Crystal Phases Formed in a CaO-Fe 2 O 3 System Under a High Cooling Rate in Air
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I.
INTRODUCTION
THE phase equilibria in a Ca-Fe-O system were discussed energetically in 1960s.[1–5] Hillert et al.[6] reported on the calculation of the phase equilibria in the Ca-Fe-O system in 1990. They could fit most of the experimental information by applying a thermodynamic model, and presented a phase diagram with a thermodynamic description more reliable than those reported in previous studies. However, discrepancies among the works of various researchers continue to exist. Many compounds have been reported to exist in the CaO-Fe2O3 system; they are summarized in Figure 1. However, in their study, Hillert et al. considered only four kinds of ternary compounds: CWF, CW3F, C4WF4, and C4WF8[6] (where C: CaO, W: FeO, and F: Fe2O3). One of the difficulties encountered in a Ca-Fe-O system is associated with kinetics before the equilibrium
YOSHIAKI KASHIWAYA is with the Department of Energy Science and Technology, Graduate School of Energy Science, Kyoto University, Yoshida Honmachi, Sakyoku, Kyoto, Kyoto Fu 606-8501 Japan. Contact e-mail: [email protected] Manuscript submitted April 5, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
state is reached. Most of the discrepancies debated among researchers involve the time-dependent factor. For example, Chessin and Turkdogan[7] discovered that the diffraction data claimed to be for C4F7 (Braun and Kwestroo),[8] C4WF8 (Phillips and Muan),[9,10] and C3WF7 (Holmquest)[11] were actually for CF2 (calcium diferrite). Given the results of the present experiment, these results were all correct in some way. However, studies have not focused on the effect of the cooling rate. When a CaO-Fe2O3 system is heated up to liquid phase, the binary phase (CaO-Fe2O3) moves to the ternary phase of CaO-FeO-Fe2O3 in an air atmosphere (PO2 = 0.21). Depending on the cooling rate, some amount of FeO remains in the sample. The amount of FeO will vary based on the oxygen partial pressure (PO2 ) in the atmosphere, and the mass of the sample in which the distance of oxygen diffusion in the sample melt will also vary. These varying conditions produce different results in studies, and many kinds of metastable compounds have been reported to form in the course of cooling. Although this study adopts a simple CaO-Fe2O3 binary system as the starting substance, the system was regarded as a CaO-FeO-Fe2O3 ternary system when the temperature increased to the region of melt formation. The compounds formed after subjecting the system to high cooling rates (maximum of 7900 K/s) were analyzed by X-ray diffraction (XRD), and the precipitation
mechanism was discussed by examining the thermodynamics and kinetics.
II.
EXPERIMENTAL
A. Quenching Method In this study, the single hot thermocouple (HT) technique[12–14] was used to melt and quench the sample. As mentioned above, the diffusion of oxygen into the sample melt controls the phase transformation during solidification; hence, a lesser weight of sample would be suitable for the study. The weight of the sample was between 10 and 20 mg. In addition to
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