Plasticity Of Glasses Research Articles

Plasticity of borosilicate glasses under uniaxial tension

AbstractThe mechanical response of multicomponent borosilicate glasses has drawn significant attention in the design of damage‐resistant glasses. In this work, we investigate the plasticity of two borosilicate glasses, Borofloat®33 (Boro33) and N‐BK7®, by implementing a uniaxial tension test using molecular dynamics simulations. A bond‐switching mechanism is found to be responsible for the plastic response of both glasses and is governed by the increasing rate of non‐bridging oxygen (NBO) production during the uniaxial tension. We found that the amount of B4OSi4 linkages in the glass governs the stress drop after yielding, due to its higher tendency to create NBOs compared to Si4OSi4. Also, the initial existence of NBOs weakens the critical stress for breaking the B4‐O bond in B4OSi4, which in turn lowers the yield strength of the glass. The local atomic constraints are analyzed in the two glasses, and high anti‐correlation between the concentration of rigid constraints and plastic deformation is observed.

Room temperature activated slow β relaxation and large compressive plasticity in a LaCe-based bulk metallic glass

Activation of the β relaxation of metallic glasses is usually related to the activation of the structure units which correlates with the activation of shear transformation zones. In the present work, a pronounced slow β relaxation behavior was observed around ambient temperature in (La25Ce75)65Al11Co24 metallic glass. Based on the compression tests at 223 K and 300 K, the results demonstrated that compression achieves large compressive plasticity with true plastic strain over 160% at room temperature, while the compressive plastic strain less than 1% at 223 K. From the perspective of structural heterogeneity, the (La25Ce75)65Al11Co24 metallic glass provides a model alloy to probe the mechanisms of deformation at room temperature induced shear transition behaviors.

Study on stochastic nature of plasticity of Cu/Zr metallic glass micropillars

Abstract Micro-sized metallic glass pillars with a diameter ranging 2.5∼7.5 μm were subjected to compression experiments using a nanoindentation tester equipped with a flat punch, to characterize their deformation behaviors. The strain-stress response and scattering mechanical properties were studied in statistics. The intermittent and trigger-aftershock shear avalanches exhibited a stochastic nature of plasticity of metallic glass specimens under microscale. The size distribution of strain bursts versus the stress where they occurred indicated the flow dynamics as a self-organized behavior with collective motion of multiple shear bands, and the maximum burst size increased with applied stress following in a power-law manner independent of burst order and pillar size. According to the experimental observation, universal features were proposed to characterize the current jerky deformation, meanwhile a Monte Carlo simulation method was developed to predict the stochastic plasticity of metallic glass under microscale.

Size dependent hidden serration behaviors of shear banding in metallic glass thin films

Serration usually observed in metallic glass (MG) is attributed to the shear band propagation and reflects the heterogeneous deformation in a large-length scale. However, the knowledge concerned small length scale heterogeneous deformation intrinsically related to shear transformation zones (STZ) in metallic glass is inadequate, for the difficulty to probe the variable in deformation process on small spatial scales. In this study, we investigated the small length scale hidden serration upon nanoindentation on metallic glass film. The hidden serration was attributed to STZs evolution, instead of shear band propagation. More heterogeneous structure which contains more STZs with larger volume, getting more possibilities to be activated and percolated, result in small-scale serration in the slope curve of load-displacement. The hidden serration patterns established a correlation between load-displacement curve and shear banding process down to microscopic level. It offers a deep understanding of heterogeneous structures dependent plasticity in metallic glasses.

Aspect ratio effects on the serrated flow dynamic of TiZrHfCuNiBe high entropy metallic glass

The effects of aspect ratio on serrated flow dynamic and plasticity of TiZrHfCuNiBe high entropy bulk metallic glass were investigated. With increasing of the aspect ratio, the stress drop time decreases and a ductile-to-brittle transition was observed, which indicates the motion of shear bands will berestricted in the specimens with smaller aspect ratio. Thus, the shear band velocity can be estimated based on the stick-slip dynamics, which enhances with increasing of the aspect ratio, while the critical shear band velocity is almost identical. Therefore, the specimens with larger aspect ratio will achieve the critical shear band velocity faster until fracture. When the aspect ratio is relatively small, the value of shear-band stability index is larger and promotes stable propagation of the shear bands. In addition, the plasticity and serration size are not monotonic correlated. Benefitting from the abundant secondary shear bands and the interactions at various scales, the plastic deformation can be regarded as the self-organized critical state for samples with better plasticity and lower aspect ratio. However, the self-organized critical state will switch to a chaotic state with increasing of the aspect ratio.

