Atom probe tomography on metallic glasses

Metallic glasses can exhibit good mechanical properties such as high compresive strength and large elastic strains, making them attractive for structural applications. Also, some metallic Fe based glases are very attractive soft magnetic materials. 

For better understanding their underlying structure-property relationships we conduct systematic atom probe tomography experiments on these materials, both in the full metallic glass state and also in the partially crystalline state.


What is the structure and local composition of rapidly quenched and partially crystallized Cu Zr Ag metallic glasses?

In this project the influence of Ag addition on the microstructure of rapidly quenched (Cu0.5Zr0.5)-Ag melts was investigated. Fully glassy alloys were obtained for <20 at.% Ag, which are characterized by a homogeneous microstructure without any phase separation. For 30-40 at.% Ag a composite structure is formed consisting of fcc-Ag nano-crystallites 5 nm in size and an amorphous matrix phase Cu40Zr40Ag20. With higher Ag-content the volume fraction of the fcc-Ag phase becomes increased mainly due to crytal growth during quenching. The primary formation of fcc-Ag for 30-40 at.% Ag is confirmed by the analysis of the microstructure of mold cast bulk samples which
were fully crystalline. From the experimental results we conclude that the miscibility gap of the liquid phase of the ternary Ag–Cu–Zr system may occur only for x > 40 at.% Ag. For the bulk glass forming quaternary Cu40Zr40Al10Ag10 alloy a homogeneous element distribution is observed in accordance with the microstructure of ternary (Cu0.5Zr0.5)-x Ag glasses (x = 10, 20 at.%).

Structure of rapidly quenched Cu Zr Ag metallic glasses
Journal of Alloys and Compounds 607 (2014) 285–290
J Alloys and Compunds 2014 APT metallic [...]
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How does ultrahigh nanocrystallization work in FeSiNbBCu soft magnetic amorphous alloys upon rapid annealing ?

In this project rapid annealing (4–10 s) induced primary crystallization of soft magnetic Fe–Si nanocrystals in a Fe73.5Si15.5Cu1Nb3B7 amorphous alloy has been systematically studied by atom probe tomography in comparison with conventional annealing (30–60 min). It was found that the nanostructure obtained after rapid annealing is basically the same, irrespective of the different time scales of annealing. This underlines the crucial role of Cu during structure formation. Accordingly, the clustering of Cu atoms starts at least 50 °C below the onset temperature of primary crystallization. As a consequence, coarsening of Cu atomic clusters also starts prior to crystallization, resulting in a reduction of available nucleation sites during Fe–Si nanocrystallization. Furthermore, the experimental results explicitly show that these Cu clusters initially induce a local enrichment of Fe and Si in the amorphous matrix. These local chemical heterogeneities are proposed to be the actual nuclei for subsequent nanocrystallization. Nevertheless, rapid annealing in comparison with conventional annealing results in the formation of 30% smaller Fe–Si nanocrystals, but of identical structure, volume fraction and chemical composition, indicating the limited influence of thermal treatment on nanocrystallization, owing to the effect of Cu.

Atom probe tomography study of ultrahigh nanocrystallization rates in FeSiNbBCu soft magnetic amorphous alloys on rapid annealing
Acta Materialia 68 (2014) 295–309
Atom probe tomography ultrahigh nanocrys[...]
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Phase selection and nanocrystallization in Cu-free soft magnetic FeSiNbB amorphous alloy upon rapid annealing
Morsdorf Pradeep et al J Appl Phys 119 ([...]
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Relationships between chemistry, topology and stiffness of ultrastrong Co-based metallic glass thin films studied by a combinatorial approach

Here we present an efficient way to study the relationship between chemical composition and mechanical properties of thin films by utilizing a combinatorial approach, where spatially resolved mechanical property measurements
are conducted along a concentration gradient. However, for thin film glasses many properties including the mechanical response are affected by chemical topology. Here a novel method is introduced which enables spatially resolved short range order analysis along concentration gradients of combinatorially
synthesized metallic glass thin films. For this purpose a CoZrTaB metallic glass film of 3 mm thickness is deposited on a polyimide foil, which is investigated by high energy X-ray diffraction in transmission mode. Through the correlative chemistry-topology-stiffness investigation, we observe that an increase in metalloid concentration from 26.4 to 32.7 at% and the associated formation of localized (hybridized) metal e metalloid bonds induce a 10% increase in stiffness. Concomitantly, along the same composition gradient, a metalloid-concentration-induced increase in first order metal - metal bond distances of 1% is observed, which infers itinerant (metallic) bond weakening. Hence, the metalloid concentration induced increase in hybridized bonding dominates the corresponding weakening of metallic bonds.

