Major Shear Band Research Articles

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.

Rate-dependent inhomogeneous creep behavior in metallic glasses

Creep deformation can be classified as homogeneous flow and inhomogeneous flow in bulk metallic glass (BMG). In order to understand the conversion conditions of the two types of creep deformation, the effect of loading rate on the creep behavior of a Ti40Zr10Cu47Sn3 (at.%) BMG at ambient temperature was investigated using nanoindentation and molecular dynamic simulation. Results indicate that at low loading rates, many serrations appear in loading stage, leading to inhomogeneous serrated flow in the creep stage. When the loading rate is high enough, the creep deformation tends to be homogeneous. The related mechanism responsible for the rate-dependent creep behavior is attributed to the number of pre-existing major shear bands which is influenced significantly by the loading rate.

Notch Effect on Plastic Deformation of Metallic Glass: A Numerical Study by Revised Free‐Volume Theory

By means of a revised free‐volume theory, the notch effect on metallic glass was systematically investigated by the numerical method. Simulations on specimens without notches demonstrated that the parameters being determined in this work could reasonably describe the strength asymmetry of tension and compression. Moreover, four samples with different notches were used to numerically investigate the notch effect on global strength and plasticity. A better agreement could also be achieved between current simulations with existing experimental results, compared with another free‐volume model. Combined with the free‐volume distribution during deformation process, it was proven that the intersection of two major shear bands is the cause for the strength and plasticity enhancement found in sample with two symmetric notches. Besides, strength asymmetry between tension and compression was also found for notched samples. Compressive strengths are accordingly higher than tensile ones. Moreover, with the augment of the aspect ratio, the plasticity for specimens with two symmetric notches was found to increase firstly and then decrease afterwards.

Revisiting the effect of aspect ratio on shear band evolution by hydrostatic-stress embedded free-volume theory

The shear band evolution process induced by different aspect ratios was investigated by experiments and hydrostatic-stress embedded free-volume theory. It was found that specimen with lower aspect ratio displayed a higher compressive plasticity since the shear band propagation was confined by the machine head in a way of preventing the sample from fracturing along one major shear band. Hydrostatic-stress embedded free-volume theory was incorporated into ABAQUS as a user material subroutine UMAT. Numerical method was performed to reveal the shear band evolution caused by different aspect ratios. There was a good agreement between experiments and simulations. Furthermore, simulations demonstrated that sample with smaller aspect ratios yielded a relatively lower strain localization level, leading to a correspondingly stable plastic deformation process. Compared with the viscoelastic model, it showed that the current model could better describe shear band evolution history.

High impact toughness of CrCoNi medium-entropy alloy at liquid-helium temperature

We show that face-centered-cubic CrCoNi medium-entropy alloy sustains a Charpy V-notch impact toughness (AK) as high as 340 J at 4 K. Deformation twinning (DT) is rendered exceptionally profuse by this alloy's low stacking fault energy, combined with liquid-helium temperature and high strain rates that both incur high stresses to favor DT across a large plastic zone. This efficiently dissipates imposed mechanical energy, accumulates defects to sustain high strain hardening, and restrains strain localization events from evolving into major shear bands. Martensitic transformation to hexagonal phase is insignificant. CrCoNi is therefore a strong competitor to the best existing cryogenic alloys.

Dual self-organised shear banding behaviours and enhanced ductility in phase separating Zr-based bulk metallic glasses

The multiplication and interaction of self-organised shear bands often transform to a stick-slip behaviour of a major shear band along the primary shear plane, and ultimately the major shear band becomes runaway and terminates the plasticity of bulk metallic glasses (BMGs). Here, we examined the deformation behaviours of the nanoscale phase-separating Zr65–xCu25Al10Fex (x = 5 and 7.5 at.%) BMGs. The formation of multi-step phase separation, being mainly governed by nucleation and growth, results in the microstructural inhomogeneity on a wide range of length-scales and leads to obviously macroscopic and repeatable ductility. The good deformability can be attributed to two mechanisms for stabilizing shear banding process, i.e. the mutual interaction of multiple shear bands away from the major shear band and the delaying slip-to-failure of dense fine shear bands around the major shear band, both of which show a self-organised criticality yet with different power-law exponents. The two mechanisms could come into effect in the intermediate (stable) and later plastic deformation regime, respectively. Our findings provide a possibility to enhance the shear banding stability over the whole plastic deformation through a proper design of microstructure heterogeneities.

