Shear Bands In Metallic Glasses Research Articles

Uncovering the structural origin of shear banding in metallic glasses via tracing back to the critical glass transition process

Uncovering the structural origin of shear banding in metallic glasses via tracing back to the critical glass transition process

How to catch a shear band and explain plasticity of metallic glasses with continuum mechanics

Capturing a shear band in a metallic glass during its propagation experimentally is very challenging. Shear bands are very narrow but extend rapidly over macroscopic distances, therefore, characterization of large areas at high magnification and high speed is required. Here we show how to control the shear bands in a pre-structured thin film metallic glass in order to directly measure local strains during initiation, propagation, or arrest events. Based on the experimental observations, a model describing the shear banding phenomenon purely within the frameworks of continuum mechanics is formulated. We claim that metallic glasses exhibit an elastic limit of about 5% which must be exceeded locally either at a stress concentrator to initiate a shear banding event, or at the tip of a shear band during its propagation. The model can successfully connect micro- and macroscopic plasticity of metallic glasses and suggests an alternative interpretation of controversial experimental observations.

Quantifying the Size-Dependent Shear Banding Behavior in High-Entropy Alloy-Based Nanolayered Glass.

Extensive research has shown that nanolayered structures are capable of suppressing the shear banding in metallic glass in nanoindentation experiments. However, the specific mode and mechanism of the shear banding underneath the indenter remains unknown. Also, the quantification of shear banding-induced strain localization is still a challenge. Herein, the size-dependent shear banding behavior of a CuTiZrNb high-entropy alloy-based nanolayered glass with individual layer thicknesses (h) ranging from 5 to 80 nm was systematically investigated by nanoindentation tests. It was found that the hardness of the designed structure was almost size-independent. Yet, a clear transition in the deformation modes from the cutting-like shear bands to the kinking-like ones was discovered as h decreased to 10 nm. Moreover, multiple secondary shear bands also appeared, in addition to the primary ones, in the sample with h = 10 nm. The transition leads to an obvious strain delocalization, as clearly illustrated by the proposed theoretical model, which is based on the assumption of a pure shear stress state to quantify the shear banding-induced strain localization. The strain delocalization results from the higher density of amorphous/amorphous interfaces that exhibit the change in morphology with a refined layered glass structure.

Substantially enhanced homogeneous plastic flow in hierarchically nanodomained amorphous alloys

To alleviate the mechanical instability of major shear bands in metallic glasses at room temperature, topologically heterogeneous structures were introduced to encourage the multiplication of mild shear bands. Different from the former attention on topological structures, here we present a compositional design approach to build nanoscale chemical heterogeneity to enhance homogeneous plastic flow upon both compression and tension. The idea is realized in a Ti-Zr-Nb-Si-XX/Mg-Zn-Ca-YY hierarchically nanodomained amorphous alloy, where XX and YY denote other elements. The alloy shows ~2% elastic strain and undergoes highly homogeneous plastic flow of ~40% strain (with strain hardening) in compression, surpassing those of mono- and hetero-structured metallic glasses. Furthermore, dynamic atomic intermixing occurs between the nanodomains during plastic flow, preventing possible interface failure. Our design of chemically distinct nanodomains and the dynamic atomic intermixing at the interface opens up an avenue for the development of amorphous materials with ultrahigh strength and large plasticity.

Direct observation of quadrupolar strain fields surrounding Eshelby inclusions in metallic glasses.

For decades, scanning/transmission electron microscopy (S/TEM) techniques have been employed to analyze shear bands in metallic glasses and understand their formation in order to improve the mechanical properties of metallic glasses. However, due to a lack of direct information in reciprocal space, conventional S/TEM cannot characterize the local strain and atomic structure of amorphous materials, which are key to describe the deformation of glasses. For this work, we applied 4-dimensional STEM to map and directly correlate the local strain and the atomic structure at the nanometer scale in deformed metallic glasses. We observe residual strain fields with quadrupolar symmetry concentrated at dilated Eshelby inclusions. The strain fields percolate in a vortex-like manner building up the shear band. This provides a new understanding of the formation of shear bands in metallic glass. This article is protected by copyright. All rights reserved.

