Interaction Of Shear Bands Research Articles

Influence of cold plasma on material removal behavior during diamond grit scratching single crystal silicon

As a typical brittle crystal material, single crystal silicon has high hardness and brittleness, which makes the machined surface prone to microscopic cracks and other subsurface damage. In addition, the anisotropic crystal structure may cause large differences in the material removal behavior along different crystal orientations. To solve these problems, researchers have proposed the methods of machining along the easy-to-cut direction and inducing external energy fields assisted machining, but there remain problems like cracks and surface thermal damage. In this study, we propose to modulate the deformation process of single crystal silicon by atmospheric pressure cold plasma and elucidate the influence of plasma on the material removal behavior based on diamond grit scratching experiments along different directions. The results show that before plasma treatment, the toughness along <110> direction is higher than that along <100> direction, and plasma has significant improvement effects on the plastic deformability in both directions. The critical load for plastic-brittle transition along <110> direction can be increased from 76.2 mN to 107.1 mN, and from 69.6 mN to 109.8 mN along <100> direction. The plasticity regulation mechanism is that the interaction of multiple shear bands dissipates the energy of crack propagation and inhibits the premature generation of cracks. Compared with the conventional scratching along <110> direction, the material removal rate of plasma-assisted scratching under normal loads of 20 mN, 40 mN and 80 mN can be increased by 30.2 %, 24.2 % and 21.9 %, respectively; while that along <100> direction can also be increased by 33.7 % and 19.5 % under 20 mN and 40 mN, respectively. This study may provide a novel approach for realizing high-quality and efficient processing of hard and brittle materials.

Shear localization as a damage mechanism in pore collapse under shock compression

The effect of porosity in shock-compressed materials is widely studied in the literature, accounting for its contribution to the materials' compressibility and to shock attenuation. Yet, the role of porosity in damage or failure under shock compression is limitedly addressed. In this work, shear localization resulting from pore collapse is studied, in potential relation to damage and failure under shock compression. Ti-6Al-4V specimens, with cylindrical and spherical pores, were manufactured using additive manufacturing (AM), and a soft catch set-up was used to enable post-mortem analysis of the impacted specimens. Under plate impact experiments, at shock pressures of 3–8 GPa, the 1–2 mm voids demonstrated collapse, followed by the evolution of shear bands (SB), emanating from the pore's surface. Samples with multiple pores were also tested to examine the interaction of shear bands between adjacent pores, to coalesce to larger damage surfaces. Varying the impact velocity and corresponding impact pressures, protracted states of SB evolution were studied. Numerical simulations using a material damage model reproduced many of the experimentally demonstrated phenomena. The presented results suggest evidence that shear localization around pores could be a damage mechanism under shock compression, identified as a threshold effect, making it a significant mechanism to be well characterized. The applicability of this mechanism is examined in relation to damage behavior under shock compression, extrapolating the experimental findings to μm-sized pores at high pressures.

Subsequent growth and sequential twinning induced by the interaction of {332} twins in Ti[sbnd]15Mo alloy

The mechanisms associated with the interaction of {332} twins are investigated in metastable Ti15Mo alloy. There are 12 possible twin-twin junctions (TTJs) induced by {332} twins that can be classified into 5 types according to the crystallographic relation. Twin-twin interactions are accompanied with the formation of twin-twin boundaries (TTBs) on the acute side and obtuse side. Experimental results based on transmission electron microscopy (TEM) and procession electron diffraction (PED) show that subsequent growth of the TTBs is asymmetric with the twin thickening preferred only on one side for each type. Besides, it is observed that the twin-twin interaction could stimulate sequential {332} twinning inside the primary twin with a specific twin variant promoted. To account for the subsequent growth of the TTBs and the preferred selection of the sequential twinning variant associated with twin-twin interactions, the elastic energy analysis of the resultant dislocations at TTBs indicates that the growth of the TTBs is energetically favorable only on one side. Displacement gradient based analysis shows that the active sequential twin variant could maximumly accommodate the accumulated strain induced by the twin-twin interaction. This work is useful for predicting the preferential growth of the interfaces and the associated accommodation mechanisms induced by the interactions of localized shear bands.

