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Chain-like structure elements in Ni40Ta60 metallic glasses observed by scanning tunneling microscopy.

Pawlak R, Marot L, Sadeghi A, Kawai S, Glatzel T, Reimann P, Goedecker S, Güntherodt HJ, Meyer E - Sci Rep (2015)

Bottom Line: The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging.In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable.The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Basel, Klingelbergstr. 82, 4056 Basel, Switzerland.

ABSTRACT
The structure of metallic glasses is a long-standing question because the lack of long-range order makes diffraction based techniques difficult to be applied. Here, we used scanning tunneling microscopy with large tunneling resistance of 6 GΩ at low temperature in order to minimize forces between probe and sample and reduce thermal fluctuations of metastable structures. Under these extremely gentle conditions, atomic structures of Ni40Ta60 metallic glasses are revealed with unprecedented lateral resolution. In agreement with previous models and experiments, icosahedral-like clusters are observed. The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging. In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable. The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

No MeSH data available.


Related in: MedlinePlus

Tip-induced surface relaxations at short-range scale (a-b) Constant-current STM image of chains surrounded by icosahedral clusters. While scanning with extremely gentle conditions, numerous tip-induced displacements of clusters depicted by white arrows (see also movie in Supp. Infos.) occur at icosahedron areas while chain-like structure remain stable. The white circles show the initial position of the clusters obtained from (a) and moved in (b) (I = 10 pA, Vt = 30 mV). (c) Model of the surface morphology: areas of icosahedrons which are weakly bound (dark blue) present high rate of relaxation and might thus act as local shear transition zones. In contrast, chain structures (pale blue) limit these relaxations, Ni and Ta atoms are in green and blue respectively.
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f4: Tip-induced surface relaxations at short-range scale (a-b) Constant-current STM image of chains surrounded by icosahedral clusters. While scanning with extremely gentle conditions, numerous tip-induced displacements of clusters depicted by white arrows (see also movie in Supp. Infos.) occur at icosahedron areas while chain-like structure remain stable. The white circles show the initial position of the clusters obtained from (a) and moved in (b) (I = 10 pA, Vt = 30 mV). (c) Model of the surface morphology: areas of icosahedrons which are weakly bound (dark blue) present high rate of relaxation and might thus act as local shear transition zones. In contrast, chain structures (pale blue) limit these relaxations, Ni and Ta atoms are in green and blue respectively.

Mentions: To study the interplay between these two metastable structures and the surface dynamics, we investigated the tip-induced relaxation processes obtained with extremely gentle conditions over several STM images while scanning the glass surface. Figure 4 shows two successive STM images extracted from a STM movie (see Supp. Materials). In Fig. 4a, a chain aggregate is atomically resolved and surrounded by icosahedrons. Successive STM images taken at constant-current mode with a tunneling resistance of ≈6 GΩ reveal that numerous relaxation processes occur under the tip action around the chain aggregate. For clarity, we marked few clusters in Fig. 4a with white circles which were moved by the STM tip in the following image (Fig. 4b). The white circles in Fig. 4b show the initial position of these clusters obtained from Fig. 4a and thus reveals the surface dynamics (see also movie in Supp. Infos.). The white arrows (Fig. 4b) point out few of the displacement of clusters which corresponds to a translation mediated by the tip scan. In contrast to the chains remaining immobile during all the STM scans, areas consisting of icosahedrons (dark blue in Fig. 4c) show numerous relaxations of the clusters along few atomic sites. Their preferential motion suggests that icosahedrons are weakly interacting to each other and allows large relaxation processes in contrast to chain area (pale blue Fig. 4c) which are much less sensitive to the tip-induced deformation. We think that these displacement processes can be interpreted as a signature of Johari-Goldstein relaxations (secondary or β-relaxation) attributed to translation motion of atoms on a short-range scale3233 and appearing well below Tg (here at 4.8 K). Importantly, β-relaxation is predicted to be correlated with the volume of shear transformation zones (STZ) of the metallic glass. Because icosahedron areas are weakly interacting and have high β relaxation rate, we propose that STZs might locally initiate at these locations where important deformations are allowed. In this picture, incorporating nano-sized chain-like structures in these areas limits these relaxations and thus increases the overall strength of the material3435.


