Austenite decomposition structures in the gibeon meteorite
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METALLOGRAPHY began with the study of meteorite structures[1] and these structures have never ceased to fascinate. The so-called ‘‘iron meteorites’’ consist predominantly of iron and nickel, the proportions of which vary quite widely.[2] Their microstructures show evidence of very slow cooling, which cannot be reproduced under normal laboratory conditions. As new experimental techniques have become available, they have been applied to gain a better understanding of the origins of meteorite structures. Major advances were made, beginning in the 1960s, using electron probe microanalysis (EPMA); the more recently developed electron backscattering diffraction (EBSD) technique offers additional possibilities. The present work was carried out to see what could be learned using these techniques. A sample of the Gibeon meteorite was available from the collection at the Swedish Institute for Metals Research; a small specimen was cut off for examination (Figure 1). Gibeon is an example of a Group IVA, or fine octahedrite, having a Widmansta¨tten structure that is visible to the naked eye. The continuity of this structure over the entire surface indicates that the material had earlier consisted of a single crystal of austenite. With its average nickel content of about 8 pct, solidification should produce a single-phase austenite structure (termed ‘‘taenite’’ in most meteorite literature). On cooling to lower temperatures, the austenite undergoes a transformation to Widmansta¨tten ferrite (or ‘‘kamacite’’), accompanied by rejection of the excess nickel. Measurements of the nickel profile at the transformation boundaries have been used to calculate the cooling BEVIS HUTCHINSON, Department Head, Mechanical Metallurgy, and ¨ M, Specialist Researcher, are with KIMAB, Swedish JOACIM HAGSTRO Institute for Metals Research, Stockholm, Sweden. Contact e-mail: bevis@ kimab.com This article is based on a presentation made in the ‘‘Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,’’ December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden. METALLURGICAL AND MATERIALS TRANSACTIONS A
rate, which has been estimated by Narayan and Goldstein to be approximately 1 °C per thousand years for this class of meteorite.[3] The transformation to Widmansta¨tten ferrite is not complete, however, and almost half of the structure consists of more complex phase mixtures of ferrite and austenite that have been generically termed ‘‘plessite.’’ Since there is no equivalent structure in other ferrous metallography, we will retain this name in the present article. Plessites appear to have attracted less research interest in the literature than have other structures existing in meteorites such as the Widmansta¨tten. The name is used as a collective term for a number of quite varied structures seen in meteorites, which have different nickel content levels and even within different areas of the same samples.[2,4–6] In the Gibeon meteorite, most of this structure is the type that h
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