On the Blocking and Shearing Interaction of Shear Bands in Pd, Zr, and Cu-Based Monolithic Metallic Glasses

On the Blocking and Shearing Interaction of Shear Bands in Pd, Zr, and Cu-Based Monolithic Metallic Glasses

Large deflection deformation behavior of a Zr-based bulk metallic glass for compliant spinal fixation application

A novel compliant spinal fixation designed based on the concept of compliant mechanisms can reduce the stress-shielding effect and adjacent segment degeneration (ASD) effectively, but propose higher requirements for the properties of the used materials. Bulk metallic glasses (BMGs), as a kind of young biomaterials, exhibiting excellent comprehensive properties, which are attractive for compliant spinal fixation. Here, according to the practical service condition of the basic elements in compliant spinal fixation, large deflection deformation behaviors of Zr61Ti2Cu25Al12 (at.%, ZT1) BMG beam, including elastic, yielding and plastic were investigated systematically. It was shown that the theoretical nonlinear analytical solution curve as the benchmark not only with the capacity to predict the nonlinear load-deflection relation within the elastic deformation regime, but also assists to capture the yielding event roughly, which can be used as a powerful design tool for engineers. To capture the beginning of the yielding event exactly, bending proof strength (σp,0.05%) accompanied with tiny permanent strain of 0.05% was proposed and determined for BMGs in biomedical implant applications, which is of significance for setting the allowable operating limits of the basic flexible elements. By approach of interrupted loading-unloading cycles, plastic deformation driven by the bending moment can be classified into two typical stages: the initial stage which mainly characterized by the nucleation and intense interaction of abundant shear bands when the plastic strain below the critical value, and the second stage which dominated by the progressive propagation of shear bands and coupled with the emergence of shear offsets on tensile side. The plasticity of BMG beam structures depends on the BMG's inherent plastic zone size (rp). When the half beam thickness less than that of the rp, the plastic deformation of BMGs will behave in a stable manner, which can be acted as the margin of safety effectively.

Achieving strength-ductility synergy in metallic glasses via electric current-enhanced structural fluctuations

Structural applications of metallic glasses are limited by their room-temperature brittleness and strain-softening, associated with the extreme localization of plastic flow in shear bands. Herein, we alleviate this dilemma by the structural fluctuation induced by electric currents, achieving simultaneous improvement of strengths, compression plasticity, and strain hardening capacity. The electric current enhances the structural fluctuations at the atomic scale, introducing more soft zones while densifying their surroundings, which differs from previously reported rejuvenation strategies. This unique structural pattern featuring soft zones surrounded by hard zones leads to extensive sprouts and interactions of shear bands, capturing substantial atoms to participate in the deformation. The plausible mechanism behind the electric current-influenced dynamic evolution of amorphous clusters is proposed. Electric current introduces interatomic electrostatic forces between shell atoms of amorphous clusters through charge transfer. This force decreases the coordination of Zr- and Cu-centered clusters and increases the Al-centered icosahedra in the glassy matrix. Differential evolution of clusters under electric currents enhances atomic structural fluctuations and originates from the cohesion mechanism determined by electron distribution. These findings are suggestive of vanishing room-temperature brittleness and shed atomic-scale light on modulations of the structure and properties of metallic glasses by electric currents.

Research progress on the shear band of metallic glasses

Metallic glasses have received considerable attention in the fields of materials science and condensed matter physics. The disordered structure of metallic glasses gives rise to unique mechanical properties, such as high strength, large elasticity and a distinct deformation mechanism in contrast to conventional crystalline alloys. The plastic deformation of metallic glasses at temperatures well below the glass transition is generally carried out through the formation of highly localized shear bands. Therefore, a thorough understanding of shear banding behavior is vital for improving the mechanical properties of metallic glasses and tailoring the properties of post-deformed metallic glasses. This review article aims to provide an up-to-date overview of the atomic mechanisms underlying shear band nucleation, propagation and interaction. Additionally, the dynamics of shear bands and their correlation with plasticity are also presented. This article also focuses on the progress in the fundamental structure and properties of mature shear bands, such as density fluctuation, medium-range order, shear-band affected zone, and diffusivity. Furthermore, the fracture mechanism based on the development of cavities is elaborated. Finally, we identify a number of key unsolved issues on this subject.

