Plasticity Of Metallic Glasses Research Articles

Hardened core of bilayer shear bands in a Zr-based metallic glass

Bilayer shear bands with a spacing of hundreds of nanometers were prepared in a Zr-based metallic glass. Elastic properties of the bands were detected using a contact-resonance atomic force microscope. Compare with an elastic modulus reduction in each single shear band, the core of the bilayer shear bands is hardened. One possible reason for the forming of the bilayer shear bands is the load releasing and position adjustment during the severe plastic deformation process. Our results reveal a new plastic deformation mechanism of the shear bands in the form of layer by layer, and provide a promising strategy to control the stability of shear bands and enhance the plasticity of metallic glasses.

A new continuum model for viscoplasticity in metallic glasses based on thermodynamics and its application to creep tests

In this study, we developed a new continuum model for the time-dependent plasticity of metallic glasses based on the laws of thermodynamics. In this model, the free energy of a material point is formulated as not only the local strain state but also the local atomic concentration (or the free volume). Atomic kinetics, controlled by the gradient of the chemical potential, is linked directly with the plastic deformation by a new plastic flow rule. Finite element implementation of our model is validated through the classical uniaxial tension and simple shear tests in which the shear band instability and shear-dilatation phenomena are reproduced. Applied to the creep of Cu-Zr metallic glass, our model captures the transition of two distinct power laws for the creep strain rate vs. applied stress relations around a critical stress. These two power laws, below and above the critical stress, correspond to two different diffusion mechanisms activated by the thermal energy gradient and strain energy gradient, respectively. At last, we carry out a linear perturbation analysis to explain the origin of the critical stress as well as its dependence on the sample size.

Ultrasonic plasticity of metallic glass near room temperature

Bulk metallic glasses (BMGs) are well-known for their superb strength (1–4 GPa) (Ashby and Greer, 2006) [1] but poor/localized plasticity when deformed at low temperatures or high strain rates (Inoue and Takeuchi, 2011; Kumar et al., 2009) [2,3]. Therefore, processing of BMGs, such as forming and shaping for various important applications, is usually performed above their glass transition temperatures (Tg) – also known as “thermo-plastic” forming (Geer, 1995) – for which the selection of alloy composition and the protocol for thermal treatment is demanding in order to promote extensive homogeneous plastic flows while avoiding crystallization (Geer, 1995). In stark contrast, here we demonstrate that homogeneous super-plasticity can occur rapidly in different BMGs below their Tg when subjected to ultrasonic agitations. Through atomistic simulations and nanomechanical characterization, we provide the compelling evidence to show that this super-plasticity is attributed to dynamic heterogeneity and cyclic induced atomic-scale dilations in BMGs, which leads to significant rejuvenation and final collapse of the solid-like amorphous structure, thereby leading to an overall fluid-like behavior. Our finding uncovers that BMGs can undergo substantial plastic flows through unusual liquefaction near room temperature and, more importantly, it leads to the development of a facile and cost-effective “ultrasonic-plastic” forming method to process a wide range of BMGs at low temperatures.

Influence of composition on the mechanical properties of metallic nanoglasses: Insights from molecular dynamics simulation

The introduction of a glass–glass interface is an effective way to improve the plasticity of metallic glass. However, the strength–plasticity trade-off has not still been effectively overcome. Here, the effect of the composition on the mechanical properties and deformation behavior of the CuZr nanoglass (NG) is investigated under tensile loading by a molecular dynamics simulation. The results indicate that high-performance NGs can be obtained by adjusting the percentage of Cu atoms. There is a critical Cu content (i.e., 75%), which makes the NGs have both high strength and high plasticity. The results show that with the increase in the Cu content, the deformation mechanism of the NGs changes from necking to uniform plastic deformation and then to the nucleation and the growth of the main shear band. Our results underscore the importance of the composition in the design and preparation of high-performance metallic glass.

Pressure-induced maximum shear strength and transition from shear banding to uniform plasticity in metallic glass

The effect of pressure on the mechanical and physical properties of metallic glasses (MGs) has been widely reported. In experiments, it remains challenging to mechanically loading the materials under extreme hydrostatic pressures and carrying out the corresponding measurements. Here, we investigate the pressure dependence of shear response of a typical brittle FeP MG by using molecular dynamics (MD) simulations. We show that the plastic deformation of this MG loaded at a pressure below 10 GPa is highly localized in the form of a dominant shear band. The plastic flow stress during shear banding is measured to increase with the applied pressure, leading to a pressure-induced strengthening behavior at pressures in the range of 0–10 GPa. As the pressure is increased to a critical pressure of approximately 15 GPa, there occurs a transition in the deformation mechanism from localized shear banding to a delocalized shear deformation mechanism governed by the uniform activation of individual shearing events in the material. At this critical pressure, both the yield strength and the average flow stress are maximized. Further increasing the pressure lowers the strength of the material. The pressure effect on the evolution of different types of atomic clusters is analyzed, revealing that the pressure-sensitivity of atomic clusters in MGs is related to their structural symmetry. Under high hydrostatic pressures, low-symmetry clusters become sluggish due to their larger pressure sensitivity. In such circumstances, high-symmetry clusters with lower pressure sensitivity are relatively more active during shear deformation and evenly distributed in the whole sample, leading to the uniform plastic deformation at high pressures. Our findings calls for further experimental investigations of the effect of pressure on mechanical properties and deformation mechanisms of MGs.

