Prevalence Of Shear Banding In Compression Of Zr41Ti14Cu12.5Ni10Be22.5 Pillars As Small As 150 Nm In Diameter

Recently, the size dependence of mechanical behaviors, particularly the yield strength and plastic deformation mode, of bulk metallic glasses (BMG) has created a great deal of interest. Contradicting conclusions have been drawn by different research groups, based on various experiments on different BMG systems. Based on in situ compression transmission electron microscopy (TEM) experiments on Zr41Ti14Cu12.5Ni10Be22.5 (Vit 1) nanopillars, this paper provides strong evidence that shear banding still prevails at specimen length scales as small as 150 nm in diameter. This is supported by in situ and ex situ images of shear bands, and by the carefully recorded displacement bursts under load control its well as load drops under displacement control. Finite element modeling of the stress state within the pillar shows that the unavoidable geometry constraints accompanying such experiments impart a strong effect on the experimental results, including non-uniform stress distributions and high level hydrostatic pressures. The seemingly improved compressive ductility is believed to be due to such geometry constraints. Observations underscore the notion that the mechanical behavior of metallic glasses, including strength and plastic deformation mode, is size independent at least in Vit 1. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Formation Mechanism Of Lamellar Chips During Machining Of Bulk Metallic Glass

The uniqThe unique lamellar chips formed in turning–machining of a Vit 1 bulk metallic glass (BMG) are found to be due to repeated shearband formation in the primary shear zone (PSZ). A coupled thermomechanical orthogonal cutting model, taking into account force, free volume and energy balance in the PSZ, is developed to quantitatively characterize lamellar chip formation. Its onset criterion is revealed through a linear perturbation analysis. Lamellar chip formation is understood as a self-sustained limit-cycle phenomenon: there is autonomous feedback in stress, free volume and temperature in the PSZ. The underlying mechanism is the symmetry breaking of free volume flow and source, rather than thermal instability. These results are fundamentally useful for machining BMGs and even for understanding the physical nature of inhomogeneous flow in BMGs.ue lamellar chips formed in turning–machining of a Vit 1 bulk metallic glass (BMG) are found to be due to repeated shearband.

Prediction Of Shear-Band Thickness In Metallic Glasses

We derive an explicit expression for predicting the thicknesses of shear bands in metallic glasses. The model demonstrates that the shear-band thickness is mainly dominated by the activation size of the shear transformation zone (STZ) and its activation free volume concentration. The predicted thicknesses agree well with the results of measurements and simulations. The underlying physics is attributed to the local topological instability of the activated STZ. The result is of significance in understanding the origin of inhomogeneous flow in metallic glasses. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. 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.

Bonding Of Alumina And Metal Using Bulk Metallic Glass Forming Alloy

Metal-alumina joints have found various practical applications in electronic devices and high technology industry. However, making of sound metal ceramic brazed couple is still a challenge in terms of its direct application in the industry. In this work we successfully braze copper with Al2O3 ceramic using Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass forming alloy as filler alloy. The shear strength of the joints can reach 140 MPa, and the microstructrural analysis confirms a reliable chemical boning of the interface. The results show that the bulk metallic glass forming alloys with high concentration of active elements are prospective for using as filler alloy in metal-ceramic bonding.

Smaller Deborah number inducing more serrated plastic flow of metallic glass

Spherical nano-indentations of Cu46Zr54 bulk metallic glass (BMG) model systems were performed using molecular dynamics (MD) computer simulations, focusing specifically on the physical origin of serrated plastic flow. The results demonstrate that there is a direct correlation between macroscopic flow serration and underlying irreversible rearrangement of atoms, which is strongly dependent on the loading (strain) rate and the temperature. The serrated plastic flow is, therefore, determined by the magnitude of such irreversible rearrangement that is inhomogeneous temporally. A dimensionless Deborah number is introduced to characterize the effects of strain rate and temperature on serrations. Our simulations are shown to compare favorably with the available experimental observations.

On the origin of shear banding instability in metallic glasses

To uncover the physical origin of shear-banding instability in metallic glass (MG), a theoretical description of thermo-mechanical deformation of MG undergoing one-dimensional simple shearing is presented. The coupled thermo-mechanical model takes into account the momentum balance, the energy balance and the dynamics of free volume. The interplay between free-volume production and temperature increase being two potential causes for shear-banding instability is examined on the basis of the homogeneous solution. It is found that the free-volume production facilitates the sudden increase in the temperature before instability and vice versa. A rigorous linear perturbation analysis is used to examine the inhomogeneous deformation, during which the onset criteria and the internal length and time scales for three types of instabilities, namely free-volume softening, thermal softening and coupling softening, are clearly revealed. The shear-banding instability originating from sole free-volume softening takes place easier and faster than that due to sole thermal softening, and dominates in the coupling softening. Furthermore, the coupled thermo-mechanical shear-band analysis does show that an initial slight distribution of local free volume can incur significant strain localization, producing a shear band. During such a localization process, the local free-volume creation occurs indeed prior to the increase in local temperature, indicating that the former is the cause of shear localization, whereas the latter is its consequence. Finally, extension of the above model to include the shear-induced dilatation shows that such dilatation facilitates the shear instability in metallic glasses.

