On Investigating the Shock History of Ataxites

Abstract

It is suggested that transmission electron microscopy studies of ataxites and shock-loaded artificial nickel-iron alloys could develop criteria whereby the shock history of this class of meteorites might be established.The extensive investigations of Lipschutz and co-workers on iron meteorites have shown that it is often possible to reveal that a meteorite has undergone moderate shock, i.e., shock-loading in the 130-750 kb range, by applying X-ray diffraction techniques to its kamacite phase. This can be done even in the case of a meteorite in which careful metallographic examination has failed to detect any traces of the epsilon-iron transformation. The X-ray techniques have been highly successful in establishing the previously unknown shock histories of a number of octahedrites and hexahedrites, but they apparently are not applicable to the fine-grained two phase microstructure of the ataxites (Jain and Lipschutz, 1970).Curiously, photomicrographs of ataxites published by Perry (1944) show no visible evidence of the epsilon-iron transformation structure, yet there seems to be little reason for believing that none of the meteorites in this classhas been shock-loaded above 130 kb.Recent work on carbon steels (Koepke, et al., 1971) indicates that higher shock loads are required to produce the epsilon-iron transformation in steels having large proportions of a second phase, cementite, in the microstructure than is the case when the proportion of the second phase is small. This suggests that the presence of large amounts of fine-grained taenite may have suppressed the epsilon-iron transformation in the surrounding kamacite in shocked ataxites -a possible explanation for the lack of visible transformation structure in Perry's photomicrographs.In consideration of the preceding statements it appears that establishment of the shock histories of ataxites will depend upon the development of new techniques for detecting shock relics in meteorites.Fortunately, recent investigations (Leslie, Stevens, and Cohen, 1965;Brown and Dash, 1966) using selected area diffraction techniques in transmission electron microscopy studies of austenitic iron-nickel alloys have shown that moderate shock produces deformation twins and bands of closely spaced dislocations on III planes of austenite. The spacing of the twins, seen in electron micrographs, is inversely related to the shock intensity; and