The role of entropy and enthalpy in high entropy carbides
The role of entropy and enthalpy in high entropy carbides
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DOI：
10.1016/j.commatsci.2022.111474
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发表时间：
2022-07
影响因子：
3.3
通讯作者：
Xiaochuan Tang;G. Thompson;Kaka Ma;C. Weinberger
中科院分区：
文献类型：
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作者：
Xiaochuan Tang;G. Thompson;Kaka Ma;C. Weinberger
通过相图计算（CALPHAD）方法研究了由IVB族和VB族过渡金属碳化物以及碳化钨组成的等原子混合碳化物（通常称为高熵碳化物（HEC））的热力学稳定性。B1结构的混合碳化物的吉布斯自由能的计算使用的复合能量形式主义，而BH结构的混合碳化物的使用点缺陷模型进行评估。从密度泛函理论计算和Debye-Grüneisen模型中获得了CALPHAD方法所需的热力学数据。通过数值计算和分析方法确定了HEC在热力学平衡下混合的温度下限。我们发现，在这些混合过渡金属碳化物化合物的混合焓至少是重要的构型混合熵。存在等原子固溶体的下限温度在很大程度上与组分的数量无关，唯一的例外是含有碳化钨的溶液，其中注意到弱的温度依赖性。此外，在低于约1000 K时热稳定的唯一等原子固溶体是仅通过焓稳定的固溶体，这表明许多目前制造的HEC在室温下不处于平衡。总的来说，我们的研究结果表明，这些碳化物的形成是由熵和焓之间的竞争，或焓单独控制，因此这些材料应被称为多主成分碳化物，因为前者的术语可能会误导。
The thermodynamic stability of equiatomic mixed carbides, commonly referred to as high entropy carbides (HECs), is investigated via the CALculation of PHAse Diagrams (CALPHAD) approach for mixed carbides consisting of the group IVB and VB transition metal carbides as well as tungsten carbide. The Gibbs free energy of the B1-structured mixed carbides is computed using the compound energy formalism while that of the Bh-structured mixed carbides is evaluated using a point-defect model. The required thermodynamic data for the CALPHAD approach are obtained from density functional theory calculations and the Debye-Grüneisen model. The lower temperature limit at which the HECs mix in thermodynamic equilibrium is determined via numerical and analytical approaches. We find that enthalpy of mixing is at least as important as configurational mixing entropy in these mixed transition metal carbide compounds. The lower limit temperature where an equiatomic solid solution is present is largely independent of the number of components with the only exception being solutions containing tungsten carbide, where a weak temperature dependence is noted. Furthermore, the only equiatomic solid solutions that are thermodynamically stable below approximately 1000 K are those stabilized by enthalpy alone, indicating that many currently fabricated HECs are not at equilibrium at room temperature. Collectively, our results demonstrate that the formation of these carbides is controlled by the competition between entropy and enthalpy, or enthalpy alone, and thus these materials should be referred to as multi-principal component carbides since the former terminology can be misleading.