Shear-band blunting governs superior mechanical properties of shape memory metallic glass composites

Metallic glass composites (MGCs) containing crystals can display tensile ductility, overcoming the catastrophic failure of bulk metallic glasses. However, MGCs containing crystals with dislocation-mediated plasticity generally undergo strain-softening, but MGCs containing shape memory crystals can exhibit strain-hardening. The origin of large ductility and strain-hardening of shape memory MGCs as well as the interactions of shear bands (SBs) with crystals remains elusive. Here, two kinds of MGCs containing crystals with dislocation-mediated plasticity or shape memory crystals with martensitic transformations are investigated. It is found that the behavior and properties of SBs in glass matrix can be significantly altered by deformation characteristics of crystals. If crystals deform via dislocations, SBs are narrow, sharp and become mature. In comparison, SBs in shape memory MGCs continuously broaden without maturing by following the growth of thick martensitic plates. Broad SBs tend to bifurcate during propagation, and bifurcated SBs further induce the formation of more martensitic variants in crystals, benefiting strain delocalization. Broadening and bifurcation of SBs cause the SB blunting, which governs the superior mechanical properties of shape memory MGCs. These findings not only deepen the understanding of SBs in glass materials, but also provide a fundamental basis to enhance their mechanical properties by engineering of blunt SBs.

Brittle Creep and Failure: A Reformulation of the Wing Crack Model

AbstractWe propose a reformulation of the wing crack model of brittle creep and failure. Experimental studies suggest that the mechanical interactions of sliding and tensile wing cracks are complex, involving formation, growth, and coalescence of multiple tensile, shear and mixed‐mode cracks. Inspired by studies of failure in granular media, we propose that these complex mechanical interactions lead to the formation of micro shear‐bands, which, in turn, develop longer wing cracks and interact with a wider volume of rock to produce larger shear bands. This process is assumed to indefinitely continue at greater scales. We assume that the original wing crack formalism is applicable to micro shear‐band formation, with the difference that the half‐length, a, of the characteristic micro shear band is allowed to increase with wing crack growth. In this approach, the functional relationship of a with the wing crack length l embodies the entire process of shear band formation, growth and interaction with other shear bands and flaws. We found that the function a(l)/a(0) = 1 + (l/λ)q, where λ and q are constant parameters, generated creep curves consistent with published creep data of rocks. Similar accord was also obtained with experimental brittle failure data. Furthermore, we found that the Mohr‐Coulomb behavior emerged from our model, allowing estimation of the cohesion and angle of internal friction in materials for which λ and q are independently known.

Interactions between multiple rigid lamellae in a ductile metal matrix: Shear band magnification and attenuation in localization patterns

A ductile matrix material containing an arbitrary distribution of parallel and stiff lamellar (’rigid-line’) inclusions is considered, subject to a prestress state provided by a simple shear aligned parallel to the inclusion lines. Because the lamellae have negligible thickness, the simple shear prestress state remains uniform and its amount can be high enough to drive the matrix material on the verge of ellipticity loss. Close to this critical stage, a uniform remote Mode I perturbation realizes shear band formation, growth, interaction, thickening or thinning. This two-dimensional problem is solved through the derivation of specific boundary integral equations, holding for a nonlinear elastic matrix material uniformly prestressed; the related numerical treatment is specifically tailored to capture the stress singularity present at the inclusion tips. Results show how complex localized deformation patterns form, so explaining features related to the failure mechanisms of ductile materials reinforced with stiff and thin inclusions. In particular, the influence of the inclusion distribution on the shear bands pattern is disclosed. Conditions for the magnification (the attenuation) of the localized deformations are revealed, fostering the progress (the setback) of the failure process.

The characterization of cold welding process in CuZr metallic glasses with dissimilar alloying compositions

This work aimed to investigate the cold welding process of dissimilar Cu60Zr40/Cu50Zr50 metallic glass (MG) system through the molecular dynamics (MD) simulation. The mechanical properties and plasticity behavior of the joint zone were also studied with the consideration of stress variations and shear atomic strains under the nanoindentation. The results indicated that the joining evolution was accompanied by the accumulation of shear events in the joint zone. However, one side of the joint was more affected by the atomic rearrangements, which were due to the dissimilar mechanical features of Cu60Zr40 and Cu50Zr50 alloys. The nanoindentation results also showed that the joint zone was considerably softer than the base alloys; however, no sudden shear propagation appeared in this part of the assembly. The strain maps of shear atomic strain under the nanoindentation indicated that the atomic rearrangement and shear band interactions were unsymmetrical in the joint zone so that the Cu50Zr50 side tended to form finer shear bands with more interactive features. Finally, it was concluded that a sound metallurgical bond was attained in the Cu60Zr40/Cu50Zr50 system; however, the good plasticity may come at the expense of strength in the joint zone.

