Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.604y&link_type=abstract
Meteoritics, vol. 30, no. 5, page 604
Computer Science
Computer Simulation, Cooling Rates, Metallographic, Meteorites, Composition, Phase Diagrams, Phase Transformation
Scientific paper
The metallographic cooling rate methods [1][2][3] have been used for some 30 years to determine the cooling rates of the metal phases in meteorites. Of the cooling rate methods that have been used, two valid procedures [4] are the "central Ni content vs. taenite size method [1]" and the "profile matching method [2]". The cooling rate of meteorites is determined by matching numerically calculated data with experimental data measured with the electron probe microanalyzer (EPMA). Both cooling rate methods strongly depend on the accuracy of the numerical calculation of the Ni composition profile in taenite. The numerical calculation is based on diffusion theory, experimentally determined diffusion coefficients and the Fe-Ni (P) phase diagram. _ Recently, the chemistry and microstructure of the metallic phases in meteorites have been investigated by Yang et al. [5] using analytical electron microscopy (AEM) and high resolution scanning electron microscopy (SEM). It has been observed that the final microstructure and Ni composition of the metallic phases are formed by a series of complex phase transformations at low temperatures (<400 degrees C), which can not be explained using the current phase diagram, Reuter et al. [6]. Therefore, a new Fe-Ni phase diagram at low temperatures has been proposed by Yang et al. [5] using the AEM results of metallic phases in meteorites. In the new phase diagram, alpha (low Ni bcc kamacite) and g' (Ni3Fe) phases are in equilibrium at low temperatures. The g" (FeNi, tetrataenite) phase is present as a metastable phase. During the cooling process, first a monotectoid reaction (g1 > a + g2, where g1 is a low Ni paramagnetic fcc taenite and g2 is a high Ni ferromagnetic fcc taenite) occurs at about 400 degrees C, and then at lower temperature (about 345 degrees C) a eutectoid reaction (g2 > a + g', where g' is Ni3Fe) occurs. Because of these low temperature reactions, the computer program which has been used for numerical calculation of the Ni composition profile based on the phase diagram of Reuter et al. has to be modified according to the new phase diagram. In addition, it has been observed, using the EPMA, that a discontinuity in the Ni composition profile is present in the taenite region of meteoritic metal [7][8]. This discontinuity can be explained by the new phase diagram. Several parameters and assumptions used in the computer simulation have to be modified in order to accurately model the formation of the Ni composition profile. The Ni diffusion coefficient for the low temperature ordered phases, such as g" (FeNi) phase and g' (Ni3Fe) phase, has to be considered in order to calculate the Ni composition profile in the tetrataenite rim (clear taenite I) region. The assumption of a linear continuous cooling history also has to be reconsidered because there is a time interval between the monotectoid and the eutectoid reaction which occurs during cooling. References: [1] Wood J. A. (1964) Icarus, 3, 429-459. [2] Goldstein J. I. and Ogilvie R. E. (1965) GCA, 29, 893-920. [3] Goldstein J. I. and Short J. M. (1967) GCA, 31, 1733-1770. [4] Saikumar V. and Goldstein J. I. (1988) GCA, 52, 715-726. [5] Yang C. W. (1995) Ph.D. dissertation, Lehigh Univ. [6] Reuter K. B. et al. (1989) Metall. Trans., 11A, 719-725. [7] Scott E. R. D. (1973) GCA, 37, 2283-2294. [8] Reuter K. B. et al. (1988) GCA, 52, 617-626. _
Goldstein Joseph I.
Williams Brian D.
Yang Chih-Wen
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