Revealing the relationships between chemistry, topology and stiffness of ultrastrong Co-based metallic glass thin films: A combinatorial approach
Acta Materialia 107 (2016) 213-219
Acta Materialia 107 (2016) 213 combinato[...]
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Ultra-stiff metallic glasses through bond energy density design
J. Phys.: Condens. Matter 29 (2017) 265502 (11pp)
PDF-Dokument [1.6 MB]
Electronic hybridisation implications for the damagetolerance of thin film metallic glasses
Scientific Reports | 6:36556 | DOI: 10.1038/srep36556
Scientific Reports 2016 damage tolerant [...]
PDF-Dokument [1.9 MB]

How do crystallization, phase evolution and corrosion work in Fe-based metallic glasses?

Understanding phase changes, including their formation and evolution, is critical for the performance of functional as well as structural materials. We analyze in detail microstructural and chemical transformations of the amorphous steel Fe50Cr15Mo14C15B6 during isothermal treatments at temperatures ranging from 550 to 800 °C. By combining high-resolution transmission electron microscopy and Rietveld analyses of X-ray diffraction patterns together with the local chemical data obtained by atom probe tomography, this research provides relevant information at the atomic scale about the mechanisms of crystallization and the subsequent phases evolution. During the initial stages of crystallization a stable (Fe,Cr)23(C,B)6 precipitates as well as two metastable intermediates of M3(C,B) and the intermetallic v-phase. When full crystallization is reached, only a percolated nano-scale Cr-rich (Fe,Cr)23(C,B)6 and Mo-rich g-Fe3Mo3C
structure is detected, with no evidence to suggest that other phases appear at any subsequent time. Finally, the corrosion behavior of the developed phases is discussed from considerations of the obtained atomic information.

Element-Resolved Corrosion Analysis of Stainless-Type Glass-Forming Steels
26 JULY 2013 VOL 341 SCIENCE
SCIENCE Duarte et al 2013 metallic glass[...]
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Crystallization, phase evolution and corrosion of Fe-based metallic glasses: An atomic-scale structural and chemical characterization study
Acta Materialia 71 (2014) 20–30
Acta Materialia 71 (2014) 20 metallic gl[...]
PDF-Dokument [1.5 MB]

How does the modulation of plastic flow in metallic glasses via nanoscale networks of chemical heterogeneities work?

In this project we systematically investigate the microstructures of metallic glasses with nanoscale networks of chemical heterogeneities introduced by the presence of a metastable miscibility gap, and their effects on modulating plastic flow of the alloys. Microstructural analysis of as-quenched alloys and the associated thermodynamic assessment in Cu-Zr-Al-Y metallic glass-forming system suggest that the existence of a metastable miscibility gap can induce not only phase-separated microstructures with sharp phase interfaces but also compositional fluctuations without a clear interface ranging from atomic scale to a fewnanometer scale in the fully amorphous alloys. The statistical analysis of shear avalanches in such compositionally heterogeneous metallic glasses reveals that chemical heterogeneities extending over a few nanometers promote a relatively large population of shear deformation units jammed before the
nucleation of mature shear bands. This leads to the multiple nucleation of shear bands and sluggish deformation behavior along them. However, phase interfaces formed by phase separation inside the miscibility gap promote rapid propagation of shear bands at low flow stress, while compositional fluctuations
creating non-sharp interfaces emerging at the outside of miscibility gap have relatively high resistance against shear band propagation. We hence suggest that the optimization of nanoscale compositional fluctuations in metallic glasses in terms of topology, percolation and magnitude can be an effective route for improving the materials' damage tolerance upon plastic flow.

Modulation of plastic flow in metallic glasses via nanoscale networks of chemical heterogeneities
Acta Materialia 140 (2017) 116-129
Kim Oh Raabe Park Metallic Glass chemica[...]
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Hows does shear-induced mixing govern codeformation of crystalline-amorphous nanolaminates?

Deformation of ductile crystalline-amorphous nanolaminates is not well understood due to the complex interplay of interface mechanics, shear banding, and deformation-driven chemical mixing. Here we present indentation experiments on 10 nm nanocrystalline Cu–100 nm amorphous CuZr model multilayers to study these mechanisms down to the atomic scale. By using correlative atom probe tomography and transmission electron microscopy we find that crystallographic slip bands in the Cu layers coincide with non-crystallographic shear bands in the amorphous CuZr layers. Dislocations from the crystalline layers drag Cu atoms across the interface into the CuZr layers. Also, crystalline Cu blocks are sheared into the CuZr layers. In these sheared and thus Cu enriched zones the initially amorphous CuZr layer is rendered into an amorphous plus crystalline nanocomposite.

Shear-Induced Mixing Governs Codeformation of Crystalline-Amorphous Nanolaminates
PRL 113, 035501 (2014)
PHYSICAL REVIEW LETTERS vol 113 - page 0[...]
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Intrinsic and extrinsic size effects in the deformation of amorphous CuZr/nanocrystalline Cu nanolaminates
Acta Materialia 80 (2014) 94–106
Acta Materialia 80 (2014) 94 Nanolaminat[...]
PDF-Dokument [3.9 MB]