Deformation and fracture behaviors of Zr-based metallic glasses under different constraint shear angles

The deformation and fracture behaviors of Zr35Ti30Be27.5Cu7.5 metallic glasses (MGs) under different shear angles were systematically investigated via shear tests with different constraint shear angles of 30°, 45° and 60°. The fracture strength of the MGs subjected to constraint shear tests increases gradually with increasing shear angles. This positive relationship between the fracture strength and the applied shear angles was verified by the Mohr-Coulomb criterion, suggesting a good agreement between experimental results and theoretical predictions. Due to the large shear stress, the content of free volume generation is greater than the content of free volume annihilation in MGs with the small constraint shear angles and then, a large amount of free volume aggregates to form multiple shear bands, manifesting as a large number of serration events. The formation and interaction of multiple shear bands can prevent the shear bands from rapidly expanding into cracks, significantly improves the plasticity of MGs.

Effect of graphene on the mechanical properties of metallic glasses: Insight from molecular dynamics simulation

The effect of the graphene on the mechanical properties of the Cu50Zr50 metallic glass (MG) is investigated by molecular dynamics simulation method. The results show that the introduction of graphene can increase the strength of the MG and also enhance its plastic deformation ability. The results also indicate that the mechanical properties and deformation behavior of the MG/graphene nanolaminates (MGGNLs) are closely related to the graphene embedded position. It is worth highlighting that there is a threshold for the embedding position of graphene, which makes the plastic deformation of the MGGNL reach uniform deformation. With the increase of graphene insertion distance, the plastic deformation mode of the MGGNL changes from the interaction of multiple shear bands (SBs) to uniform deformation, and ultimately to a dominant SB propagation. The results indicate that the high-strength and high-plasticity MGGNLs can be obtained by introducing graphene and optimizing its insertion position.

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.

Size dependence of ductile to brittle transition of Zr-based metallic glasses under multiaxial loading

The size dependence of ductile to brittle transition of Zr-based metallic glasses was systematically investigated by small punch test. It was found that the metallic glasses with thickness ranging from 50 to 700 μm displayed an obvious transition from ductility to brittleness. Compared with thick samples, thin samples exhibit higher strength and better plasticity, which is attributed to the size-dependent transformation from soft state to hard state dominated by the deformation factor ( α = τ / σ ). Specifically, two significant transitions in deformation behavior at sample thickness of 200 and 500 μm are attributed to two critical deformation factors of α c 1 and α c 2 , which are responsible for the initiation of circumferential and radial shear bands, respectively. Specifically, only several micro-cracks can be observed on sample surface with α ≤ α c 2 . While α c 1 ≥ α ≥ α c 2 , multiple radial shear bands are activated and uniformly distributed on the sample surface. Further, when α ≥ α c 1 , multiple circumferential shear bands primarily initiate under high shear stress and large number of radial and circumferential shear bands can be observed on sample surface. The initiation and interaction of multiple radial and circumferential shear bands significantly enhance the ductility of the samples with α ≥ α c 1 .

Effect of strain rates on the plastic deformation behavior and serrated flow of Zr55.7Cu22.4Ni7.2Al14.7 bulk metallic glass

The effects of strain rates on the plastic deformation behavior and serrated flow of Zr55.7Cu22.4Ni7.2Al14.7 bulk metallic glass (BMG) were studied by compression test at room temperature. The results show that the plastic deformation behavior of metallic glass at room temperature is highly sensitive to strain rates. The plastic strain, yield strength, compressive strength and fracture strength were found to increase with decreasing of strain rates. For example, the plasticity and the compressive strength increased from 0.4 ± 0.1%–8.3 ± 0.4 %, and from 1684 ± 11 MPa to 2000 ± 23 MPa, respectively. In addition, the number and density of shear bands on the fracture side also increased gradually. The fracture morphology, which is mainly composed of vein-like patterns, can be understood by the fact that the number and density of vein-like patterns increase with decreasing of strain rates. However, the mechanism of serrated flow was analyzed and discussed from the perspective of the initiation, expansion, and interaction of multiple shear bands correspondingly. With decreasing strain rates, the uniformity of the serrated flow in the first stage is not sensitive to the change of strain rates, while the uniformity of the second stage serrated flow changes. The change of serrated flow from near uniformity to non-uniformity in the second stage is attributed to the interaction between multiple shear bands. In comparison, with decreasing strain rates, the stress drops in the second stage first decreases and then increases due to the interaction of shear bands. The plastic deformation behavior and serrated rheology of bulk metallic glasses (BMGs) at room temperature, which lay a foundation for promoting the engineering application of BMGs, were discussed in details in this study.

