Deformation Behavior Of Metallic Glass Research Articles

Deformation behavior of thermally rejuvenated Zr-Cu-Al-(Ti) bulk metallic glass

The deformation behavior of metallic glasses has been shown in prior studies to be often dependent on its structural state, namely higher energy “rejuvenated” state versus lower energy “relaxed” state. Here, the deformation behavior of thermally rejuvenated Zr-Cu-Al-(Ti) bulk metallic glasses (BMGs) was evaluated. Rejuvenation was achieved by cryogenic thermal cycling with increase of free volume measured in terms of enthalpy of relaxation. Hardness, stiffness, and yield strength of the BMGs were all found to decrease while plasticity increased after rejuvenation. More free volume in the rejuvenated BMG resulted in homogeneous plastic deformation as was evident from the high strain rate sensitivity and more pronounced shear band multiplication during uniaxial compression. Shear transformation zone (STZ) volume was calculated by cooperative shear model and correlated well with the change in structural state after rejuvenation. The enhanced plasticity with the addition of 1 at. % Ti as well as after cryogenic thermal cycling was explained by lower activation energy for shear flow initiation due to increased heterogeneity induced in the system. Molecular dynamics simulation demonstrated that the variation in plastic deformation behavior is correlated with local atomic structure changes.

Effect of Strain Rate on Mechanical Deformation Behavior in CuZr Metallic Glass.

Tensile tests were performed on Cu64Zr36 metallic glass at strain rates of 107/s, 108/s, and 109/s via classical molecular dynamics simulations to explore the underlying mechanism by which strain rate affects deformation behavior. It was found that strain rate has a great impact on the deformation behavior of metallic glass. The higher the strain rate is, the larger the yield strength. We also found that the strain rate changes the atomic structure evolution during deformation, but that the difference in the atomic structure evolution induced by different strain rates is not significant. However, the mechanical response under deformation conditions is found to be significantly different with the change in strain rate. The average von Mises strain of a system in the case of 107/s is much larger than that of 109/s. In contrast, more atoms tend to participate in deformation with increasing strain rate, indicating that the strain localization degree is more significant in cases of lower strain rates. Therefore, increasing the strain rate reduces the degree of deformation heterogeneity, leading to an increase in yield strength. Further analysis shows that the structural features of atomic clusters faded out during deformation as the strain rate increased, benefiting more homogeneous deformation behavior. Our findings provide more useful insights into the deformation mechanisms of metallic glass.

Micromechanical response of an electrodeposited NiP metallic glass by pillar compression under extreme conditions

In this work, the mechanical properties of pulse electrodeposited amorphous nickel phosphorous (NiP), with ∼16 wt% P, was investigated for an improved understanding of the deformation behavior of metallic glasses at extreme conditions, particularly at the microscale. Upon decreasing the temperature to sub-ambient, both the yield strength and the shear band density increase. While the change of yield strength with temperature can be well explained by a cooperative shear model, the increase in the number of shear bands can be related to an increase in potential sites for activation of shear transformation zones due to the enhanced structural heterogeneity at lower temperatures. This, however, hinders the percolation of deformation units and leads to a decrease in the shear band propagation speed, from ∼50 nm/s at 295 K to ∼20 nm/s at 195 K. By testing micropillars across seven orders of magnitude in strain rate, a two-stage strain rate sensitivity is observed, from nearly negligible at the quasi-static range (m1 = 0.001 ± 0.002 for 10−3 s−1 to 10−1 s−1) to highly positive at dynamic high strain rate range (m2 = 0.02 ± 0.004 for 1 s−1 to 1000 s−1). This transition implies a change in the yield-controlling process, from the diffusion-assisted relaxation at quasi-static conditions to the rate of energy barrier crossing of shear transformation zones at high strain rates. These findings highlight several distinctive features of microscale metallic glasses, including the reduced shear band velocity at sub-ambient temperature and the two-stage strain rate sensitivity.

