The formation of plessite in meteoritic metal

Abstract

Plessite is a mixture of body-centered cubic (bcc) kamacite (alpha), face-centered cubic (fcc) taenite (gamma), and/or ordered FeNi-tetrataenite (gamma-") phases and is observed in the metal of iron, stony-iron, 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 gamma (fcc) --> alpha2 (bcc) in which the alpha2 phase forms during cooling below the martensite start temperature, Ms, on the close-packed 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 alpha-2 (martensite) laths, is the same as that of the single crystal parent taenite phase of the meteorite. Therefore, the parent taenite was retained at the interfaces of martensite laths during cooling after the formation of martensite. The formation of plessite is described by the reaction gamma --> alpha2 + gamma --> alpha + gamma. This reaction is inconsistent with the decomposition of martensite laths to form phase as described by the reaction gamma --> alpha2 --> alpha -+ gamma, 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 alpha2 (martensite) laths at low temperature and are formed by the reaction alpha2 --> alpha + gamma.