The Characterization Of Creep And Time-Dependent Properties Of Bulk Metallic Glasses Using Nanoindentation

Viscoelastic deformation and creep behavior of La- and Ce-based bulk metallic glasses (BMGs) with low glass transition temperature are investigated through nanoindentation at room temperature. Creep compliance and retardation spectra are derived to study the creep mechanism. The time-dependent displacement can be well described by a generalized Kelvin model. A modification is proposed to determine the elastic modulus from the generalized Kelvin model. The results are in excellent agreement with the elastic modulus determined by uniaxial compression tests. (c) 2007 Published by Elsevier B.V.

Effect of Fe content on the thermal stability and dynamic mechanical behavior of NdAlNiCu bulk metallic glasses

The dependence of microstructure and thermal stability on Fe content of bulk Nd60Al10Ni10Cu20-xFex (0 less than or equal to x less than or equal to 20) metallic glasses is investigated by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and high-resolution transmission electron micrograph (HRTEM). All samples exhibit typical amorphous feature under the detect limit of XRD, however, HRTEM results show that the microstructure of Nd60Al10Ni10Cu20-xFex alloys changes from a homogeneous amorphous phase to a composite structure consisting of clusters dispersed in amorphous matrix by increasing Fe content. Dynamic mechanical properties of these alloys with controllable microstructure are studied, expressed via storage modulus, the loss modulus and the mechanical damping. The results reveal that the storage modulus of the alloy without Fe added shows a distinct decrease due to the main a relaxation. This decrease weakens and begins at a higher temperature with increasing Fe content. The mechanism of the effect of Fe addition on the microstructure and thermal stability in this system is discussed in terms of thermodynamics viewpoints. (C) 2004 Elsevier B.V. All rights reserved.

Microstructure and mechanical properties of Zr-Cu-Al bulk metallic glasses

Zr49Cu46Al5 and Zr48.5Cu46.5Al5 bulk metallic glasses(BMGs) with diameter of 5 mm were prepared through water-cooled copper mold casting. The phase structures of the two alloys were identified by X-ray diffractometry(XRD). The thermal stability was examined by differential scanning calorimetry(DSC). Zr49Cu46Al5 alloy shows a glass transition temperature, T, of about 689 K, an crystallization temperature, T-x, of about 736 K. The Zr48.5Cu46.5Al5 alloy shows no obvious exothermic peak. The microstructure of the as-cast alloys was analyzed by transmission electron microscopy(TEM). The aggregations of CuZr and CuZr2 nanocrystals with grain size of about 20 nm are observed in Zr49Cu46Al5 nanocrystalline composite, while the Zr48.5Cu46.5Al5 alloy containing many CuZr martensite plates is crystallized seriously. Mechanical properties of bulk Zr49Cu46Al5 nanocrystalline composite and Zr48.5Cu46.5Al5 alloy measured by compression tests at room temperature show that the work hardening ability of Zr48.5Cu46.5Al5 alloy is larger than that of Zr48.5Cu46.5Al5 alloy.

Simple phenomenological determination of contact stiffness and elastic modulus of Ce-based bulk metallic glasses through nanoindentation

The viscoelastic deformation of Ce-based bulk metallic glasses (BMGs) with low glass transition temperature is investigated at room temperature. Contact stiffness and elastic modulus of Ce-based BMGs cannot be derived using the conventional Oliver-Pharr method [W. C. Oliver and G. M. Pharr, J. Mater. Res. 7, 1564 (1992)]. The present work shows that the time dependent displacement of unloading segments can be described well by a generalized Kelvin model. Thus, a modified Oliver-Pharr method is proposed to evaluate the contact stiffness and elastic modulus, which does, in fact, reproduce the values obtained via uniaxial compression tests. (c) 2007 American Institute of Physics.

The characterization of plastic flow in three different bulk metallic glass systems

Plastic deformation behaviors of Zr52.5Al10Ni10Cu15Be12.5, Mg65Cu25Gd10 and Pd43Ni10Cu27P20 bulk metallic glasses (BMGs) are studied by using the depth-sensing nanoindentation, macroindentation and uniaxial compression. The significant difference in plastic deformation behavior cannot be correlated to the Poisson's ratio or the ratio of shear modulus to bulk modulus of the three BMGs, but can be explained by the free volume model. It is shown that the nucleation of local shear band is easy and multiple shear bands can be activated in the Zr52.5Al10Ni10Cu15Be12.5 alloy, which exhibits a distinct plastic strain during uniaxial compression and less serrated flow during nanoindentation. (c) 2006 Elsevier B.V. All rights reserved.