Effects of He ion irradiation on the structural stability and deformation behavior in Zr63.5Cu23Al9Fe4.5 and Fe80B13Si7 metallic glasses

The effects of He2+ irradiation on the structural stability and deformation behavior in Zr63.5Cu23Al9Fe4.5 and Fe80B13Si7 metallic glasses (MGs) were investigated by TEM and nano-indenter. He bubbles in Fe80B13Si7 MG distributed more narrowly along the depth direction, packed more densely and exhibited larger size. At 4 × 1017/cm2 dose, Fe3B nanocrystals generated in Fe80B13Si7 MG caused by enhanced atomic diffusion due to increased excess free volume and larger compressive stress on atoms of bubbles gaps, while Zr63.5Cu23Al9Fe4.5 MG could maintain the amorphous structure. According to serrated flow phenomenon and bubbles distribution, the underlying mechanism of the transformation in deformation behavior of MGs with increasing He dose was analyzed as follows: localized inhomogeneous deformation in as-cast MGs, homogeneous deformation in the rejuvenated stage, localized inhomogeneous deformation in the stage where small bubbles act as weak pinning, homogeneous deformation in the stage where large bubbles act as strong pinning, localized fracture in the stage where bubbles collapse.

Atomistic Study on Microstructure Control and Deformation Behaviors of Metallic Glasses

Mechanical properties of metallic glasses depend on their microstructure. Rejuvenation recently has attracted much attention because it is a potential tool for controlling the microstructure and properties of metallic glasses. Here I explain molecular dynamics studies on microstructure control and deformation behaviors of metallic glasses. From atomistic simulation approaches, I revealed the thermal rejuvenation process in the heterogeneous microstructure of metallic glasses, the formation of nanocrystals in thermal processing for rejuvenation, and the effects of rejuvenation and nanocrystallization on deformation behaviors.

Uncovering the Inherent Size Dependence of Yield Strength and Failure Mechanism in Micron-Sized Metallic Glass.

The sample size effect on the deformation behavior of metallic glasses (MGs) has recently become research of intense interest. An inverse sample size effect is observed in previous experimental studies; where the yield strength decreases with decreasing sample size, rather than increasing. We propose a theoretical analysis based on the shear banding process to rationalize the inherent size dependence of yield strength, showing an excellent agreement with experimental results. Our model reveals that the anomalous inverse size effect is, in fact, caused by a transition in failure mode; from a rapid shear banding process with a shear band (SB) traversing the entire sample in bulk MGs, to an immature shear banding process with propagated SBs only at the surface in micron-sized MGs. Our results fill the gap in the current understanding of size effects in the strength and failure mechanism of MGs at different length scales.

Interface-related deformation phenomena in metallic glass/high entropy nanolaminates

The deformation behavior of metallic glass (MG)-high entropy alloy (HEA) nanolaminate is explored through molecular dynamics simulations using nanolaminate models of FeCoCrNiAl1.7 MG and FeCoCrNiAl0.5 HEA. Quantitative analysis of the atomic strain and dislocation dynamics among competing and cooperative deformation mechanisms is accomplished through the implementation of uniaxial tensile deformation. The combination of glassy and crystalline nanolayers biases the plastic deformation to regions near the glass-crystalline interface at lower strains, which lowers the activation barrier for the onset of dislocation nucleation and propagation. With increasing applied strain, dislocations are absorbed into the amorphous plate via slip transfer across the glass-crystalline interface, in turn triggering the activation of homogeneously distributed shear transformation zones (STZs) in amorphous plate. The competitive deformation mechanism suppresses the formation of localized shear bands and increases the resistance to dislocation motion, thereby promoting enhanced ductility in MG-HEA nanolaminates. Additionally, due to the high strength of the HEA, the laminate structures exhibit a much higher strength than conventional MG-crystalline laminates. The combination of high strength HEAs and MGs and the complex deformation behavior may overcome the typical strength-ductility trade-off and make MG-HEA laminates promising candidates for a variety of structural and functional applications.

The atomistic mechanism of notch sensitivity on the deformation mode in metallic glasses

In previous studies, the influence of flaws on macro-scale and nano-scale metallic glasses (MGs) has always been a topic of great interest, with important applications in engineering. Moreover, through experiments and simulations, many researchers draw contradictory conclusions of the notch sensitivity, including the notch strengthening, weakening, and insensitivity in different MGs. In order to better understand its atomic mechanism, classical molecular dynamics simulations have been performed on the nano-scale notched MGs in the present work to investigate the deformation modes under the uniaxial tension. According to the results, after setting notches, the MG samples exhibit a significant change from the shear to necking failure, which indicates a notch strengthening phenomenon. At the same time, the evolution of atomic structures suggests that the introduction of notches will increase the degree of shear localization and further disturb the structural order of MGs. The atomic displacement, which is different from shear strain, can also be used as an index to visually describe the deformation behavior of MGs and shows the high notch sensitivity.

