Propagation Of Shear Bands Research Articles

Strong resistance to shear instability in multilayered metallic composites by nanoscale amorphous-BCC crystalline interfaces

Experiments have shown that high strength and high plastic stability are difficult to be simultaneously obtained in crystalline-amorphous layered composites. Here we report a complete suppression of plastic instability in a strong multilayered composite via nanoscale BCC Nb crystalline-amorphous CuNb interface architecting. Specifically, the size-dependent hardness and shear instability of the designed nanolayered composites are systematically investigated with individual layer thickness (h) ranging from 2 to 100 nm by nano/micro-indentation tests. A mechanics model is also proposed to quantify the shear instability. A non-monotonic shear instability behavior is revealed with decreasing h. When h > 40 nm, a cutting-like shear banding prevails in the composites and the shear instability reduces with decreasing h. Finally, a homogeneous plastic deformation without shear banding is achieved at h of 40 nm. For h < 40 nm, a kinking-like shear banding occurs in the composites and the shear instability increases with reducing h. Further theoretical analysis reveals a critical layer thickness for activating homogeneous deformation in the composites, i.e., 45 nm, above and below which the shear deformation would be dominated by cutting-like and kinking-like shear banding, respectively. The cutting-like shear banding is attributed to the propagation of the mature shear band formed in the amorphous phase that penetrates the crystalline layer. The homogeneous plastic deformation can be ascribed to the activation of the confined layer slip mechanism in the crystalline layer and the non-clustered shear transformation zones initiated in the amorphous layer. The kinking-like shear banding below 40 nm may originate from the cooperative kinking of the constituent layers with decreasing h. The potential advantage of the ductile BCC metals over FCC ones for suppression of shear instability in crystalline-amorphous composites is discussed based on the unique dislocation activities in nanoscale BCC crystals (e.g., self-multiplication and significant cross-slipping mechanisms). The present study sheds some light on designing strong and ductile crystalline-amorphous composites by selecting an appropriate BCC crystalline phase and an optimal interface spacing to eliminate plastic instability.

Fretting wear resistance of amorphous/amorphous (AlCrFeNi)N/TiN high entropy nitride nanolaminates

The application of amorphous high entropy ceramics as wear-resistant materials is limited due to their inherent brittleness at room temperature and strain softening during deformation. In order to overcome this limitation, we constructed amorphous/amorphous (AlCrFeNi)N/TiN nanolaminates with varying modulation period thickness by introducing a second amorphous phase through reactive radio frequency (RF) magnetron sputtering, along with corresponding monolithic amorphous films. Microstructure, mechanical properties, and tribological behaviors of the films were characterized wear in detail. Fretting wear results show that the nanolaminates with an average modulation period of 6 nm exhibited a wear rate of 2.8 times lower than that of the (AlCrFeNi)N film and 8.4 times lower than that of the TiN film. Further analysis using FIB-TEM revealed that the enhanced wear resistance of (AlCrFeNi)N/TiN nanolaminates was attributed to the high-density heterointerfaces. These interfaces inhibited the initiation and propagation of mature shear bands and acted as barriers to stress distribution. Additionally, the oxide composite layer at the interface demonstrated a synergistic effect through a mechanically induced tribo-chemical reaction, resulting in slight plastic deformation. For the amorphous (AlCrFeNi)N film, moderate wear resistance was achieved through the formation of transfer layer at the interface. For the amorphous TiN film, the dimensional stability of the film deteriorates due to the significant strain softening that occurs during deformation. This study deepens our understanding of the friction mechanisms involved in amorphous high entropy ceramics, offering valuable insights for the design of high damage-resistant materials.

Effect of microstructure on the effectiveness of hybridization on additively manufactured Inconel718 lattices

This work aims to investigate the effect of the material microstructure on the effectiveness of hybridization in additively manufactured Inconel718strut-based lattices. