Deformation Of Metallic Glass Research Articles

Emergence of localized plasticity and failure through shear banding during microcompression of a nanocrystalline alloy

Microcompression testing is used to probe the uniaxial stress–strain response of a nanocrystalline alloy, with an emphasis on exploring how grain size and grain boundary relaxation state impact the complete flow curve and failure behavior. The yield strength, strain hardening, strain-to-failure and failure mode of nanocrystalline Ni–W films with mean grain sizes of 5, 15 and 90nm are studied using taper-free micropillars that are large enough to avoid extrinsic size effects. Strengthening is observed with grain refinement, but catastrophic failure through strain localization is found as well. Shear banding is found to cause failure, resembling the deformation of metallic glasses. Finally, we study the influence of grain boundary state by employing heat treatments that relax nonequilibrium boundary structure but leave grain size unchanged. A pronounced strengthening effect and increased strain localization are observed after relaxation in the finer grained samples.

Localized shear deformation and softening of bulk metallic glass: stress or temperature driven?

Metallic glasses due to their unique combination of physical and chemical properties have a great potential in various applications: materials for construction, medical, MEMs devices and so on. The deformation mechanism in metallic glasses is very much different from that in conventional crystalline materials and not yet fully understood. Here we are trying to find out what drives shear deformation in metallic glasses. The compression experiments of the bulk metallic glassy (BMG) samples coated with tin, Rose metal and indium were performed. There were no melting sites of the coating observed near individual shear bands. Melting occurred only near fracture surface, near microcracks and in the places of shear band concentrations. The results indicate that shear banding is rather a stress driven process while the temperature rise that was observed takes place due to friction forces in the viscous supercooled liquid thin layer in the shear bands.

Densification and Strain Hardening of a Metallic Glass under Tension at Room Temperature

The deformation of metallic glasses involves two competing processes: a disordering process involving dilatation, free volume accumulation, and softening, and a relaxation process involving diffusional ordering and densification. For metallic glasses at room temperature and under uniaxial loading, disordering usually dominates, and the glass can fail catastrophically as the softening process runs away in a localized mode. Here we demonstrate conditions where the opposite, unexpected, situation occurs: the densifying process dominates, resulting in stable plastic deformation and work hardening at room temperature. We report densification and hardening during deformation in a Zr-based glass under multiaxial loading, in a notched tensile geometry. The effect is driven by stress-enhanced diffusional relaxation, and is attended by a reduction in exothermic heat and hardening signatures similar to those observed in the classical thermal relaxation of glasses. The result is significant, stable, plastic, extensional flow in metallic glasses, which suggest a possibility of designing tough glasses based on their flow properties.

Anelasticity-induced increase of the Al-centered local symmetry in the metallic glass La50Ni15Al35

The mechanism of anelastic deformation of metallic glasses is a fundamental issue of materials physics. A critical step toward atomic level understanding is the identification of measurable atomic level structural parameters that respond to anelastic deformation. We demonstrate that the electric-field-gradient tensor measured by means of 27Al nuclear magnetic resonance in glassy La50Ni15Al35 is such a parameter and it reveals that anelasticity induces atomic processes that lead to increases of local site symmetry at Al sites. Such atomic processes could play an important role in the reversible slow β process.

Shear transformation zone dynamics model for metallic glasses incorporating free volume as a state variable

A mesoscale model, shear transformation zone dynamics (STZ dynamics), is employed to investigate the connections between the structure and deformation of metallic glasses. The present STZ dynamics model is adapted to incorporate a structure-related state variable, and evolves via two competing processes: STZ activation, which creates free volume, vs. diffusive rearrangement, which annihilates it. The dynamical competition between these two processes gives rise to an equilibrium excess free volume that can be connected to flow viscosity via the phenomenological Vogel–Fulcher–Tammann relation in relaxed structures near the glass transition temperature. On the other hand, the excess free volume allows glasses to deform at low temperatures via shear localization into shear bands, even in the presence of internal stress distributions that arise upon cooling after processing.

On the plasticity event in metallic glass

Based on a systematic molecular dynamics analysis, this study reveals that plastic deformation of metallic glass is not through a uniform configuration change but via many localized plasticity events. These events are manifested by the atomic clusters of high kinetic energy and high strain rate, emerging even in the elastic deformation regime. The life of such a plasticity event is on the order of 10−12 s, during which the distribution of kinetic energy follows a power law. The study shows that yielding in metallic glass occurs at the sudden surge point of the number of plasticity events. In the steady plastic deformation regime, the continuous nucleation and annihilation of the plastic events lead to a steady flow stress and stabilized total potential energy.

