Фазовая диаграмма системы Ca-Fe

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Ca-Fe (Calcium-Iron) D.J. Kahan and L.J. Swartzendruber The assessed Fe-Ca phase diagram is based on review of the limited experimental data and was obtained by thermodynamic modeling. The limited liquidus data of [64Spo] and [75Sch] were used to obtain thermodynamic parameters for a regular solution model, from which extrapolations were made to construct the rest of the phase diagram. The transformation enthalpies suggested by [86Itk] were used to construct the Ca lattice stabilities. Ca and Fe are virtually insoluble in each other, in both the liquid and solid states. The equilibrium solid phases, all based on the elemental allotropes, are (1) (dFe), bcc, stable from the melting point (1538 C) down to 1394 C; ( 2) (gFe), fcc, stable from 1394 down to 912 C; (3) (aFe), bcc, stable below 912 C, ferromagnetic below 770 C; (4) (bCa), bcc, stable from the melting point (842 C) down to 443 C; and (5) (aCa), fcc, stable below 443 C. Ca has been widely noted as insoluble in liquid Fe. [64Spo] published the only experimental data-a solubility of 0.032 wt.% (0.045 at.%) Ca in liquid Fe at 1607 C was determined from the time needed for a melted sample to reach equilibrium. Their samples were melted under an Ar pressure of 1.4 MPa (14 atm) , said to be above the pressure at which Ca would vaporize at this temperature. The solubility was said to be virtually pressure-independent, as long as Ca was constrained from evaporating. The thermal analysis data of [75Sch] are the only information on the solubility of Fe in liquid Ca. The solubility was reported as <0.01 wt.% Fe (< 0.007 at.% Fe) from 900 to 1200 C. [75Sch] reported a Ca-rich eutectic, and although neither the composition nor temperature were given, limits can be assigned to both from the liquidus data. The eutectic composition can be no richer in Fe than the reported liquidus (<0.007 at.% Fe at 900 C), and to avoid an implausibly deep eutectic, the temperature can be no more than a fraction of a degree below the Ca melting point. In the assessed diagram, the estimated monotectic and eutectic points, 0.0347 at.% Ca at 1537.75 C (Fe-rich monotectic) and 99.998 at.% Ca at 841.8 C (Ca- rich eutectic), are consistent with the limited experimental values. There are no data on solid-state alloy equilibria. A discontinuity in the heating curve of an ~16 wt.%(?) Ca sample, between 380 and 540 C, was thought to indicate an intermetallic phase [27Tam], although this has never been confirmed. The solubility of Ca in Fe is so small that its effect on the Curie temperature of pure Fe (770 C) is negligible. The industrial importance of Ca as a scavenger in steel production and as a spheroidizing agent in the production of cast iron [64Spo] suggests that further measurement of its solubility in both liquid Fe and its solid allotropes would be useful. Determination of the liquid miscibility gap to higher solute concentrations and temperatures would allow a more confident estimation of thermodynamic parameters and prediction of the phase boundaries. 27Tam: G. Tamman and K. Schaarwachter, Z. Anorg. Chem., 167, 401-410 (1927) in German. 64Spo: D.L. Sponseller and R.A. Flinn, Trans. Metall. Soc. AIME, 230, 876-888 ( 1964). 75Sch: E. Schurmann, H. Litterscheidt, and P. Funders, Arch. Eisenhuttenwes., 46(10), 619-622 (1975) in German. 86Itk: V. Itkin and C.B. Alcock, private communication (Apr 1986). Submitted to the APD Program. Complete evaluation contains 3 figures, 3 tables, and 12 references. Special Points of the Fe-Ca System