Effect of Carbon on the Volume of Solid Iron at High Pressure: Implications for Carbon Substitution in Iron Structures and Carbon Content in the Earth’s Inner Core
Effect of Carbon on the Volume of Solid Iron at High Pressure: Implications for Carbon Substitution in Iron Structures and Carbon Content in the Earth’s Inner Core
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DOI：
10.3390/min9120720
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发表时间：
2019-11
期刊：
影响因子：
2.5
通讯作者：
Jing Yang;Y. Fei;Xiaojun Hu;E. Greenberg;V. Prakapenka
中科院分区：
文献类型：
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作者：
Jing Yang;Y. Fei;Xiaojun Hu;E. Greenberg;V. Prakapenka
了解碳对六边形致密铁-碳合金密度的影响对于模拟地球内核中的碳含量至关重要。以往的研究主要集中在铁碳化物的状态方程上，这些方程可能不适用于固体内核，因为金属铁中可能以溶解碳的形式含有碳。hcp-Fe中的碳取代及其对密度的影响尚无实验研究。我们研究了含碳0.31和1.37 wt %的Fe- c合金的压缩行为，并以纯铁为参考，通过原位x射线衍射测量，纯铁为135 GPa， Fe-0.31 c为87 GPa， Fe-1.37 c为109 GPa。结果表明，碳在hcp-Fe中的掺入导致了晶格的膨胀，这与已知的体心立方(bcc)-Fe中的作用相反，表明取代机制或局部环境发生了变化。轴向压缩性数据表明，碳含量的增加可能会增强地球内核的地震各向异性。新的热弹性参数使我们能够建立一个热弹性模型来估计当碳以溶解碳hcp-Fe的形式掺入内核时的碳含量。解释地球内核密度缺陷所需的碳含量在内核边界温度为5000k和7000k时分别为1.30%和0.43%。
Understanding the effect of carbon on the density of hcp (hexagonal-close-packed) Fe-C alloys is essential for modeling the carbon content in the Earth’s inner core. Previous studies have focused on the equations of state of iron carbides that may not be applicable to the solid inner core that may incorporate carbon as dissolved carbon in metallic iron. Carbon substitution in hcp-Fe and its effect on the density have never been experimentally studied. We investigated the compression behavior of Fe-C alloys with 0.31 and 1.37 wt % carbon, along with pure iron as a reference, by in-situ X-ray diffraction measurements up to 135 GPa for pure Fe, and 87 GPa for Fe-0.31C and 109 GPa for Fe-1.37C. The results show that the incorporation of carbon in hcp-Fe leads to the expansion of the lattice, contrary to the known effect in body-centered cubic (bcc)-Fe, suggesting a change in the substitution mechanism or local environment. The data on axial compressibility suggest that increasing carbon content could enhance seismic anisotropy in the Earth’s inner core. The new thermoelastic parameters allow us to develop a thermoelastic model to estimate the carbon content in the inner core when carbon is incorporated as dissolved carbon hcp-Fe. The required carbon contents to explain the density deficit of Earth’s inner core are 1.30 and 0.43 wt % at inner core boundary temperatures of 5000 K and 7000 K, respectively.