Inherent Shear-Dilatation Coexistence In Metallic Glass

Shear deformation can induce normal stress or hydrostatic stress in metallic glasses [ Nature Mater. 2 ( 2003) 449, Intermetallics 14 ( 2006) 1033]. We perform the bulk deformation of three-dimensional Cu46Zr54 metallic glass (MG) and Cu single crystal model systems using molecular dynamics simulation. The results indicate that hydrostatic stress can incur shear stress in MG, but not in crystal. The resultant pronounced asymmetry between tension and compression originates from this inherent shear-dilatation coexistence in MG.

Nanoscale periodic corrugation to dimple transition due to "beat" in a bulk metallic glass

We report an intriguing observation that the interaction of brittle nanoscale periodic corrugations (NPCs) can lead to the formation of ductile dimples on the dynamic fracture surface of a tough Vit 1 bulk metallic glass (BMG) under high-velocity plate impact. A “beat” phenomenon due to superposition of simple harmonic vibrations, approximately characterizing NPCs, is proposed to explain this unusual brittle-to-ductile transition. The present results agree well with our previously revealed energy dissipation mechanism in the fracture of BMGs.

Short-range-order effects on intrinsic plasticity of metallic glasses

Through a systematical analysis of the elastic moduli for 137 metallic glasses (MGs) and 56 polycrystalline metals, we use a simple model developed by Knuyt et al. [J. Phys. F: Met. Phys. 16 (1986) p.1989; Phil. Mag. B 64 (1991) p.299] based on a Gaussian distribution for the first-neighbor distance to reveal the short-range-order (SRO) structural conditions for plasticity of MGs. It is found that the SRO structure with dense atomic packing, large packing dispersion and a significant anharmonicity of atomic interaction within an MG is favorable for its global plasticity. Although these conditions seem paradoxical, their perfect matching is believed to be a key for designing large plastic bulk MGs not only in compression but also in tension.

Frequency upconversion properties of Er3+/Yb3+-codoped lead-germanium-bismuth oxide glasses

The frequency upconversion properties of Er3+/Yb3+-codoped heavy metal oxide lead-germanium-bismuth oxide glasses under 975 mn excitation are investigated. Intense green and red emission bands centered at 536, 556 and 672 run, corresponding to the H-2(1/2) --> I-4(15/2), S-4(3/2) --> I-4(15/2) and F-4(9/2) -->I-4(15/2) transitions of Er3+, respectively, were simultaneously observed at room temperature. The influences of PbO on upconversion intensity for the green (536 and 556 nm) and red (672 nm) emissions were compared and discussed. The optimized rare earth doping ratio of Er3+ and Yb3+, is 1:5 for these glasses, which results in the stronger upconversion fluorescence intensities. The dependence of intensities of upconversion emission on excitation power and possible upconversion mechanisms were evaluated and analyzed. The structure of glass has been investigated by means of infrared (IR) spectral analysis. The results indicate that the Er3+/Yb3+-codoped heavy metal oxide lead-germanium-bismuth oxide glasses may be a potential materials for developing upconversion fiber optic devices. (C) 2006 Published by Elsevier Ltd.

Improved fluorescence from Tm3+/Er3+/Ce3+ triply doped bismuth-silicate glasses for S+C-bands amplifiers

Fluorescence of Tm3+/Er3+ codoped bismuth-silica (BS) glasses and the sensitization of Ce3+ are investigated. It shows that Ce3+ codoping with Tm3+/Er3+ in BS glasses results in a quenching of Tm3+ ion emission from F-3(4) to the H-3(6) level. Consequently, the 1.47 mu m emission occurs after the population inversion between the H-3(4) and F-3(4) levels. Furthermore, the codoped glasses show the broad emission spectra over the whole S and C bands with full-width at half-maximum (FWHM) up to about 119nm, as it combines 1.55 mu m emission band of Er3+ with 1.47 mu m emission band of Tm3+ under 800nm excitation.

