Showing papers on "Shear band published in 2023"

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

Abstract: 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, 4-dimensional-STEM (4D-STEM) is applied to map and directly correlate the local strain and the atomic structure at the nanometer scale in deformed metallic glasses. Residual strain fields are observed 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.

Role of particle rotation in sheared granular media

Abstract: Abstract When granular assemblies are subject to external loads or displacements, particles interact with each other through contact and may exhibit translations and rotations. From a micromechanical perspective, particle rotations are an essential mechanism influencing the macroscopic behavior of granular materials. In this study, biaxial shearing tests were conducted on assemblies of dual-sized circular particles at different confining pressures. A high-precision image analysis method was developed to extract the particle-level motion of all the particles, including the rotational behavior. Experimental results showed that most of the particles exhibited rotations. Particles within the shear band exhibited more significant rotations and were characterized by low connectivity (number of contacts per particle). In contrast, the particles outside the shear band rotated lesser, only in the beginning stage of shearing. Every rotation in either direction is accompanied by an opposite rotation of almost the same magnitude in the neighboring region, and rotation clusters have been observed. Rotations in both directions are normally distributed within the assembly, and the average particle rotation is zero. The average rotations in both directions evolve symmetrically with major principal strain. Generally, the rotation rate (degrees per incremental strain) is observed to be maximum at the start of the shearing, and gradually it becomes constant toward the end of the shearing. The average value of the absolute cumulative rotation observed for whole particles is 18.6° at the end of shearing, i.e., 20% deviatoric strain. Smaller size particles tend to exhibit 67% higher rotations than bigger particles. Confining pressures have no significant effect on the rotational behavior of circular particles.

Heterogeneity of deformation, shear band formation and work hardening behavior of as-printed AlSi10Mg via laser powder bed fusion

Abstract: The deformation heterogeneities and micro-to macro-structural evolution of shear bands were thoroughly characterized and assessed in correlation with the work hardening behavior of laser powder bed fused as-built AlSi10Mg using compression testing at 78 and 293 K in a wide range of constant strain rates. Using the Kocks-Mecking model, work hardening behavior was correlated with the evolved microstructure and rate-controlling deformation mechanisms. It was found that initial work hardening (athermal hardening rate) is dependent on the strain rate at both 78 and 293 K, where it increases by a transition from shearing to looping of nano-precipitates in the 10−2-10−1 s−1 strain rate range. Stages II (hardening stage) and Stage III (dynamic recovery stage) of work hardening were also observed and their dependence on temperature and strain rate was assessed. The former includes the dislocation-obstacle interactions resulting in work hardening, whereas the latter consists of recovery processes where density of dislocations decreases. Shear banding was reported for higher strain rates manifesting in micro- and then evolving into macro-shear bands with deformation, which led to the increase in the dislocation recovery rate in Stage III of work hardening. Compressive work hardening mechanisms were discussed for both 78 and 293 K temperatures.

Theoretical study of the void collapse and shear band formation mechanism for β ‐HMX

Abstract: Based on molecular dynamics simulation, we obtain a shear band formation mechanism for β-HMX. The shock wave reflected at void face induces rarefaction wave, and the velocity difference between rarefaction wave zone and shock wave zone induces slipping. A set of physical models are developed to describe the wave reflection, shear band expansion and energy dissipation. The orientation angle, face indices, expansion velocity and formation energy of shear bands are calculated. A complete physical picture for shear band formation is obtained.

Effects of pore diameter and B2-NiTi crystal on plasticity of amorphous Ni-Ti alloy based on molecular dynamics simulation

Abstract: The poor plasticity of metallic glasses (MGs) limits its applicability as engineering materials. Therefore, it is particularly important to find ways to improve the plasticity of metallic glasses. In this study, the effects of pore diameter and B2–NiTi crystal on the compressive deformation of Ni50Ti50 MGs are studied by molecular dynamics simulation. The results demonstrate that pores effectively inhibit rapidly stress reduction. Pore experiences pressure in the direction of the load under compression process, which induce the formation of high shear strain atoms around the pores. The presence of pore induces the formation of shear transition zones (STZs) and shear bands. Specifically, larger pore diameters result in lower yield strengths, flatter stress-strain curves, and increase plasticity of MGs. As the B2–NiTi crystal is highly resilient in MGs, it could not be easily destroyed. In the compression process, the B2–NiTi crystal reduce the deformation of the system and impede the propagation of shear bands. As a result, the shear bands only propagate in the cracks of crystals. The larger number of crystals lead to a larger inelastic deformation interval in the nanocomposites and improve the ability to block the propagation of shear bands. These findings elucidate the relationship between yield strength, shear band, plasticity, pore diameter, and B2-phase crystals, and provide theoretical guidance for the development of MGs with improved plasticity.