Martensite transformation govern serration deformation of a metastable metallic glass matrix composite

A metastable Ti-based metallic glass matrix composite (MGMC) exhibits deformation behavior that is closely related to the temperature and strain rate in the supercooled liquid region. The state of the amorphous matrix varies with the temperature, resulting in different responses to the overall mechanical properties. The remarkable strain rate sensitivity makes the strength of this MGMC reach 1,355 MPa, which is more than that of most MGMCs at diverse strain rates. In addition, serration deformation induced by martensite transformation is found in MGMC, which is different from serrated flow caused by the movement of shear bands in metallic glasses. The serration behavior changes synchronously with the martensite transformation.

The dynamics of shear band propagation in metallic glasses

Understanding the dynamics of shear band propagation in metallic glasses remains elusive due to the limited temporal and spatial scales accessible in experiments. In micron-scale molecular dynamics simulations on two model metallic glasses, we studied the propagation of a dominant shear band under uniaxial tension with a macroscopic strain of 3-5%. For both materials, the shear band can be intersonic with a propagation speed exceeding their respective shear wave speeds. The propagation exhibits intrinsic instability that manifests itself as microbranching and considerable fluctuations in velocity. The shear strain singularity ahead of propagating shear band tip scales as 1/r (r is the distance away from the tip), independent of the macroscopic tensile strain. In addition, we studied the intersection of two shear bands under uniaxial tension, during which path deflection, speed slowing-down, and temperature rise at the junction region were observed. The dynamics of propagating shear band shown here indicate that shear band in metallic glasses can be viewed as shear crack under the framework of weakly nonlinear fracture mechanics theory.

Percolation-like transition from nanoscale structural heterogeneities to shear bands in metallic glass detected by static force microscopy

Structural heterogeneities of metallic glasses at the nanoscale have been confirmed by many experiments. Some simulations indicate that these structural heterogeneities show a direct correlation with the initiation of shear bands. However, due to the limited experimental conditions, the detailed evolution from nanoscale heterogeneities to shear bands during deformation is still unclear. Here, a tensile platform coupled with in situ static force microscopy is designed to detect the evolution of nanoscale heterogeneous viscoelasticity during deformation in a CuZr metallic glass. We observe that the dominant viscoelastic mode changes from solid-like to liquid-like with increasing tensile strain, which displays the percolation-like character. Meanwhile, the correlation length of structural heterogeneities increases with strain, and the stripe-like shear bands form when the correlation length reaches a critical value. The current findings not only present a clear picture of the evolution of structural heterogeneities with stress but also reveal the microscopic deformation mechanism of amorphous materials.

Experimental evidence that shear bands in metallic glasses nucleate like cracks

Highly time-resolved mechanical measurements, modeling, and simulations show that large shear bands in bulk metallic glasses nucleate in a manner similar to cracks. When small slips reach a nucleation size, the dynamics changes and the shear band rapidly grows to span the entire sample. Smaller nucleation sizes imply lower ductility. Ductility can be increased by increasing the nucleation size relative to the maximum (“cutoff”) shear band size at the upper edge of the power law scaling range of their size distribution. This can be achieved in three ways: (1) by increasing the nucleation size beyond this cutoff size of the shear bands, (2) by keeping all shear bands smaller than the nucleation size, or (3) by choosing a sample size smaller than the nucleation size. The discussed methods can also be used to rapidly order metallic glasses according to ductility.

Towards commonality between shear banding and glass-liquid transition in metallic glasses

Despite the high attendance of shear banding in metallic glasses and other disordered materials, the nature of the emergence of shear band is still mysterious. Using molecular dynamics simulations, a set of detailed characterizations of shear band in a typical ${\mathrm{Cu}}_{50}{\mathrm{Zr}}_{50}$ metallic glass is obtained. Then we uncover a large number of robust and intriguing commonalities between the emergence of shear bands and the glass-to-liquid transition, including strong similarities on viscosity drop, enthalpy discontinuity, breakdown of hard backbone network, as well as relaxation process. Such observations indicate that shear banding in metallic glasses is a consequence of deformation-controlled glass transition, as further quantitatively validated via the compelling overlap between the venerable Vogel-Fulcher-Tammann law (and Adam-Gibbs relation) and the evolving glass state of shear band controlled by configurational temperature. These results provide a direct bridge between shear banding and glass-to-liquid transition and are instrumental to build the unified framework of flow behavior induced either by thermal or stressed stimuli in disordered materials.