Chain-like structure elements in Ni40Ta60 metallic glasses observed by scanning tunneling microscopy.

Pawlak R, Marot L, Sadeghi A, Kawai S, Glatzel T, Reimann P, Goedecker S, Güntherodt HJ, Meyer E - Sci Rep (2015)

Tip-induced surface relaxations at short-range scale (a-b) Constant-current STM image of chains surrounded by icosahedral clusters. While scanning with extremely gentle conditions, numerous tip-induced displacements of clusters depicted by white arrows (see also movie in Supp. Infos.) occur at icosahedron areas while chain-like structure remain stable. The white circles show the initial position of the clusters obtained from (a) and moved in (b) (I = 10 pA, Vt = 30 mV). (c) Model of the surface morphology: areas of icosahedrons which are weakly bound (dark blue) present high rate of relaxation and might thus act as local shear transition zones. In contrast, chain structures (pale blue) limit these relaxations, Ni and Ta atoms are in green and blue respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4542518&req=5

f4: Tip-induced surface relaxations at short-range scale (a-b) Constant-current STM image of chains surrounded by icosahedral clusters. While scanning with extremely gentle conditions, numerous tip-induced displacements of clusters depicted by white arrows (see also movie in Supp. Infos.) occur at icosahedron areas while chain-like structure remain stable. The white circles show the initial position of the clusters obtained from (a) and moved in (b) (I = 10 pA, Vt = 30 mV). (c) Model of the surface morphology: areas of icosahedrons which are weakly bound (dark blue) present high rate of relaxation and might thus act as local shear transition zones. In contrast, chain structures (pale blue) limit these relaxations, Ni and Ta atoms are in green and blue respectively.
Mentions: To study the interplay between these two metastable structures and the surface dynamics, we investigated the tip-induced relaxation processes obtained with extremely gentle conditions over several STM images while scanning the glass surface. Figure 4 shows two successive STM images extracted from a STM movie (see Supp. Materials). In Fig. 4a, a chain aggregate is atomically resolved and surrounded by icosahedrons. Successive STM images taken at constant-current mode with a tunneling resistance of ≈6 GΩ reveal that numerous relaxation processes occur under the tip action around the chain aggregate. For clarity, we marked few clusters in Fig. 4a with white circles which were moved by the STM tip in the following image (Fig. 4b). The white circles in Fig. 4b show the initial position of these clusters obtained from Fig. 4a and thus reveals the surface dynamics (see also movie in Supp. Infos.). The white arrows (Fig. 4b) point out few of the displacement of clusters which corresponds to a translation mediated by the tip scan. In contrast to the chains remaining immobile during all the STM scans, areas consisting of icosahedrons (dark blue in Fig. 4c) show numerous relaxations of the clusters along few atomic sites. Their preferential motion suggests that icosahedrons are weakly interacting to each other and allows large relaxation processes in contrast to chain area (pale blue Fig. 4c) which are much less sensitive to the tip-induced deformation. We think that these displacement processes can be interpreted as a signature of Johari-Goldstein relaxations (secondary or β-relaxation) attributed to translation motion of atoms on a short-range scale3233 and appearing well below Tg (here at 4.8 K). Importantly, β-relaxation is predicted to be correlated with the volume of shear transformation zones (STZ) of the metallic glass. Because icosahedron areas are weakly interacting and have high β relaxation rate, we propose that STZs might locally initiate at these locations where important deformations are allowed. In this picture, incorporating nano-sized chain-like structures in these areas limits these relaxations and thus increases the overall strength of the material3435.

Bottom Line: The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging.In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable.The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Basel, Klingelbergstr. 82, 4056 Basel, Switzerland.

ABSTRACT
The structure of metallic glasses is a long-standing question because the lack of long-range order makes diffraction based techniques difficult to be applied. Here, we used scanning tunneling microscopy with large tunneling resistance of 6 GΩ at low temperature in order to minimize forces between probe and sample and reduce thermal fluctuations of metastable structures. Under these extremely gentle conditions, atomic structures of Ni40Ta60 metallic glasses are revealed with unprecedented lateral resolution. In agreement with previous models and experiments, icosahedral-like clusters are observed. The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging. In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable. The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

No MeSH data available.


Related in: MedlinePlus