Cu50Zr50 Metallic Glass Research Articles

Interaction between parallel shear bands in a metallic glass

We present molecular dynamics simulations in a large CuZr metallic glass sample that allows the linking of the structural properties of a single shear band to the presence of other subsequently forming parallel shear bands. In a closely controlled study, we observe that the subsequent formation of parallel shear bands restores the disrupted icosahedral matrix in the core of previously formed shear bands, i.e., the shear-band core structure is partially relaxed to the original bulk level. The relaxation caused by the interaction between shear bands also manifests in a reduction of the potential energy and in an eventual return of previous plastic regions called shear transformation zones.

Cu50Zr50 metallic glass flakes reinforced Al composites: Experimental and molecular dynamics nanoindentation response of matrix, interface, and reinforcement

In this study, we report experimental and molecular dynamics (MD) studies on the sintering and nanoindentation behavior of Cu50Zr50 metallic glass (MG) flakes reinforced Al composites. The mechanical response (yielding, elastic modulus, and hardness) of the individual components has been investigated in detail. Spark plasma sintering technique is used to synthesize Al powder (70 Vol. %) and Cu50Zr50 (30 Vol.%) MG flakes composite. The values of elastic modulus (Es) are found to be 71 GPa (Es, Al), 97 GPa (Es, MG), and 80.3 (Es, Al-MG interface). The values of hardness (H) are 8.5GPa in MG, 3.1 3.5 GPa in the Al-MG interface region, and 2.4 GPa in the Al region. MD simulations of sintering and nanoindentation are carried out comprising 583882 atoms. Embedded atom method (EAM) potential is used to model the atomic interactions between Al, Cu and Zr atoms. Simulation results are in good agreement with the experiments.

Enhanced ductility in Cu64Zr36 metallic glasses induced by prolonged low-energy ion irradiation: A molecular dynamics study

The irradiation induced variations in microstructure and mechanical properties of the Cu64Zr36 metallic glasses (MGs) are investigated via large-scale molecular dynamics simulations. The irradiation condition is modeled by sequential and prolonged collision cascades. The simulation results show a simultaneous decrease of the density and the fraction of Cu-centered full icosahedron (CCFI) clusters in the irradiated samples. This microstructural evolution induced by the prolonged irradiation is proposed to be responsible for the change of deformation mode of tested samples under uniaxial tensile loading. When increasing the irradiation dose, the ultimate tensile strength of MG samples decreases gradually, while their deformation mechanism switches from localized shear banding to ductile necking and finally to ideally plastic flow, enhancing the MG ductility. Careful atomic-scale examinations on the initiation and development of shear transformation zones (STZs) are carried out for a better understanding of the irradiation-induced ductility enhancement in MGs. Two related effects of the irradiation are unveiled. On the one hand, the irradiation releases the stored strain energy by promoting the STZ activation within the MG matrix. On the other hand, the irradiation damages restrain the propagation of shear band through the innovatively proposed two-unit “STZ-vortex” mechanism. Under the combined effect of these two mechanisms, the low-temperature ductility of irradiated MGs is improved and the brittle-like failure driven by single shear banding is prevented.

Atomistic Simulation on the Mechanical Properties of Diffusion Bonded Zr-Cu Metallic Glasses with Oxidized Interfaces

A novel welding technique of diffusion bonding for Zr-Cu metallic glass with pre-oxidized surfaces is proposed in this study, which is systematically investigated by molecular dynamics (MD) simulation. Compared with the conventional welding technique, the diffusion bonding process can be well implemented below the crystallization temperature of metallic glass. The obtained structure possesses glass–glass interfaces (GGIs) similar with those in nano-glasses. As revealed by MD simulation, the diffusion bonded metallic glasses possess enhanced mechanical strength and ductility that generally do not exist in nano-glasses and their bulk metallic glass counterparts. The GGIs are found to hinder the propagation of shear bands, where there is strong bonding between Zr and O and the segregated Cu and ZrO2 clusters could induce extra plasticity. The results demonstrate that the diffusion bonding of metallic glass with pre-oxidized surfaces could provide an alternative approach in solving the longstanding issue of size limitation on metallic glasses.

