Four-Dimensional Scanning Transmission Electron Microscopy (4D-STEM) for Advance Characterization of Grain Boundaries at the Nanoscale in Copper Bicrystals

Interfaces, including grain boundaries, have been well-established for their prominent role in influencing mechanical properties in materials [1]. It remains technically challenging to characterize interfacial behaviors in the nanoscale under different conditions such as heat. Standard measurement techniques often provide only global averages and local determination such as strain across grain boundaries or evolution of lattice parameters remain technically challenging [2,3]. Here, we use four-dimensional scanning transmission electron microscopy (4D-STEM) with advanced post-processing to produce strain and lattice parameter maps across a grain boundary in copper (Cu) bicrystals as a function of temperature. Determining the local interfacial behavior aids our understanding of structure-dependent properties that can be significant in polycrystalline materials.

Electron diffraction is a convenient method to determine structure but interpretation can become quickly complicated due to multiple/dynamical scattering such as in the case of thick sample or overlapping grains. Overcoming these challenges is possible with 4D-STEM which is a relatively recent advancement in which electron diffraction patterns are collected at every position in the acquired scan. This remarkable tool enables statistical structural analysis, producing strain information at the nanometer scale in addition to extraction of the crystalline structure and orientation [4-6]. In combination with postprocessing algorithms, challenges of single pattern analysis can be overcome to provide a wealth of information.

In order to obtain a fundamental understanding of local interfacial behavior in response to heating, we investigate the difference between grains of Cu bicrystals from two different background processes (annealed and additively manufactured) as a function of temperature. We use 4D-STEM and post-processing to probe the development of strain across grain boundaries and differences in the change in unit cell parameters. These results could expand our knowledge on the development of strain across grain boundaries in the nanoscale as a response to heating which has important implications for understanding the structure/interface-property connection.

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