Propagation Of Shear Bands Research Articles

Structure effect of ENPs on mechanical properties of amorphous CuCo alloys

Nanoparticles play an important role in the properties of metallic glasses (MGs) due to their diversified structures; however, their structure–property relationship is unclear. In this paper, three ex situ metallic glass matrix composites were assembled by three kinds of nanoparticles and Cu50Co50 MG obtained by rapid cooling, and their structural evolution under uniaxial compression is investigated by molecular dynamic simulation. It is found that the activated atoms always preferentially accumulate in the amorphous region near the embedded nanoparticles (ENPs). ENPs hinder the propagation of shear bands and lead to strain-hardening behavior. The fractal structures convert the HCP and tDh atoms into atoms of other structures to improve the anti-deformation ability, and the parallel-twin structure improves the anti-deformation ability through the mutual conversion of the FCC and HCP atoms. These findings provide a new idea for improving the mechanical properties of MGs. The change in the ENP structure provides theoretical support for the design of composite materials with specific requirements for structural evolution.

Microstructural evolution and interfaces in cold sprayed hybrid aluminum alloy powders of different hardness

The development of thick cold sprayed deposits of hard amorphous/nanocrystalline Aluminum High Entropy Alloys (Al HEA) is limited by their poor plastic deformability. To address this, Al HEA powder (Al90.05Ni4.3Y4.4Co0.9Sc0.35 (at. %)) is blended with soft Al 6061 in a 1:1 wt% ratio and cold sprayed. The microstructure of the composite deposits reveals that while Al HEA splats undergo limited deformation, Al 6061 splats deform extensively. Extreme deformation of Al 6061 captures the hard Al HEA particles, resulting in a composite deposit of 5 mm thickness, compared to 0.2 mm for pure Al HEA. Within the composite, Al HEA particles deform through shear band propagation and viscous flow, whereas Al 6061 splats deform via dislocation slip motion. These deformation mechanisms induce interface nanocrystallization, leading to a 123 % increase in lattice strain, a 67 % reduction in crystallite size, and an 800 % increase in dislocation density compared to the initial feedstock. This microstructural evolution, combined with the presence of hard Al HEA regions, results in a mean hardness of 2.56 GPa and an elastic modulus of 76 GPa. These values represent a 47 % and 6 % improvement over conventional polycrystalline cold-sprayed aluminum deposits. Thus, this study provides insights into the microstructural and interface evolution and its effect on indentation characteristics for making high-strength Al HEA deposits.

A failure model for bulk metallic glass based on strain rate-correlated softening mechanism

Due to the amorphous nature of the bulk metallic glasses (BMGs), catastrophic fractures occur once the nucleation and propagation of shear bands occur, which is quite different from crystal materials. So far, the underlying mechanism of BMG failure behavior and its strain rate effect are still controversial. In this work, the quasi-static and dynamic compression experiments of Vit-1 BMG were carried out in a wide range of strain rates from 0.001 s−1 to 5000 s−1. The results show that the variation tendency of failure stress with strain rate can be divided into three stages. The quasi-static failure stress is insensitive to the change of strain rate with only almost constant free volume softening effect inside the material during the failure process. When entering dynamic loading stage, the failure stress decreases significantly, which is caused by a combination of increasing thermal softening and free volume softening effects. When the strain rate increases to the third stage, that is, over 2000 s−1, the failure stress and the softening effects basically remain unchanged. The transformation of failure mechanism is well confirmed by the micromorphology of fracture surfaces. Based on the experimental results, a concise empirical failure model is established, in which the failure stress is decomposed into softening and non-softening parts. The verification experiments show that the model can effectively predict the compression failure stress of BMG under different strain rates.

Nucleation mechanisms of shear bands in amorphous alumina

The origin of the plastic deformation in amorphous solids is a challenging issue unlike that of the plasticity in crystals. Plasticity in amorphous systems can occur by the nucleation and propagation of shear bands (SBs). In this work, we investigate the mechanical response of an amorphous alumina sample subjected to shear deformation via extensive molecular dynamics simulations. We find SBs form only within a certain range of parameter values, e.g., low quenching rates, low temperatures and high strain rates. In our proposed atomic-scale model shear transformation zones (STZs) first nucleate and act as precursors of SBs due to their high stress concentration. The results showed that STZs generate an anti-symmetric stress that is responsible for the vortex-like motion of atoms that facilitate the percolation of STZs, finally forming an SB. These results provide insight into the mechanisms of plastic deformation of similar amorphous solids.

Novel Class of Crystal/Glass Ultrafine Nanolaminates with Large and Tunable Mechanical Properties.

The control of local heterogeneities in metallic glasses (MGs) represents an emerging field to improve their plasticity, preventing the propagation of catastrophic shear bands (SBs) responsible for the macroscopically brittle failure. To date, a nanoengineered approach aimed at finely tuning local heterogeneities controlling SB nucleation and propagation is still missing, hindering the potential to develop MGs with large and tunable strength/ductility balance and controlled deformation behavior. In this work, we exploited the potential of pulsed laser deposition (PLD) to synthesize a novel class of crystal/glass ultrafine nanolaminates (U-NLs) in which a ∼4 nm thick crystalline Al separates 6 and 9 nm thick Zr50Cu50 glass nanolayers, while reporting a high density of sharp interfaces and large chemical intermixing. In addition, we tune the morphology by synthesizing compact and nanogranular U-NLs, exploiting, respectively, atom-by-atom or cluster-assembled growth regimes. For compact U-NLs, we report high mass density (∼8.35 g/cm3) and enhanced and tunable mechanical behavior, reaching maximum values of hardness and yield strength of up to 9.3 and 3.6 GPa, respectively. In addition, we show up to 3.6% homogeneous elastoplastic deformation in compression as a result of SB blocking by the Al-rich sublayers. On the other hand, nanogranular U-NLs exhibit slightly lower yield strength (3.4 GPa) in combination with enhanced elastoplastic deformation (∼6%) followed by the formation of superficial SBs, which are not percolative even at deformations exceeding 15%, as a result of the larger free volume content within the cluster-assembled structure and the presence of crystal/glass nanointerfaces, enabling to accommodate SB events. Overall, we show how PLD enables the synthesis of crystal/glass U-NLs with ultimate control of local heterogeneities down to the atomic scale, providing new nanoengineered strategies capable of deep control of the deformation behavior, surpassing traditional trade-off between strength and ductility. Our approach can be extended to other combinations of metallic materials with clear interest for industrial applications such as structural coatings and microelectronics (MEMS and NEMS).

Dynamic mechanical response and constitutive model of (Ti37.31Zr22.75Be26.39Al4.55Cu9)94Co6 high-entropy bulk metallic glass

In this work, the mechanical response and fracture characteristics of (Ti37.31Zr22.75Be26.39Al4.55Cu9)94Co6 high-entropy bulk metallic glass (HE-BMG) were investigated in detail over a wide range of strain rates (10−4–105 s−1). The HE-BMG exhibited a negative strain rate sensitivity under uniaxial compression, with the strength showing more significant rate dependence under dynamic conditions. The shear band behavior translated from the dominance of multiple shear bands propagations under quasi-static compression to the rapid propagation of a single shear band to form cracks under dynamic compression. Under dynamic loading, the shearing velocity increased along an arc-shaped displacement path, and the local stress state on the shear fracture surface shifted from compressive-shear to tensile-shear, accompanied by changes in fracture morphologies. The spall strength of HE-BMG decreased as flyer impact speed increased, while the long-term dependence of spall strength on strain rate may be positive. With the increase of impact speed, the main microstructural features of the spalling surface translated from flat regions accompanied by dimples to cup-cone structures. Furthermore, the complete parameters of the Johnson–Holmquist II (JH-2) model for HE-BMG were obtained based on experimental data. Numerical simulations of planar impact, penetration, and hypervelocity impact are in good agreement with experimental results, demonstrating the validity of the JH-2 model parameters. The current work has important guiding value for the application of HE-BMG in space debris protection.

