Cu50Zr50 Metallic Glass Research Articles

Bicontinuous nanoporous design induced homogenization of strain localization in metallic glasses

Bicontinuous nanoporous metallic glasses (MG) synergize the outstanding properties of MGs and open-cell nanoporous materials. The low-density and high-specific-surface-area of bicontinuous nanoporous structures have the potential to enhance the applicability of MGs in catalysis, sensors, and lightweight structural designs. Here, we report molecular dynamics simulations of tensile loading deformation and failure of bicontinuous nanoporous Cu64Zr36 MG with 55% porosity and 4.4 nm ligament size. Results indicate an anomalous mechanical behavior featuring delocalized plastic deformation preceding ductile failure. The deformation follows two mechanisms: i) Necking of ligaments aligned with the loading direction and ii) progressive alignment of randomly oriented ligaments. Failure occurs at 0.16 strain, following massive rupture of ligaments. This work indicates that a bicontinuous nanoporous design is able to effectively delocalize strain localization in a MG due to a combination of size effect on the ductility of MGs resulting in nano ligaments necking and progressive asynchronous alignment of ligaments.

Composition-Dependent Effects of Oxygen on Atomic Structure and Mechanical Property of Metallic Glasses

Although minor alloying in metallic glasses (MGs) has been extensively investigated, the effect of O doping is still a debatable topic. In the present study, the atomic-level structures and mechanical properties of Cu-Zr MGs doped with different O contents have been analyzed using ab initio molecular dynamics simulations. It is found that O atoms prefer to bond with Zr atoms due to their low mixture enthalpy. The key finding is that we reveal the compositional dependence of O doping. For O-doped Cu64Zr36 MGs, the fraction of full icosahedra, size of the medium-range-order clusters, Young’s modulus, and shear modulus decrease sharply with O content, accompanied by a sharp increase of the non-Frank-Kasper polyhedral, ratio of bulk modulus to shear modulus, and Poisson's ratio, indicating decreased strength and improved plasticity. For O-doped Cu47Zr53 MGs, however, the above-mentioned structural and mechanical features experience little change or only change slowly after O doping, showing low oxygen sensitivity. It is reasoned that the high Zr content in Cu47Zr53 MGs weakens the effect of Zr-O bonding to some extent. The present study not only sheds light on the atomic-level structures of O-doped MGs that may provide guidelines for designing MGs with low-grade materials and low processing cost, but also help to explain the previous conflicting results based on the composition-dependence effect.

An atomic-level perspective of shear band formation and interaction in monolithic metallic glasses

Understanding the relationship between nanoscale structural heterogeneities or elastic fluctuations and strain localization in monolithic metallic glasses remains a long-standing underlying issue. Here, an atomic-level investigation of the correlation between elastic and structural heterogeneities and the mechanisms of shear banding in CuZr metallic glass is conducted using molecular dynamics simulations. The shear band formation and propagation processes and the intersection mechanism of multiple shear bands are evaluated by means of local entropy-based structural identification and von Mises stress calculation. The shear band follows the path of lower order and high entropy while shear deflection and branching occur when approaching regions of low entropy. The local von Mises stress calculation allows predictions on the shear band direction and the propensity for activation and propagation prior to yielding and sheds light on shear band branching and multiplication processes.

Effects of pre-strain on the nanoindentation behaviors of metallic glass studied by molecular dynamics simulations

Nanoindentation simulations have been performed on pre-compressed and pre-stretched Cu64Zr36 metallic glasses (MGs) samples via molecular dynamics methods, to investigate the effects of pre-stress on the mechanical behavior, deformation performance and shear bands forming morphologies of the MG material. The results have shown that pre-tensile stress has a more intuitive “weaken” impact on the mechanical performance of the MG. While for pre-compressive cases, with increasing compressive strain, the pre-stress has a “harden” to “weaken” effects transition. Based on Hertz’s contact, the analysis of simulation results indicated both the stress state of the material and the structural change induced by applied stress contribute to the “harden/weaken” effects. Besides, the shear bands morphologies under indentations are also found to be influenced by the sign and magnitude of the pre-stress. And if the pre-stress is large enough, the shear banding would propagate spontaneously and rapidly, which may result in brittle deforming mode happened in the indentation process. Furthermore, By using the energy analysis method, this plastic to brittle transition is inferred to be connected with the stress state and potential energy stored in the material. If the stored potential energy exceeds the essential forming energy of shear bands, the fracture would happen once local area is triggered by external stress.

