Thermodynamics of TiCN and TiC in Fe-C sat melts
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the metal surface were displayed on a monitor and recorded on videotape. In this technique, we observed the formation of TiC and Ti(C,N) on the surface. However, the activities of components on the surface are the same as the bulk, and therefore, for the present purpose observing the surface is meaningful. Precipitation of Ti(C,N) from Fe-Csat-Ti-N: Precipitation was observed at the surface of the liquid metal during cooling. Figure 1 shows a typical confocal laser image of the precipitates observed during cooling. Two types of crystals were observed: one with rough surfaces and another with smooth surfaces. The crystals with rough surfaces tend to agglomerate, but the bond is rather weak and can be broken by the metal movement. The crystals with smooth surfaces did not stick together even if they touched. Energy-dispersive X-ray element mapping of the quenched sample confirmed that the crystals are titanium carbonitride. The compositions of the crystals with rough and smooth surfaces appear to be identical. However, the exact compositions of the crystals cannot be analyzed using EDX because of the overlapping of the peaks among CK␣, NK␣, and L peaks of Ti. The precipitation temperature was determined as the temperature at which the first Ti(C,N) crystal was observed during cooling. It should be pointed out that the crystal at precipitation temperature is very small and difficult to reprint due to limited image resolution. Larger and clearer images of the crystals were obtained at lower temperatures, as shown in Figure 1. Once the precipitation temperature was determined for a particular titanium concentration, several experiments were carried out with the same metal composition. In those repeat experiments, the temperature was dropped step by step and kept for 5 minutes after each drop. Gradually, finer steps were used when the temperature approached the precipitation temperature determined previously. This was done in order to “pinpoint” the precipitation temperature. The results are listed in Table I. The different precipitation temperatures for the same sample demonstrate the reproducibility. Determination of the precipitation temperature through the reverse direction, i.e., by observing the dissolving of the crystals, was attempted. The crystals were found to be hard to dissolve; or, more precisely, the rate of dissolution was extremely slow. Because of the gradual smearing of the optical window in the image furnace by iron vapor, the CSLM is not suitable for observing a process taking place over many hours or days. Precipitation of TiC from Fe-Csat-Ti: As discussed later, there appears to be some uncertainty in the activity coefficient of Ti in Fe-Csat melts. Therefore, the precipitation temperature of TiC from Fe-Csat-Ti was determined in a similar way. In these experiments, ultrahigh purity argon was used during the entire experiment. The results are listed in Table II. Discussion: As discussed recently by Bergsma and Fruehan,[2] titanium cabonitride may form in a Fe-Csat melt if both Ti and N exist, and whe
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