[Paper Review] Determining the three-dimensional atomic structure of a metallic glass

Yao Yang, Jihan Zhou+13 more|arXiv (Cornell University)|Apr 5, 2020

Metallic Glasses and Amorphous Alloys77 references396 citations

TL;DR

This study presents the first direct experimental determination of the three-dimensional atomic structure in a metallic glass using atomic electron tomography (AET), achieving 21 pm precision. By analyzing short-range and medium-range order, the authors identify coexisting face-centered cubic, hexagonal close-packed, body-centered cubic, and simple cubic crystal-like networks with translational but no orientational order, confirming the efficiency of the cluster packing model in amorphous solids.

ABSTRACT

Amorphous solids such as glass are ubiquitous in our daily life and have found broad applications ranging from window glass and solar cells to telecommunications and transformer cores. However, due to the lack of long-range order, the three-dimensional (3D) atomic structure of amorphous solids have thus far defied any direct experimental determination without model fitting. Here, using a multi-component metallic glass as a proof-of-principle, we advance atomic electron tomography to determine the 3D atomic positions in an amorphous solid for the first time. We quantitatively characterize the short-range order (SRO) and medium-range order (MRO) of the 3D atomic arrangement. We find that although the 3D atomic packing of the SRO is geometrically disordered, some SRO connect with each other to form crystal-like networks and give rise to MRO. We identify four crystal-like MRO networks - face-centred cubic, hexagonal close-packed, body-centered cubic and simple cubic - coexisting in the sample, which show translational but no orientational order. These observations confirm that the 3D atomic structure in some parts of the sample is consistent with the efficient cluster packing model. Looking forward, we anticipate this experiment will open the door to determining the 3D atomic coordinates of various amorphous solids, whose impact on non-crystalline solids may be comparable to the first 3D crystal structure solved by x-ray crystallography over a century ago.

Motivation & Objective

• To directly determine the three-dimensional atomic structure of a metallic glass without relying on model fitting.
• To characterize the short-range order (SRO) and medium-range order (MRO) in amorphous solids at atomic resolution.
• To validate the efficiency of the cluster packing model in describing the atomic arrangement in metallic glasses.
• To demonstrate the capability of atomic electron tomography (AET) for resolving complex amorphous structures with high precision.

Proposed method

• Acquisition of a 55-image tomographic tilt series using a scanning transmission electron microscope with an annular dark-field detector on a multi-component metallic glass nanoparticle.
• Application of advanced iterative reconstruction algorithms and multislice simulations to refine the 3D atomic model and correct for angular errors in the tilt series.
• Use of atom classification based on image contrast to assign elements into three groups (Co/Ni, Ru/Rh/Pd/Ag, Ir/Pt) based on atomic number sensitivity.
• Employment of Voronoi tessellation and local bond orientational order (BOO) parameters to identify and classify SRO and MRO structures.
• Quantitative analysis using radial distribution functions (RDF), partial pair distribution functions (PDFs), and normalized BOO parameters to distinguish amorphous regions from crystal nuclei.
• Validation of the reconstructed 3D model by comparing experimentally reconstructed images with multislice-simulated images, achieving 97.37% atom identification accuracy.

Experimental results

Research questions

• RQ1 Can the three-dimensional atomic structure of a metallic glass be determined experimentally without model fitting?
• RQ2 What is the nature of the short-range and medium-range order in the amorphous structure of a multi-component metallic glass?
• RQ3 Do crystal-like networks exist in metallic glasses, and if so, what are their geometric and symmetry characteristics?
• RQ4 To what extent does the cluster packing model describe the observed atomic arrangements in amorphous solids?
• RQ5 How accurate is atomic electron tomography in resolving atomic positions in disordered materials with sub-25 pm precision?

Key findings

• The 3D atomic structure of a metallic glass was directly determined for the first time using atomic electron tomography with a precision of 21 pm.
• 15.46% of atoms were identified as forming crystal nuclei, which were distinguished using a normalized BOO parameter threshold of 0.5.
• Four crystal-like medium-range order (MRO) networks—face-centered cubic (fcc), hexagonal close-packed (hcp), body-centered cubic (bcc), and simple cubic (sc)—were identified, showing translational but no orientational order.
• The partial pair distribution function for Ir/Pt (type 3) atoms showed a second peak higher than the first, indicating that these atoms extend beyond the short-range order.
• The radial distribution function (RDF) of the amorphous structure exhibited peak ratios of 1.74, 1.99, 2.64, and 3.51 for the second to fifth peaks relative to the first, consistent with metallic glass behavior.
• The 3D atomic model was validated by multislice simulations, showing 97.37% accuracy in atom identification and a root mean square deviation of atomic positions of less than 21 pm.