Localization model description of the interfacial dynamics of crystalline Cu and Cu64Zr36 metallic glass films.

Recent studies of structural relaxation in Cu-Zr metallic glass materials having a range of compositions and over a wide range of temperatures and in crystalline UO2 under superionic conditions have indicated that the localization model (LM) can predict the structural relaxation time τα of these materials from the intermediate scattering function without any free parameters from the particle mean square displacement ⟨r2⟩ at a caging time on the order of ps, i.e., the "Debye-Waller factor" (DWF). In the present work, we test whether this remarkable relation between the "fast" picosecond dynamics and the rate of structural relaxation τα in these model amorphous and crystalline materials can be extended to the prediction of the local interfacial dynamics of model amorphous and crystalline films. Specifically, we simulate the free-standing amorphous Cu64Zr36 and crystalline Cu films and find that the LM provides an excellent parameter-free prediction for τα of the interfacial region. We also show that the Tammann temperature, defining the initial formation of a mobile interfacial layer, can be estimated precisely for both crystalline and glass-forming solid materials from the condition that the DWFs of the interfacial region and the material interior coincide.

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.

Quasi-work-hardening at sites of shear band interactions in a Cu50Zr50 metallic glass

Using a powerful method of contact-resonance atomic force microscope, we studied the elastic modulus at shear band (SB) interactions in a Cu50Zr50 metallic glass (MG). A quasi-work-hardening effect is observed: the interaction site is harder than the SB itself, but softer than the undeformed matrix. Therefore, the interaction site is not the weakest position during the deformation process of MGs. Large-scale molecular dynamics simulation suggests that this quasi-work-hardening effect is correlated to a locally configurational evolution of shear-resistant cluster. Our findings are useful for understanding the deformation mechanism of multi-SBs in MGs.

Aspect ratio-dependent nanoindentation behavior of Cu64Zr36 metallic glass nanopillars investigated by molecular dynamics simulations

In this paper, we study nanoindentation in Cu64Zr36 metallic glass (MG) nanopillars with different aspect ratios by molecular dynamics simulations. The activation of shear transformation zones (STZs) and the deformation behavior of MG pillars are discussed during nanoindentation loading and unloading processes. Buckling and serrated flow are the two types of deformation behaviors observed during nanoindentation. For large aspect ratio pillars, a sudden stress drop in the load–displacement curve is found that relates to the buckling process, while smaller aspect ratio pillars exhibit large stress fluctuations. The serrated flow is associated with STZ activation. STZs are locally activated, and their number gradually increases with increasing indentation depth during loading, whereas their number decreases during unloading. For pillars with a large aspect ratio, no new STZs are activated and their number decreases rapidly once the indenter has left the sample because of the buckling deformation. In contrast, new STZs are activated for pillars with smaller aspect ratio during the unloading process. Analysis of STZ activation and shear localization reveals an inhomogeneous deformation process and an increase in the degree of structural heterogeneity as the aspect ratio of the pillars increases for both loading and unloading stages. The present work provides an insight into the atomic-scale plastic deformation behavior of MG nanopillars during nanoindentation loading and unloading processes.

Creep Deformation of a Cu-Zr Nanoglass and Interface Reinforced Nanoglass-Composite Studied by Molecular Dynamics Simulations

Using molecular dynamics simulations, we compare the creep properties of a homogeneous Cu64Zr36 metallic glass, a nanoglass with the same nominal composition, and a nanoglass-crystal composite, where the amorphous grain boundary phase has been reinforced with the high-temperature stable Cu2Zr Laves phase. While the nanoglass architecture is successful at preventing shear band formation, which typically results in a brittle failure mode at room temperature and conventional loading conditions, we find that the high fraction of glass-glass grain boundary phase therein is not beneficial to its creep properties. This can be amended by reinforcing the glass-glass interphase with a high-temperature stable crystalline substitute.