Improving fatigue performance of metallic glasses with crystalline metal coating revealed by atomistic simulations

In this work, a comparative study was undertaken to investigate the effects of Cu coating on the deformation behaviour of metallic glass (MG) and crystalline Cu-coated MG under monotonic tensile loading and cyclic compressive-tensile loading by means of atomistic simulations. The results showed that under monotonic tensile loading, yielding first occurs in the Cu coating due to its low elastic limit, while plasticity in the MG core is directly triggered by the transfer of crystal plasticity from the Cu coating across the well-bonded crystalline-amorphous interfaces. The Cu coating serves as an effective medium for nucleating multiple shear bands, thus resulting in an improvement of tensile ductility. However, under cyclic loading, yielding tends to initiate via shear band activation in the interior of the Cu-coated MG. The Cu coating serves as effective barrier to suppress shear band initiation on the free surfaces, thus giving rise to enhanced fatigue life of the Cu-coated MG. The interlay bounding condition is also verified as playing a crucial role on the fatigue performance in the Cu-coated MG. The findings provide valuable insights on the effect of crystalline metal coating on the tensile and cyclic deformation mechanisms of MGs from an atomistic perspective.

Inverse size effects in un-notched and notched metallic glass thin films

The sample size effect on the deformation behaviors of metallic glasses (MGs) attracts a lot of interest. Here, a series of molecular dynamics simulations have been performed on un-notched and notched MG thin films to fully investigate the influence of sample thickness on their fracture strengths and failure mechanisms. With decreasing sample thickness, the failure strength of un-notched MG thin films decreases slightly and the failure mode changes from shear banding to homogeneous deformation. In contrast to this brittle-to-ductile transition observed in un-notched MG thin films, shear banding prevails for notched MG thin films regardless of sample thickness. Concurrently, a notch strengthening effect is observed in the notched MG thin films when the sample thickness falls close to the shear band (SB) thickness. This significant notch strengthening is due to the failure mode transition from homogeneous deformation in un-notched MGs to shear banding in notched MGs. Based on the energetic model, a theoretical analysis is proposed to rationalize the size effect on failure mechanism transitions of both un-notched and notched MG thin films. The present study offers an understanding of size effect on the un-notched and notched MG thin films, as well as useful guidelines for the design, testing, and engineering of MG for microelectromechanical applications.

Effect of strain rates on the plastic deformation behavior and serrated flow of Zr55.7Cu22.4Ni7.2Al14.7 bulk metallic glass

The effects of strain rates on the plastic deformation behavior and serrated flow of Zr55.7Cu22.4Ni7.2Al14.7 bulk metallic glass (BMG) were studied by compression test at room temperature. The results show that the plastic deformation behavior of metallic glass at room temperature is highly sensitive to strain rates. The plastic strain, yield strength, compressive strength and fracture strength were found to increase with decreasing of strain rates. For example, the plasticity and the compressive strength increased from 0.4 ± 0.1%–8.3 ± 0.4 %, and from 1684 ± 11 MPa to 2000 ± 23 MPa, respectively. In addition, the number and density of shear bands on the fracture side also increased gradually. The fracture morphology, which is mainly composed of vein-like patterns, can be understood by the fact that the number and density of vein-like patterns increase with decreasing of strain rates. However, the mechanism of serrated flow was analyzed and discussed from the perspective of the initiation, expansion, and interaction of multiple shear bands correspondingly. With decreasing strain rates, the uniformity of the serrated flow in the first stage is not sensitive to the change of strain rates, while the uniformity of the second stage serrated flow changes. The change of serrated flow from near uniformity to non-uniformity in the second stage is attributed to the interaction between multiple shear bands. In comparison, with decreasing strain rates, the stress drops in the second stage first decreases and then increases due to the interaction of shear bands. The plastic deformation behavior and serrated rheology of bulk metallic glasses (BMGs) at room temperature, which lay a foundation for promoting the engineering application of BMGs, were discussed in details in this study.

Trimodal shear band nucleation distribution in a Gd-based metallic glass via nanoindentation

Nucleation of shear bands is of critical importance in the deformation behaviors of metallic glasses, but the detailed examination of nucleation kinetics and the correlation with the structure and relaxation kinetics has been few. Here, based upon instrumented nanoindentation with spherical indenter and a great deal of the observed first pop-in events, one unique trimodal distribution of shear band nucleation events in a Gd-based metallic glass was reported. Four different loading rates were employed to study the evolution of the nucleation distribution with external loading. In the analyses of nucleation kinetics, the site density, nucleation rate and activation barrier for three nucleation events were obtained based on the cooperative deformation model, which clearly display the trimodal character. The discovery of a trimodal shear band nucleation distribution provides new insights for understanding the relationship between the heterogeneous structure, relaxation kinetics and deformation mechanism and opportunities for controlling the ductility of amorphous alloys.

