Discontinuous precipitation reactions in alpha and gamma phases of meteoritic metal

Details Discontinuous precipitation reactions in alpha and gamma phases of meteoritic metal Discontinuous precipitation reactions in alpha and gamma phases of meteoritic metal

Nov 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982jmsl....1..485g&link_type=abstract

Journal of Materials Science Letters, vol. 1, Nov. 1982, p. 485-488.

Physics

Condensed Matter

Materials Science

Chemical Reactions, Iron Compounds, Meteoritic Composition, Meteoritic Microstructures, Nickel Compounds, Precipitation (Chemistry), Close Packed Lattices, Face Centered Cubic Lattices, Hexagonal Cells, Reaction Products, Recrystallization

Scientific paper

Evidence is presented for interrupted discontinuous precipitation (DP) reactions in meteoritic FeNi metal at about 350 C. Whereas pseudo-euctectoid alpha + gamma structures are well known in meteoritic metal when the reaction has gone effectively to completion (Buchwald, 1975) the recognition of these as DP reaction products reported here arises from observations on interrupted reactions. In explaining these microstructures, it is pointed out that slow continuous cooling through the equilibrium diagram allows alpha to equilibrate to its maximum percentage of nickel at 500 C by the usual process of volume (lattice) diffusion. With further cooling, this alpha becomes supersaturated as lattice diffusion rates become progressively inadequate. At this stage, recrystallization of the supersaturated solid solution begins, starting from local positions at the silicate-metal contact surfaces. It is noted that diffusion rates would be enhanced during the recrystallization process, allowing local precipitation of high gamma-Ni. The normal process of discontinuous precipitation proceeds from this condition.

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