Cyclic cryogenic pretreatments influencing the mechanical properties of a bulk glassy Zr‐based alloy

AbstractIn this study, cryogenic thermal cycling was applied to bulk glassy Zr52.5Cu17.9Al10Ni14.6Ti5 (Vitreloy 105) bending samples. After that, significantly improved plasticity and better fatigue properties at three‐point bending as well as higher impact toughness were measured in dependence of the duration and the cooling rate of the thermal cycling process. This improved material behaviour is caused by reinforced shear banding, visible in scanning electron microscopy at the side faces of the specimens after fracture. A comprehensive analysis of the fracture surfaces of both quasi‐static and cyclic bent specimens revealed intense crack propagation along major shear bands and increased fatigue fracture regions, respectively. It is supposed that there is a structural rejuvenation due to the local variations of the thermal expansion coefficients within the multicomponent alloy affecting the mechanical properties.

Gradual shear band cracking and apparent softening of metallic glass under low temperature compression

Metallic glasses (MGs) usually exhibit synchronously enhanced plasticity and strength with decreasing the testing temperature. Although great efforts have been made, why MGs show better plasticity at low temperature remains unclear. In this work, the shear band cracking and fracture mechanism of a relatively brittle TiZr-based MG was investigated through methods of low temperature compression and 3D X-ray tomography. Pronounced apparent softening and profuse internal shear-band cracks were observed along with the improved plasticity, enhanced yield strength, decreased average softening rate, and reduced area of vein pattern as decreasing the testing temperature. Moreover, the fracture features can be correlated well with the compressive properties; and the true rupture stress that is carried by the still-bonded part of major shear band was found to be very close to the yield strength, demonstrating that the apparent softening should mainly originate from the cracking rather than dilation of shear band. The decreased softening rate can be fitted by a diffusion model, implying reduced atomic mobility and increased cracking resistance. Consequently, the improved plasticity of TiZr-MG at low temperature was attributed to the suppression of instant fracture and the enhanced resistance to shear band cracking, rather than the change of shear band density.

Failure of Zr61Ti2Cu25Al12 bulk metallic glass under torsional loading

The torsional properties of the Zr61Ti2Cu25Al12 BMG have been tested using cylinder samples, including the shear yield strength, shear elastic strain limit and shear modulus. Under torsional loading, the BMG fails via a major shear band, without obvious macroscopic plasticity on the specimen surface. The shear band maintained stable propagation by a distance of ∼300 μm (∼20% of cylinder radius) before final catastrophic failure, owing to the constraint of stress gradient along the radial direction. The combined tensile, compressive and torsional properties of the Zr61Ti2Cu25Al12 BMG suggest that recent ellipse criterion and eccentric ellipse criterion are more appropriate than other well-known ones in describing the yield behavior of this BMG. The cooperative shear model underestimates the shear elastic strain limit, because of its default assumption that the yielding behavior follows the Tresca yield criterion.

Seeing the unseen: uncover the bulk heterogeneous deformation processes in metallic glasses through surface temperature decoding

Deformation processes in various materials are inhomogeneous in space and jerky in time, with the shear banding in bulk metallic glasses (BMGs) as a quintessential example, but there is a lack of in situ, nondestructive observations of such processes on the appropriate spatio-temporal scales. This work solves this long-lasting difficulty by the integration of in situ infrared (IR) measurements and innovative signal processing algorithms. A spatio-temporal unmixing method is developed to identify the discrete surface ‘hot-spots’ that are responsible for the initiation and propagation of macroscopic shear bands during the serrated flow. The use of a thermal-electric analogy further identifies depths of these hot-spots, whose magnitudes and locations evolve as the successive shearing process repeats on the major shear band. Seeing the previously ‘unseen’ localized heat sources and their 3D evolution patterns, both in situ and inside the bulk, reveals for the first time how the coupled structural/thermal softening mechanisms govern the heterogeneous deformation processes in BMGs.

The influence of particle rolling and imperfections on the formation of shear bands in granular material

This paper examines the development and evolution of shear bands in granular assemblies when particle rolling and imperfections are taken into account. Simulated biaxial tests in two-dimension are conducted using the discrete element method. The progressive development of rotational angles and effective strain are presented to describe the emergence and evolution of shear bands in biaxial tests. The simulated results reveal that when rolling resistance is taken into account in DEM, the development of shear bands is more distinct as the evolution of the minor shear bands is limited while the major shear bands are preferably promoted in granular materials, and that the local rotating bearings not only influence the onset of shear bands and the width of the shear bands, but also decrease the resistance and reduce the strength of the granular material. Also, it is demonstrated that the primary shear bands initiate from the imperfect areas and develop preferentially along the direction of imperfections. Therefore, the emergence and development of shear bands, which will result in a decline in strength and eventually lead to instability and destruction of the material, can be effectively simulated when rolling resistance is incorporated in DEM and the initial distribution of imperfections in the granular material is defined.

