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J Eckert - One of the best experts on this subject based on the ideXlab platform.

  • influence of the ag concentration on the medium range Order in a cuzralag bulk metallic glass
    Scientific Reports, 2017
    Co-Authors: Christoph Gammer, J Eckert, B Escher, Catharina Ebner, Andrew M Minor, H P Karnthaler, Simon Pauly, C Rentenberger
    Abstract:

    Fluctuation electron microscopy of bulk metallic glasses of CuZrAl(Ag) demonstrates that Medium-Range Order is sensitive to minor compositional changes. By analyzing nanodiffraction patterns Medium-Range Order is detected with crystal-like motifs based on the B2 CuZr structure and its distorted structures resembling the martensitic ones. This result demonstrates some structural homology between the metallic glass and its high temperature crystalline phase. The amount of Medium-Range Order seems slightly affected with increasing Ag concentration (0, 2, 5 at.%) but the structural motifs of the Medium-Range Ordered clusters become more diverse at the highest Ag concentration. The decrease of dominant clusters is consistent with the destabilization of the B2 structure measured by calorimetry and accounts for the increased glass-forming ability.

  • mapping of residual strains around a shear band in bulk metallic glass by nanobeam x ray diffraction
    Acta Materialia, 2016
    Co-Authors: Hamed Shakur Shahabi, J Eckert, I Kaban, M Stoica, B Escher, Sergio Scudino, Siegfried Menzel, Gavin Vaughan, Uta Kuhn
    Abstract:

    Abstract Despite recent progress in understanding the outstanding role of shear bands in plastic deformation of metallic glasses, the details of structural changes in the shear-induced zone is not yet known. In Order to probe such changes, we determined the distribution of residual strains at short- and Medium-Range Order around a single shear band in cold-rolled Vit105 bulk metallic glass using a nano-focused high energy X-ray beam. Plastic deformation results in significant residual normal and shear strains at distances of more than 15 μm around the shear band. Based on a detailed analysis of the distribution profile, the magnitude and the direction of the residual shear strain, it is suggested that the shear strain plays a dominant role, compared to the normal strains, for triggering nucleation of further shear bands from a mature shear band.

  • structural aspects of elasto plastic deformation of a zr based bulk metallic glass under uniaxial compression
    Acta Materialia, 2015
    Co-Authors: Shakur H Shahabi, S Scudino, I Kaban, M Stoica, U Rutt, U Kuhn, J Eckert
    Abstract:

    Abstract The structural rearrangements occurring during compressive deformation of a plastically deformable Zr 52.5 Ti 5 Cu 18 Ni 14.5 Al 10 bulk metallic glass have been investigated in situ using high energy synchrotron X-rays. It was found that in the elastic regime, the atomic distances at both short and medium range Order vary linearly with macroscopic stress where the atomic bonds in short range Order appear significantly stiffer than medium range Order. Upon elastic loading, a small fraction of bonds in the first shell is broken in the loading direction whereas some new bonds are formed in the transverse direction. Atomic strain–stress correlation at medium range Order deviates from linearity at the onset of plastic deformation which was correlated to the activation of irreversible STZs. This was confirmed by quantifying the amount of atomic shear strain value during loading. The length scale of 12.5 A indicated the largest shear strain and is thought to be the most effective length scale in the formation of STZs. The typical fracture angle of this BMG was explained by the orientation of maximum atomic shear strain at the onset of major shear band formation.

  • hierarchical densification and negative thermal expansion in ce based metallic glass under high pressure
    Nature Communications, 2015
    Co-Authors: Qiang Luo, Gaston Garbarino, Baoan Sun, Dawei Fan, Y Zhang, Zhi Wang, Yajuan Sun, Jin Jiao, N Mattern, J Eckert
    Abstract:

    The polyamorphsim in amorphous materials is one of the most fascinating topics in condensed matter physics. In amorphous metals, the nature of polyamorphic transformation is poorly understood. Here we investigate the structural evolution of a Ce-based metallic glass (MG) with pressure at room temperature (RT) and near the glass transition temperature by synchrotron X-ray diffraction, uncovering novel behaviours. The MG shows hierarchical densification processes at both temperatures, arising from the hierarchy of interatomic interactions. In contrast with a continuous and smooth process for the low- to medium-density amorphous state transformation at RT, a relatively abrupt and discontinuous transformation around 5.5 GPa is observed at 390 K, suggesting a possible weak first-Order nature. Furthermore, both positive and abnormal-negative thermal expansion behaviours on Medium-Range Order are observed in different pressure windows, which could be related to the low-energy vibrational motions and relaxation of the weakly linked solute-centred clusters.

