Characterizing Local Chemical Order in Multi-Principal Element Alloys by Femtosecond Time-Resolved Ultrafast Electron Diffraction

合金化是提高金属材料力学性能的重要途径。最近发现的多主元素合金，其中等原子组成元素同时充当主元素和合金元素，为寻找和开发具有前所未有性能的新型金属材料开辟了一个新的领域，可用于低温、冲击载荷、恶劣化学环境和超高温等极端条件下。多主元素合金的独特性能源于组成元素之间复杂的化学相互作用。了解复杂合金中组成原子的局部静态排列和动力学（通常称为声子）对于设计和建模这类新型材料至关重要。由于局部静态结构和声子的散射信号在衍射光谱中重叠，使用传统的x射线和电子衍射方法来表征复杂合金的结构和动力学具有挑战性。本项目正在开发一种利用飞秒时间分辨MeV电子衍射表征多主元素合金的新方法。超快时间下的电子衍射可以有效地解耦静态和动态原子散射，揭示多主元素合金结构和动力学的原子特征。该项目在了解复杂合金的结构-性能关系方面取得了实质性进展，并有助于开发用于广泛工业应用的新型金属材料，以加强美国在先进和高性能材料方面的竞争力。在多主元素合金（mpea）中，局部化学有序对合金的结构性能和相稳定性起着至关重要的作用，但由于局部化学有序的静态扩散散射与相干声子和非相干声子的动态扩散散射之间存在纠缠，因此实验表征具有挑战性。本项目将利用MeV超快电子衍射（MeV UED）研究单晶mpea中的局部化学秩序。利用MeV UED的飞秒时间分辨率和高空间互反分辨率，实现了静态和动态散射之间的纠缠解耦。通过飞秒MeV电子衍射的全散射分析，测量了局部化学有序度。定量表征方法还可以研究局部化学顺序随退火的演变。为了模拟mpea化学不稳定性的潜在物理机制和优化mpea的性能，确定了局部化学顺序的温度依赖性。此外，MeV UED测量了不同化学有序度的单晶mpea的动态扩散散射，揭示了局部化学有序度与声子色散之间的关系，有助于深入理解局部化学有序度与mpea的力学、热学和电子性能之间的关系。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARYAlloying is an important approach to improve the mechanical performance of metallic materials. The recent discovery of multi-principal element alloys in which equiatomic constituent elements act as both principal and alloying elements opens a new field to search and develop new metallic materials with unprecedented properties for applications under extreme conditions of cryogenic temperatures, shock loading, harsh chemical environments, and ultrahigh temperatures. The distinctive properties of multi-principal element alloys originate from complex chemical interactions amongst constituent elements. It is essential to understand local static arrangements and dynamics (usually termed as phonons) of constituent atoms in the complex alloys for designing and modeling the new class of materials. As the scattering signals of the local static structure and phonons overlap in diffraction spectra, it is challenging to characterize the structure and dynamics of the complex alloys using traditional X-ray and electron diffraction methods. This project is developing a new approach to characterize multi-principal element alloys using femto-second time-resolved MeV electron diffraction. Electron diffraction at ultrafast time regimes can effectively decouple the static and dynamic atomic scattering and unveil the atomic insights of structure and dynamics of multi-principal element alloys. This project is making substantial progress towards understanding the structure-property relations of complex alloys and is helping enable the development of new metallic materials for a wide range of industry applications to strengthen the U.S. competitiveness in advanced and high-performance materials.TECHNICAL SUMMARYIn multi-principal element alloys (MPEAs) local chemical order plays a crucial role in determining structural properties and phase stability, but it is challenging to characterize experimentally due to the entanglement between static diffuse scattering from local chemical order and dynamic diffuse scattering from coherent and incoherent phonons. This project will employ MeV ultrafast electron diffraction (MeV UED) to investigate local chemical order in single-crystal MPEAs. By utilizing the femto-second time resolution and the high reciprocal spatial resolution of MeV UED, the entanglement between the static and dynamic diffuse scatterings is being decoupled. The degree of local chemical order is being measured via the total scattering analysis of femto-second MeV electron diffraction. The quantitative characterization method also enables investigation of the evolution of local chemical order with annealing. The temperature dependence of local chemical order is being determined for modeling the underlying physical mechanism of chemical instability in MPEAs and for optimizing the properties of MPEAs. Moreover, the MeV UED measurements of the dynamic diffuse scattering from the single-crystal MPEAs with different degrees of chemical order are unveiling the correlation between local chemical order and phonon dispersions for developing deep understanding of the relationship between local chemical order and the mechanical, thermal, and electronic properties of MPEAs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.