Dynamic properties of major shear bands in Zr–Cu–Al bulk metallic glasses

We present a systematic investigation of shear-band dynamics as a function of chemical composition in the ZrxCu90−xAl10 (x=45–65) metallic glass system. We investigate aging dynamics in the non-serrated flow regime, shear-band velocities in the serrated flow regime, the transition between these two flow modes, and the transition from ductile to brittle behavior. We find that the activation energy for shear-band propagation is largely determined by the underlying time scales of the shear process, and that temperature-dependent stress drops only play a minor role. The activation energy as a function of composition can be linked to the bonding strength between the fastest diffusor, Cu, and its coordinating atoms, represented by the ratio of strong Cu–Zr to weaker Cu–Cu bonds. This indicates that the resistance to accelerated shear, i.e. the apparent activation barrier, is primarily controlled by a chemical nearest-neighbor effect.

Structural aspects of elasto-plastic deformation of a Zr-based bulk metallic glass under uniaxial compression

The structural rearrangements occurring during compressive deformation of a plastically deformable Zr52.5Ti5Cu18Ni14.5Al10 bulk metallic glass have been investigated in situ using high energy synchrotron X-rays. It was found that in the elastic regime, the atomic distances at both short and medium range order vary linearly with macroscopic stress where the atomic bonds in short range order appear significantly stiffer than medium range order. Upon elastic loading, a small fraction of bonds in the first shell is broken in the loading direction whereas some new bonds are formed in the transverse direction. Atomic strain–stress correlation at medium range order deviates from linearity at the onset of plastic deformation which was correlated to the activation of irreversible STZs. This was confirmed by quantifying the amount of atomic shear strain value during loading. The length scale of 12.5Å indicated the largest shear strain and is thought to be the most effective length scale in the formation of STZs. The typical fracture angle of this BMG was explained by the orientation of maximum atomic shear strain at the onset of major shear band formation.

Progressive shear band propagation in metallic glasses under compression

Shear band plays a key role in dominating the strength and plasticity of metallic glasses, and exhibits two kinds of propagation modes under compression, i.e., progressive and simultaneous propagation. These two different propagation modes of shear bands lie in different stages of plastic deformation. Prior to macroscopic yielding, shear bands have already been initiated yet not penetrated through the sample. These inserting shear bands exhibit linearly decreasing plastic strain from the end to tip, demonstrating a progressive propagation mode. Once the macroscopic yielding occurs, the major shear band fully transects the sample and propagates in a simultaneous sliding manner. The progressive propagation of shear bands causes an apparent work-hardening behavior, which can be well explained by assuming a higher critical stress for shear band initiation than propagation. The results demonstrate that metallic glasses with a smaller difference between critical conditions for initiation and propagation of shear band should have better plastic deformability, which can be reflected by the plastic strain to macroscopic yielding read from stress–strain curves.

Localization of plastic deformation along shear bands in Vitreloy bulk metallic glass during high pressure torsion

Microscopic plastic behavior of high purity commercial Vitreloy 1b glassy disks subjected to high pressure torsion was investigated by analyzing the distortion of a marker grid produced by focused ion beam milling. Scanning electron microscopy and atomic force microscopy are applied to measure offsets of shear bands at the surface. Unlike in other macroscopic deformation tests, short and wavy shear bands with submicron offsets and substantial normal offset components are observed indicating concurrent formation of numerous bands. Material pile-ups along major shear bands result in healing of the glass counterparts.

Compressive fracture morphology and mechanism of metallic glass

We quantitatively investigated the fracture morphologies of Zr52.5Cu17.9Ni14.6Al10Ti5 and Pd78Cu6Si16 metallic glasses (MGs) under compression. The characteristic features of the compressive fracture morphology were captured, and the shear vein patterns were found to be not a one-to-one correspondence between two opposing fracture surfaces in an identical sample. This finding experimentally confirms that the compressive failure behaves in a fracture mode of pure shear (mode II). Quantitative measurements show that a ∼1 μm thickness layer with materials not only inside but also adjacent to the major shear band contributes to the formation of shear vein patterns. The critical shear strain to break a shear band was found to be more than 105% and higher in more ductile MGs under compression than tension. Estimation on the temperature rise at the fracture moment indicates that only ∼5% of the total elastic energy stored in the sample converts into the heat required for melting the layer to form the vein patterns. The mode II fracture toughness was also estimated based on the quantitative measurements of shear vein pattern and found larger than the mode I fracture toughness. Finally, the deformation and fracture mechanisms of MGs under tension and compression were compared and discussed. These results may improve the understanding on the fracture behaviors and mechanisms of MGs and may provide instructions on future design for ductile MGs with high resistance for fracture.

