Abstract text (incl. figure legends and references)
Introduction:
Properties of nanomaterials are not only dependent on chemistry but also on the manufacturing process. Every structural change induced by thermal or mechanical stresses alters the material properties. In metallic glasses, a material class exhibiting a disordered amorphous structure, the fundamental deformation mechanisms are still not yet fully understood [1]. Therefore, they are subject to intensive research. The introduction of secondary crystalline phases or highly rejuvenated regions renders transformation mechanisms in metallic glasses even more complex. Due to the emergence of novel dedicated specimen holders, in-situ TEM measurements are a powerful tool allowing to perform direct measurements on the deformation behavior of these tailored nanocomposites [2].
Objectives:
In this work, we study the effect of local heterogeneities on the structural transformation mechanisms in metallic glass composites by combined in-situ TEM techniques. A direct electron detector is used allowing to perform the experiments with high temporal resolution.
Materials and Methods:
The as-cast specimen material was prepared using a Suction Cast Arc Melter with 1.6 mm plate thickness. To provide tailored composites a combination of severe plastic deformation using high pressure torsion deformation and controlled heating is used. Quantitative in-situ nanomechanical deformation experiment were performed using a Hysitron PI 95 PicoIndenter from Bruker. Scanning transmission electron microscopy (STEM) images were taken on a JEOL 2200FS equipped with Omega energy filter A QuantumDetectors MerlinEM pixelated 512x512 direct electron detector was used to acquire 4D-STEM datasets during the in-situ experiments [3]. The analysis was performed using custom Python scripts and Digital Micrograph software.
Results & Conclusions:
The metallic glass composites produced in the present work show highly complex microstructures, including a heterogeneous disordered amorphous phase and multiple homogeneously distributed crystalline phase precipitates. It is shown that the nanocomposites exhibit a strain localization that correlates with the scaling of local heterogeneities. Advanced in-situ TEM experiments using a fast direct electron detector are able to unravel quantitative local deformation mechanisms on the nanoscale. Specifically, we use 4D-STEM to measure the strain at the nanoscale. This provides new insights into the evolution of local elastic and global plastic strains during the in situ experiments and thus helps to draw a more direct link between structure and properties of the metallic glass composites.
The authors gratefully acknowledge the financial support from the Austrian Science Fund (FWF): Y1236-N37.
Fig. 1: STEM HAADF image showing complex strain softened regions before deformation
References
[1] A.L. Greer, Metallic glasses on the threshold, Mater. Today 12 (1-2) (2009);
[2] H. Sheng, L. Zhang, H. Zhang, J. Wang, J. Eckert, C. Gammer In-situ TEM observation of phase transformation in bulk metallic glass composites, Materials Research Letters, 9 (2021) 190-195;
[3] H. Sheng, D. Şopu, S. Fellner, J. Eckert, C. Gammer PHYSICAL REVIEW LETTERS 128 (2022) 245501.