Origin of Chinese ancient glasses;study on the earliest Chinese ancient glasses

The earliest Chinese ancient glasses before the West Han Dynasty (200 BC) from different regions are studied. The glass samples were unearthed from Hunan, Hubei, Yunnan, Sichuan, Guizhou, Guangdong and Xinjiang of China. The chemical composition of these glasses samples is analyzed by proton induced X-ray emission (PIXE) technique, energy dispersive X-ray fluorescence (EDXRF) method and inductively coupled plasma atomic emission spectrometry (ICP-AES). It is shown that the glass chemical compositions belong to barium-lead silicate BaO-PbO-SiO2, potash soda lime silicate K2O (Na2O)-CaO-SiO2 (K2O/Na2O > 1), soda potash lime silicate Na2O (K2O)-CaO-SiO2 (K2O/Na2O < 1) and potash silicate K2O-SiO2 glass systems, respectively. The origins of the earliest Chinese ancient glasses are discussed from the archaeological and historical points of view. These four types of Chinese ancient glasses were all made in Chinese territory using local raw materials. The glass preparation technology was related to the Chinese ancient bronze metallurgy and proto-porcelain glaze technology. The glass technology relationship between the East and the West is analyzed at the same time.

Effect of network modifiers on spectroscopic properties of erbium-doped phosphate glasses

The integrated absorption cross section Sigma(abs), I peak emission cross section sigma(cmi), Judd-Ofeld intensity parameters Omega(iota) ( t = 2,4,6), and spontaneous emission probability A(R) of Er3+ ions were determined for Erbium doped alkali and alkaline earth phosphate glasses. It is found the compositional dependence of sigma(emi) 5 almost similar to that of Sigma(abs), which is determined by the sum, of Omega(1) (3 Omega(2) + 10 Omega(4) + 21 Omega(6)). In addition, the compositional dependence of Omega(1) was studied in these glass systems. As a result, compared with. Omega(4) and Omega(6) the Omega(2) has a stronger compositional dependence on the ionic radius and content of modifers. The covalency of Er-O bonds in phosphate glass is weaker than that in silicate glass, germanate glass, aluminate glass, and tellurate glass, since Omega(6) of phosphate glass is relatively large. A(R) is affected by the covalency of the Er3+ ion sites and corresponds to the Omega(6) value.

Effect of lithium-potassium mixed alkali on spectroscopic properties of Er3+-doped aluminophosphate glasses

Er3+-doped lithium-potassium mixed alkali aluminophosphate glasses belonging to the oxide system xK(2)O-(15x)Li2O-4B(2)O(3)-11Al(2)O(3)-5BaO-65P(2)O(5) are obtained in a semi-continuous melting quenching process. Spectroscopic properties of Er3+-doped glass matrix have been analysed by fitting the experimental data with the standard Judd-Ofelt theory. It is observed that Judd-Ofelt intensity parameters-Omega(t)(t=2, 4 and 6) of Er3+ change when the second alkali is introduced into glass matrix. The variation of line strength S-ed[I-4(13/2),I-4(15/2)] follows the same trend as that of the Omega(6) parameter. The effect of mixed alkali on the spectroscopic properties of the aluminophosphate glasses, such as absorption cross-section, stimulated emission cross-section, spontaneous emission probability, branching ratio and the radiative lifetime, has also been investigated in this paper.

Near infrared broadband emission from bismuth-dysprosium codoped chalcohalide glasses

We investigate the broadband infrared emission of bismuth doped and bismuth/dysprosium codoped chalcohalide glasses. It is found that the bismuth/dysprosium codoping can drastically enhance the fluorescence as compared with either bismuth or dysprosium doped glasses. Meanwhile, the full width at half maximum of bismuth/dysprosium codoped glasses is over 170 nm, which is the largest value among all the reported rare-earth doped chalcohalide glasses. An ideal way for energy consumption between bismuth and dysprosium ions is supposed. Such improved gain spectra of both bismuth and dysprosium ions may have potential applications in developing broadband fibre amplifiers.

Judd-Ofelt intensity parameters of Er3+ doped mixed alkali aluminophosphate glasses

Lithium sodium mixed alkali aluminophosphate glasses of the composition xNa(2)O-(15-x)Li2O-4B(2)O(3)-11Al(2)O(3)-5BaO-65P(2)O(5) (where x=0, 3.75, 7.5, 11.25 and 15 mol%) containing 0.5 mol% Er2O3 were prepared by melt quenching. The absorption spectra of Er3+ were studied from the experimental oscillator strengths and the Judd-Ofelt intensity parameters were obtained. The variations of Judd-Ofelt intensity parameters (Omega(2), Omega(4) and Omega(6)), experimental oscillator strengths of certain excited states of Er3+ and hypersensitive band positions with different mixed alkali content have been discussed in detail. It was found that there were similar effects of mixed alkali on both Judd-Ofelt intensity parameter 02 and the experimental oscillator strength of the hypersensitive transition, I-4(15/2) -> H-2(11/2). No shifts in the peak wavelength of the studied transitions were found in different glasses. (c) 2006 Elsevier B.V. All rights reserved.