Bi-metallic lattice structures manufactured via an intralayer multi-material powder bed fusion method

The laser powder bed fusion technology is widely utilized for manufacturing intricate metallic components, overcoming design limitations inherent in traditional subtractive methods. Recent advancements in powder bed fusion have extended to the creation of metal multi-material structures, allowing for functionally graded and tailored properties within a single component. While functionally graded lattice structures are commonly achieved by spatially varying the relative density and/or topology of the lattice, multi-material lattices are rarely studied particularly via metal additive manufacturing where multi-material methods are a relatively recent development. This study applies an accessible enhancement to a standard powder bed fusion system for multi-material additive manufacturing, and investigates printed and heat-treated bi-metallic lattices containing regions of both 316L and 17–4PH stainless steel. The bi-material interface is examined in lattice and bulk samples, and the compression behaviour of bi-metallic lattices is investigated experimentally and computationally. The material interface is shown to be robust as built and under loading, and the bi-metallic lattices exhibit greater energy absorption than single-material samples. The inclusion of the ductile 316L in bi-metallic lattices also delays the propagation of local failure and shear bands initiated in the more brittle 17–4PH. Finite element analysis simulates the lattice compression which includes strut fracture and as-built diameters informed by computed tomography, enabling further investigation and analysis of multi-material lattice designs. This work demonstrates the benefit of multi-material additive manufacturing by providing improved performance over single-material designs.

Nanoindentation and relaxation behavior of nitrogen doped zirconium based bulk metallic glass

In this paper, the effect of nitrogen doping on the mechanical properties of Zr-based bulk metallic glasses (BMGs) was studied by nano indentation experiment, and the correlation mechanism between microstructure evolution and mechanical properties was analyzed. The results show that the serrations events exhibited by metallic glasses during nanoindentation are related to the loading rates. With the decreasing loading rate, more obvious serrations events occur in the loading segment, which is attributed to the sufficient time for shear bands to multiply and propagate at low rates. The addition of nitrogen will induce the generation of more flow defects. Compared with BMGs without nitrogen, the nitrogen-doped Zr-based BMGs exhibit more serration events due to introducing more flow defects to form the shear transformation zones (STZs) and subsequently facilitating the shear bands to multiply. The possible relationships between the structural evolution, shear band initiation and propagation, and relaxation behavior of nitrogen doped and non doped metallic glasses were discussed.

Kink‐band Kinematic Analysis and its Implications for Late‐stage Deformation in the Internal Parts of the Zagros Collision (Sanandaj‐Sirjan Zone) in West Iran

AbstractIn the internal parts of the Zagros collision zone, several deformation phases have been superimposed. The early deformation phase caused the development of a penetrative foliation. The late‐stage deformation phase was preferentially accommodated within shear zones and caused the generation of shear bands, implying a non‐coaxial component of deformation, the end of this stage deformation was marked by the development of kink‐bands. In the vicinity of Zagros suture zone, the kink angle increased from 40° to 60°, and the kink‐bands was converted to chevron folds. In this region, the external (α) and internal (β) angular ratio is α/β ≠ 1 and kink angle increased, and deformation occurred with 10% to 30% volume loss. Farther from the suture zone in the east, α/β = 1; and total volume was constant or increased by 5% to 10%. Kink‐bands kinematic analysis in the study area revealed this structures were sensitive to deformation conditions and components such that, with decreasing distance to the Zagros suture zone, shearing and rotation increased, a high kinematic vorticity dominated, and volume loss occurred during deformation.

An atomic-scale insight into mechanical enhancement and frictional properties of amorphous/graphene multilayers

This study utilizes molecular dynamics simulation to explore the mechanical and friction properties of CuTa/Graphene A/GR multilayers during nanoscratching. By investigating the influence of amorphous layer thickness, substrate temperature, and scratching speed valuable insights into scratching behaviors are gained. Reducing the amorphous layer thickness enhances the normal force and contact pressure, effectively strengthening the A/GR multilayer, while decreasing the frictional force owing to the lubricating properties of graphene, resulting in a notable reduction in friction coefficient. Analysis of the deformation mechanism reveals that the graphene layer impedes shear band propagation, especially noticeable at thinner amorphous layer thicknesses. Additionally, the A/GR multilayer exhibits outstanding wear resistance at high temperature and speed, as indicated by a slightly increased wear atomic ratio.