Intrinsic and extrinsic effects on the brittle-to-ductile transition in metallic glasses

The effects of cooling rate, temperature, and applied strain rate on the tensile deformation behavior of a Cu64Zr36 metallic glass (MG) are investigated using large-scale molecular dynamics simulations. An increase in the quenching rate during sample preparation, as well as an increase of the temperature or the applied strain rate, affects the activation of shear transformation zones (STZs) and, consequently, the shear-banding processes, which ultimately causes a brittle-to-ductile transition in the deformation behavior of MGs. A quantitative interpretation for the observed enhanced ductility in MGs with an increasing quenching rate is obtained by sampling the saddle points on the potential energy surface. High quenching rates lead to lower energy barriers for activation of a local atomic rearrangement (STZ) as compared to those MGs obtained at low quenching rates. Although the glassy structure does not show significant variations with increasing temperature, the kinetic energy of the atoms increases dramatically, which allows the atoms to rearrange easily; therefore, the probability of homogeneous thermal activation of STZs increases. Finally, a large number of STZs can also be activated by deformation at high strain rates when a large amount of elastic energy is stored in the glassy matrix. Consequently, a high density of STZ events and, therefore, a more complex percolation process results in a low probability for strain localization and formation of critical shear bands. Our results provide an atomistic understanding for the strain localization mechanisms in metallic glasses and shed more light on the brittle-to-ductile transition.

Signature of local stress states in the deformation behavior of metallic glasses

The design of ductile heterogeneous metallic glasses (MGs) with enhanced deformability by purposely controlling the shear-band dynamics via modulation of the atomic-scale structures and local stress states remains a significant challenge. Here, we correlate the changes in the local atomic structure when cooling to cryogenic temperature with the observed improved shear stability. The enhanced atomic-level structural and elastic heterogeneities related to the nonaffine thermal contraction of the short-range order (SRO) and medium-range order (MRO) change the characteristics of the activation process of the shear transformation zones (STZs). The experimental observations corroborated by Eshelby inclusion analysis and molecular dynamics simulations disclose the correlation between the structural fluctuations and the change in the stress field around the STZ. The variations in the inclination axes of the STZs alter their percolation mechanism, affect the shear-band dynamics and kinetics, and consequently delay shear failure. These results expand the understanding of the correlation between the atomic-level structure and elementary plastic events in monolithic MGs and thereby pave the way for the design of new ductile metallic alloys.

Achieving pronounced β-relaxations and improved plasticity in CuZr metallic glass

Johari-Goldstein secondary relaxation known as β-relaxation has long been considered to play a key role in mechanical behaviors of metallic glasses (MGs). CuZr alloys are typical MG system exhibits non-pronounced β-relaxation and poor ductility. In the present work, we showed that pronounced β-relaxation that only observed in few La- and Y-based MG systems could be achieved in Cu50Zr50 MG and effectively improve the plasticity of the CuZr alloys. Other than the well-documented strain-like motions, the simulation results and analysis indicated, higher degree heterogeneity generated in severe deformed region is responsible to the unusual pronounced β-relaxation. The outcomes of current work not only deepen understanding of the microstructural origin of β-relaxation, also path a new way to alter the β-relaxation and plastic deformation behaviors of MGs.

Simulations of Shear bands in Metallic Glasses with Mesoscale Modeling

Metallic glasses (MGs) have many attractive advantages such as high yield strength, high hardness, high wear resistance, high fracture toughness, and low friction coefficient. However, the localized deformation and poor ductility due to shear bands prohibit the further applications of MGs. The localized deformation could be controlled by the residual stresses in MGs. In this study, a mesoscale model which combines the kinetic Monte Carlo algorithm (kMC) and the finite element method (FEM) is constructed to investigate the effects of residual stresses on deformation behaviors of MGs. The developed computational framework is implemented by integrating in-house Matlab program for kMC and commercial finite element software, Abaqus. In the mesoscale model, a shear transformation zone (STZ) with nanoscale volume is the basic deformation unit and is represented by an element in the FEM model. Each element whether is transformed into an STZ is determined by the kMC algorithm according to its energy state. The developed mesoscale model can simulate the deformation process of MGs with the time and length scales which are greater than those by atomistic modeling. Finally, a sanity check was performed to verify the accuracy of FEM in solving Eshelby’s inclusion problem. Then the developed computational framework is applied to simulate the uniaxial tension test of the metallic glass specimen with different residual stresses. Simulation results demonstrate that the residual stresses could dominate the occurrences of STZs and change the stress–strain curve of MGs.