Gradient microstructure induced shear band constraint, delocalization, and delayed failure in CuZr nanoglasses

The mechanical properties of metallic glasses (MGs) can be significantly tuned by introducing a nanoscale glass-glass interface microstructure. Here, we design three types of Cu64Zr36 MG seamless gradient microstructures by combining glassy grain sizes from 3 to 15 nm. We perform tensile loading molecular dynamics simulations on the named gradient nanoglasses (GNGs) to characterize their mechanical behavior. In all cases, results indicate the generation of gradient plasticity following the gradient in grain size, i.e. local plasticity increases from large to small grain size regions. Loading parallel to the gradient direction results in initial broad necking followed by catastrophic failure. In contrast, loading perpendicular to the gradient direction results in diffused shear band propagation from large to small grain size regions, delaying the generation of a dominant shear band and further delocalizing plastic deformation. The results demonstrate the synergistic effects generated by heterogeneous gradient designs in the mechanical behavior of MGs.

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.

Applicability of cutting theory to nanocutting of metallic glasses: Atomistic simulation

Using molecular dynamics simulation, we perform simulations of cutting a CuZr metallic glass at a wide range of rake angles. Plasticity in the metallic glass is governed by the formation of parallel shear bands in the chip. The chip is clearly separated from the uncut workpiece by a primary shear zone, from which the shear angle can be easily determined. The dependence of the shear angle on the rake angle and the friction angle follows closely Merchant’s theory of cutting. This agreement is particularly pronounced in the case that the force exerted on the workpiece is far from the cutting direction; this occurs for negative rake angles.

Atomistic investigation of aging and rejuvenation in CuZr metallic glass under cyclic loading

The structural evolution and mechanical response of CuZr metallic glass (MG) under cyclic loading are investigated by molecular dynamics simulations. Either aging or rejuvenation was observed in the MG under cyclic loading with different cyclic strain amplitudes. Aging is more remarkable in the surface, while rejuvenation is correlated with the nucleation and development of shear transformation zone in the bulk of MG. The MG models with more structural heterogeneity were prepared by increasing the cooling rate during cooling process to exhibit larger degrees of structural evolution and aging during cyclic loading. While the fraction of ordered coordination polyhedron increases monotonically with increasing Cu content, the degree of aging during cyclic loading is not significantly impacted by Cu content. The glass-forming ability is the intrinsic character that governs the change in energy state and structural evolution of MG under cyclic loading. The critical cyclic strain amplitude for the transition from aging to rejuvenation is closely related to the initial structure of MG.

Unusual internal friction and its size dependence in nanoscale metallic glasses

The internal friction of Cu50Zr50 metallic glass nano-pillars was investigated by using molecular dynamics simulations. An unusual non-monotonic variation of internal friction is revealed against the size of the specimen, which differs significantly from that of the bulk metallic glass. Meanwhile, by analyzing the rearranged atoms with high mobility, which play a vital role in affecting the internal friction, it is found that the rearrangement of surface atoms is more significant than that of the bulk ones, and their fraction depends on the sample size as well. With reducing the sample size, the fraction of rearranged atoms in the surface region increases, which could be described by an exponential equation. This finding suggests that the size dependence of internal friction originates directly from the different fractions of the rearranged atoms in the surface region of nano-pillars. Furthermore, a phenomenological model was established to describe the internal friction of the nano-pillars against their diameters. The presented results provide a quantitative insight into the size effect on internal friction in nanoscale metallic glasses, also shedding light on the atomistic mechanism of surface relaxation of amorphous solids.

Shear transformation zones structure characterization in Cu50Zr50 metallic glasses under tensile test

The atomic structure of Cu50Zr50 metallic glasses under tensile test was investigated using molecular dynamics simulations. Voronoi polyhedra analysis was employed to calculate the number of neighbors for each atom, obtained as the total number of faces of the polyhedron, in order to inspect their relationship with plasticity. An inspection based on the atoms that contribute to shear transformation zones revealed that Cu atoms with 12 neighbors played the main role during the plastic regime. During the onset of plasticity, it was observed that the population of Cu species with initially 12 neighbors, denoted as Cu12, increased their number of neighbors to 13 and 14. Then, during the softening of the sample, the population of Cu12 decreased, equaling the population of Cu13 at 0.11 strain onwards. We hope that these results can help to further unveil the structure transformation driven by shear transformation zones nucleation in metallic glasses.