Tensile Plastic Deformation Behavior of Zr<sub>70</sub>Ni<sub>16</sub>Cu<sub>6</sub>Al<sub>8</sub> Bulk Metallic Glass at Cryogenic Temperature

A hypoeutectic Zr­Ni­Cu­Al bulk metallic glass (BMG) shows a clear tensile plastic deformation under a high strain rate condition at room temperature. In this study, the effect of cryogenic temperature and strain rate on the tensile plastic deformation behavior of the hypoeutectic Zr­Ni­Cu­Al BMG was investigated. Tensile tests were performed for the hypoeutectic Zr­Ni­Cu­Al BMG specimen with gauge part dimensions of 2.3mm in length and 0.8mm in diameter at cryogenic temperature (133K) under different strain rate conditions (1014 and 1011 s11). As the results, the tensile plastic deformation occurs under both strain rate conditions. Especially, in low strain rate condition (1014 s11), the amount of plastic deformation is clearly larger than that under the high strain rate condition (1011 s11). Furthermore, it was found that there is no positive correlation between the amount of tensile plastic deformation and the number of shear band. In addition, it was inferred that the tensile plastic deformation of the hypoeutectic Zr­Ni­Cu­Al BMG at cryogenic temperature was induced by specimen sliding along only one major shear band. [doi:10.2320/matertrans.M2012102]

Development of Ni- and Cu-Free Zr-Based Bulk Metallic Glasses for Biomedical Applications

Two Ni- and Cu-free bulk metallic glasses (BMGs) of Zr65Pd17:5Fe10Al7:5 and Zr65Pd12:5Ag5Fe10Al7:5, with a critical casting diameter of 6 mm and a compressive strength above 1500 MPa, were developed. While the Zr65Pd17:5Fe10Al7:5 alloy showed little plastic strain before fracture, the Zr65Pd12:5Ag5Fe10Al7:5 alloy exhibited much larger plasticity by the relatively stable, intermittent propagation of the major shear band under compression. This unusual plastic deformation behavior was interpreted on the basis of the contribution of the icosahedral shortrange order. Potentiodynamic polarization tests revealed that the Zr65Pd12:5Ag5Fe10Al7:5 alloy has a higher corrosion resistance than the Zr65Pd17:5Fe10Al7:5 alloy in phosphate buffered saline at 37 � C. X-ray photoelectron spectroscopy analysis revealed that the passive film of Zr65Pd12:5Ag5Fe10Al7:5 alloy consisted mostly of ZrO2, which may be responsible for the excellent corrosion resistance of this alloy. Ni- and Cufree BMGs with a critical diameter of 6 mm, together with enhanced mechanical properties and corrosion resistance, may be promising in biomedical applications. [doi:10.2320/matertrans.M2010068]

Modeling of shear band multiplication and interaction in metallic glass matrix composites

Shear band multiplication and interaction in Zr-based bulk metallic glass (BMG) composites are investigated based on the phase-field modeling of shear banding. Shear band multiplication is closely related to the bonding conditions at the interface between reinforcement and BMG matrix and is quantitatively characterized. We analyze the mechanisms of deflection and integration of shear bands. It is found that the intersection of major shear bands would generate secondary shear bands, which are responsible for the enhancement of plasticity of BMG–matrix composite.

Cold versus hot shear banding in bulk metallic glass

We present an analysis of the shear-banding dynamics in a bulk metallic glass (BMG), including the temperature rise in the band, the sliding speed of the band, and the time elapsed as well as the step size of the shear offset growth in a stop-and-go cycle. This model analysis quantitatively demonstrates that the major shear band can remain cold and slide in a stick-slip manner. We predict that the shear step (distance covered by a stop-and-go cycle) scales with the sample size and machine stiffness. We also illustrate the conditions when such serrated shear is unsustainable and a hot shear band directly develops in a runaway instability (catastrophe). These findings provide physical insight into the shear-instability processes and offer useful information for improving the plasticity of BMGs. The calculation results are used to explain several intriguing recent experimental observations, including the stick slip of the dominant shear-band and the sample-size effects on the plastic-flow behavior of BMGs.