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.

Characterization Of Shear Bands In Two Bulk Metallic Glasses With Different Inherent Plasticity

Shear banding characterization of Zr64.13Cu15.75Ni10.12Al10 and Zr65Cu15Ni10Al10 bulk metallic glasses (BMGs) with significant difference in inherent plasticity and quite similar chemical composition was studied by depth sensitive macroindentaion tests with conical indenter. Well-developed shear band pattern can be found for both BMGs after indentation. Distinct difference in the shear band spacing, scale of plastic deformation region and the shear band branching in the two BMGs account for the different plasticity.

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.

Intrinsic correlation between fragility and bulk modulus in metallic glasses

A systematic study on the available data of 26 metallic glasses shows that there is an intrinsic correlation between fragility of a liquid and bulk modulus of its glass. The underlying physics can be rationalized within the formalism of potential energy landscape thermodynamics. It is surprising to find that the linear correlation between the fragility and the bulk-shear modulus ratio exists strictly at either absolute zero temperature or very high frequency. Further analyses indicate that a real flow event in bulk metallic glasses is shear dominant, and fragility is in inverse proportion to shear-induced bulk dilatation. Finally, extension of these findings to nonmetallic glasses is discussed.

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.

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.

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.

Complex Transition of Double-Diffusive Convection in a Rectangular Enclosure with Height-to-Length Ratio Equal to 4: Part I

This is the first part of direct numerical simulation (DNS) of double-diffusive convection in a slim rectangular enclosure with horizontal temperature and concentration gradients. We consider the case with the thermal Rayleigh number of 10^5, the Pradtle number of 1, the Lewis number of 2, the buoyancy ratio of composition to temperature being in the range of [0,1], and height-to-width aspect ration of 4. A new 7th order upwind compact scheme was developed for approximation of convective terms, and a three-stage third-order Runge-Kutta method was employed for time advancement. Our DNS suggests that with the buoyancy ratio increasing form 0 to 1, the flow of transition is a complex series changing fromthe steady to periodic, chaotic, periodic, quasi-periodic, and finally back to periodic. There are two types of periodic flow, one is simple periodic flow with single fundamental frequency (FF), and another is complex periodic flow with multiple FFs. This process is illustrated by using time-velocity histories, Fourier frequency spectrum analysis and the phase-space rajectories.

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.

High Order Multi-Moment Constrained Finite Volume Method. Part I: Basic Formulation

A new high-order finite volume method based on local reconstruction is presented in this paper. The method, so-called the multi-moment constrained finite volume (MCV) method, uses the point values defined within single cell at equally spaced points as the model variables (or unknowns). The time evolution equations used to update the unknowns are derived from a set of constraint conditions imposed on multi kinds of moments, i.e. the cell-averaged value and the point-wise value of the state variable and its derivatives. The finite volume constraint on the cell-average guarantees the numerical conservativeness of the method. Most constraint conditions are imposed on the cell boundaries, where the numerical flux and its derivatives are solved as general Riemann problems. A multi-moment constrained Lagrange interpolation reconstruction for the demanded order of accuracy is constructed over single cell and converts the evolution equations of the moments to those of the unknowns. The presented method provides a general framework to construct efficient schemes of high orders. The basic formulations for hyperbolic conservation laws in 1- and 2D structured grids are detailed with the numerical results of widely used benchmark tests. (C) 2009 Elsevier Inc. All rights reserved.

Electromagnetic fields and transmission properties in tapered hollow metallic waveguides

We analyze the electromagnetic spatital distributions and address an important issue of the transmission properties of spherical transverse-electric (TE) and transverse-magnetic (TM) eigenmodes within a tapered hollow metallic waveguide in detail. Explicit analytical expressions for the spatital distributions of electromagnetic field components, attenuation constant, phase constant and wave impedance are derived. Accurate eigenvalues obtained numerically are used to study the dependences of the transmission properties on the taper angle, the mode as well as the length of the waveguide. It is shown that all modes run continuously from a propagating through a transition to an evanescent region and the value of the attenuation increases as the distance from the cone vertex and the cone angle decrease. A strict distinction between pure propagating and pure evanescent modes cannot be achieved. One mode after the other reaches cutoff in the tapered hollow metallic waveguide as the distance from the cone vertex desreases. (C) 2008 Optical Society of America