–Plessite is a mixture of body-centered cubic (bcc) kamacite (α), face-centered cubic (fcc) taenite (γ), and/or ordered FeNi-tetrataenite (γ′′) phases and is observed in the metal of iron, stonyiron, and chondritic meteorites. The formation of plessite was studied by measuring the orientation of the bcc and fcc phases over large regions of plessite using electron backscatter diffraction (EBSD) analysis in five ataxites, the Carlton IAB-IIICD iron, and zoneless plessite metal in the Kernouve H6 chondrite. The EBSD results show that there are a number of different orientations of the bcc kamacite phase in the plessite microstructure. These orientations reflect the reaction path γ (fcc) → α2 (bcc) in which the α2 phase forms during cooling below the martensite start temperature, Ms, on the closepacked planes of the parent fcc phase according to one or more of the established orientationPlessite is a mixture of body-centered cubic (bcc) kamacite (α), face-centered cubic (fcc) taenite (γ), and/or ordered FeNi-tetrataenite (γ′′) phases and is observed in the metal of iron, stonyiron, and chondritic meteorites. The formation of plessite was studied by measuring the orientation of the bcc and fcc phases over large regions of plessite using electron backscatter diffraction (EBSD) analysis in five ataxites, the Carlton IAB-IIICD iron, and zoneless plessite metal in the Kernouve H6 chondrite. The EBSD results show that there are a number of different orientations of the bcc kamacite phase in the plessite microstructure. These orientations reflect the reaction path γ (fcc) → α2 (bcc) in which the α2 phase forms during cooling below the martensite start temperature, Ms, on the closepacked planes of the parent fcc phase according to one or more of the established orientation relationships (Kurdjumov-Sachs, Nishiyama-Wasserman, and Greninger-Troiano) for the fcc to bcc transformation. The EBSD results also show that the orientation of the taenite and/or tetrataenite regions at the interfaces of prior α2 (martensite) laths, is the same as that of the single crystal parent taenite γ phase of the meteorite. Therefore, the parent taenite γ was retained at the interfaces of martensite laths during cooling after the formation of martensite. The formation of plessite is described by the reaction γ → α2 + γ → α + γ. This reaction is inconsistent with the decomposition of martensite laths to form γ phase as described by the reaction γ → α2 → α + γ, which is the classical mechanism proposed by previous investigators. The varying orientations of the fine exsolved taenite and/or tetrataenite within decomposed martensite laths, however, are a response to the decomposition of α2 (martensite) laths at low temperature and are formed by the reaction α2 → α + γ.