Microstructural observations of spheroidization from a lamellar structure in iron meteorites

The Ocotillo and Arispe iron meteorites have a coarse Widmanstaten pattern of primary ferrite crystals, typical of many iron meteorites. These crystal s are separated by thin sheets or plates of retained austenite, some of whi ch have decomposed into various structures upon very slow extraterrestrial cooling and subsequent unknown thermal histories. One structure is a lamell ar product, probably formed by a monotectoid reaction. Some regions in thes e two meteorites show that the lamellar structure has begun to change to a structure of spheroids of a nickel-rich phase in ferrite, and other regions have completely spheroidized. In some regions, the spheroidization process occurs by geometric instability of the lamellar plates. However, most of t he spheroidization appears to be occurring at morphological defects in the lamellar structure (e.g., branches), or at the end of the plates of the lam ellar structure. There is evidence that the processes are controlled by int erphase surface diffusion. (C) 2000 Elsevier Science Inc. All rights reserv ed.