Intrinsic correlation between dilatation and pressure sensitivity of plastic flow in metallic glasses

Taking shear-induced dilatation into consideration in shear transformation zone (STZ) operations, we derive a new yield criterion that reflects the pressure sensitivity in plastic flow in metallic glasses (MGs), which agrees well with experiments. Furthermore, an intrinsic theoretical correlation between the pressure sensitivity coefficient and the dilatation factor is revealed. It is found that the pressure sensitivity of plastic flow of MGs originates in the dilatation of microscale STZs.

Synthesis Of Plastic Zr-Based Bulk Metallic Glass Matrix Composites By The Copper-Mould Suction Casting And The Bridgman Solidification

Zr-based bulk metallic glass matrix composites with the composition of Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.(5) were synthesized by the copper-mould suction casting and the Bridgman solidification. The composite, containing a well-developed flowery beta-Zr dendritic phase, was obtained by the Bridgman solidification with the withdrawal velocity of 0.8 mm/s and the temperature gradient of 45 K/mm, and the ultimate strength of 2050 MPa and fracture plastic strain of 14.6% of the composite were achieved, which was mainly interpreted by the homogeneous dispersion of bcc beta-Zr phase in the glass matrix. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.

The mechanism of PEO process on Al–Si alloys with the bulk primary silicon

This study focuses on mechanism of ceramic coating on Al-Si alloys with bulk primary Si using plasma electrolytic oxidation (PEO) technology. Al-Si alloys with 27-32% Si in weight were used as substrates. The morphologies, composition and microstructure of PEO coatings were investigated by scanning electron microscopy (SEM) with energy dispersive X-ray system (EDX). Results showed that the PEO process had four different stages. The effect of bulk Si is greatly on the morphology and composition of coatings at first three stages. Anodic oxide films formed on Al and Si phases, respectively. When the voltage exceeded 40 V, glow appeared and concentrated on the localized zone of interface of Al and Si phase. Al-Si-O compounds formed and covered on the dendrite Si phase surface, and the coating on bulk Si, which was silicon oxide, was rougher than that on other phase. If the treatment time was long enough, the coatings with uniform surface morphologies and elements distribution will be obtained but the microstructure of inner layer is looser due to the bulk Si.

Pulse self-compression in normally dispersive bulk media

We experimentally demonstrate that high-power femtosecond pulses can be compressed during the nonlinear propagation in the normally dispersive solid bulk medium. The self-compression behavior was detailedly investigated under a variety of experimental conditions, and the temporal and spectral characteristics of resulted pulses were found to be significantly affected by the input pulse intensity, with higher intensity corresponding to shorter compressed pulses. By passing through a piece of BK7 glass, a self-compression from 50 to 20 fs was achieved, with a compression factor of about 2.5. However, the output pulse was observed to be split into two peaks when the input intensity is high enough to generate supercontinuum and conical emission. (c) 2005 Elsevier B.V. All rights reserved.

Infrared luminescence properties of bismuth-doped barium silicate glasses

Infrared (IR) luminescence covering 1.1 to similar to 1.6 mu m wavelength region was observed from bismuth-doped barium silicate glasses, excited by a laser diode at 808 nm wavelength region, at room temperature. The peak of the IR luminescence appears at 1325 nm. A full width half-maximum (FWHM) and the lifetime of the fluorescence is more than 200 nm and 400 mu s, respectively. The fluorescence intensity increases with Al2O3 content, but decreases with BaO content. We suggest that the IR luminescence should be ascribed to the low valence state of bismuth Bi2+ or Bi+, and Al3+ ions play an indirect dispersing role for the infrared luminescent centers.

Precipitation of nanocrystals in glasses by electron irradiation: An alternative path to form glass ceramics?

This letter demonstrates an alternative method to form gallium silicate glass ceramics using high-energy electron irradiation. Compared with glass ceramics obtained from the conventional thermal treatment method, the distribution and crystal sizes of the precipitated Ga2O3 nanoparticles are the same. An advantage of this method is that the spatial distribution of the precipitated nanoparticles can be easily controlled. However, optically active dopants Ni2+ ions do not participate in the precipitation during electron irradiation. (c) 2007 American Institute of Physics.

Microstructures induced in the bulk of SrTiO3 crystal by a femtosecond laser

Microstructures with the total length of hundreds of mu m were induced by fixing the focal point of the femtosecond laser at a certain depth in the bulk of SrTiO3 crystal. By different combination of the focusing conditions with the laser parameters, different morphologies have been observed, such as void array, necklace-shaped structures, continuous/segmental filaments and etc. The possible mechanism of the formation of those diversiform structures is discussed.

Orientation-controllable self-organized microgratings induced in the bulk SrTiO3 crystal by a single femtosecond laser beam

Self-organized microgratings were induced in the bulk SrTiO3 crystal by readily scanning the laser focus in the direction perpendicular to the laser propagation axis. The groove orientations of those gratings could be controlled by changing the irradiation pulse number per unit scanning length, which could be implemented either through adjusting the scanning velocity at a fixed pulse repetition rate or through varying the pulse repetition rate at a fixed scanning velocity. This high-speed method for fabrication of microgratings will have many potential applications in the integration of micro-optical elements. The possible formation mechanism of the self-organized microgratings is also discussed. (C) 2007 Optical Society of America.