Tailoring Mechanical Properties and Shear Band Propagation in ZrCu Metallic Glass Nanolaminates Through Chemical Heterogeneities and Interface Density

The design of high‐performance structural thin films consistently seeks to achieve a delicate equilibrium by balancing outstanding mechanical properties like yield strength, ductility, and substrate adhesion, which are often mutually exclusive. Metallic glasses (MGs) with their amorphous structure have superior strength, but usually poor ductility with catastrophic failure induced by shear bands (SBs) formation. Herein, we introduce an innovative approach by synthesizing MGs characterized by large and tunable mechanical properties, pioneering a nanoengineering design based on the control of nanoscale chemical/structural heterogeneities. This is realized through a simplified model Zr24Cu76/Zr61Cu39, fully amorphous nanocomposite with controlled nanoscale periodicity (Λ, from 400 down to 5 nm), local chemistry, and glass–glass interfaces, while focusing in‐depth on the SB nucleation/propagation processes. The nanolaminates enable a fine control of the mechanical properties, and an onset of crack formation/percolation (>1.9 and 3.3%, respectively) far above the monolithic counterparts. Moreover, we show that SB propagation induces large chemical intermixing, enabling a brittle‐to‐ductile transition when Λ ≤ 50 nm, reaching remarkably large plastic deformation of 16% in compression and yield strength ≈2 GPa. Overall, the nanoengineered control of local heterogeneities leads to ultimate and tunable mechanical properties opening up a new approach for strong and ductile materials.

Tensile deformation of metallic glass and shape memory alloy nanolaminates

Mechanical deformation of Cu50Zr50 metallic glasses (MGs) with CuZr B2 layers was explored using tensile tests under iso-strain and iso-stress conditions using molecular dynamics simulations. Atomic structure identification was conducted through machine learning techniques, enabling a distinct examination of the deformation exhibited by each phase. Our results revealed early-stage plasticity driven primarily by shear transformation zones at the amorphous/crystalline interface, rather than shear band formation and propagation observed in the monolithic MG, leading to nanolaminates with enhanced strength. Furthermore, under iso-strain deformation, B2 layers underwent martensitic transformation promoted by the nucleation of voids. However, void growth quickly supersedes these effects under both iso-strain and iso-stress conditions, culminating in sample fracture. In summary, this work advances our understanding of amorphous/crystalline nanolaminates, providing valuable insights into their mechanical complexities.

Study on strain localization of frozen sand based on uniaxial compression test and discrete element simulation

Strain localization has always been an important subject in frozen soil mechanics and engineering. To evaluate the development of local strain and the formation of shear bands in frozen soil, uniaxial compression tests have been conducted on frozen sand at various temperatures and particle grades. The strain localization evolution law of the frozen soil is analyzed utilizing the digital image correlation (DIC) method. The test results reveal that the entire process of shear band generation, development, and formation in frozen soil can be well captured. Within the testing temperature and particle grade range in this study, in comparison to particle grade, temperature exerts a more pronounced influence on the shear band angle which increases as temperature decreases. It is discovered that the width of the shear band increases with the decrease in temperature and the increase in mean particle diameter d50. Subsequently, a discrete element method (DEM) model is developed to examine the microscopic mechanical characteristics of frozen sand in uniaxial compression tests. The reliability of the DEM model is verified through comparative analysis with test results. Besides, the development law of the strain localization of the frozen soil obtained by the numerical simulation is revealed based on the quantitative analysis of the rotational angle, bonding state, and displacement of the soil particle during the shearing process.

Modulation of plasticity by crystalline precipitates in CuZr metallic glasses

Amorphous/crystalline composites consist of crystalline phases embedded in an amorphous matrix that offer the possibility for synthesizing materials with tailored properties. Here, molecular dynamics simulations were performed to study the plastic behavior of CuZr metallic glasses (MGs) with CuZr B2 spherical precipitates under tensile tests. Shear transformation zones were observed to nucleate at the MG/B2 interface, a phenomenon that was more remarkable at larger precipitate sizes. Furthermore, large precipitates acted as barriers for shear band (SB) propagation. To overcome such barriers, and owing to the lack of phase transformation and plastic activity in the CuZr B2 structures, SBs displaced the precipitates, leading to increased stress in the sample. In addition, the degree of strain localization was higher when the precipitates were closer to each other. Thus, the plasticity in our MG composites was modulated by the size and distribution of the precipitates.