Enhancing strength and plasticity by pre-introduced indent-notches in Zr36Cu64 metallic glass: A molecular dynamics simulation study

The deformation behavior in Zr36Cu64 metallic glasses with pre-introduced indent-notches has been studied by molecular dynamics simulation at the atomic scale. The indent-notches can trigger the formation of densely-packed clusters composed of solid-like atoms in the indent-notch affected zone. These densely-packed clusters are highly resistant to the nucleation of shear bands. Hence, there is more tendency for the shear bands to nucleate outside the indent-notch affected zone, which enlarges the deformation region and enhances both the strengthening effect and the plastic deformation ability. For indent-notched MGs, when determining the initial yielding level, there is a competition process occurring between the densely-packed clusters leading to the shear band formation outside the indent-notch affected zone and the stress-concentration localizing deformation around the notch roots. When the indent-notch depth is small, the stress-concentration around the notch root plays a dominant role, leading to the shear bands initiating from the notch root, reminiscence of the cut-notches. As the indent-notch depth increases, there are many densely-packed clusters with high resistance to deformation in the indent-notch affected zone, leading to the shear band formation from the interface between the indent-notch affected zone and the matrix. Current research findings provide a feasible means for improving the strength and the plasticity of metallic glasses at room temperature.

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.

Local elasticity and macroscopic plasticity in homogeneous and heterogeneous bulk metallic glasses

To understand why heterogeneity leads to improved ductility in bulk metallic glasses (BMGs), we derived the local elastic moduli and energy barriers for the activation of shear transformation zones (STZs) in homogeneous and heterogeneous BMGs using high-energy x-ray diffraction. In contrast to the homogeneous glass, STZ activation dynamics in the heterogeneous BMG are spatially nonuniform and the activation of STZs with low energy barriers is favored. Using qualitative arguments, we propose that there is an alternative deformation pathway involving multiple shear bands, which makes the material plastically deformable.

Serrated Behaviors and Plasticity of Nb-Alloyed Cu-Based Bulk Metallic Glasses

This paper is to disclose the plastic deformation mechanism of 3 Nb-alloyed bulk metallic glasses and composites via the analysis of serration dynamics. The as-cast alloys with 1.0 at% and 2.0 at% of Nb element display the large time window of serrated events, implying that the serrated behavior can dynamically retain in a long-time scale and thus the alloys are endowed the largest compressive plasticity. The statistic results suggest that the serrated flow of both alloys with 1.0 at% and 2.0 at% Nb element achieves the self-organized critical state. The normal probability plots were utilized to further evaluate serrated dynamics and demonstrate that large deviations from the mean value of stress drops facilitate the self-organized critical state of serrations. This work suggests that large plastic bulk metallic glasses and composites are characterized by non-normal probability of serration statistics during plastic deformation.

Effect of Different Cooling Rates in High Rheological Rate Forming Process on Mechanical Properties of Zr57Cu20Al10Ni8Ag5 Bulk Metallic Glass

The influence of cooling rates on the mechanical properties of a Zr-based bulk metallic glass prepared with high rheological rate forming (HRRF) was investigated and compared with traditional suction cast methods. Amorphous samples of Zr57Cu20Ni8Al10Ag5 were prepared in copper molds with different sizes in order to obtain different cooling rates for both HRRF and traditional cast methods. These specimens were subjected to compression experiments, including microhardness testing, X-ray diffraction testing and differential scanning calorimetry analysis. The results indicate that the plasticity of the samples formed by HRRF are higher than that of the as-cast ones at the same cooling rates, while the microhardness manifests the opposite principle. As the cooling rate increases further, the difference in plasticity further increases between two methods, indicating that the plasticity of metallic glasses is more sensitive to cooling rates during the HRRF process. At the core of this phenomenon is the fact that HRRF methods can introduce more free volume into glasses than traditional cast methods with an elevated cooling rate are able to.