Tension-compression asymmetry of grain-boundary sliding: A molecular dynamics study

• A pressure-dependent behavior of grain-boundary sliding is found. • A new contribution to the explanation of the tension–compression asymmetry of mechanical properties in nanomaterials. • A novel strengthening mechanism of compression-induced GB bending is found. The underlying mechanisms of the tension–compression asymmetry of grain boundary (GB) sliding are studied by molecular dynamics (MD) simulations. The results show that the compressive stress has a suppression effect on GB sliding, while the tensile stress has a promotion effect. The pressure-dependent behaviors of GB sliding are attributed to two aspects. The first one is the tension–compression asymmetry of local shear events in GBs, which is similar to shear banding in metallic glasses. The other is due to a novel strengthening mechanism of compression-induced GB bending, which increases the plastic flow stress.

Mapping Shear Bands in Metallic Glasses: From Atomic Structure to Bulk Dynamics.

A deep understanding of the mechanisms controlling shear banding is of fundamental importance for improving the mechanical properties of metallic glasses. Atomistic simulations highlight the importance of nanoscale stresses and strains for shear banding, but corresponding experimental proofs are scarce due to limited characterization techniques. Here, by using precession nanodiffraction mapping in the transmission electron microscope, the atomic density and strain distribution of an individual shear band is quantitatively mapped at 2nm resolution. We demonstrate that shear bands exhibit density alternation from the atomic scale to the submicron scale and complex strain fields exist, causing shear band segmentation and deflection. The atomic scale density alternation reveals the autocatalytic generation of shear transformation zones, while the density alternation at submicron scale results from the progressive propagation of shear band front and extends to the surrounding matrix, forming oval highly strained regions with density consistently higher (∼0.2%) than the encapsulated shear band segments. Through combination with molecular dynamic simulations, a complete picture for shear band formation and propagation is established.

Predicting the location of shear band initiation in a metallic glass

We report atomistic simulation results which indicate that the location of shear banding in a metallic glass (MG) can be ascertained with reasonably high accuracy solely from the undeformed static structure. Correlation is observed between the location of the initiation of shear bands in a simulated MG and the initial distribution of the density of fertile sites (DFS) for stress-driven shear transformations identified a priori based on a deep learning model devised in our recent work [Fan and Ma, Nat. Commun. 12, 1506 (2021)]. In addition, we demonstrated that one can judge whether a glass is brittle or ductile solely based upon its initial DFS distribution. These validate that shear bands in MG arise from non-linear instabilities, and that the as-quenched glass structure contains inhomogeneities that influence these instabilities. This work also reveals an important subtlety regarding the non-deterministic nature of athermal quasistatic shear simulations.

Visualization of indentation induced sub-surface shear bands of Zr-based metallic glass by nanosecond pulse laser irradiation

Characterizing the shear band of metallic glasses (MGs) is meaningful to understand its plastic deformation mechanism. However, the direct method for detecting the internal shear bands of deformed MGs with large field of view is still lacking. Inspired by the fact that surface defects have the enhanced absorption of laser energy, this study attempted to visualize the indentation induced sub-surface shear bands of Zr-based MG by nanosecond pulse laser irradiation in argon. The principle was first verified by laser irradiation of the surface scratch, and then implemented on the visualization of sub-surface shear bands. The experimental results demonstrated that the indentation induced sub-surface shear bands were successfully visualized by nanosecond pulse laser irradiation, and meanwhile, the laser irradiated surface maintained the amorphous characteristic. This study may open a new window for exploring the shear band characteristics of MGs upon external loads.

The formation and propagation mechanism of shear band in bulk metallic glasses under dynamic compression

Shear banding behavior plays a significant role in the deformation and failure mechanisms of metallic glasses (MGs). However, due to the limits of dynamic loading and associated characterization methodologies, relevant studies of shear band in dynamic loading remain hazy. Using digital image correlation technologies, the spatial–temporal evolutions of shear bands such as formation, propagation, effective thickness, and instability are in-situ revealed and discussed during the SHPB test. Specific determinism exists for the nucleated locations and orientations of the primary shear band, which is responsible for final failure. The progressive propagation front of the shear band is found to be strongly correlated to the primary shear stress profile, suggesting that the importance of the principal shear stress in driving shear-band propagation. Additionally, a long-range stress zone of the shear band on the order of hundreds of microns is found, which exhibits uniform shear viscosity. The findings of this article shed light on the viability of artificially created shear bands in metallic glasses.