Mechanisms of bulk and surface diffusion in metallic glasses determined from molecular dynamics simulations

The bulk and surface dynamics of Cu50Zr50 metallic glass were studied using classical molecular dynamics (MD) simulations. As the alloy undergoes cooling, it passes through liquid, supercooled, and glassy states. While bulk dynamics showed a marked slowing down prior to glass formation, with increasing activation energy, the slowdown in surface dynamics was relatively subtle. The surface exhibited a lower glass transition temperature than the bulk, and the dynamics preceding the transition were accurately described by a temperature-independent activation energy. Surface dynamics were much faster than bulk at a given temperature in the supercooled state, but surface and bulk dynamics were found to be very similar when compared at their respective glass transition temperatures. The manifestation of dynamical heterogeneity, as characterized by the non-Gaussian parameter and breakdown of the Stokes-Einstein equation, was also similar between bulk and surface for temperatures scaled by their respective glass transition temperatures. Individual atom motion was dominated by a cage and jump mechanism in the glassy state for both the bulk and surface. We utilize this cage and jump mechanisms to separate the activation energy for diffusion into two parts: (i) cage-breaking barrier (Q1), associated with the rearrangement of neighboring atoms to free up space and (ii) the subsequent jump barrier (Q2). It was observed that Q1 dominates Q2 for both bulk and surface diffusion, and the difference in activation energies for bulk and surface diffusion mainly arose from the differences in cage-breaking barrier Q1.

Enhanced Mechanical Properties of Metallic Glass Thin Films via Modification of Structural Heterogeneity.

The structure of Cu50Zr50 and Co56Ta35B9 metallic glass thin films (MGTF) was effectively tailored via various applied substrate temperatures by means of the magnetron sputtering technology. Obviously enhanced hardness and elastic modulus are achieved by different compositional MGTFs by increasing the substrate temperature. Compared with the CuZr MGTFs, the CoTaB MGTF deposited at 473 K displays the smaller strain-rate sensitivity exponent, m, and a weaker spectrum intensity based on the nanoindentation creep test, suggesting its better creep resistance. In addition, the STZ volume of the CoTaB MGTF significantly decreases after depositing at higher temperature. According to the nano-scratch analysis, the CoTaB MGTF at the substrate temperatures of 473 K performs the shallower scratch width and the larger H3/E2 value, indicating its better tribological property.

Nanoscale periodic distribution of energy dissipation at the shear band plane in a Zr-based metallic glass

The local energy dissipation near the shear band plane was characterized by using a method of amplitude-modulation atomic force microscope, in a Cu50Zr50 metallic glass. The shear band plane has higher energy dissipation than the undeformed metallic glass surface. Along the propagating direction of shear bands, the energy dissipation is periodically distributed, arising from density changes and discontinuous growth of the shear offset during the sliding process. Our experimental findings of the fluctuated distribution of energy dissipation at the plane uncover some dynamic information about the shear banding process and explain some key features of shear bands in metallic glasses.

Molecular dynamics study on the nanovoid collapse and local deformation in shocked Cu50Zr50 metallic glasses

The nanovoid evolution is an important part of understanding the physical and mechanical properties of metallic glasses (MGs). Here, the void-included Cu50Zr50 MGs under different shock intensities are investigated by the molecular dynamics simulations, focusing on the characteristics of void collapse, the evolution and correlation of clusters in the collapsed region. The results show that there are two typical mechanisms for the void collapse along the shock direction: the vertical collapse mechanism and the horizontal collapse mechanism. The vertical collapse mechanism is dominated by the hoop stress under the weak shock intensity, while the horizontal collapse mechanism is led by the principal stress along the shock direction under the strong shock intensity. In terms of shock-induced void collapse in the MGs, the plastic deformation mainly concentrates around the void and forms local shear transformation zones. There is very weak activity of the Zr-centered clusters in the shear transformation zones under the weak shock intensity, whereas the Cu-centered clusters appear a strong spatial correlation, and several of them tend to strongly aggregate. When the shock velocity exceeds 1.0 km/s, the Zr-centered clusters also begin to correlate intensely in the shear transformation zones, which corresponds to the horizontal collapse of the void. The results also show that the void defects in the Cu50Zr50 MGs can be almost fixed by long-term shock compression.

Atomistic understanding of creep and relaxation mechanisms of Cu64Zr36 metallic glass at different temperatures and stress levels

In this paper, the tensile creep behavior of Cu64Zr36 metallic glass (MG) is investigated by molecular dynamics simulations. The effects of stress and temperature on the evolution of activated states during nanoscale creep tests are discussed, the creep failure time is predicted by linear regression analysis and three creep mechanisms are found. The creep mechanisms at the low stress and high temperature regime is dominated by thermally activated atomic diffusional creep; the low temperature and low stress regime is dominated by anelastic deformation and homogeneous activation of elastic events, while the low temperature and high stress regime is governed by local inhomogeneous shear deformation through shear transformation zones (STZs) percolation and embryonic shear band formation. These mechanisms help to further understand the creep behavior and derive an atomistic description of the relationship between the creep behavior and the three mechanisms of mechanical relaxations in MGs: atomic diffusion via cooperative atomic motion or α relaxation, stress-induced STZs activation such as slow β relaxation and anelastic deformation (fast β’ relaxation), respectively.

