npj: 相稳定性预测—看看熵的脸色
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高熵合金含有多种元素而且比例很大，容易导致相分离，通常具有浅混合焓和相似级别的熵。因此，合金的相稳定性既取决于混合焓又取决于混合熵，必须等量齐观地了解它们各自对合金热力学性质的贡献，才能预测合金的稳定性。
来自美国怀俄明大学的Dilpuneet Aidhy教授等，采用密度泛函理论计算，检测了不同类型的熵在7种具有浅混合焓的二元金属合金中的贡献。他们发现依靠混合焓来预测固溶体稳定性是不准的。就各种熵来说，电子熵与振动熵和位形熵相比，贡献很小，对任何合金的相稳定性没有显著贡献；位形熵能够稳定固溶体；振动熵却在不同合金系统中表现出不同作用，或促进或破坏固溶体的稳定性。振动熵是衡量原子键柔软度的指标，研究它在不同体系中的变化方式，可能有助于我们更好地预测单相多组分合金的稳定性。据此他们认为，与有序金属间化合物不同，高熵合金的稳定性需要综合考虑各种熵的贡献。
该文近期发表于npj Computational Materials 4: 47 (2018) ，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。
Entropy contributions to phase stability in binary random solid solutions
Anus Manzoor, Shubham Pandey, Debajit Chakraborty, Simon R. Phillpot & Dilpuneet S. Aidhy
High entropy alloys contain multiple elements in large proportions that make them prone to phase separation. These alloys generally have shallow enthalpy of mixing which makes the entropy contributions of similar magnitude. As a result, the phase stability of these alloys is equally dependent on enthalpy and entropy of mixing and understanding the individual contribution of thermodynamic properties is critical. In the overall vision of designing high entropy alloys, in this work, using density functional theory calculations, we elucidate the contributions of various entropies, i.e., vibrational, electronic and configurational towards the phase stability of binary alloys. We show that the contribution of electronic entropy is very small compared to the vibrational and configurational entropies, and does not play a significant role in the phase stability of alloys. The configurational and vibrational entropies can either destabilize or can collectively contribute to stabilize the solid solutions. As a result, even those systems that have negative mixing enthalpy can show phase instability, revealed as a miscibility gap; conversely, systems with positive mixing enthalpy can be phase stable due to entropic contributions. We suggest that including entropic contributions are critical in the development of theoretical framework for the computational prediction of stable, single-phase high entropy alloys that have shallow mixing enthalpies, unlike ordered intermetallics.
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