  • fabrication and characterization of co40fe22ta8 xyxb30 x 0 2 5 4 6 and 8 metallic glasses with high thermal stability and good soft magnetic properties
    Journal of Applied Physics, 2014
    Co-Authors: Amir Hossein Taghvaei, Hamed Shakur Shahabi, J Bednarcik, J Eckert
    Abstract:

    Atomic structure and thermal behavior of Co40Fe22Ta8-xYxB30 (x = 0, 2.5, 4, 6, and 8) metallic glasses with good soft magnetic properties have been investigated by high-energy synchrotron X-ray diffraction and differential scanning calorimeter, respectively. It has been shown that the extension of the supercooled liquid region first increases and reaches a large value of 95 K and subsequently decreases as a function of Y content. Analysis of the structure factors and pair correlation functions in the reciprocal-space and real-space have indicated that the addition of Y noticeably changes the atomic structure and reduces the degree of the Medium-Range Order. Magnetic measurements have implied that the introduction of Y enhances both saturation magnetization and Curie temperatures of the ribbons, while keeping their coercivity very small. The underlying mechanisms for changes in the atomic structure, improving the thermal stability and magnetic properties upon Y addition have been discussed.

Hellmut Eckert - One of the best experts on this subject based on the ideXlab platform.

  • Short- and Medium-Range Order in Photothermal Refractive Glass Revealed by Solid-State NMR Techniques
    2019
    Co-Authors: Lena Marie Funke, Oliver Janka, Rainer Pöttgen, Leonid Glebov, Michael Ryan Hansen, Hellmut Eckert
    Abstract:

    Photo-thermo-refractive (PTR) glass is an optically transparent photosensitive Na2O–ZnO–Al2O3–SiO2 glass, containing NaF and KBr additives, along with cerium, silver, tin, and antimony oxide dopants. After heating above 500 °C, UV-exposed regions of this glass produce permanent refractive index changes, resulting from precipitation of NaF nanocrystals. Short- and Medium-Range Order of this glass system is studied via multinuclear single- and double-resonance solid-state nuclear magnetic resonance (NMR) spectroscopy in regular PTR glass and in model glasses with simplified compositions. The results, when combined with data from energy-dispersive X-ray spectroscopy, indicate that the NaF component modifies the standard aluminosilicate framework, producing small amounts of F-bonded five- and six-coordinated aluminum species. The fluoride speciation is obtained from 19F magic-angle spinning NMR spectra, supported by 19F­{27Al} and 19F­{23Na} dipolar recoupling experiments. The majority of fluoride within the PTR glass is found within Na-dominated local environments, which also interact strongly with the aluminum. 23Na­{19F} rotational echo double resonance reveals that about 1/3 of the Na+ ions have fluoride ions in their first coordination spheres

  • Superstructural Units Involving Six-Coordinated Silicon in Sodium Phosphosilicate Glasses Detected by Solid-State NMR Spectroscopy
    2018
    Co-Authors: Jinjun Ren, Hellmut Eckert
    Abstract:

    Sodium phosphosilicate glasses with high phosphate contents represent an unusual case in glass science as they are known to contain large amounts of six-coordinated silicon species (SiO6/2 units, Si(6)). Although the network connectivity of these units has been previously investigated, the overall structural organization of this system at the Medium-Range Order level is still incompletely understood. In the present study, this issue is addressed by using a comprehensive suite of homo- and heteronuclear dipolar recoupling studies involving the nuclear isotopes 23Na, 29Si, and 31P on isotopically enriched glasses in the systems xSiO2–(1 – x)­(0.45Na2O–0.55P2O5) (x = 0.0, 0.1, and 0.2) and 0.2SiO2–0.2Na2O–0.6P2O5. Four- and six-fold coordinated silicon species (Si(4) and Si(6)) coexist in these glasses. Invariably, Si(6) selectively connects with phosphate species, which are exclusively of the P(3) type, whereas the Si(4) species are coordinated to both P(2) and P(3) units. 29Si ↔ 23Na and 31P ↔ 23Na rotational echo double resonance experiments indicate much closer sodium/phosphorus than sodium/silicon proximities. Evidently, the bond valence gradient of the Si(6)–O–P(3) linkages redistributes the anionic charge onto the phosphate nonbridging oxygen species, which attract Na+ cations. The Si(6)(P(3))6Na2 superstructural units (stoichiometry Na2SiP6O18) thus formed represent an exceptionally high degree of Medium-Range Order, accounting for high thermal and mechanical stability of these glasses. The Na+ ions neutralize the negative charge located at nonbridging oxygen atoms of P(3) tetrahedra, whereas the bridging oxygen atoms in the Si(6)–O–P bonds are electrically neutral. Finally, we show that the structural speciations of silicon and phosphorus in SiO2–Na2O–P2O5 glasses can be predicted straightforwardly from the elemental glass compositions in excellent agreement with experimental results

  • Medium-Range Order in Sol–Gel Prepared Al2O3–SiO2 Glasses: New Results from Solid-State NMR
    2014
    Co-Authors: Jinjun Ren, Long Zhang, Hellmut Eckert
    Abstract:

    The Medium-Range Order of 0.5Al2O3–xSiO2 glasses (1 ≤ x ≤ 6) prepared via a new sol–gel route from the Al lactate precursor has been studied by 29Si and 27Al single- and double-resonance solid-state NMR techniques. For high-alumina samples Si–O–Al connectivities are detected by 29Si MAS NMR as well as by 29Si­{27Al} rotational echo adiabatic passage double-resonance (REAPDOR) spectroscopy. To boost the signal-to-noise ratio, the REAPDOR experiment was combined with a Carr–Purcell–Meiboom–Gill (CPMG) echo train acquisition. While all five silicon units Q(4)mAl (0 ≤ m ≤ 4) are detectable in appreciable concentrations for x = 1, the spectra indicate that the average number of Al species bound to silicon, ⟨mAl⟩, gradually decreases toward higher x values, as expected. The 27Al MAS NMR spectra reveal four-, five-, and six-coordinated aluminum in these glasses. For x ≥ 3, the Al species detected are essentially independent of sample composition indicating a constant structural environment of Al. In contrast, for x = 1 and 2, an increase in the 27Al isotropic chemical shifts suggests an increasing number of Al···Al proximities. Consistent with this finding, two-dimensional 27Al–27Al double-quantum/single-quantum correlation spectroscopy reveals spatial proximities among and between all types of aluminum species present. On the basis of the complementary evidence from these single- and double-resonance experiments, a model for the Medium-Range Order of these glasses is developed

  • medium range Order in sodium phosphate glasses a quantitative rotational echo double resonance solid state nmr study
    Physical Chemistry Chemical Physics, 2006
    Co-Authors: Wenzel Strojek, Hellmut Eckert
    Abstract:

    Sodium ultraphosphate glasses (Na2O)x(P2O5)1−x show a strongly non-linear dependence of the glass transition temperatures Tg(x) on composition. To explore the structural origins of this behaviour, local and medium range Ordering processes have been investigated by state-of-the-art 23Na high-resolution and dipolar NMR spectroscopies. In particular, 31P{23Na} and 23Na{31P} rotational echo double resonance (REDOR) experiments have been analyzed to yield quantitative constraints for the structural description of these glasses. The sodium ions are found to be randomly distributed and, for x 0.25), the magnitude of the 31P{23Na} dipolar interaction increases markedly, indicating a significantly increased extent of Q(2)-Na-Q(2) crosslinking. Based on these results, a comprehensive description of Medium-Range Order in sodium ultraphosphate glasses is developed, suggesting that the Tg(x) dependence is closely linked to changes in the relative phosphorus/sodium distance distributions.