Structural Toughness of Fluorinated-Nitrided Interphase Passivation Layers for Lithium-Ion Batteries: A Reactive Molecular Dynamics Study.

The performance of lithium-ion batteries largely depends on the stability of the solid electrolyte interphase (SEI) layer formed on the anode surface. Strategies to improve SEI robustness often rely on optimizing its composition through electrolytic additives. Recently, the amalgamation of fluorinated cosolvents with nitride sources as additives has been shown to enable the construction of sustainable fluorinated-nitrided SEI layers (FN-SEI). Furthermore, the presence of lithiophilic nitrides embedded in lithium fluoride (LiF) was found to contribute toward stability of a beneficial amorphous phase for interfacial passivation. However, there is a lack of understanding on how key indicators of mechanical longevity, like plasticity and fracture resistance, may evolve in such multiphase SEI building blocks. Herein, in conjunction with first-principles calculations, a reactive force field (ReaxFF) has been developed for deriving new mechanistic insights into the intriguing FN-SEI. Our studies demonstrate that owing to a significant elasticity mismatch, the hard nitride phases have a propensity to affect the native deformation modes when embedded in a soft amorphous LiF-rich matrix. Impact of the volume fraction and distribution of the nitride (Li3N) phases are discussed from the perspective of how they interfere with the propagation of shear bands. Interestingly, brittle-ductile-brittle regimes are recognized along the nitride infusion window, providing a glimpse into the effect of phase distribution on the structural toughness of the LiF-Li3N-enhanced SEI.

Micromechanical response of an electrodeposited NiP metallic glass by pillar compression under extreme conditions

In this work, the mechanical properties of pulse electrodeposited amorphous nickel phosphorous (NiP), with ∼16 wt% P, was investigated for an improved understanding of the deformation behavior of metallic glasses at extreme conditions, particularly at the microscale. Upon decreasing the temperature to sub-ambient, both the yield strength and the shear band density increase. While the change of yield strength with temperature can be well explained by a cooperative shear model, the increase in the number of shear bands can be related to an increase in potential sites for activation of shear transformation zones due to the enhanced structural heterogeneity at lower temperatures. This, however, hinders the percolation of deformation units and leads to a decrease in the shear band propagation speed, from ∼50 nm/s at 295 K to ∼20 nm/s at 195 K. By testing micropillars across seven orders of magnitude in strain rate, a two-stage strain rate sensitivity is observed, from nearly negligible at the quasi-static range (m1 = 0.001 ± 0.002 for 10−3 s−1 to 10−1 s−1) to highly positive at dynamic high strain rate range (m2 = 0.02 ± 0.004 for 1 s−1 to 1000 s−1). This transition implies a change in the yield-controlling process, from the diffusion-assisted relaxation at quasi-static conditions to the rate of energy barrier crossing of shear transformation zones at high strain rates. These findings highlight several distinctive features of microscale metallic glasses, including the reduced shear band velocity at sub-ambient temperature and the two-stage strain rate sensitivity.

Microstructure evolution during fracture process of gradient twinned ag using molecular dynamics

In recent years, the materials with high strength and good toughness have become a focus of intense research. In this paper, FCC Ag with three different microstructures: single crystal (SC), uniformly arranged twin boundaries (UTB), and gradient arranged twin boundaries (GTB), are designed and compared during the fracture process through molecular dynamics simulation. The result shows that both strength and toughness are the highest in GTB, followed by UTB, and lowest in SC. Differences in three microstructural evolutions are compared to explain macroscopic strength and toughness differences. For SC, brittle cleavage dominates the fracture process, accompanied by the generation of a large number of shear bands. While in UTB, brittle cleavage and crack deflection occur successively, and the propagation of numerous shear bands is segmented by twin boundaries, resulting in the improvement toughness and strength. For GTB, crack tip bluntness and dislocation transmission occur sequentially during the crack initiation stage, and the emitted dislocations discontinuously bridge the gradient arranged twin boundaries and crack tip, which causes the reduction of the local stress and strain experienced by the crack tip. In addition, the shear bands formed from the crack tip in GTB are impeded by the gradient arranged twin boundaries, effectively slowing down the propagation of the plastic zone and the crack tip. The above microstructural mechanisms work together to make GTB exhibit high strength and toughness during the fracture process. Finally, the role of dislocation types is analyzed, Hirth dislocations are activated in advance in GTB, leading to crack tip blunting. The Stair-rod dislocations near the stress peak impedes the plastic deformation in GTB, effectively improves toughness and suppresses the strength decline trend in GTB. This study helps to understand the relationship between microscopic deformation and macroscopic properties of gradient nanotwinned materials.

