Flow Of Glass Research Articles

From elastic excitations to macroscopic plasticity in metallic glasses

Shear transformation zone (STZ) remains the fundamental unit to explain plastic flow in metallic glasses (MGs). Although STZs have been known and characterized for decades now the morphological and dynamical characteristics of STZs are not fully understood yet. Here, simulating athermal quasistatic shearing processes, the atomic-level mechanisms underlying elastic and plastic deformation in MGs are disclosed. Given the highly heterogeneous nature of glassy materials and the related rugged energy landscape the activation of STZs is observed from an early level of elastic deformation until the final shearing stage. The STZs identified within the elastic and plastic ranges have similar fingerprints but different magnitude and dynamics. Long-range overlapping STZ fields constrain atomic rearrangements and limit quantifying structure-dynamics correlations that explains the difficulty in establishing an ideal structure-property relationships for MGs.

Size-temperature equivalence in tensile deformation of metallic glass

Abstract Tensile fracture of irradiation-free metallic glass specimens with diameters ranging from 100 nm to 500 μm is investigated at different temperatures. A gradual change in fracture morphology from vein-pattern to completely smooth fracture surface to necking is observed with decreasing sample size and testing temperature. The size-temperature equivalence in the entire length scale can be described by considering the thermal effects in shear localization of metallic glasses. We construct an empirical model based on the shear band heating and velocity formulations to qualitatively describe the size and temperature effects on the fracture morphology. Our results suggest that the widely reported size-dependent transition from shear-localized to homogeneous flow in metallic glasses is fundamentally different from the high temperature homogeneous viscous flow. The plastic deformation in nanoscale samples is spatially localized in embryonic shear bands, which never mature to the propagation stage due to lack of heat content.

Temperature dependence of serrated flow, strain-rate sensitivity and activation volume in a Ti-based bulk metallic glass

In this study, the influence of temperature on serrated flow behavior, strain rate sensitivity (SRS) and effective activation volume in a Ti-based bulk metallic glass (BMG) bent at different strain rates has been investigated. Results reveal that the fundamental origin of shear deformation is related to the time-dependent structural relaxations, which occur during deformation at special temperatures and strain rates. Furthermore, the localized structural relaxation processes at intermediate temperatures cause an inelastic shuffling within cages and mitigate the activation of STZs. The main reason of negative SRS at intermediate temperatures is due to the insufficient time for structural relaxation at high strain rates, which leads to a larger free volume concentration within shear bands and makes the BMG softer. It is concluded that the activation volume increases with temperature in the inhomogeneous deformation region, whilst its value at high temperatures, where the deformation regime is homogeneous, decreases with temperature rise.

Anelastic and viscoplastic deformation in a Fe-based metallic glass

The micro-creep behavior of the Fe66Tb5B23Nb6 metallic glass and its anelastic and viscoplastic deformation mechanism were investigated by using nanoindentation. The Maxwell-Voigt model with one or two Kelvin units were utilized to describe three stages of creep deformations at different loading rates and temperatures during the constant load holding process. The creep displacement of the indenter into the Fe66Tb5B23Nb6 sample at a constant load obeys the classic relaxation kinetics at loading rates ranging from 5 to 100 mN s−1. The activation energy for the relaxation process increases with increasing loading rate, which leads to the pronounced relaxation with longer characteristic times under a low-velocity impact mode observed at room temperature. On the other hand, the relaxation process with a short characteristic time plays a dominant role in determining viscoplastic flow and softening behavior at the temperature approaching the glass transition temperature. This work reveals the important role of the anelasticity on the plastic flow of metallic glasses under dynamic loading conditions.

Beyond Serrated Flow in Bulk Metallic Glasses: What Comes Next?

This manuscript is based on an oral contribution to the TMS 2020 annual meeting and is dedicated to Prof. Peter Liaw, who for decades has shown great interest in serrated plastic flow. Here we will focus on the case of bulk metallic glasses, and begin with briefly summarizing some aspects of serrated and non-serrated inhomogeneous flow—a phenomenon that has perplexed materials scientists for decades. Four directions of research are identified that emerged out of the desire to fundamentally understand the intermittent inhomogeneous flow response. These research directions gear away from the phenomenological stress–strain behavior but put the underlying shear defect into focus. Unsolved problems and future research topics are discussed.

