Prediction of the thermodynamic properties of multicomponent liquid alloys by binary infinite dilute activity coefficien
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samples of equilibrium experiments. Titanium carbide crystals were found in those samples. It is arguable whether the Ti in the TiC crystals should be taken into account. In the earlier studies by Delve et al.[11] and Narita et al.,[13] only acid-soluble titanium content of the metal, the portion that is soluble in H2SO4 or HCl, was considered as the titanium originally in solution in the liquid iron. Morizane et al.[9] and Sumito et al.[10] considered also the titanium in the TiC crystal as part of titanium in solution in the liquid iron. They found that most of the TiC crystals are located on the basal planes of graphite flakes. Therefore, they considered that the TiC crystals were formed during solidification and should be taken into account as titanium originally in solution in the liquid metal. This is probably the main reason that titanium solubility reported by Morizane et al. and Sumito et al. is higher than that from the earlier studies. While it is certainly possible that TiC will precipitate during quenching, and the amount will vary with different quenching practices, it is possible that some of the TiC crystals originate from the liquid metal considering that a TiC tablet, not a TiC single crystal, was used in the experiments by Morizane et al. and Sumito et al. Accordingly, the titanium solubility from the studies by Morizane et al. and Sumito et al. may be considered too high, while data from earlier studies[11–14] may be too low. In the present study, the precipitation of TiC was directly observed, which eliminates the uncertainties resulted from quenching. Taking the free energy of Reaction [3] as Eq. [6], the activity coefficient of titanium in the Fe-Csat-Ti system is recalculated using results from the present study and is given by log fTi ⫽
6890(⫾50) ⫺ 5.41(⫾0.01) T
[8]
The range of error was estimated from an uncertainty of ⫾5 K in the TiC precipitation temperature. At 1773 K, the activity coefficient of Ti is 0.03, slightly higher than 0.023 obtained by Morizane et al.[9] but the temperature dependence differs considerably with that obtained by Sumito et al.[10] Using Eqs. [6], [5], and [8] for the calculation of KTiC, KTiN, and fTi, respectively, the minimum Ti concentration was recalculated using Eq. [2]. The result is presented in Figure 2 as a broken line. From METALLURGICAL AND MATERIALS TRANSACTIONS B
Figure 2, it can be seen that the recalculated values are closer to the experimental results. Delve et al.[11] measured the solubility of titanium in carbon-saturated iron under 1 atm of nitrogen. Their results showed a lower titanium solubility. In their analysis, only the acid-soluble portion of titanium was considered. However, part of titanium originally in solution in the liquid iron would inevitably be transformed into Ti(C,N) during solidification. Therefore, their data are likely lower than the actual titanium solubility. Summary: A confocal scanning laser microscope with an infrared image furnace was successfully used for in-situ observation of TiC and Ti(C,N) precipitation from
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