Shear bands in amorphous alloys and their role in the formation of nanocrystals

The processes of evolution of the structure and surface morphology of Al87Ni8La5 and Fe76Si13B11 amorphous alloys under deformation have been studied. It is shown that the deformation occurs through the formation and propagation of shear bands, which form steps when they reach the surface. The formation of nanocrystals in shear bands was noted. It is shown that steps on the surface are formed under the combined action of several elementary shear bands. Shear bands have a variable thickness in the range from 5 to 20 nm. An elementary step has a thickness of about 15 nm. Shear bands can be combined into zones. The transverse size of the zones is about 1 μm. The formation of nanocrystals in zones can lead to anisotropy in the orientational position of nanocrystals in an amorphous matrix. With an increase in the degree of deformation, nanocrystals are formed not only in shear bands, but also in areas adjacent to them. There is a difference in the kinetics of the formation of nanocrystals in an alloy based on aluminum and iron.

Distinct avalanche dynamics detected in metallic glasses with high energy state revealing the crack-like shear banding mechanism

When a sufficiently high stress is applied to a metallic glass, causing plastic deformation, the material undergoes structural reconfiguration through dissipative slip avalanche events that release local stresses. By utilizing isothermal annealing and cold rolling techniques to tune the energy levels of metallic glasses, it has been observed that structural rejuvenation is accompanied by structural relaxation, as evidenced by distinct changes in avalanche dynamics. We present detailed statistics of the avalanche dynamics during shear band formation in energy-tuned metallic glasses, ranging from structurally relaxed to rejuvenated states. By analyzing shear band characteristics and examining scaling exponents, avalanche durations, and stress relaxation rates, we can establish a connection between the local activation of shear transformation zones and the formation of macroscopic shear bands. The statistics of avalanche duration indicate that an increase in soft zones within metallic glasses can alleviate stress release and stabilize plastic flow, as evidenced by the characteristics of shear bands. We attribute the significant transition of serrated flow, observed at different energy levels (i.e., as-cast, relaxed, and rejuvenated states) to the variations in nucleation and multiplication of shear bands that originate from local weak spots. Analysis of the distinct avalanche dynamics suggests that in lower energy level metallic glasses, the nucleation and propagation of shear bands exhibit localized crack-like behavior, while in higher energy level metallic glasses, they display diffused crack-like characteristics. Indeed, our results strongly support that the decreased avalanches observed in the high energy level metallic glasses originate from the nucleation of numerous small shear bands, which directly compete with the propagation of the main local shear band. These findings deepen our fundamental understanding of the relationship between the microscopic mechanism of slip avalanche dynamics and shear banding, providing a pathway to control the plasticity of metallic glasses.

Roots seeking of multiple shear-bands in amorphous alloys at the atomic scale

The propagation of shear-bands is of utmost importance in understanding the macro and microscopic deformations of bulk metallic glasses (BMGs). In this study, using molecular dynamics simulations to elucidate the intricate atomic-scale mechanisms governing dynamic continuum shear-bands and microstructural evolution during deformation. This work provides compelling evidence of self-organized critical behavior, as evidenced by the observation of serrated flow patterns and distributions of elastic energy density. Moreover, we unveil the influence of notch length on the principal shear-band based on the steep stress drop (i.e., Δτ =τy-τs, distinct from serrated flows), strength-normalized difference (i.e., Δτ/τy), and probability distribution of shear-strain. The effective blocking of shear bands by nanocrystal second phase can be attributed to the synergistic reinforcement effect of dislocations and nano-stacking faults. Finally, the analysis of the serrated flow behavior and stress-increment distribution during nanoindentation establishes the connection between micromechanical behavior, shear-band evolution, and macroscopic mechanical properties.