Interfacial bonding of CuZr metallic glass via oxide: A molecular dynamics study

A layer of oxygen is introduced into a Cu64Zr36 MG to achieve interfacial bonding. The oxidation kinetics, diffusion behavior, local structure, and mechanical performance of the oxide-bonded MG were investigated via molecular dynamics simulations. The growth of the oxide layer follows the inverse-logarithmic law, indicating that this process is controlled by the ionic drift through the oxide in response to electric fields. The introduction of oxygen atoms seriously deteriorates the short-range ordering of MG by suppressing the formation of icosahedral clusters. Uniaxial tensile tests indicate that the oxide-bonded MG has favorable ductility, while its ultimate tensile strength is reduced.

Enhanced strain rate sensitivity in thermal-cycling-rejuvenated metallic glasses

Strain rate sensitivity (SRS) is an effective parameter to demonstrate the deformation mechanism of metallic materials. However, despite extensive research, the meanings of the SRS of metallic glasses (MGs) were still vague and controversial results largely remained. In the present study, with altering the microstructural features of the magnetron sputtering CuZr MG before and after thermal cycling treatment, the effects of size of both positive and negative flow units on SRS were evaluated by amplitude-modulation atomic force microscopy (AM-AFM) and nanoindentation testing. Positive SRS index m was derived in the CuZr MG, and m increased from 0.014 to 0.040 after thermal cycling. The mechanism of the enhanced SRS and underlying microstructural evolution were explored and proposed. It shows the competition between time scales of atom rearrangement within flow units and applied loading strain rate causes the change of SRS, and both the size of positive and negative flow units play crucial roles in determining the magnitude of SRS.

Dynamic mechanical relaxation behavior of binary metallic glasses

Secondary β relaxation is a fundamental issue to understand the diffusion, plasticity and glass transition behavior of metallic glasses. Binary metallic glasses provide a simple glassy system to probe the dynamic mechanical processes. In the current research, dynamic mechanical behavior of the typical binary metallic glasses was investigated by dynamic mechanical analysis (DMA). The dynamic mechanical relaxation behavior of Cu50-xTi50+x (x = 0, 7 and 9) and CuxZr100-x (x = 30, 50, 56, 61.8 and 64) binary metallic glass system was studied. It is found that the β relaxation becomes pronounced with the increase of copper, which is analyzed in the framework of the principle of mixing enthalpy. However, the β relaxation of CuZr or CuTi binary metallic glasses is much more modest than that of Y65Co35, Dy65Co35 and Y67Cu33.

Molecular dynamics simulation of the effect of temperature and size on the mechanical behavior of Cu50Zr50 metallic glass

Abstract Amorphous materials such as metallic glasses have been a recent area of research because of their more or less defined position of atoms at the molecular level. Because of their superior strength and corrosion resistance properties, metallic glasses can be used as structural materials in applications such as in nuclear reactors, marine vessels, etc. This work presents a Molecular Dynamics (MD) simulation of the mechanical behavior of Copper-Zirconium (Cu-Zr) metallic glass under the application of temperature (at a constant quenching rate (1014 K/s) for all specimens) on different sizes. The volume-temperature and temperature–time plots were drawn and the results analyzed for various sizes to estimate their mechanical behavior.

A concurrent high strength and ductility of 3D gyroidal nanoporous metallic glasses

By using molecular dynamics simulations, we investigate the tensile response of nanoporous (NP) Cu50Zr50 metallic glasses (MGs) with 3D gyroidal structure. Similar to nanoporous crystalline metals, NP–MGs fail by the plastic necking of ligaments, but shear bandings (SBs) prevail in those thick–ligament specimens. Particularly, with ligament diameter decreasing to 2.34 nm, SBs are hardly observed due to the limit of translation from shear transformation zones, resulting in superior strength and ductility of NP–MGs. The critical ligament size to suppress the formation of SBs is much smaller than those calculated value in MGs. The observations reveal a design strategy that a concurrent high strength and ductility of MGs could be obtained by designing 3D nanoporous structure with supra–nano size. The scaling laws for Young's modulus and strength as functions of relative density and ligament diameters are addressed, consistent with those in nanoporous crystalline materials.