Deformation stability transition of notched metallic glasses

Abstract: Systematical experiments, simulations and microscopic characterizations on double edge notched samples manifested a transition from brittle instability to steady plasticity. Effects of ligament length, geometrical notch parameters as well as thickness were studied. It was found that ligament length is essentially the space provider for intersection of shear bands. With a narrow ligament, it is unlikely to form V-shaped double shear bands because of the limited region between notch tips. Meanwhile, the plastic deformation ability is determined by stress triaxiality. With a wider ligament, V-shaped double shear bands become predominant in controlling deformation stability, accompanying with much higher plasticity than previous investigations because of large-regions of shear band entanglements. Such plasticity enhancement is proven to have weak linkage with the restriction of crosshead of machines, and it is the resultant response of notch configuration. Besides, thickness is less influential to global plasticity. On another hand, by means of hydrostatic stress embedded free volume model, shear banding process is simulated by FEM model, by which the shear band evolution patterns in tests could be well understood and explained. The current findings could further enrich the plastic stability mechanism of metallic glasses under complex stress states. As well, those parameters on notch configurations could provide the state-of-the-art parameters used for fabrication of porous metallic glasses.

Microstructural Evolution of Shear Localization in High-Speed Cutting of CoCrFeMnNi High-Entropy Alloy

Abstract: Shear localization is one of the most important failure mechanisms subjected to high-strain-rate deformation and has significant effects on the process, plastic deformation, and catastrophic failure of a material. Shear localization was observed in serrated chips produced during the high-speed cutting of the CoCrFeMnNi high-entropy alloy. Electron backscatter diffraction was performed to systematically investigate microstructural evolution during shear banding. The elongation and subdivision of the narrow grains were observed in the areas adjacent to the shear band. The microstructure inside the shear band was found to be composed of equiaxed ultrafine grains. The results reveal that grain subdivision and dynamic recrystallization might have significant roles in the microstructural evolution of shear bands. These results offer key insights into our understanding of shear localization and high-speed machining behavior for high entropy alloys.

Fatigue failure of amorphous alloys under cyclic shear deformation

Abstract: The accumulation of plastic deformation and flow localization in amorphous alloys under periodic shear are investigated using molecular dynamics simulations. We study a well-annealed binary mixture of one million atoms subjected to oscillatory shear deformation with strain amplitudes slightly above a critical value. We find that upon approaching a critical strain amplitude from above, the number of shear cycles until the yielding transition is well described by a power-law function. Remarkably, the potential energy at the end of each cycle as a function of the normalized number of cycles is nearly independent of the strain amplitude, which allows for estimation of the fatigue lifetime at a given strain amplitude. The analysis on nonaffine displacements of atoms elucidates the process of strain localization, including irreversible rearrangements of small clusters until the formation of a system-spanning shear band.

An atomistic study of plastic deformation of SmCo5 by amorphous shear bands

Abstract: Recently, SmCo5 was found to be capable of forming amorphous shear bands to induce dislocation-free plastic deformation at large strains. The formation of shear band is energetically more favorable compared to cracking and the small density variation in shear bands is not likely to induce high local stress that assists crack opening. However, the related mechanism of crystalline-to-amorphous transition during shear-band formation at the atomic level remains unclear. In this paper, the shear deformation of SmCo5 is investigated by molecular dynamics simulations and we discuss the behavior of shear-band formation by exploring the atomic packing in the interfacial zone between crystal matrix and shear band. The stress-strain curves of SmCo5 show anisotropy with respect to formation of amorphous shear band. The analysis of atomic packing indicates that the behavior of Sm is critical to the atomistic amorphization path and thus accounts for the mechanical anisotropy of the material. Temperature calculations show that the material is not subject to shear melting during deformation that would otherwise lead to embrittlement.

