Model Metallic Glass Research Articles

Activation-relaxation technique study on β-relaxation in La55Ni20Al25 and Cu46Zr46Al8 metallic glasses

Activation-relaxation technique combined with molecular dynamics simulations was employed to investigate β-relaxation behavior in La55Ni20Al25 and Cu46Zr46Al8 model metallic glasses (MGs) for exploring the underlying mechanism. It is found that the activation energies are much smaller in La55Ni20Al25 MG, indicating that β-relaxation can be more easily activated in La55Ni20Al25 MG. It is also revealed that in both MGs, string-like diffusion dominates the atomic motions in β-relaxation. However, in different MGs, the string-like motions are found to involve atoms with different structure features, which could be the underlying structural basis of β-relaxation mechanism. In addition, because of very low activation energies, atomic hopping and cooperative rearrangements in La55Ni20Al25 MG also have significant impact on β-relaxation. However, such atomic motions are much weaker in Cu46Zr46Al8 MG, due to higher activation energies. These findings establish a physical link for activation energy and atomic motions in β-relaxation to atomic structures, and provide new understanding of β-relaxation mechanism in MGs.

Correlation between initial structure and athermal quasi-static compressive deformation in a metallic glass

Athermal quasi-static compressive deformation for a model metallic glass is investigated by molecular dynamics simulations. A new structural parameter, quasi-nearest atom (QNA), is used to characterize local atomic packing of initial structure. We find that regions with higher nonaffine displacement have more atoms with more QNA. The coarse grained number of QNAs exhibits significant spatial heterogeneities and these structural fluctuations show close correlation with the nonaffine displacement. The correlation between the coarse grained number of QNA and nonaffine displacement does not decrease as the strain increases but show the maximum at a certain strain, indicating that strain localized regions are activated individually. Therefore, our results provide a direct evidence of the correlation between atomic structure and deformation in metallic glass.

Correlation between Viscoelastic Moduli and Atomic Rearrangements in Metallic Glasses.

Dynamical moduli, such as storage and loss moduli, characterize the viscoelasticity of materials (i.e., time-dependent elasticity) and convey important information about the relaxation processes of glasses and supercooled liquids. A fundamental question is what ultimately determines them in glassy materials. Here, for several model metallic glasses, we demonstrate that both the storage and loss moduli are uniquely determined by the most probable atomic nonaffine displacements, regardless of temperature or frequency. Moreover, the fast-moving atoms (which contribute to dynamical heterogeneity) do not contribute explicitly to the moduli. Our findings provide a physical basis for the origin of viscoelasticity in metallic glasses.

Bulk Mechanical Properties Testing of Metallic Marginal Glass Formers

We developed a unique three-point bend testing apparatus to measure bulk mechanical properties of a model metallic glass alloy (SAM2X5 with nominal composition Fe49.7Cr17.1Mn1.9Mo7.4W1.6B15.2C3.8Si2.4) prepared by spark plasma sintering. The relatively large sample sizes in the present work allowed for the preparation of test specimens with a macroscale cross section (in the millimeter range) with well-controlled sample dimensions closer to standardized tests. Wire saw cutting allowed for a relatively sharp notch radius (3x smaller than previous studies) and minimal sample damage. We determined that Young’s modulus and notch fracture toughness measured by our three-point bending apparatus are 230 GPa and 4.9 MPa·m1/2. Also, Vickers indentation and flexure testing provided consistent results for Young’s modulus. Indentation fracture toughness measured by Vickers indentation produced values at least 50% lower than by flexure. The microscale mechanical properties testing technique presented in this work and subsequent analyses are applicable to specimens of other compositions or ones prepared by other methods.

