High Pressure Melting Curve of Fe Determined by Inter-Metallic Fast Diffusion Technique
The use of inter-metallic diffusion as a melting criterion for a given metal say “B” involves the monitoring of the interface behavior of the metal with another metal assigned “A” across the melting transition of the metal B. The metal designated as A must have a higher melting temperature than metal B. In principle, these two metals should be immiscible when B is in solid state. On melting of B, the two metals must react to form a uniform solution. Hence, this process can be adopted as a melting criterion. Although, it is difficult to find an ideal candidate metal that would be immiscible with solid Fe at high temperature, the slow diffusion of solid W in solid Fe would enable the use of W-Fe interface behavior across Fe melting transition as a melting criterion if there is enough Fe material to accommodate the formation of the intermetallic boundary layer with pure Fe remaining for a given interaction time. Also, the formed solid FeW and Fe2W alloys must have higher melting temperature than Fe. This implies that the melting transition could be derived from the reaction at the interface between the intermetallic alloy of FeW/Fe2W compound and the pure Fe metal. We tested and established this technique at fixed pressure of ~ 15 GPa. The duration of all experiments is 25 minutes. Shown in Figure 1 (A) is the chemical analyses of the recovered run at 15 GPa and 2043 K. The BSE and EDS analyses show a formation of a thin intermetallic composition of FeW (grey color) at the W and Fe interface. EPMA analysis was also performed on the sample as shown in Figure 1 (C). The results indicate that the intermetallic layer was formed by a solid-state diffusion process with a concentration gradient of ~ 0.5 wt% per unit distance (Fig. 2C). The overall percentage composition of W lies between 20-30 wt% in this boundary (~30µm wide). Judging from the phase diagram of Fe-W (Goldbeck, 1982), the intermetallic boundary would have a higher melting temperature than pure Fe. The quench texture and composition at the interface confirm the solid state at 15 GPa and 2043 K. With increasing temperature to 2073 K at 15 GPa, we show melting of the sample as indicated in Figure 1 (B). The BSE image and EDS analyses of the sample recovered show dendritic quench texture and distribution of W in the Fe sample region that are consistent with melting. The EPMA results shown in Figure 1 (D) demonstrate an even dissolution of W in the range of 60-30 wt% in molten Fe with an average composition of 50 wt%. The variation of the measured W concentration is due to the spot analysis of the sample with dendritic texture (See figure S2). By taking the average of the measured temperatures between the solid and melting runs, we determine the melting temperature of Fe to be 2058 K±15 K at 15 GPa. Table S1 lists the experimental conditions for each run, the identified phase, and the determined melting temperature at each given pressure. The raw EPMA data can be found in this publication.