Mechanisms of two-step yielding in attractive colloidal glasses

A combination of experiments and Brownian Dynamics simulations is utilized to examine the mechanisms of yielding and flow in attractive colloidal glasses during start-up shear flow. In both experiments and simulations, the transient stress exhibits two stress peaks indicative of two-step yielding processes. The first yield depends largely on details of interparticle potential whereas the second yield is independent of the potential and takes place at strain (≃20%), at which a purely repulsive glass yields. The stress decomposition into repulsive (hard sphere, HS) and attractive contributions reveals that there are strong contributions of both types of stresses into the first stress peak whereas the second stress peak is mainly linked with HS stresses. The transient stress during start-up shear originates from the change in the averaged pair orientation. At the first stress peak, bonded particles (causing attractive stresses) show the maximum orientation along the extension axis with colliding particles (causing HS stresses) being locally oriented along the compression axis. However, at the second stress peak, collided particles show the maximum orientation along the compression axis with particles escaping their cages along the extension axis similar to a HS glass. Analysis of particle dynamics shows that yielding takes place through a two-step shear-activated hopping process in which first shear flow takes particles out of their attractive constraints. The length scale associated to this process is at the order of attraction range (bond length). Subsequently, cage escape of particles sets the second process which leads to a complete yielding and flow.

Ultrasonic-assisted plastic flow in a Zr-based metallic glass

Ultrasonic vibration can be used for the micro-molding of metallic glasses (MGs) due to stress-softening and fast surface-diffusion effects. However, the structural rearrangement under ultrasonic vibration and its impact on the mechanical response of metallic glasses remain a puzzle. In this work, the plastic flow of the Zr35Ti30Cu8.25Be26.75 metallic glass with the applied ultrasonic-vibration energy of 140 J was investigated by nanoindentation. Both Kelvin and Maxwell-Voigt models have been adopted to analyze the structural evolution during the creep deformation. The increase of the characteristic relaxation time and the peak intensity of relaxation spectra can be found in the sample after ultrasonic vibration. It effectively improves the activation energy of atomic diffusion during the glass transition ( E g) and the growth of the crystal nucleus ( E p). A more homogenous plastic deformation with a weak loading-rate sensitivity of stress exponent is observed in the ultrasonic-vibrated sample, which coincides with the low pile-up and penetration depth as shown in the cross profile of indents. The structural rearrangement under resonance actuation demonstrated in this work might help us better understand the defect-activation mechanism for the plastic flow of amorphous systems.

Molecular dynamics simulation on structure evolution of silica glass in nano-cutting at high temperature

ABSTRACT Machining of silica glass is indeed a challenge due to its extreme brittleness and hardness. In recent years, laser-assisted machining (LAM) is becoming a powerful method for cutting silica glass. However, as a typical non-crystalline structure, the deformation mechanism of silica glass in the cutting process at high temperature has not been explored clearly. In this paper, classical molecular dynamics simulation was conducted to investigate the structure evolution of silica glass during the nano-cutting process at high temperature. Firstly, a uniaxial tension test was carried to investigate the brittle-to-ductile transition with increasing temperature. Then, the cutting simulation was conducted at 300 and 1500 K. The results showed that the plastic deformation is promoted at 1500 K. The atomic flow of silica glass is different from crystal material during the cutting process. No rotational flow and upward motion of atoms are observed. Moreover, microstructure evolution like bond-switch and rings distribution was discussed in detail. Furthermore, although the densification scope is almost unaffected, the extent of densification near the machined surface is greatly decreased at a high temperature. These results contribute to the theoretical research on the precision machining of silica glass with LAM.