Intense shear band plasticity in metallic glass as revealed by a diametral compression test

Abstract: Shear band initiation, development and mutual interaction are studied in a Zr55Cu30Al10Ni5 metallic glass by means of an innovative experimental technique associating diametral compression test (or Brazilian test), scanning electron microscopy and digital image correlation analysis. The intense strain occurring in shear bands and the deformation map of their overall pattern are both estimated with a high resolution (∼5μm/pixel), offering a better understanding of the phenomenon. Finite element simulations based on a new and original plasticity model, the compartmentalized model, makes it possible to reproduce the shear band development observed experimentally, as well as the interlocking mechanism occurring between the shear bands.

Microscopic shear deformation characteristics of the Lüders front in a metastable austenitic transformation-induced-plasticity steel

Abstract: To elucidate the mechanisms of deformation and a state of plastic stability at the front of Lüders bands during a tensile test, metastable austenitic transformation-induced plasticity (TRIP) steels with different dislocation densities and ferritic steels were characterized via macroscopic-DIC-based stress–strain investigations and scanning electron microscopy (SEM). A direct correlation between stress–strain curves and measured strain distributions in the tensile specimen indicated that the Lüders front represents a transition region from a state of plastic instability to one of stability, whereby a general rule relating the Lüders strain ∆𝜀𝐿∗ and increments in the true stress in the Lüders band ∆𝜎𝐿∗ to a lower yield stress (𝜎𝑦0∗) can be described as 𝜎𝑦0∗=∆𝜎𝐿∗∆𝜀𝐿∗ irrespective of the amount of deformation-induced martensite in the band or crystal structure of the steel. The inclination angle of the Lüders front with respect to the tensile direction changed from 55° to 90° with a reduction in the measured strain ratio (-𝜀𝑦𝑦/𝜀𝑥𝑥) in the Lüders band, and the change agreed with the tendency calculated by the plasticity model, assuming the pure shear occurs under the minimum shear strain criterion. SEM observations of the sheet surface and the front cross-section in the TRIP steel showed the formation of multiple inclined ~20 μm-wide shear deformation zones that accompanied a reduction in thickness. All the observed geometrical characteristics of the Lüders front were qualitatively described by a mechanism involving minimizing the misalignment from the fixed tensile axis caused by ‘shear' deformation.

Characterization of Plastic Deformation in CuZr Metallic Glasses Subjected to the Rolling Process

Abstract: Using the rolling process, it is possible to induce multiple shear bands in the microstructure of metallic glasses (MGs) and improve the overall plasticity in the subsequent mechanical loadings. Hence, it is crucial to understand the mechanism of shear banding and plastic deformation under the rolling process. In this work, molecular dynamics (MD) simulation was applied to evaluate the formation and generation of shear bands in a CuZr MG under cold- and hot-rolling processes. Based on the results, it is found that the shear bands are formed with secondary branches in the cold rolling, while the shear events are scattered in the bulk of material in the hot rolling. Considering Voronoi analysis, it is revealed that the hot rolling is accompanied by the recovery of crystalline-like clusters provided that rolling process continues for subsequent passes. On the other hand, the cold-rolled sample shows a stable behavior in the evolution of crystalline-like clusters; however, the population of main icosahedral polyhedrons decreases in the system.

A cavity-based micromechanical model for the shear band failure in metallic glasses under arbitrary stress states

Abstract: Deformation and fracture of metallic glasses are often modeled by stress-based criteria which often incorporate some sorts of pressure dependence. However, detailed mechanisms that are responsible for the shear band formation and the entire damage initiation and evolution process are complex and the origin of such a pressure dependence is obscure. Here we argue that the shear band formation results from the constitutive instability, so that the shear-band angle and arrangements can be easily related to the macroscopic constitutive parameters such as internal friction and dilatancy factor. This is one reason for the observed tension-compression asymmetry in metallic glasses. The free volume coalescence leads to precipitous formation of voids or cavities inside the shear bands, and the intrinsic “ductility” is therefore governed by the growth of these cavities. Based on a generalized Stokes-Hookean analogy, we can derive the critical shear-band failure strain with respect to the applied stress triaxiality, in which the cavity evolution scenarios are sharply different between tension-controlled and shear/compression-dominated conditions. This is another possible reason for the tension-compression asymmetry. It is noted that diffusive-controlled cavity growth could also be the rate-determining process, as suggested by the recent measurements of shear-band diffusivity and viscosity that turn out to satisfy the Stokes-Einstein relationship. This constitutes the third possible reason for the tension-compression asymmetry.