Mechanical behavior of metallic glasses with pressure-promoted thermal rejuvenation

The pressure-promoted thermal rejuvenation is a promising approach for improving the macroscopic plasticity of metallic glasses (MGs). Here, molecular dynamics simulations have been performed to investigate the atomic structure and mechanical behavior of the rejuvenated MGs with pressure-promoted thermal processing. The effects of annealing temperatures and pressures are investigated, with results suggesting that the pressure has a significant influence on the deformation and failure mechanism of the rejuvenated MGs. The MGs can be rejuvenated either by the application of negative pressures with the low annealing temperature, or by the application of positive pressures with the high annealing temperature. Accompanied by the rejuvenation, a transition in failure mode from localized shear banding to homogeneous plastic deformation occurs due to the higher-energy glassy state induced by the thermal-pressure loading process. The present study provides important insights into the atomic-level structures of the rejuvenated MGs, as well as useful guidelines for the design of strong and high plastic MGs.

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.

Composition dependence of metallic glass plasticity and its prediction from anelastic relaxation – A shear transformation zone analysis

The mechanical relaxation behavior of metallic glasses (MGs) is composition sensitive. For example, in dynamic mechanical analysis, La70Ni15Al15 shows a pronounced secondary peak, termed β relaxation, of the normalized loss modulus at high frequency and/or low temperature, while La70Cu15Al15 only exhibits a shoulder. We have determined relaxation-time spectra at room temperature for these alloys over eleven orders of magnitude of time from quasi-static anelastic relaxation measurements. These are employed to characterize shear transformation zone (STZ) properties in both the α and β regimes. The pronounced β relaxation peak, observed for La70Ni15Al15, is a result of both larger volume fraction of fast and small potential STZs and smaller volume fraction of slow and large potential STZs as compared to La70Cu15Al15. Room-temperature tensile tests show increasing plasticity with decreasing strain rates, characteristic of thermally activated processes. La70Cu15Al15 exhibits greater plasticity than La70Ni15Al15, a negative correlation with the intensity of the β relaxation that is opposite to that previously suggested. The STZ spectra are used to explain the plasticity difference between the two alloys. Plasticity predictions from loss modulus measurements need to be revised, and require absolute measurements.

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.

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.

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.

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.

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.

Enhancing the plasticity of noncrystalline Cu[sbnd]Zr multilayer: Insights from molecular dynamics simulations

Amorphous alloys with pre-fabricated defective structure have been proved to have splendid ductility, making it a prospective second phase for metallic glass (MG) matrix composites. Here, the plastic deformation behavior of amorphous-amorphous (A/A) dual-phase nano-multilayers is successfully simulated by performing molecular dynamics method. The results reveal that the soft phase with structural defects in A/A nano-multilayers plays a key role in improving plasticity of MG. Specifically, when the density of structural defects is small, most of plastic deformation dominated by a single shear band is mainly confined to the flabby soft phase. A suitable density makes the clusters with large atomic shear strain more likely to be activated at the A/A interfaces, thereby linking the shear deformation between the two amorphous phases and promoting a relatively uniform global plasticity. Nevertheless, the free volume gradient caused by excessive density difference between two amorphous phases will create the rapid release of large stress at the A/A interfaces. The results indicate that the introduction of A/A interfaces and soft MG phase is an effective approach for significantly improving the plasticity without damage and expense the strength of MG.

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.

Design strategy for high plasticity and strength in metallic glasses: A molecular dynamics simulation study

The poor plasticity of metallic glasses (MGs) limits their application as structural materials. How to enhance dramatically the plasticity of the MGs without compromising the strength is a highly attractive topic. Here, the effect of layer thicknesses and analogous flow defects concentrations (AFDCs) on the mechanical behavior of amorphous/amorphous (A1/A2) nano-multilayers is investigated by molecular dynamics method. The results indicate, when layer thickness is small (from 1.8 to 8.5 nm), for any given layer thickness, the plastic deformation of A1/A2 nano-multilayer evolves from a “shear localization” fashion to a “uniform deformation” mode, and eventually to a “shear localization” deformation again with the increase of AFDC. Comparing with the monolithic MGs, the A1/A2 nano-multilayers with optimum AFDC manifesting uniform deformation achieve a perfect combination of high strength and superior plasticity. What is more, the optimum AFDC could always be found for the sample with arbitrary layer thickness, and shows an upward tendency with increasing layer thickness. When layer thickness is large (13.0 and 25.0 nm), no matter how the AFDC changes, the optimum AFDC disappears for determined layer thickness. In this case, the plastic deformation behavior is arrested in A1 soft layer, resulting in intense shear localization and the reduced plasticity.