高熵或多主元合金的发展极大地拓展了物理冶金学的探索空间。这种成分复杂的新型材料设计理念， 带来了优异的机械性能和热力学性质， 引起了极大的关注。 学界对多主元合金的普遍认识是其具有高的构型（化学） 熵，即原子的理想混合在体系的热力学稳定性中起到了重要作用。然而近年来，诸多实验观测发现多主元合金中存在大量化学短程有序结构，这些发现对熵主导热力学稳定性这一观点提出了极大的挑战。目前为止， 多主元合金的构型熵尚未实现直接测量或进行定量数值分析。 由于组成元素的多样性和分布的随机性， 多主元合金的势能曲面相较于传统金属变得异常复杂，相邻势能极小值点之间的路径更加崎岖和多样，导致材料塑性失稳事件难以明确地识别。 集体振动模式（声子） 与材料的力学不稳定性之间存在关联。 此外， 元素分布的有序度对振动模式也起到了重要的作用，从而对材料的塑性产生影响。 化学短程序的存在， 使得材料经历了从熔融的理想混合到室温的短程有序分布，其内部驱动力（自由能函数） 的计算尚未有全面的报道。因此，基于原子分辨率建立多主元合金的有序度与熵、振动的对应关系，以及“有序-无序” 转变的自由能判据， 从而定量刻画热力学熵、短程序和塑性事件发生的振动起源， 以及“有序-无序” 转变的物理图像尤为重要。 为此，我们开展了如下研究：
（1） 结合分子动力学、 蒙特卡罗、 热力学积分与晶格动力学的计算， 我们确定了一系列平衡态 CoCrNi 多主元合金的构型熵、振动熵，并建立了热力学量与短程有序的关联。 我们成功解耦了构型熵和振动熵在热力学中的作用。 与振动熵相比，构型熵并不像通常认为的在所谓的“高熵” 材料中那样重要。 在环境温度或更高温度下，决定多主元合金热力学稳定性的是振动熵而不是构型熵。通过引入原子尺度香农熵启发的有序参数， 我们量化了构型熵演化时对应的结构有序度的改变。 我们发现构型熵和退火温度相关， 原子的理想混合仅在某些特定高温或接近熔融情况下存在。在高温下，元素的占位几乎是随机的。但是，一个有趣的发现是晶格畸变（由局部化学不均匀性引起） 对构型无序产生了额外的贡献。在固定的温度下，即使构型熵发生显著变化，振动熵的变化也很小， 但是振动与构型空间的确存在物理关联。 引入适当的短程有序， 可以实现对振动熵的调节，说明声子和短程结构同样存在一定的关联。
（2） 基于非平衡热力学积分的 Frenkel-Ladd 路径，实现直接计算绝对自由能，预测了多主元合金的相稳定性，并且给出了温度诱导“无序-有序”转变发生的直接判据。 给出了具有不同短程有序结构的合金相演化过程的自由能依据， 发现了自由能和结构的一致性。 我们提出了一组新的物理量， 包括非谐度、化学短程序和香农熵相关的有效无序温度，用以描述多主元合金可能的“有序-无序” 转变。进一步，我们揭示了非谐效应在驱动随机固溶体向化学短程有序结构转变中的关键作用。对无序合金自由能的定量认识， 有助于理解理想混合固溶体局部有序结构的形成机制， 为理解多主元合金的热力学稳定性提供了新的思路。
（3） 通过原子模拟表征了多主元合金的声子特征及其不稳定性路径。 根据量化的振动模态 Grüneisen 常数的变化，我们发现与传统金属相比多主元合金存在着强非谐效应， 尤其低频振动的非谐性更高。 在低频或高频振动中均出现了局域和扩展的振动模式，但极高频率振动倾向于局域模式。多主元合金中存在低频局域振动模式，挑战了学界对固体振动的传统认识。 短程有序的引入， 增加了高频的声子态密度，有效降低了非谐度， 从而增强了多主元合金的热力学与力学稳定性。随着外部应变增加至临界屈服应变， 声子失稳通过几种低频模式协同软化发生，从而导致塑性萌生与位错形核。 这与以往认为的材料失稳来源于最软振动模式的崩塌不同，为多主元合金的力学性质提供了新见解。

