The Metallurgical Interpretation of the Structures found in Meteoritic Irons

Abstract

Two crystallographic mechanisms have been established for the transformation from the high-temperature, face-centered cubic lattice of γ-iron to the low-temperature, body-centered cubic lattice of α-iron. Published researches on meteorites were not adequate to establish which of these two mechanisms operated in the formation of the coarse Widmanstatten patterns found in meteorites. It was felt that a study of these patterns in meteoritic irons would contribute to the understanding of these mechanisms. An investigation of the macro-and micro-crystalline perfection of an Amalia Farm [= Bethany, Great Namaqualand], South Africa, and a Canyon Diablo, Arizona, meteorite established that the latter was far better suited for an X-ray study of the orientation relationships in the structure. In the course of the research it was established that one of the mechanisms operated at high transformation temperatures, while the other operated at much lower temperatures. The orientations in the Canyon Diablo sample corresponded to those predicted by the high-temperature mechanism. This, and other metallurgical evidence, is in accord with the theory that the Widmanstatten patterns in meteorites are formed by extremely slow cooling from high temperatures. Recent advances in physical metallurgy have developed very useful correlations between rates of solid reactions and the resultant structures (coarseness of pattern) and between speed and temperature of plastic deformation and the type of deformation which occurs (slip, twinning, and cleavage). The application of this knowledge, in conjunction with adequate experiments on alloys of meteoritic composition, would undoubtedly lead to definite knowledge concerning the origin and the history of meteorites.