Improvement of intrinsic plasticity and strength of Zr55Cu30Ni5Al10 metallic glass by tuning the glass transition temperature

We report the tuning of the glass transition temperature (Tg) in Zr55Cu30Ni5Al10 metallic glass by rolling at room temperature. The precise thermo-analytical measurements using step-scan modulated temperature differential scanning calorimeter (MTDSC) revealed a decrease of Tg by 16.9 K in ribbons and 7.1 K in bulk glassy plates upon 40% and 70% thickness reduction, respectively. The cold rolled plates exhibit higher yield strength up to σy = 1.66 GPa and larger plastic strain (εp = 8.0%) before failure than that of as-cast plates (σy = 1.45 GPa). Transmission electron microscopy and DSC studies suggest that a relaxed structure has evolved due to the deformation induced structural change upon rolling, which reduces the activation energy of the shear transformation zone and improves the inherent plasticity of the glassy phase.

Severe Plastic Deformation of Amorphous Alloys

Bulk metallic glasses having a disordered amorphous structure have been in the focus of recent intensive materials research due to their special mechanical properties, however, these materials exhibit brittleness in conventional unconstrained deformation modes. High-pressure torsion as a special severe plastic deformation method, which applies constraints on the material, can induce significant plasticity in metallic glasses. Apart, the deformation can promote structural changes in the glass, such as anisotropy and nanocrystallization. The inhomogeneity generated by torsional shear deformation in bulk metallic glasses can be detected in internal surfaces. The final structure and morphology of the deformed material depend on the processing parameters (deformation rate, shear strain, temperature and pressure) of the high-pressure torsion apparatus.

Selective Laser Melting (SLM) of in-situ beta phase reinforced Ti/Zr-based bulk metallic glass matrix composite

In-situ ductile dendrite reinforcement is the most effective method to improve the plasticity of bulk metallic glasses. In this study, an in-situ beta phase reinforced Ti/Zr-based bulk metallic glass matrix composite (BMGC), (Ti0.65Zr0.35)90Cu10, was deposited via selective laser melting. Its microstructural characteristics and indentation deformation behaviors were investigated. There were fine and coarse microstructures in the BMGC, due to the existence of heat-affected zone, and they were finer than the microstructures of the BMGCs prepared using traditional technologies; the amorphous and beta phases could be coordinately deformed during the indentation process.

Significantly improved plasticity of bulk metallic glasses by introducing quasicrystal within high energy glass matrix

Many studies consistently report that quasicrystal-BMG (bulk metallic glass) composites usually exhibit limited plasticity. In this work, however, we demonstrate that quasicrystal embedded in high energy glass matrix could remarkably improve the plasticity of BMGs. By the method, a Zr-based quasicrystal-BMG composite successfully exhibits >50.0% compressive plasticity and >6.6% bending strain, which is much higher than ∼3.0% compressive plasticity in the monolithic as-cast BMG. After introducing quasicrystal within high energy glass matrix, the lower formation energy and difficulty to propagate of shear bands contribute to the enhanced plasticity. This work provides a new strategy to overcome the brittleness of BMGs and greatly promotes the practical value of quasicrystal in improving plasticity of BMGs.

Investigation of shear transformation zone and ductility of Zr-based bulk metallic glass after plasma-assisted hydrogenation

The shear transformation zone (STZ) and ductility of Zr-based bulk metallic glass with varying hydrogen content were analyzed using serrated flow in nanoindentation, with each serration representing the formation and expansion of one or more shear bands. An exponential model was employed to extract the serrations and obtain a certain range of shear stress, and STZ size was estimated via thorough statistical data analysis. The experimental results demonstrated a striking decrease in the serration number and width of the load-displacement curve after hydrogenation, which is similar to the effects of the nanoindentation loading rate. The calculated STZ volume and atomic number were in good agreement with those reported in the literature. The STZ volume, size, activation energy, and activation volume also increased with the increased hydrogen content, leading to shear band formation and proliferation as well as increased shear band density, plastic zone, and plasticity. At the microscopic level, the hydrogen addition accelerated free volume formation and enhanced flow ability between the atoms, as characterized by the improvement of plasticity. At the macroscopic level, hydrogen addition promoted shear band nucleation and multiplication and increased shear band density, plastic zones, and plasticity. The metallic glass plasticity was greatly improved after hydrogenation as assessed via compression tests at room temperature, confirming the rationale of the experimental results.