The development of high entropy or multi-principal element alloys has greatly expanded the exploration space of physical metallurgy. This new concept of material design with complex composition brings excellent mechanical and thermodynamic properties, which has attracted great attention. The general understanding of multiprincipal element alloy is that it has high configurational (chemical) entropy, that is, the ideal mixing of atoms plays an important role in the thermodynamic stability of the system. However, in recent years, many experimental observations have found that a large number of chemical short-range order exist in multi-principal element alloys. These findings have challenged the idea that entropy dominates thermodynamic stability. So far, the configurational entropy of multi-principal element alloys has not been measured directly or analyzed numerically. Due to the diversity of component elements and the randomness of distribution, the potential energy landscape of multiprincipal element alloys becomes more complex than that of traditional metals, and the paths between adjacent potential energy minima are more rugged and diversified, which makes it difficult to identify plasticity of materials clearly. There is an association between the collective vibrational mode (phonon) and the plasticity of materials. In addition, the degree of order of the distribution of elements also plays an important role in the vibrational mode, thus affecting the plasticity of the materials. Chemical shortrange order allows the materials to undergo a short-range ordered distribution from melting ideal mixing to room temperature. The calculation of the internal driving force (free energy function) has not been fully reported. Therefore, it is particularly important to establish the corresponding relationship between the degree of order, entropy and vibration of multi-principal element alloys based on atomic resolution, as well as the free energy criterion of "order-disorder" transition, so as to quantitatively characterize the vibrational origin of thermodynamic entropy, short-range order and the occurrence of plastic event, and the physical view of "order-disorder" transition. To this end, we carried out the following research:

(1) Combined with the calculation of molecular dynamics, Monte Carlo, thermodynamic integration and lattice dynamics, we determine the configurational entropy and vibrational entropy of a series of equilibrium CoCrNi multi-principal element alloys, and establish the correlation between thermodynamic quantity and short-range order. We have successfully decoupled the role of configurational entropy and vibrational entropy in thermodynamics. In contrast to vibrational entropy, configurational entropy is not as important as commonly assumed in so-called "high entropy" materials. At ambient temperature or higher, the vibrational entropy rather than configurational entropy determines the thermodynamic stability of multi-principal element alloys. By introducing order parameters inspired by Shannon entropy, weMolecular dynamics simulation of chemical order and elastoplastic deformation of multi-principal element alloy quantify the degree of order during the evolution of configurational entropy. We find that configurational entropy is related to annealing temperature and the ideal mixing of atoms only exists at certain high temperatures or near melting. At high temperatures, element occupancy is almost random. However, an interesting finding is the additional contribution of lattice distortion (caused by local chemical inhomogeneity) to configurational disorder. At a fixed temperature, vibrational entropy changes very little even if configurational entropy changes significantly. But it does have a physical relationship with the configurational space. By introducing an appropriate short-range order, the vibrational entropy can be adjusted, which indicates that phonons and shortrange order are also related to some extent.

(2) Based on the Frenkel-Ladd path of non-equilibrium thermodynamic integration, the absolute free energy can be calculated directly. It is predicted that the phase stability of multi-principal element alloys and the direct criterion of temperature induced orderdisorder transition. It is found that the free energy of the phase evolution with different short-range order and the consistency of free energy and structure. We propose a new set of physical quantities, which include the degree of anharmonicity, chemical shortrange order parameter and effective disorder temperature derived from Shannon entropy to describe the possible "order-disorder" transition of multi-principal element alloys. Furthermore, we propose that anharmonic effects play a key role in driving the transition of random solid solutions to chemical short-range order. The quantitative understanding of the free energy of disordered alloys is helpful to understand the formation mechanism of the local ordered structure of the solid solutions, and provides a new way to understand the thermodynamic stability of multi-principal element alloys.

(3) The phonon characteristics and instability paths of multi-principal element alloys are characterized by atomic simulation. According to the quantized Grüneisen parameter of vibrational mode, it is found that the multi-principal element alloy has strong anharmonic effect compared with the traditional metal, especially the anharmonic effect of low frequency vibration is higher. Local and extended vibrational modes appear in both low frequency and high frequency, but extremely high frequency tends to be localized. The existence of local vibrational mode of low frequency in multiprincipal element alloys challenges the traditional understanding of solid vibration. The introduction of short-range order increases the density states of high frequency phonon and reduces the anharmonicity effectively, thus enhancing the thermodynamic and mechanical stability of the multi-principal element alloy. As the external strain increases to the critical yield strain, phonon instability occurs through softening of several low-frequency modes, which leads to plastic initiation and dislocation nucleation. It is different from the previous concept that the instability comes from the collapse of the softest vibrational mode, which prov