Interaction of dislocations and shear bands in cutting of an amorphous-crystalline bilayer: An atomistic study

Composites of crystalline and amorphous metals combine high strength with ductility. Using molecular dynamics simulation, we analyze the plastic processes occurring at the interface of a bilayer system consisting of a crystalline Cu and an amorphous CuZr sample. When cutting this bilayer parallel to the interface, a top glass layer forms a series of parallel shear bands, while a top crystal layer deforms by the coordinated action of dislocation activity. Zones of localized shear strain – dislocations in the crystal and shear bands in the glass – arriving at the interface may transport the localized strain through it by exciting dislocations and shear bands on the other side of the crystal. In particular, dislocations in the underlying crystal trigger a more homogeneous deformation in the top glass layer as compared to cutting a pure glass.

Free volume evolution dominated by glass forming ability determining mechanical performance in Zr Ti65-Be27.5Cu7.5 metallic glasses

The shear deformation and fracture behaviors of Zr x Ti 65- x Be 27.5 Cu 7.5 metallic glasses (MGs) were systematically investigated in the range x = 20, 25, 30, 35, 40, 45, 50 at% under multi-axial loading mode by small punch test (SPT). Results show that enhanced plasticity can be obtained in MGs with composition ranging from 30 to 45 Zr at%, which is attributed to the large free volume dependent on their better glass forming ability (GFA). It is reasonably concluded that the total free volume consists of two parts: intrinsic free volume ( V 0 ) and excess free volume ( V G ). Intrinsic free volume, involved in an ideal amorphous structure, is defined as the free volume of MGs undergoing fully structural relaxation, which depends on the atomic size of each element. On the basis of atomic dense packing model, intrinsic free volume of the studied MGs is basically same due to a little difference in the atomic size between Zr and Ti. Excess free volume is crucially dominated by the GFA of MGs. In the case of little difference in atomic size of Zr and Ti, the MGs with large GFA possess a much looser atomic stacking structure, leading to the larger content of excess free volume. The large content of free volume not only favor the better atomic mobility, but also facilitate the nucleation of multiple shear bands, which can dramatically improve the plasticity of MGs. Thus, the studied MGs with Zr content ranging from 30 to 45 at% possess the larger content of total free volume than that with 20, 25 and 50 Zr at% due to their better GFA, depicting the enhanced plasticity induced by the initiation and interaction of multiple shear bands. In addition, the MGs with large content of free volume exhibits the lower driving force of shear bands nucleation, showing a soft state.

Role of Glass Composition on Mechanical Properties of Shape Memory Alloy‐Metallic Glass Composites

In this work, the molecular dynamics (MD) simulation was applied to design a laminated composite structure comprised of the shape memory alloy (SMA) and Cu‐Zr metallic glasses (MGs). A wide range of MG compositions was considered to tune the mechanical features and improve the homogenous plastic deformation during the tension loading. The results indicated that the martensitic transformation in the SMA inhibited the sudden shear band propagation in the composite for all the samples. Moreover, it was revealed that the mechanism of plasticity was significantly affected by the change of MG composition. In the Cu‐rich MGs, the formation and propagation of thick shear bands occurred at the end of the tension loading; however, the increase in Zr content induced the interaction of multiple shear bands with finer configurations in the system. Nevertheless, the excessive Zr addition in the MG composition facilitated the aggregation of nanopores at the interface of SMA and MGs, which may be due to the softening effect in the Zr‐rich MGs. Finally, it is concluded that an optimized MG composition is required for the trade‐off between the plasticity and the strength in the SMA‐MG composites.

A plastic FeNi-based bulk metallic glass and its deformation behavior

The strength and plasticity of Fe39Ni39B12.82Si2.75Nb2.3P4.13 bulk metallic glass (BMG) are improved simultaneously by modulating atomic-scale structure through fluxing treatment. The compression strength increases from 3074 to 4220 MPa, and the plastic strain is enlarged from 10.7 % to more than 50 %. The increased mechanical properties of the fluxed FeNiBSiNbP BMG originate from the optimization of atomic-scale structure. More icosahedral-like clusters (ILCs) and crystal-like clusters (CLCs) are found in this FeNi-based BMG with fluxing treatment, and the ILCs are usually surrounded by CLCs. Furthermore, phase separation and a sandwich-like heterogeneous structure of SB are also observed during deformation, indicating the multiscale deformation mechanism and a stable shear-band evolution. The unique “ILC surrounded by CLCs” structure and phase separation lead to a stable plastic deformation process with strong interactions of multiple shear bands, thereby the improved plasticity and strength. This work provides useful guidelines to develop strong and plastic Fe-based BMGs from a structural aspect.