Mapping nanoscale strain fields in metallic glass composites via precession nanobeam electron diffraction

Abstract text (incl. figure legends and references)

Introduction: Accurate strain determination in both crystalline and amorphous materials is crucial for a fundamental understanding of a materials response to applied loads at the atomic level. Using the widely adopted nanobeam electron diffraction (NBED) technique, it is possible to accurately measure in-plane strains in crystalline TEM samples by peak detection using phase- or cross-correlation techniques as demonstrated by many authors before. For amorphous materials such as metallic glasses, the in-plane strains can be determined by fitting an elliptic shape function to the amorphous halo in the diffraction space. However, the desired experimental conditions for these two experiments have a key difference. Peak detection in crystalline samples works best for large diffraction disks up to the point where they almost start to overlap. For amorphous materials, such large convergence angles would result in a very diffuse, low intensity halo making fitting an ellipse impossible. Therefore, strain mapping experiments on non-crystalline materials must be conducted using an, ideally, parallel beam.

Objectives: The objective of this study is to show the possibility to measure strain in both crystalline and amorphous samples in a single experiment with high precision and accuracy by using a small convergence angle as recently shown by Sheng et al. [1].

Materials & methods: This study was carried out on a CuZrAl glass forming alloy. The sample was prepared via suction mold casting. Homogenously distributed Cu10Zr7 crystals precipitated while the matrix transformed into the glassy state, resulting in a metallic glass composite (MGC). An electron transparent lamella of this sample, prepared via FIB, was subsequently investigated via 4DSTEM. A fast CCD camera was used to capture the full diffraction patterns. For the reasons stated above, a semi-convergence angle of about 0.7 mrad and an illumination time of 200 ms were used to ensure strong signals. The analyzed region was chosen to be at the phase boundary between the amorphous matrix and a crystal in [001] zone axis.

Results: The results show that strain determination in the crystalline region via correlation methods works well even when using a small convergence angle. The strain fields show a very gradual transition between regions of different strain levels, indicating the low level of noise in the maps. Thus, it can be assumed that a disk size of 46 pixel in diameter is sufficient for accurate peak detection. In the amorphous region, the strain maps show greater deviations across the scan area. The standard deviations of 0.63, 0.08 and 0.66 % for exx, exy and eyy indicate significant inhomogeneities in the material. Nevertheless, the fitting resulted in an average RMS value of 2∙10-9 for a mean ring intensity of 663 counts. Therefore, it can be stated that the fitting procedure was successful and the results in the amorphous region are accurate.

Conclusion: We have determined the atomic level elastic strains in a MGC by using cross-correlation techniques in the crystalline and ellipse fitting in the amorphous region in the same experiment. This opens the possibility for future in-situ deformation experiments on MGCs on the quest for better understanding of the deformation mechanism and the interplay between the two phases.

[1] H. Sheng, D. Sopu, S. fellner, J. Eckert and C. Gammer, Physical Review Letters 218, 245501 (2022)

We acknowledge support from the Austrian Science Fund (FWF):Y1236-N37