Chaotic dynamics in shear-band-mediated plasticity of metallic glasses

Metallic glasses response to the mechanical stress in a complex and inhomogeneous manner with plastic strain highly localized into nanoscale shear bands. Contrary to the well-defined deformation mechanism in crystalline solids, understanding the mechanical response mechanism and its intrinsic correlation with the macroscopical plasticity in metallic glasses remains long-standing issues. Through a combination of experimental and theoretical analysis, we showed that the shear banding process in metallic glasses exhibits complex chaotic dynamics, which manifests as the existence of a torus destroyed phase diagram, a positive Lyapunov exponent and a fractional Lyapunov dimension. We also demonstrated that the experimentally observed large plasticity fluctuation of metallic glasses tested at the same conditions can be interpreted from the chaotic shear-band dynamics, which could leads to an uncertainty on the appearance of the critical condition for runaway shear banding. Physically, the chaotic shear-band dynamics arises from the interplay between structural disordering and temperature rise within the shear band. By tuning the deformation parameters, the chaotic dynamics can be transformed to a periodic orbit state corresponding to a smaller plasticity fluctuation in metallic glasses. Our results suggest that the plastic flow of metallic glasses is a complex dynamic process, which is highly sensitive to initial conditions and reminiscent of the "butterfly effect" as observed in many complex dynamic systems.

Origin of the Bauschinger Effect in Amorphous Solids.

We study the structural origin of the Bauschinger effect by accessing numerically the local plastic thresholds in the steady state flow of a two-dimensional model glass under athermal quasistatic deformation. More specifically, we compute the local residual strength, Δτ^{c}, for arbitrary loading orientations and find that plastic deformation generically induces material polarization, i.e., a forward-backward asymmetry in the Δτ^{c} distribution. In steady plastic flow, local packings are on average closer to forward (rather than backward) instabilities, due to the stress-induced bias of barriers. However, presumably due to mechanical noise, a significant fraction of zones lie close to reverse (backward) yielding, as the distribution of Δτ^{c} for reverse shearing extends quasilinearly down to zero local residual strength. By constructing an elementary model of the early plastic response, we then show that unloading causes reverse plasticity of a growing amplitude, i.e., reverse softening, while it shifts away forward-yielding barriers. This result in an inversion of polarization in the low-Δτ^{c} region and, consequently, in the Bauschinger effect. This scenario is quite generic, which explains the pervasiveness of the effect.

Investigation on low-temperature reactive viscous flow sintering behavior of lanthanum-borate glass-ceramic with BaTi4O9 ceramic filler

In this paper, the sintering behavior of the lanthanum-borate (B2O3-La2O3-MgO-TiO2,BLMT) glass-ceramic with BaTi4O9 filler composite was investigated in terms of the wetting behavior, interfacial reaction, sinter-shrinkage process, sintering activation energy, as well as phase and microstructure evolution with change in filler contents and sintering temperature. Our research suggested that the glass is unable to wet the filler material within a temperature up to 1000 °C, indicating that the densification process of composites is dominated by viscous flow of glass matrix. The increase in filler content that performs as a rigid particle in composite causes the rise in the sinter-shrinkage-onset and -end temperature, thereby proving that the viscous-flow densification of the composites with x≤ 30 wt% filler content is accomplished before the crystallization of BLMT glass, whereas the composites with x≥ 40 wt% cannot. After densification, a chemical reaction that almost synchronizes with the glass crystallization occurred between glass and ceramic, which not only imposes significant influence on the crystallization behavior, but eradicates the closed pore formed by the viscous flow and the induced crystallization porosity. The densification process of the BaTi4O9 filler-BLMT glass composite was referred to as two-stage reactive assisted viscous flow sintering process which consists of viscous flow of glass, of the crystallization process of glass, and/or of the chemical reaction between glass and filler.