  • Short and Medium Range Order in Sodium Aluminoborate Glasses. 2. Site Connectivities and Cation Distributions Studied by Rotational Echo Double Resonance NMR Spectroscopy
    The Journal of Physical Chemistry B, 2000
    Co-Authors: Marko Bertmer, Lars Züchner, And Jerry C. C. Chan, Hellmut Eckert
    Abstract:

    A quantitative NMR strategy is developed for the study of the network connectivity (intermediate range structure) and the cation distribution in the sodium aluminoborate glass system. The strategy is based on the analysis of rotational echo double resonance (REDOR) spectroscopy applied to the glasses and the crystalline model compounds Li6Al2(BO3)4, NaBO2, and Na2B4O7. The heterodipolar multispin interaction between the quadrupolar spin systems 27Al ↔ 11B and 23Na ↔ 11B is analyzed in terms of approximate second moments, which are extracted from REDOR data measured at short dipolar evolution times. On the basis of 27Al{11B} and 11B{27Al}-REDOR results, the framework connectivity distribution is extracted and compared to statistical and preferential bonding distribution scenarios, respectively. The sodium aluminoborate system is characterized by a large degree of bonding regularity in the framework. Four-coordinated aluminum is preferentially linked to BO3/2 and BO2/2O- units and vice versa, while linking ...

Akihisa Inoue - One of the best experts on this subject based on the ideXlab platform.

  • correlation of atomic packing with the boson peak in amorphous alloys
    Journal of Applied Physics, 2014
    Co-Authors: Akihisa Inoue, Chuntao Chang, Weiming Yang, Guofu Chen, Y C Zhao, Baolong Shen, Renwen Li
    Abstract:

    Boson peaks (BP) have been observed from phonon specific heats in 10 studied amorphous alloys. Two Einstein-type vibration modes were proposed in this work and all data can be fitted well. By measuring and analyzing local atomic structures of studied amorphous alloys and 56 reported amorphous alloys, it is found that (a) the BP originates from local harmonic vibration modes associated with the lengths of short-range Order (SRO) and Medium-Range Order (MRO) in amorphous alloys, and (b) the atomic packing in amorphous alloys follows a universal scaling law, i.e., the ratios of SRO and MRO lengths to solvent atomic diameter are 3 and 7, respectively, which exact match with length ratios of BP vibration frequencies to Debye frequency for the studied amorphous alloys. This finding provides a new perspective for atomic packing in amorphous materials, and has significant implications for quantitative description of the local atomic Orders and understanding the structure-property relationship.

  • atomic scale heterogeneity of a multicomponent bulk metallic glass with excellent glass forming ability
    Physical Review Letters, 2009
    Co-Authors: Takeshi Fujita, Akihisa Inoue, Kazuya Konno, Wei Zhang, Vijay Kumar, Makoto Matsuura, T Sakurai, Mingwei Chen
    Abstract:

    We report the atomic structure of a multicomponent Cu 45 Zr 45 Ag 10 bulk metallic glass investigated by state-of-the-art experimental and computational techniques. In comparison with a binary Cu 50 Zr 50 metallic glass, Zr-rich interpenetrating clusters centered by paired and stringed Ag atoms and Cu-rich icosahedra are widely observed in the ternary Cu 45 Zr 45 Ag 10 alloy. The atomic-scale heterogeneity caused by chemical short- and Medium-Range Order is found to play a key role in stabilizing the liquid phase and in improving the glass forming ability of the multicomponent alloy.