Localized plastic deformation in a model metallic glass: a survey of free volume and local force distributions

Plastic deformation of a model glass is investigated via large scale molecular dynamics simulations. The role of microscopic fluctuations of the structure for the deformation behavior is highlighted by demonstrating that statistically independent samples prepared via an identical protocol develop qualitatively distinct deformation paths. As a quantitative measure, the spatial distribution of the particle based excess volume is monitored via Voronoi tesselation. While the fluctuations of the thus defined single-particle based excess volume do not seem to show any signature of the strain field, a non-local definition of the excess volume clearly correlates with the observed shear deformation field. The distribution of the force acting on individual particles also shows a pattern strongly similar to that of the strain. In line with other studies, these results underline the importance of both the structural heterogeneities as well as the fluctuations of the locally acting forces and stresses for plastic deformation in amorphous solids.

Crack initiation in metallic glasses under nanoindentation

Simulated nanoindentation tests on a model metallic glass reveal that the crack initiates inside a shear band via cavitation. The load-displacement curve was shown to be insensitive to the crack initiation but sensitive to subsequent crack propagation. The critical conditions for crack initiation were identified at both the macroscopic and microscopic levels. At the macroscopic level, the indenter geometry affects the overall critical load for crack initiation. Interestingly, the indentation volume at crack initiation appears to be a constant for different indenter geometries, based on which an analytical formula of the critical load as a function of the indenter geometry was derived. At the microscopic level, cavitation occurs once the normal stress perpendicular to the shear band exceeds a temperature-dependent critical cavitation stress. This critical cavitation stress was shown to reduce significantly upon shear deformation.

Competition between shear band nucleation and propagation across rate-dependent flow transitions in a model metallic glass

Shear transformation zone (STZ) dynamics is used to examine the transition between different regimes of flow serration in the strain rate dependent deformation of metallic glass. To capture the strain rate independent yield strength of Vitreloy 1 at low to moderate strain rates, the model is adapted to include STZ volume and activation energy that decrease with increasing strain rate. The different stages of shear banding are examined in a statistical fashion over six different strain rates ranging from 10−5 to 100 s−1, with twelve replicates at each strain rate. Examination of flow serration, shear band nucleation rates, propagation rates, and sliding rates in each simulation find support for the hypothesis that the flow transition is caused by high shear band propagation and sliding rates at low strain rates, and high shear band nucleation rates at high strain rates. The underlying cause for the flow transition is hypothesized to be a strain rate dependent critical shear band nucleus size that increases with increasing strain rate. This critical shear band nucleus size results from the strain rate dependent STZ volume and activation energy, in which very small variations can cause a large change in shear banding behavior.

A model metallic glass exhibits size-independent tensile ductility

Metallic glasses (MGs) usually suffer brittle fracture under uniaxial tension due to size-dependent shear band cavitation. Here we developed a concurrent multi-scale simulation method to describe the uniaxial tension of a binary Lennard-Jones (BLJ) model glass up to 88 microns in length. No cavitation or brittle fracture was found even for the longest BLJ sample. As the length increases, the shear band temperature increases, then saturates, while the elastic unloading from shear-off diminishes. We conjecture that the shear band of a BLJ sample, even with a macroscopic length, cannot reach the herein estimated critical cavitation conditions. Thus, BLJ samples appear to be free of size-induced tensile brittleness. Based on the shear band evolution and the critical cavitation conditions, we propose three classes of MGs in terms of tensile ductility: brittle-MGs (brittle for all lengths), normal-MGs (ductile for short samples, brittle for long samples), cohesive-MGs (ductile for all lengths). Our simulation results illustrate limitations of existing molecular models, and suggest that certain experimental metallic glasses may be free of size-induced tensile brittleness.

Unified Criterion for Temperature-Induced and Strain-Driven Glass Transitions in Metallic Glass.

In a model metallic glass, we study the relaxation dynamics in both the linear and the nonlinear response regimes by numerical simulations of dynamical mechanical spectroscopy and analyze the atomic displacement statistics. We find that the primary (α) relaxation always takes place when the most probable atomic displacement reaches a critical fraction (~20%) of the average interatomic distance, irrespective of whether the relaxation is induced by temperature (linear response) or by mechanical strain (nonlinear response). Such a unified scenario, analogous to the well-known Lindemann criterion for crystal melting, provides insight into the structural origin of the strain-induced glass-liquid transition.

