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.

以间金属扩散作为特定金属，例如金属“B”的熔融标准，涉及监测金属“B”在熔融过渡期间与另一指定为“A”的金属的界面行为。金属“A”的熔点必须高于金属“B”。原则上，当金属“B”处于固态时，这两种金属应互不相溶。在金属“B”熔融过程中，两种金属必须发生反应，形成均匀的溶液。因此，此过程可被采纳作为熔融标准。尽管难以找到一种理想的候选金属，该金属在高温下与固态铁不互溶，但固态钨在固态铁中的缓慢扩散将允许利用钨-铁界面行为作为熔融标准，前提是有足够的铁材料以容纳形成与纯铁共存的金相间金属层，并在给定交互时间保持。此外，形成的固态FeW和Fe2W合金必须具有比铁更高的熔点。这意味着熔融过渡可以源自FeW/Fe2W间金属合金与纯铁金属之间的界面反应。我们在约15 GPa的恒定压力下测试并建立了这项技术。所有实验的持续时间为25分钟。图1（A）展示了在15 GPa和2043 K下回收运行的化学分析。BSE和EDS分析显示，在钨和铁的界面形成了FeW（灰色）的薄层间金属组成。在图1（C）中，对样品也进行了EPMA分析。结果表明，间金属层是通过浓度梯度约为每单位距离0.5 wt%的固态扩散过程形成的（图2C）。在此边界中，钨的整体百分比含量介于20-30 wt%之间（约宽30µm）。根据Fe-W相图（Goldbeck，1982年），间金属边界将具有比纯铁更高的熔点。界面处的淬火纹理和成分证实了在15 GPa和2043 K时的固态。在15 GPa下，温度升高至2073 K时，我们展示了样品的熔融，如图1（B）所示。回收样品的BSE图像和EDS分析显示，在铁样品区域存在与熔融一致的树枝状淬火纹理和钨的分布。图1（D）中展示的EPMA结果证明，在60-30 wt%的范围内，钨在液态铁中均匀溶解，平均成分为50 wt%。测量钨浓度的变化是由于对具有树枝状纹理的样品进行点分析（见图S2）。通过取固态和熔融运行之间测量温度的平均值，我们确定在15 GPa下铁的熔融温度为2058 K±15 K。表S1列出了每次运行的实验条件、识别的相以及在每个给定压力下确定的熔融温度。原始EPMA数据可在此出版物中找到。