  • Cluster packed structures in bulk metallic glasses created from BCC derivative compounds
    Journal of Physics: Conference Series, 2009
    Co-Authors: Akira Takeuchi, Kunio Yubuta, Akihisa Inoue
    Abstract:

    Molecular dynamics simulations based on plastic crystal model were performed for the Fe69.0Nb10.3B20.7 and Zr57.1Ni28.6Al14.3 alloys to investigate their forming ability and features of the noncrystalline structures. The Fe69.0Nb10.3B20.7 and Zr57.1Ni28.6Al14.3 alloys were created from C6Cr23 and Ti4Ni2O structures, respectively, by assuming the presence of hypothetical clusters in the crystalline structures under a treatment that the clusters and glue atoms are regarded as the components. The analysis with total pair-distribution function, g(r), revealed that these alloys have high tendencies to form noncrystalline structures while their partial g(r) profiles for pairs among components showed the presence of medium range Order. Cluster packed structures from bcc derivative compounds cause high forming ability of noncrystalline structure and the medium range Order in the noncrystalline structure.

  • cooling rate structure thermal stability and crystallization behaviour of cu based bulk glass forming alloys
    Journal of Physics: Conference Series, 2009
    Co-Authors: Dmitri V Louzguineluzgin, Konstantinos Georgarakis, Guoqiang Xie, Wei Zhang, Gavin Vaughan, T Saito, A R Yavari, Akihisa Inoue
    Abstract:

    We summarize the structural features of several Cu-based alloys known so far having high glass-forming ability and investigate the influence of Ag addition on their crystallization behavior. According to synchrotron-beam X-ray radiation studies, an oC68 Cu10Zr7 phase is found to be a good approximant to the Cu-Zr-Ti and Cu-Zr-Ag glassy phases. The structure of the bulk glassy Cu36Zr48Al8Ag8 and Cu44Ag15Zr36Ti5 alloy samples studied by high-resolution TEM was found to contain well developed Medium-Range Order zones and nanoparticles, respectively. An influence of the cooling rate on the structure and properties of the Cu-based glassy alloys on heating was also studied. The crystallization kinetics of Cu55Zr45, Cu50Zr50, Cu55-xZr45Agx (x = 0, 10, 20), Cu44Ag15Zr36Ti5, Cu45Zr45Al5Ag5 and Cu36Zr48Al8Ag8 glassy alloys was also analyzed substantially. Cu35Zr45Ag20 alloy was found to exhibit possible phase separation upon heating within a supercooled liquid region.

  • structural relaxation glass forming ability and mechanical properties of mg cu ni gd alloys
    Journal of Non-crystalline Solids, 2005
    Co-Authors: Guangyin Yuan, Kenji Amiya, Akihisa Inoue
    Abstract:

    Abstract The structural relaxation and glass-forming ability (GFA) of Mg–Cu–Gd-based glassy alloys and the effect of Ni addition on the glass-forming ability, structural relaxation and mechanical properties were studied by thermal analysis, X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The Medium-Range Order (MRO) zones of about 1–2 nm size were observed in the Mg 65 Cu 25 Gd 10 glassy alloy after structural relaxation caused by annealing for 180 s at 305 K. The structural relaxation resistance increased with decreasing Cu content, although the GFA also decreased in Mg–Cu–Gd-based alloys. Appropriate substitution of Cu by Ni in Mg 75 Cu 15 Gd 10 -based alloy not only improved the glass-forming ability and mechanical properties, but also increased the stability against structural relaxation.

Weiming Yang - One of the best experts on this subject based on the ideXlab platform.

  • correlation of atomic packing with the boson peak in amorphous alloys
    Journal of Applied Physics, 2014
    Co-Authors: Akihisa Inoue, Chuntao Chang, Weiming Yang, Guofu Chen, Y C Zhao, Baolong Shen, Renwen Li
    Abstract:

    Boson peaks (BP) have been observed from phonon specific heats in 10 studied amorphous alloys. Two Einstein-type vibration modes were proposed in this work and all data can be fitted well. By measuring and analyzing local atomic structures of studied amorphous alloys and 56 reported amorphous alloys, it is found that (a) the BP originates from local harmonic vibration modes associated with the lengths of short-range Order (SRO) and Medium-Range Order (MRO) in amorphous alloys, and (b) the atomic packing in amorphous alloys follows a universal scaling law, i.e., the ratios of SRO and MRO lengths to solvent atomic diameter are 3 and 7, respectively, which exact match with length ratios of BP vibration frequencies to Debye frequency for the studied amorphous alloys. This finding provides a new perspective for atomic packing in amorphous materials, and has significant implications for quantitative description of the local atomic Orders and understanding the structure-property relationship.