Pinching a glass reveals key properties of its soft spots

It is now well established that glasses feature quasilocalized nonphononic excitations-coined "soft spots"-, which follow a universal [Formula: see text] density of states in the limit of low frequencies ω. All glass-specific properties, such as the dependence on the preparation protocol or composition, are encapsulated in the nonuniversal prefactor of the universal [Formula: see text] law. The prefactor, however, is a composite quantity that incorporates information both about the number of quasilocalized nonphononic excitations and their characteristic stiffness, in an apparently inseparable manner. We show that by pinching a glass-i.e., by probing its response to force dipoles-one can disentangle and independently extract these two fundamental pieces of physical information. This analysis reveals that the number of quasilocalized nonphononic excitations follows a Boltzmann-like law in terms of the parent temperature from which the glass is quenched. The latter, sometimes termed the fictive (or effective) temperature, plays important roles in nonequilibrium thermodynamic approaches to the relaxation, flow, and deformation of glasses. The analysis also shows that the characteristic stiffness of quasilocalized nonphononic excitations can be related to their characteristic size, a long sought-for length scale. These results show that important physical information, which is relevant for various key questions in glass physics, can be obtained through pinching a glass.

PbO-SiO2 Based Glass Coating of PbI2 Doped PbTe

Thermoelectrics is one promising way of increasing the efficiency of machines and devices by reusing some of the waste heat produced. One obstacle for commercialization is the need to coat the materials to prevent sublimation and oxidation of the thermoelectric materials. Such coatings were designed for PbI2 doped PbTe using a (SiO2)0.68(PbO)0.3(B2O3)0.01(Na2O)0.01 based glass designed for operation at 500 °C. In this research various conditions of the coating process were examined. The effect of the atmosphere on the bonding and densification of the coating was studied using argon, vacuum and air. From the three air shows, the best bonding characteristics were from a better flow of glass and increased bonding between the oxidized PbTe layer and glass. This also created a PbO rich glass in the interface between the glass and the PbTe sample. The effect of 0, 3, and 6 wt. % NaCl additive to the solution was tested and showed that NaCl achieves better coverage due to high green body density, reaction of NaCl with the glass and removal of remaining CO2 from the glass in the form of decomposing Na2CO3. In addition, when testing the time and temperature, it was shown that the temperature of 520 °C was the minimum needed for high densification of the glass, but a duration shorter than 30 min did not allow for bonding of the glass to the substrate despite adequate densification. Finely, to obtain a well bonded coating with full coverage over the sample, the glass was coated with 6% NaCl in air at 520 °C for 30 min.

Quantifying configuration-entropy change during plastic flow of metallic glasses

Based on the energy balance principle for a serrated shear event, the configuration entropy change of a plastic flow within a shear band in metallic glasses is precisely quantified. With this method, we successfully investigated the evolution of entropy change with the plastic displacement/strain. It was found that the evolution of entropy changes for various metallic glasses follows a unified linear relation. The experimental result is consistent with the thermodynamic deduction from the free volume theory, corroborating the nature of shear-induced dilation for the plastic flow of metallic glasses.

Optimum shear stability at intermittent-to-smooth transition of plastic flow in metallic glasses at cryogenic temperatures

The runaway instability of shear bands leads to catastrophic failure of metallic glasses while the link between the shear banding process and the macroscopic plasticity in a quantitative manner in metallic glasses remains a major challenge.Through a series of compression tests at cryogenic temperatures, we found that the plasticity of the metallic glass can attain a maximum value at a critical temperature at which the transition from serrated flow to non-serrated flow occurs on a Zr-based metallic glass at cryogenic temperatures. The transition point corresponds to the lowest shear-band velocity and the most stable state of plastic shearing during deformation. The results provide an insight for understanding the temperature-dependent plasticity of metallic glasses from shear-band dynamics and may help to design the plasticity/ductility of metallic glasses at cryogenic temperatures.

Optimum Shear Stability at Intermittent-to-Smooth Transition of Plastic Flow in Metallic Glasses at Cryogenic Temperatures

The runaway instability of shear bands leads to catastrophic failure of metallic glasses while the links between the shear banding process and the macroscopic plasticity in a quantitative manner in metallic glasses remains a major challenge.Through a series of compression tests at cryogenic temperatures, we found that the plasticity of the metallic glass can attain a maximum value at a critical temperature at which the transition from serrated flow to non-serrated flow occurs on a Zr-based metallic glass at cryogenic temperatures. The transition point corresponds to the lowest shear-band velocity and the most stable state of plastic shearing during deformation. The results provides an insight for understanding the temperature-dependent plasticity of metallic glasses from shear-band dynamics and may help to design the plasticity/ductility of metallic glasses at cryogenic temperatures.