The local stress state of a running shear band in amorphous solids

Abstract

Strain induced fragility transition in metallic glass.

Relaxation dynamics are the central topic in glassy physics. Recently, there is an emerging view that mechanical strain plays a similar role as temperature in altering the relaxation dynamics. Here, we report that mechanical strain in a model metallic glass modulates the relaxation dynamics in unexpected ways. We find that a large strain amplitude makes a fragile liquid become stronger, reduces dynamical heterogeneity at the glass transition and broadens the loss spectra asymmetrically, in addition to speeding up the relaxation dynamics. These findings demonstrate the distinctive roles of strain compared with temperature on the relaxation dynamics and indicate that dynamical heterogeneity inherently relates to the fragility of glass-forming materials.

Visualization of topological landscape in shear-flow dynamics of amorphous solids

A potential energy landscape (PEL) gives rise to diverse non-equilibrium dynamics in complex systems. In this study, we visualize the topological landscape of a steady-state flow under shear deformation in a model metallic glass. We use the “state space network” as the best predictor of state transitions in terms of statistical model selection. The proposed scheme visualizes the underlying PEL regarding α transitions associated with irreversible cage-breaking events as the cooperative motion of clusters. By characterizing the topological property of the state space network, we observe characteristics of a small-world network. The proposed scheme has a significant potential to enable the understanding of the underlying mechanisms of glass physics, and is analogous to biological networks.

Microstructural factors of strain delocalization in model metallic glass matrix composites

Metallic glass matrix (MGM) composites demonstrate impressive plasticity over monolithic metallic glasses due to their ability to delocalize strain and inhibit shear band propagation. This work investigates various microstructural factors (volume fraction, length scale and yield strength) influencing strain delocalization in a model MGM composite. Shear transformation zone dynamics is utilized to model the amorphous phase while a local Taylor dislocation model is used for the crystalline phase. An N-factorial experiment examines replicates for various macro- and microscopic measures of strain delocalization and regression analysis is used to identify statistically significant trends in the data. The experiment shows that strain delocalization and the consequent ductility are most strongly influenced by a crystalline phase with a substantially lower yield stress than that of the amorphous matrix. It also shows that increased crystalline volume fraction alone is insufficient to promote strain delocalization in the case of a crystalline phase with high relative yield stress, and that a lower yield stress for the crystalline phase implies lower maximum stresses supported by the composite. Examination of shear band–inclusion interactions indicate that the nucleation and diffuse transmission of numerous shear bands is critical to the strain delocalization. The regression analysis provides continuous functional forms for the various relationships between properties that can be exploited to optimize the microstructure of MGM composites.

Atomic mechanism of internal friction in a model metallic glass

Internal friction (IF) describes the ability of materials to damp out mechanical oscillations. It is a crucial engineering parameter and also conveys unique microscopic information about structural defects, transport phenomena, and phase transformations in solids. While IF predominately results from lattice defects in crystalline materials, the origin of IF remains unclear in disordered materials, like metallic glasses. In this paper, we study the atomic rearrangements that govern IF in a model metallic glass, via numerical simulations of dynamical mechanical spectroscopy together with structural analysis. We identify cooperative and avalanchelike thermal-driven excitations as an underlying mechanism and demonstrate a linearlike relation between the concentrations of these excitations and the values of IF. Structurally, these excitations can be hindered, and thus suppress IF, by slow atoms that usually associate with full icosahedral symmetry. Our results also provide practical guides in tuning IF in metallic glasses from atomistic perspectives.

