TY - THES

T1 - Probing local atomic strain of metallic glasses with nanometer resolution using TEM diffraction mapping

AU - Keckes, Julius

N1 - no embargo

PY - 2022

Y1 - 2022

N2 - This thesis elucidates the characterization of metallic glasses (MGs) by means of scanning nanobeam electron diffraction (NBED) mapping, also known as four-dimensional scanning transmission electron microscopy (4D STEM), using precession electron diffraction (PED). An emphasis lies on the evaluation of nanodiffraction datasets through fitting of a parametric ellipse equation, which enables two-dimensional determination of local elastic strains, as well as structural and compositional characterization at the nanoscale. An insight is given into the implementation of the fitting procedure, as well as dataset acquisition and subsequent data processing steps. A study is conducted on a Cu-Zr-Al bulk-metallic-glass (BMG) alloy, to determine optimal experimental parameters and a suitable evaluation approach. The results demonstrate the potential of mapping intrinsic structural heterogeneities in metallic glasses (MGs). The method is applied to a multilayered Co-Ta-B MG thin film system, demonstrating a spatial resolution of a few nanometres for the determination of elastic strains, as well as structural and compositional characterization. The results also provide unique insight into strain distribution and structure at amorphous interfaces. Additionally, an in-situ nanomechanical testing experiment is conducted on a Cu-Zr-Al BMG bending beam, during which 4D STEM strain mapping datasets are acquired. The subsequent evaluation allows the quantification of local multiaxial elastic strain distributions, occurring at different bending loads, which indicate an evolution of the stress concentrations at the beam notch position. The unprecedented resolution with which local elastic strains, structure and composition of MGs can be characterized, enables opportunities for material optimization, such as the quantitative comparison of obtained results to simulations.

AB - This thesis elucidates the characterization of metallic glasses (MGs) by means of scanning nanobeam electron diffraction (NBED) mapping, also known as four-dimensional scanning transmission electron microscopy (4D STEM), using precession electron diffraction (PED). An emphasis lies on the evaluation of nanodiffraction datasets through fitting of a parametric ellipse equation, which enables two-dimensional determination of local elastic strains, as well as structural and compositional characterization at the nanoscale. An insight is given into the implementation of the fitting procedure, as well as dataset acquisition and subsequent data processing steps. A study is conducted on a Cu-Zr-Al bulk-metallic-glass (BMG) alloy, to determine optimal experimental parameters and a suitable evaluation approach. The results demonstrate the potential of mapping intrinsic structural heterogeneities in metallic glasses (MGs). The method is applied to a multilayered Co-Ta-B MG thin film system, demonstrating a spatial resolution of a few nanometres for the determination of elastic strains, as well as structural and compositional characterization. The results also provide unique insight into strain distribution and structure at amorphous interfaces. Additionally, an in-situ nanomechanical testing experiment is conducted on a Cu-Zr-Al BMG bending beam, during which 4D STEM strain mapping datasets are acquired. The subsequent evaluation allows the quantification of local multiaxial elastic strain distributions, occurring at different bending loads, which indicate an evolution of the stress concentrations at the beam notch position. The unprecedented resolution with which local elastic strains, structure and composition of MGs can be characterized, enables opportunities for material optimization, such as the quantitative comparison of obtained results to simulations.

KW - metallic glasses

KW - scanning transmission electron microscopy

KW - electron diffraction

KW - nanodiffraction

KW - data fitting

KW - Metallische Gläser

KW - Rastertransmissionselektronenmikroskopie

KW - Elektronenbeugung

KW - Nanobeugung

KW - Ausgleichungsrechnung

M3 - Master's Thesis

ER -