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

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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.