Plastic Deformation Clusters with High Kinetic Energy in Metallic Glass

Although localized atomic rearrangements have been considered to be the underlying mechanism of plastic deformation in metallic glass, the nucleation and evolution of such plasticity events are still elusive. With the aid of molecular dynamics analysis, this study revealed that a series of localized atomic rearrangement events would occur in metallic glass as demonstrated by the formation of high-kinetic-energy clusters. It was found that atomic clusters of average sizes of 1 to 2 nm nucleate during elastic deformation, and become prevailing after yielding. The cores of these clusters contain several high-velocity atoms, which drive the local structural change and accommodate plastic strain. The nucleation and evolution of the local plasticity events are shown clearly by the strong dynamic signature, attributed to the spontaneous structural reshuffling after crossing an energy barrier.

Comparative Study of Elastoplastic Constitutive Models for Deformation of Metallic Glasses

We present and compare three elastoplastic models currently used for deformation of metallic glasses, namely, a von Mises model, a modified von Mises model with hydrostatic stress effect included, and a Drucker-Prager model. The constitutive models are formulated in conjunction with the free volume theory for plastic deformation and are implemented numerically with finite element method. We show through a series of case studies that by considering explicitly the volume dilatation during plastic deformation, the Drucker-Prager model can produce the two salient features widely observed in experiments, namely, the strength differential effect and deviation of the shear band inclination angle under tension and compression, whereas the von Mises and modified von Mises models are unable to. We also explore shear band formation using the three constitutive models. Based on the study, we discuss the free volume theory and its possible limitations in the constitutive models for metallic glasses.

Towards more uniform deformation in metallic glasses: The role of Poisson's ratio

We develop a quantitative analysis of how the plastic deformation in a metallic glass is more uniform if its Poisson ratio ν is higher. The plasticity of metallic glasses under ambient conditions is mediated by shear localized in thin bands, and can be characterized by experiments on the bending of thin plates. We extend the analysis by Conner et al. (Conner et al., J. Appl. Phys. 94 (2003), 904–911) of bands in bent plates to include the micromechanics of individual shear bands. Expressions are derived for the shear-band spacing and the offset on each band. Both these quantities are predicted to decrease as ν is increased. The predictions are tested against measurements on metallic glasses with a wide range of ν. Good agreement is found, supporting the new model for the shear-band spacing, and pointing the way towards more diffuse deformation, and consequently improved plasticity and toughness, of metallic glasses as ν increases toward the limiting value of 0.5.

Ideal elasto-plastic behavior in torsional deformation of Zr44Ti11Cu10Ni10Be25 bulk metallic glass

Large plastic strain can be attained in Zr44Ti11Cu10Ni10Be25 bulk metallic glass below the glass transition temperature by applying the free-end torsion method. The initial part of the deformation of the metallic glass can be accurately described by a homogeneous ideal elasto-plastic model. Deviation from the ideal behavior occurs when plastic shear deformation becomes macroscopically inhomogeneous and longitudinal shear bands appear on the surface in addition to shear bands perpendicular to the torsion axis. Samples with multiple sets of shear bands fracture after substantially higher macroscopic shear deformation.

Formation and properties of strontium-based bulk metallic glasses with ultralow glass transition temperature

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The deformation units in metallic glasses revealed by stress-induced localized glass transition

We report that even in quasi-static cyclic compressions in the apparent elastic regimes of the bulk metallic glasses, the precisely measured stress-strain curve presents a mechanical hysteresis loop, which is commonly perceived to occur only in high-frequency dynamic tests. A phenomenological viscoelastic model is established to explain the hysteresis loop and demonstrate the evolutions of the viscous zones in metallic glasses during the cyclic compression. The declining of the viscosity of the viscous zones to at least 1 × 1012 Pa s when stress applied indicates that stress-induced localized glass to supercooled liquid transition occurs. We show that the deformation units of metallic glasses are evolved from the intrinsic heterogeneous defects in metallic glasses under stress and the evolution is a manifestation of the stress-induced localized glass transition. Our study might provide a new insight into the atomic-scale mechanisms of plastic deformation of metallic glasses.

On the characteristic length scales associated with plastic deformation in metallic glasses

Atomistic simulations revealed that the spatial correlations of plastic displacements in three metallic glasses, FeP, MgAl, and CuZr, follow an exponential law with a characteristic length scale ℓc that governs Poisson’s ratio ν, shear band thickness tSB, and fracture mode in these materials. Among the three glasses, FeP exhibits smallest ℓc, thinnest tSB, lowest ν, and brittle fracture; CuZr exhibits largest ℓc, thickest tSB, highest ν, and ductile fracture, while properties of MgAl lie in between those of FeP and CuZr. These findings corroborate well with existing experimental observations and suggest ℓc as a fundamental measure of the shear transformation zone size in metallic glasses.