From critical behavior to catastrophic runaways: comparing sheared granular materials with bulk metallic glasses

The flow of granular materials and metallic glasses is governed by strongly correlated, avalanche-like deformation. Recent comparisons focused on the scaling regimes of the small avalanches, where strong similarities were found in the two systems. Here, we investigate the regime of large avalanches by computing the temporal profile or “shape” of each one, i.e., the time derivative of the stress-time series during each avalanche. We then compare the experimental statistics and dynamics of these shapes in granular media and bulk metallic glasses. We complement the experiments with a mean-field model that predicts a critical size beyond which avalanches turn into large runaway events. We find that this transition is reflected in a characteristic change of the peak width of the avalanche profile from broad to narrow, and we introduce a new metric for characterizing this dynamic change. The comparison of the two systems points to the same deformation mechanism in both metallic glasses and granular materials.

Avalanche statistics and the intermittent-to-smooth transition in microplasticity

Plastic flow at small scales is generally observed to be intermittent, whereas the stress-strain behavior of bulk crystals is mostly smooth. Here we find that when the external deformation rate of small-scale crystals approaches the speed of the crystallographic slip velocity, an intermittent-to-smooth transition of plastic flow is observed. By defining a rate-dependent intermittency parameter, this phenomenon can be captured with a power law covering 5.5 orders of magnitude for Au and Nb micron-sized single crystals with experiments and via simulations for Nb crystals. Our results indicate that the transition to smooth flow is driven by a gradual truncation of the underlying truncated power law that describes the intermittently evolving system. This is caused by a competition of internal and external rates, which aligns with the well-known transitions from serrated to nonserrated flow in metallic glasses or materials with dynamic strain aging.

Rheological signatures of aging in hard sphere colloidal glasses

Colloidal glasses are out-of-equilibrium in nature. When such materials are quenched from a shear-melted state into a quiescent one, their structure freezes due to entropic caging of the constituents. However, thermal fluctuations allow slow structural evolution, a process known as aging, in favor of minimizing free energy. Here, we examine the rheological signatures of aging, in a model system of nearly hard sphere colloidal glass. Subtle changes in the linear viscoelastic properties are detected with the age of the colloidal glass where viscous modulus shows a decrease with aging whereas the elastic modulus remains unaffected. This is associated with the slowing-down of long-time out-of-cage dynamics as the glass ages. On the contrary, nonlinear rheological measurements such as start-up shear flow, stress relaxation, and creep experiments show a strong dependence on sample age. Moreover, creep and stress relaxation experiments show ample evidence of avalanche type processes that occur during aging of colloidal glasses. Finally, comparison of creep and start-up shear flow measurements indicate that the latter is more energy efficient in inducing flow in colloidal glasses irrespective of aging dynamics.

Thermoplastic bonding (TPB) of TiAl- and Ni-based alloys with Zr-Al-Ni-Cu bulk metallic glass

The joining of dissimilar materials such as TiAl- and Ni-based alloys has always been a challenging work due to the larger differences in properties. In this work, a new strategy called thermoplastic bonding (TPB) was proposed and successfully applied in joining of the two alloys. Through utilizing superplastic flow of bulk metallic glasses (BMGs), a good mechanical bonding with no cracks between the base metals and filler was achieved, and after holding a time at a higher temperature lower than the melting point of the filler, the alloys were metallurgical bonded through atomic diffusion. The results showed that the phases in the joint bonded by TPB were very fine (approximately 300 nm) and uniformly distributed at the grain boundaries due to the low temperature and concomitant sluggish diffusion effect, while a series of directional intermetallics detrimental to the joint strength formed in the joint bonded by vacuum brazing. Tension tests showed that the strength of the TPB bonded joint was as high as 415 MPa, while that of the vacuum brazed joint was only approximately 315 MPa. These results verified that this new joining method can obtain a liquidus bonding effect equivalent to that obtained by a solid diffusion method.