Barkhausen noise in metallic glasses with strong local anisotropy: model and theory

.We discuss a model metallic glass in which Barkhausen noise can be studied in exquisite detail, free of thermal effects and of the rate of ramping of the magnetic field. In this model the mechanism of the jumps in magnetic moment that cause the Barkhausen noise can be fully understood as consecutive instabilities where an eigenvalue of the Hessian matrix hits zero, leading to a magnetization jump Δm which is simultaneous with a stress and energy changes Δ σ and ΔU respectively. Due to a large effect of local anisotropy, in this model Barkhausen noise is not due to movements of magnetic domain boundaries across pinning sites. There are no fractal domains, no self-organized criticality and no exact scaling behavior. We present a careful numerical analysis of the statistical properties of the phenomenon, and show that with every care taken this analysis is tricky, and easily misleading. Without a guiding theory it is almost impossible to get the right answer for the statistics of Barkhausen noise. We therefore present an analytic theory that culminates in a probability distribution function that is in excellent agreement with the simulations.

A Diagram for Glass Transition and Plastic Deformation in Model Metallic Glasses

In this paper, we report a strain rate related glass transition in model SrCaYbMg(Li)Zn(Cu) metallic glasses at room temperature. A critical strain rate, equivalent to glass transition temperature, is found for the strain rate induced glassy state to liquid-like viscoplastic state translation. The results show that the observation time, equivalent to temperature and stress, is a key parameter for the transition between the glass and supercooled liquid states. A three-dimension glass transition diagram involved in time, temperature and stress in metallic glasses is established.

Evolution of Mechanical Anisotropy in Nano-scale Metallic-glass thin Films Under Indentation

The mechanical behavior of Cu-Zr-Al metallic-glass thin films under indentation was studied with the molecular dynamics simulation methodology to investigate changes of microstructures through degrees of elastic anisotropy. In this work, the (Cu50Zr50)100-xAlx, where x is measured by atomic percentage, was studied by molecular dynamics (MD) simulation. Our metallic-glass models were first sputter-deposited on crystalline substrate by MD with consideration argon working gas. The interaction between argon working gas and metallic atoms was modeled by the pair-wise Moliere potential. The as-deposited films were amorphous after equilibration, and their mechanical properties were characterized by ultra-nano-indentation simulations by MD with a right-angle diamond conical indenter tip, as well as a strain perturbation method to identify full elastic constants under the tip. Through region-by-region probes of the elastic properties of the film after being indented, it is found that local elastic anisotropy changes in relation to locations and penetration depth.

Effects of atomic size difference and heat of mixing parameters on the local structure of a model metallic glass system

Atomic size differences between constituting elements and the heat of mixing are key factors in designing a metallic glass system. In this study, the effects of atomic size differences and the heat of mixing on the glass-forming ability and the local structure of metallic glasses were studied via molecular dynamic simulations of an ideal system known as the Lennard-Jones embedded-atom method model. The atomic size difference and the heat of mixing of the system were varied by means of the Lennard-Jones parameters. The glass transition behavior was characterized based on the chemical short-range order and by a Voronoi analysis. Our simulations lead to optimized windows of atomic size differences and heat of mixing parameters for metallic glass-forming of the model system. Both a greater negative heat of mixing and a larger atomic size difference are necessary for the enhancement of the glass-forming ability.

Inverse Meyer-Neldel behavior for activated processes in model glasses

The activation-relaxation technique is used to explore the distribution of escape times from metastable minima of a model metallic glass, estimated using the harmonic transition state theory. We investigate in particular the distribution of attempt frequencies and find that these prefactors are distributed over a range of almost ten orders of magnitude, in stark contrast with the common assumption that they are given by a typical vibrational frequency. Moreover, the typical attempt frequencies appear to decrease with increasing barrier energy, a behavior inverse to the so-called Meyer-Neldel rule observed in many processes with apparent Arrhenius behavior. When combined with a multiple-hop analysis that takes into account the multiplicity of possible transitions, a direct Meyer-Neldel behavior is recovered, albeit with a reduced characteristic prefactor.

Ordering in model metallic glass upon external homogeneous shear

Nonequilibrium molecular dynamics simulations are performed for model metallic glass upon external homogeneous shear. The results from cluster analysis reveal the positive influence of external shear on the structural ordering in the considered glassy system. It is found that homogeneous shear facilitates the appearance of crystalline clusters whose structure is characterized by face-centered cubic and hexagonal symmetries.