Low temperature uniform plastic deformation of metallic glasses during elastic iteration

Molecular dynamics simulations and dynamic mechanical analysis experiments were employed to investigate the mechanical behavior of metallic glasses subjected to iteration deformation in a nominally elastic region. It was found that cyclic deformation leads to the formation of irreversible shear transformation zones (STZs) and a permanent uniform strain. The initiation of STZs is directly correlated with the atomic heterogeneity of the metallic glass and the accumulated permanent strain has a linear relation with the number of STZs. This study reveals a new deformation mode and offers insights into the atomic mechanisms of STZ formation and low temperature uniform plastic deformation of metallic glasses.

A mean-field model for anelastic deformation in metallic-glasses

In this article, a simple mean-field model is developed for quantitatively understanding the anelastic deformation in metallic-glasses (MGs). By relating the anelastic strain of an MG under a constant stress to the constrained configurational transition of liquid-like phases, we are able to derive the dynamic equation that captures the essential features of anelastic deformation hitherto reported. The important implications of this mean-field model are also discussed with reference to the recent findings of nano-scale structural heterogeneity in MGs.

Free volume simulation for severe plastic deformation of metallic glasses

Evolution of internal parameters of metallic glasses subjected to high pressure torsion was simulated using a free volume based thermo-mechanical model. Steady state has been predicted in the whole, plastically sheared volume for large strains irrespectively from the applied revolution rate. Based on the simplification of the model equations in steady state, simple methods were obtained to determine the in situ temperature increase and stress distribution from external torque measurements.

Correlation between elastic structural behavior and yield strength of metallic glasses

Tremendous research effort has been put into the study of plastic deformation mechanisms of metallic glasses (MGs) in an attempt to elucidate the origin of their high fracture strength. Little attention has, however, been paid to the elastic deformation of MGs. In this paper, a series of MGs with different yield strengths are studied, with a focus on the fine structural evolution (at the atomic level) during elastic deformation. Our results reveal that an atomic reorientation happens in the first nearest-neighbor shell due to elastic deformation. This reorientation subsequently leads to a drop in the local stress, which further results in a cooperative shift of surrounding atoms to counterbalance this change in local stress level. A concordant region is formed as a result. The relation between this concordant region and the yield strength is thoroughly discussed in terms of its size and the stress level in this region. It is proposed that this concordant region could be the missing part that bridges the macroscopic yield strength and the microscopic atomic structure.

Correlation between relaxations and plastic deformation, and elastic model of flow in metallic glasses and glass-forming liquids

We study the similarity and correlations between relaxations and plastic deformation in metallic glasses (MGs) and MG-forming liquids. It is shown that the microscope plastic events, the initiation and formation of shear bands, and the mechanical yield in MGs where the atomic sites are topologically unstable induced by applied stress, can be treated as the glass to supercooled liquid state transition induced by external shear stress. On the other hand, the glass transition, the primary and secondary relaxations, plastic deformation and yield can be attributed to the free volume increase induced flow, and the flow can be modeled as the activated hopping between the inherent states in the potential energy landscape. We then propose an extended elastic model to describe the flow based on the energy landscape theory. That is, the flow activation energy density is linear proportional to the instantaneous elastic moduli, and the activation energy density ρE is determined to be a simple expression of ρE=1011G+111K. The model indicates that both shear and bulk moduli are critical parameters accounting for both the homogeneous and inhomogeneous flows in MGs and MG-forming liquids. The elastic model is experimentally certified. We show that the elastic perspectives offers a simple scenario for the flow in MGs and MG-forming liquids and are suggestive for understanding the glass transition, plastic deformation, and nature and characteristics of MGs

A new constitutive model for shear banding instability in metallic glass

Inelastic deformation of metallic glass is through shear banding, characterized by significantly localized deformation and emerged expeditiously under certain stress state. This study establishes a new constitutive model addressing the physical origin of the shear banding. In the modeling, the atomic structural change and the free volume generation are embodied by the plastic shear strain and the associated dilatation. The rugged free energy landscape is adopted to naturally reflect the rate-independent flow stress and flow serrations. Based on this, the conditions for the onset of shear banding instability are established, which enables the explicit calculation of the shear band inclination angle and its extension speed. The study concludes that shear band angle is significantly influenced by the diltancy factor and pressure sensitivity, that a shear band does not increase its thickness once emanated from a deformation unit, that the spreading speed of a shear band is intersonic, and that more shear bands, which lead to higher ductility, can be induced by high strain rates or by the introduction of a second material phase. The analysis also demonstrates that the ductility of metallic glass depends on the sample geometry and/or the stress state.

Stick-slip dynamics and recent insights into shear banding in metallic glasses

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