M M J Treacy - One of the best experts on this subject based on the ideXlab platform.

  • medium range Order in amorphous silicon investigated by constrained structural relaxation of two body and four body electron diffraction data
    Acta Materialia, 2012
    Co-Authors: Konstantin B Borisenko, M M J Treacy, Amelia C Y Liu, Bianca Haberl, Yixin Chen, J S Williams, J E Bradby, D J H Cockayne
    Abstract:

    Abstract The structures of four types of amorphous silicon are examined by an experimentally constrained structural relaxation method (ECSR). Experimental selected area electron diffraction data and fluctuation electron microscopy normalized diffraction variance data were used as constraints to guide a Monte Carlo relaxation procedure towards best fit models. A Tersoff potential was also used to further restrict the space of possible solutions. The materials examined were self-ion-implanted silicon and pressure-amorphized silicon, both in their as-prepared and thermally annealed states. In the fitted models for these materials regions containing two types of Medium-Range Order were identified. One type involves formation of paracrystallites with cubic and hexagonal structures, where both short-range crystalline and Medium-Range Order are present. The other type of Medium-Range Order appears in the form of extended crystalline planes without associated short-range crystalline Order. These two types can coexist. It is observed that the best fit models for both as-prepared samples contain approximately 10–15% paracrystalline Ordered regions, reducing to about 5–10% in the annealed materials. None of the models are true continuous random networks. We conclude that, with long computational times and with a suitable potential function, the ECSR procedure provides a powerful, although at present semi-quantitative, tool for determining the structural form of Medium-Range Order in thin amorphous materials.

  • medium range Order and the radial distribution function
    Journal of Non-crystalline Solids, 2006
    Co-Authors: Arun Bodapati, M M J Treacy, Michael L Falk, John Kieffer, Pawel Keblinski
    Abstract:

    Abstract We have studied the subtle differences between the radial distribution functions g(r) of several models of disOrdered silicon, which contain differing amounts of paracrystalline medium range Order. Due to the inherent averaging of diffraction data at medium range length scales r, the differences are indeed small. We find, however, that the residual function G(r) = r[g(r) − 1] exhibits an oscillatory decay, with discernibly different decay lengths. The decay lengths are found to be proportional to the radial extent of the medium range Order in the model as determined by several other computational methods. Our results indicate that the extent of medium range Order could be measured by diffraction experiments. However, to discern the nature of the Ordering, fluctuation microscopy is needed, and for models, improved methods for modeling the oscillatory part of G(r) are essential.

  • absence of an abrupt phase change from polycrystalline to amorphous in silicon with deposition temperature
    Physical Review Letters, 2001
    Co-Authors: Paul M. Voyles, M M J Treacy, J M Gibson, J. E. Gerbi, John R Abelson
    Abstract:

    Using fluctuation electron microscopy, we have observed an increase in the mesoscopic spatial fluctuations in the diffracted intensity from vapor-deposited silicon thin films as a function of substrate temperature from the amorphous to polycrystalline regimes. We interpret this increase as an increase in paracrystalline Medium-Range Order in the sample. A paracrystal consists of topologically crystalline grains in a disOrdered matrix; in this model the increase in Ordering is caused by an increase in the grain size or density. Our observations are counter to the previous belief that the amorphous to polycrystalline transition is a discontinuous disOrder-Order phase transition.

  • paracrystallites found in evaporated amorphous tetrahedral semiconductors
    Journal of Non-crystalline Solids, 1998
    Co-Authors: M M J Treacy, J M Gibson, P J Keblinski
    Abstract:

    Variable coherence microscopy shows that as-deposited amorphous germanium and silicon contain Medium-Range Order. The Ordering can be explained by a fine-grained paracrystallite material in which intergranular stress has been relaxed by deformation. On annealing, the paracrystallite structure transforms towards the lower-energy continuous random network (CRN). We present data on a variety of vacuum-evaporated samples, and on molecular dynamics simulations of candidate structures.