Identifying flow defects in amorphous alloys using machine learning outlier detection methods

Shear transformation zones (STZs) are widely believed to be the fundamental flow defects that dictate the plastic deformation of amorphous alloys. However, it has been a long-term challenge to characterize STZs and their evolutions by experimental methods due to transient nature. Here we first introduced a consistent, automated, robust method to identify STZs by linear based machine learning outlier detection algorithms. We exemplify these algorithms to identify the atoms of STZs in Cu64Zr36 metallic glass system, and verify this data-driven model with a physical based model. It is revealed that the average STZ size slightly increases with decreasing cooling rate.

Low-frequency vibrational properties and structure correlation in metallic glass

Molecular dynamic simulations were carried out to study the characteristics of the vibrational modes and structural correlation in Cu50Zr50 metallic glass. The participation ratio and the 3-dimensional eigenvector fields were analyzed to study the vibrational characteristics. The modes at low frequency with high participation ratio values show significant plane wave character, whereas the low participation ratio modes show vortex structures, consisting of localized structures embedded in a plane-wave-like background. Furthermore, the low-frequency modes with small participation ratio show strong anharmonicity which is closely correlated to the weight of the localized component in these modes. Our results also manifest that the boson peak has no direct correlation with anharmonicity and may be directly controlled by the five-fold local symmetry in local structures. These findings provide useful understanding of the vibrational properties in metallic glasses.

Atomic dynamics under oscillatory shear in metallic glasses

The influence of quenching condition on the aging and the rejuvenation effect was investigated by classical molecular-dynamics simulations on CuZr metallic glasses under oscillatory shear. Atomic dynamics underlying this procedure is focused. We found a separation of fast and slow atoms in the non-equilibrium state, with an exponential tail in the displacement distribution function contributed by the fast atoms, similar to that in the glass-forming liquids. However, we do not observe any localized relaxation process, e.g., the cage relative motion, which is prominent in the supercooled liquids. Structural relaxation in the fluidized glasses exhibits a simple exponential decay, in contrast to the stretched exponential law in liquids. This dynamic significance is ascribed to the subextensive feature of the atomic displacement in the sheared amorphous solids.

Mechanical properties and thermal conductivity of pristine and functionalized carbon nanotube reinforced metallic glass composites: A molecular dynamics approach

This work uses the molecular dynamics approach to study the effects of functionalization of carbon nanotubes (CNTs) on the mechanical properties of Cu64Zr36 metallic glass (MG). Three types of functional groups, carboxylic, vinyl and ester were used. The effect of CNT volume fraction (Vf) and the number of functional groups attached to CNT, on the mechanical properties and thermal conductivity of CNT-MG composites was analysed using Biovia Materials Studio. At lower values of Vf (from 0 to 5%), the percentage increase in Young’s modulus was approximately 66%. As the value of Vf was increased further (from 5 to 12%), the rate of increase in Young’s modulus was reduced to 16%. The thermal conductivity was found to increase from 1.52 W/mK at Vf = 0% to 5.88 W/mK at Vf = 12%, thus giving an increase of approximately 286%. Functionalization of SWCNT reduced the thermal conductivity of the SWCNT-MG composites.

A comprehensive analysis of the mechanical properties and fracture analysis of metallic glass nanocomposites reinforced by carbon nanotubes and Cu nanowires: A molecular dynamics study

Reinforced by nanowires (NWs), carbon nanotubes (CNTs) and NW encapsulated CNT (NW@CNT), tensile behavior of various types of Cu-Zr based metallic glass (MG) nanocomposites are studied using molecular dynamics (MD) simulations. It is observed that pure two-toms alloy MG and the one reinforced with bigger CNT demonstrates higher tensile properties than other types of MGs. Further, it is observed that the ultimate strength of reinforced MGs with individual CNTs is slightly higher than that of NW@CNT reinforced analogous. In this case, it is noticed that reinforced three-atoms Cu-Zr MG nanocomposites including Ti atoms demonstrate the highest ultimate strength and strain.