Distinctive Slow β Relaxation and Its Impact on Mechanical Property of LaCe Based Bulk Metallic Glasses

The slow β relaxation which occurring at temperatures far below glass transition temperature, typically playing important roles in the plastic deformation behavior of metallic glasses. Compared with polymer glasses, most of the metallic glasses do not exhibit obvious β relaxation based on dynamic mechanical analysis. In the current work, prominent β relaxation behaviors of a series (LaxCe100−x) Al10Co25 (x = 50, 60, 70, 80) bulk metallic glasses (BMGs) at low temperature are observed using dynamic mechanical analysis. Compared with other BMGs, the LaCe based BMGs show a pronounced β relaxation peak and relative low peak temperature, the activation energy of the β relaxation based on Arrhenius equation are calculated around 79.3 kJ/mol to 86.4 kJ/mol, which obey an empirical rule that Eβ ≈ (26 ± 4) RTg, with the coefficient of 23. Experimental results from room temperature compression tests for the LaCe based BMGs indicate that low temperature activated β relaxation behaviors facilitate the good plasticity of the LaCe based BMGs.

On the effect of hydrostatic stress on plastic deformation in metallic glasses

Previous experimental observations have demonstrated that large stress gradient results in high plasticity, while the underlying mechanism and constitutive laws are rarely reported. In this work, a micro-mechanics based model was proposed to account for the contribution made by hydrostatic stress. This micro-mechanics model was then incorporated into the free volume model describing the viscous softening behavior and a generalized constitutive law was established. Using a user material subroutine (UMAT) and von-Mises criterion, the new constitutive equations were implemented into a finite element code to simulate the material response and free volume evolution procedure under tensile and compressive loading when hydrostatic stress was taken into account. Furthermore, comparison was made between current simulation and previous theoretical or experimental results, good agreement was successfully obtained. Therefore, it is concluded that the current constitutive law is a good candidate for describing the deformation behavior of metallic glass under complex stress states.

Spatial correlation of elastic heterogeneity tunes the deformation behavior of metallic glasses

Metallic glasses (MGs) possess remarkably high strength but often display only minimal tensile ductility due to the formation of catastrophic shear bands. Purposely enhancing the inherent heterogeneity to promote distributed flow offers new possibilities in improving the ductility of monolithic MGs. Here, we report the effect of the spatial heterogeneity of elasticity, resulting from the inherently inhomogeneous amorphous structures, on the deformation behavior of MGs, specifically focusing on the ductility using multiscale modeling methods. A highly heterogeneous, Gaussian-type shear modulus distribution at the nanoscale is revealed by atomistic simulations in Cu64Zr36 MGs, in which the soft population of the distribution exhibits a marked propensity to undergo the inelastic shear transformation. By employing a mesoscale shear transformation zone dynamics model, we find that the organization of such nanometer-scale shear transformation events into shear-band patterns is dependent on the spatial heterogeneity of the local shear moduli. A critical spatial correlation length of elastic heterogeneity is identified for the simulated MGs to achieve the best tensile ductility, which is associated with a transition of shear-band formation mechanisms, from stress-dictated nucleation and growth to structure-dictated strain percolation, as well as a saturation of elastically soft sites participating in the plastic flow. This discovery is important for the fundamental understanding of the role of spatial heterogeneity in influencing the deformation behavior of MGs. We believe that this can facilitate the design and development of new ductile monolithic MGs by a process of tuning the inherent heterogeneity to achieve enhanced ductility in these high-strength metallic alloys.

Understanding the effects of Poisson’s ratio on the shear band behavior and plasticity of metallic glasses

In metallic glasses, a high Poisson’s ratio often corresponds to a large plasticity and ductility. Yet the physics underpinning such a connection is still poorly understood. Here through finite element simulations, we reveal that a high Poisson’s ratio could promote the inhomogeneous stress distribution in metallic glasses. The inhomogeneous stress field could cause earlier nucleation and easier arrest of shear bands, and therefore better plasticity. Experimental results also show a trend of decreasing shear limit with Poisson’s ratio. These findings suggest that the stress inhomogeneity might be a key to understand the effects of Poisson’s ratio and loading condition on the plastic deformation behaviors of metallic glasses.