Microstructural evolution in ZrCu metallic glass under neutron irradiation

A detailed numerical investigation has been conducted on the microstructural evolution of a Zr2Cu metallic glass (MG) model under neutron irradiation by extensive molecular dynamics simulations. It was discovered that at the melted zone of the irradiated MG model, an irradiation-induced cascades region was prominent, where there exists a significant atomic-to-cluster structural changes in terms of the coordination numbers, atomic packing efficiency, cluster distribution and regularity. These features effectively were responsible for the transformation of disordered local structures from a densely-packed to a loosely-packed arrangement and largely, the underlying chaos within the structure. Eventually, it was highly remarkable to discover that when the model was fully relaxed at 20 ns, the irradiation-induced structural changes completely vanished and the entire structure was effectively and fully self-recovered. The results of this study will potentially facilitate the development of advanced materials with high irradiation resistance.

Predicting the propensity for thermally activated \u03b2 events in metallic glasses via interpretable machine learning

The elementary excitations in metallic glasses (MGs), i.e., β processes that involve hopping between nearby sub-basins, underlie many unusual properties of the amorphous alloys. A high-efficacy prediction of the propensity for those activated processes from solely the atomic positions, however, has remained a daunting challenge. Recently, employing well-designed site environment descriptors and machine learning (ML), notable progress has been made in predicting the propensity for stress-activated β processes (i.e., shear transformations) from the static structure. However, the complex tensorial stress field and direction-dependent activation could induce non-trivial noises in the data, limiting the accuracy of the structure-property mapping learned. Here, we focus on the thermally activated elementary excitations and generate high-quality data in several Cu-Zr MGs, allowing quantitative mapping of the potential energy landscape. After fingerprinting the atomic environment with short- and medium-range interstice distribution, ML can identify the atoms with strong resistance or high compliance to thermal activation, at a high accuracy over ML models for stress-driven activation events. Interestingly, a quantitative “between-task” transferring test reveals that our learnt model can also generalize to predict the propensity of shear transformation. Our dataset is potentially useful for benchmarking future ML models on structure-property relationships in MGs.

Frank-Kasper Z16 local structures in Cu-Zr metallic glasses

Although previous molecular dynamics studies proposed the existence of Zr-centered Frank-Kasper Z16 structures in Cu-Zr metallic glasses, it cannot be concluded yet, owing to the degeneracy problem of the Voronoi index and the artifact in sets of potentials for Cu-Zr systems. We solve both problems by combining a recently developed Finnis-Sinclair potentials to generate realistic atomic structures and the ${p}_{3}$ code to properly differentiate local structures. In previous studies, the Voronoi index $\ensuremath{\langle}0,0,12,4\ensuremath{\rangle}$ was typically associated with Frank-Kasper Z16 local structures. However, we demonstrate that this index includes two types of polyhedra, only one of which is associated with the Frank-Kasper Z16 structures. We reveal that the Z16 structures are more frequent than the other structures and that the two have different behaviors. The tendency of Zr-centered Z16 local structures to be neighbors of Cu-centered icosahedral local structures is also confirmed. Our findings illustrate the importance of properly differentiating local structures in order to elucidate the behavior of metallic glasses at the atomic scale.

Hierarchical fivefold symmetry in CuZr metallic glasses

In this letter, we use molecular dynamics simulations to show that the CuZr metallic glasses have an intricate hierarchical fivefold symmetry that, either from the perspective of atom packing or icosahedral packing, the CuZr glasses follow the same fivefold packing symmetry in medium range. This fivefold symmetry extends up to surprisingly the eighth atomic shells, with the first five shells packed in Bergman- icosahedral scheme. These results show greater degree of order in glasses than presently envisaged and good agreement with experimental measurements which imply the intrinsic correlation between metallic glasses and quasicrystals as well.

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.

Creep behavior of polycrystalline Al (metal)-Cu50Zr50 (metallic glass) cold welds

Creep properties of cold weld polycrystalline Al (metal)-Cu50Zr50 (metallic glass) specimens are investigated by molecular dynamics (MD) simulations. The EAM (Embedded Atom Method) potential is used for modelling the interactions between Al, Cu and Zr atoms. Cold welding is carried out at a velocity of 30 m/s and interference (contact time) of 1.3 nm. The Creep behavior study of welded sample is carried out at different range of stress (100 MPa–500 MPa) and temperatures (100 K–500 K) for analysing the effect of stress and temperature on creep behavior. The creep curve shows very short primary creep and significant steady-state creep. Also with an increase in temperature or stress the steady-state creep rate increases. At lower temperature (100 K) the creep deformation mechanism is by dislocation slip and at a higher temperature (500 K) and higher stress (500 MPa) it is by diffusion. The stress exponent ‘n’ has two values, at lower stress n = 1.56 and at higher stress n = 5.82 respectively.