高温高熵合金：声子的理论研究
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与传统合金相比，高熵合金在单个“晶格”内具有五种甚至更多种元素的同数量原子，并表现出一些非凡的物理性质。声子对材料性能的影响主要体现在热力学稳定性、热力学函数、弹性性能和热导率。这彰显了构造声子激发和声子间相互作用模型的重要性。来自荷兰代尔夫特理工大学的Körmann和来自日本京都大学的池田宇池博士等采用第一性原理计算，系统研究了质量和力常数涨落对12个具有体心立方结构的随机合金的声子谱的影响。这些结构包含二元合金到五组分的高熵合金，并探讨合金化学组分的复杂性如何影响材料的晶格振动行为。结果表明，质量和力常数涨落对声子的影响都需考虑，缺一不可。随着合金中元素种类的增加，振动熵也相应变大，其变化可能与构型熵的变化一样大，并严重影响材料的相稳定性。这项基于理论计算角度的声子宽频化研究为高温高熵合金的设计开辟了道路。该文近期发表于npj Computational Materials 3:36 (2017); doi:10.1038/s41524-017-0037-8; 标题与摘要如下，论文PDF文末点击阅读原文可以获取。
Phonon broadening in high entropy alloys
（高熵合金中的声子宽频化）
Fritz Körmann, Yuji Ikeda, Blazej Grabowski & Marcel H. F. Sluiter
Refractory high entropy alloys feature outstanding properties making them a promising materials class for next-generation high-temperature applications. At high temperatures, materials properties are strongly affected by lattice vibrations (phonons). Phonons critically influence thermal stability, thermodynamic and elastic properties, as well as thermal conductivity. In contrast to perfect crystals and ordered alloys, the inherently present mass and force constant fluctuations in multi-component random alloys (high entropy alloys) can induce significant phonon scattering and broadening. Despite their importance, phonon scattering and broadening have so far only scarcely been investigated for high entropy alloys. We tackle this challenge from a theoretical perspective and employ ab initio calculations to systematically study the impact of force constant and mass fluctuations on the phonon spectral functions of 12 body-centered cubic random alloys, from binaries up to 5-component high entropy alloys, addressing the key question of how chemical complexity impacts phonons. We find that it is crucial to include both mass and force constant fluctuations. If one or the other is neglected, qualitatively wrong results can be obtained such as artificial phonon band gaps. We analyze how the results obtained for the phonons translate into thermodynamically integrated quantities, specifically the vibrational entropy. Changes in the vibrational entropy with increasing the number of elements can be as large as changes in the configurational entropy and are thus important for phase stability considerations. The set of studied alloys includes MoTa, MoTaNb, MoTaNbW, MoTaNbWV, VW, VWNb, VWTa, VWNbTa, VTaNbTi, VWNbTaTi, HfZrNb, HfMoTaTiZr.
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