In Situ Generated Shear Bands in Metallic Glass Investigated by Atomic Force and Analytical Transmission Electron Microscopy

Plastic deformation of metallic glasses performed at temperatures well below the glass transition proceeds via the formation of shear bands. In this contribution, we investigated shear bands originating from in situ tensile tests of Al88Y7Fe5 melt-spun ribbons performed under a transmission electron microscope. The observed contrasts of the shear bands were found to be related to a thickness reduction rather than to density changes. This result should alert the community of the possibility of thickness changes occurring during in situ shear band formation that may affect interpretation of shear band properties such as the local density. The observation of a spearhead-like shear front suggests a propagation front mechanism for shear band initiation here.

Nano-voids formation at the interaction sites of shear bands in a Zr-based metallic glass

Understanding the formation mechanism of voids is a significant issue in controlling the catastrophic fracture in the form of shear bands in metallic glasses. Here, using an amplitude-modulation atomic force microscope, we investigated the nano-voids formation at the mutual interaction of shear bands in a Cu50Zr50metallic glass. The results of phase shift revealed higher energy dissipation and more soft zones for the nano-voids. The formation of these nano-voids results from tensile stress concentration caused by the interaction of shear bands, based on the results of finite element simulation. The appearance of nano-voids and stress distribution at the site of shear band interaction is essential in understanding the plastic deformation and fracture of metallic glasses.

A new idea of modeling shear band in metallic glass based on the concept of distributed dislocation

Shear bands are closely linked with the plasticity and fracture behaviors of metallic glasses (MGs). This work proposes a new idea to predict shear bands by continuously distributed dislocations. The dislocations are unreal and used to model the plastic deformation of shear band. The possible positions of shear band initiation are determined based on the elastic stress field, and the direction and length of shear band propagation are determined by the distributed dislocation technique. Finite element simulations based on constitutive model are carried out to compare with the theoretical modeling. Two examples are considered, i.e., shear bands near a void and a notch under tensile loading. The results show that the theoretically predicted shear band morphology is well consistent with the finite element simulations, which verifies the validation of using distributed dislocations to predict shear bands. This work provides a new way to model shear band, and it has potential applications in predicting shear band morphology and fracture behaviors in MGs.

Giant configurational softening controls atomic-level process of shear banding in metallic glasses

While the shear banding is a ubiquitous feature for metallic glasses and other disordered solids, the underlying material softening mechanism is still an open question requiring physical interpretation at atomic level. Here, through a set of atomistic simulations, we clarify the origin of flow localization, i.e., the birth of a shear band. The local thermal temperature induced by atomic vibration and configurational temperature attributed to structural disordering are proposed to quantify critical degree of thermal and configurational softening, respectively. The comparison between configurational softening and thermal softening being two potential causes for shear banding emergence is then examined at atomic scale. Numerical evidence from atomistic simulations indicates that configurational-softening-induced strain burst is the dominant instability mode as evidenced through a very large value of configurational temperature rise that commensurate with glass transition temperature. It is also found that configurational softening takes precedence over thermally activated ones. Configurational softening is thus conceivably acting as the root cause for the onset of strain localization and shear banding, while the subsequent thermal softening is its consequence. These results provide important insights into the puzzle about the material weakening mechanism underlying shear banding.

A Comparison of in- and ex-situ Shear Bands in Metallic Glass by Transmission Electron Microsopy

Plastic deformation of metallic glasses performed at temperatures well below the glass transition proceeds via the formation of shear bands. In this contribution shear bands originating from in situ tensile tests of Al88Y7Fe5 melt-spun ribbons conducted in a transmission electron microscope are compared with ones which had formed ex situ during cold rolling. The observed contrasts for shear bands generated in situ at the edges of thin electron-transparent foils are shown to be related to a meniscus-like foil thickness reduction. In comparison with ex situ samples, alternating contrast changes due to volume changes are missing and the shear band width is a factor of 15 larger. The meniscus-like shear band profile is accounted for by the thin foil allowing volume increase due to shear deformation to annihilate via the free surfaces of the thin foil and the sheared zones to widen.