Rejuvenation, embryonic shear bands and improved tensile plasticity of metallic glasses by nanosecond laser shock wave

The structural and mechanical responses of Ti-based and Zr-based metallic glasses (MGs) treated by the nanosecond laser shock wave were investigated. A rejuvenation transition was observed, due to the structural rearrangement of neighboring atoms in the instantaneous shock process. Plenty of embryonic shear bands occurred along the shock-affected zone. A damped model of the shock pressure was established to describe the characteristic of the embryonic shear bands. The results showed that the shear band initiation in the nanosecond scale along the length direction was divided into two stages: the initial low propagation stage and the following fast propagation stage, because of the reduction of the shock pressure. The hardness of the shock-affected zone displayed a maximal decrease of 25% in the critical zone between the two propagation stages, due to the combined effect of the structural rejuvenation and the compressive residual stress. A schematic of the energy landscape was proposed to explain the relationship between the rejuvenation and the formation of the embryonic shear bands. The maximum tensile plasticity increased by 0.46%. The results demonstrated that the MGs possessed different shear band propagation modes in the initiation stages, which deepened the understanding of the deformation mechanism in the MGs.

Modulating heterogeneity and plasticity in bulk metallic glasses: Role of interfaces on shear banding

Structural heterogeneities in monolithic metallic glasses, i.e. free volume variations or stress/strain fields, are known to be pivotal for their plasticity but the mechanisms how they affect irreversible deformation remain poorly understood. We show that flash-annealing is a capable tool for modifying the free volume content and its distribution, which creates non-affine stress fields in a monolithic CuZr-based bulk metallic glass. Rather than the overall free volume, μm-scale regions of relaxed and rejuvenated glass govern plastic deformation. Complementary molecular dynamics simulations underpin the importance of the interfaces between relaxed and rejuvenated volumes for shear band proliferation. Not only can we reach unmatched states of metastability in metallic glasses with the present approach but also better understand the mechanisms underlying plasticity in monolithic but structurally heterogeneous metallic glasses.

Atom Delocalization and Fluctuation Hole Formation in Amorphous Organic Polymers and Inorganic Glasses

A viewpoint is developed that formation of a hole in amorphous substances corresponds to the atom delocalization process, i.e., its fluctuation limiting shift from the temporary equilibrium state. The molecular-kinetic processes proceeding in liquids and amorphous rigid solids do not depend on the classical van der Waals free volume, “void statistical space between atoms,” but mainly depend on the fluctuation dynamic free volume, which coincides with the fluctuation free volume caused by the atom delocalization. The atom delocalization process (local configuration structural change) plays the key role in the viscous flow of glass-forming liquids. The softening of glasses (at Tg) and their frozen reversible deformation (at 20°C) are characterized by the same molecular mechanism, namely, the atom delocalization. From the same positions as noted above, the effect of plasticity of inorganic glasses and amorphous organic polymers was considered.

Improving plasticity of metallic glass by electropulsing-assisted surface severe plastic deformation

Using either electropulsing (EP) or surface severe plastic deformation (SSPD) to process metallic glasses can improve their plasticity, however, the moderate improvement in plasticity does not warrant a commercial usage. This work, for the first time, demonstrates the integration of electropulsing and surface severe plastic deformation is much more effective in improving the plasticity of metallic glasses than EP or SSPD treatment alone, opening a new avenue towards an unprecedented combination of strength and plasticity in metallic glasses. It is found that SSPD can generate microstructure heterogeneity featured by a mixture of matrix and plastically displaced regions with increased atomic volume. When applying EP and SSPD simultaneously, a synergistic effect occurs to produce a hybrid network with excess free volume and nanocrystals uniformly embedded in amorphous matrix. Molecular dynamics simulation and fracture surface analysis further reveal that the hybrid network is able to effectively reduce shear band localization, and therefore delay the fracture of metallic glasses.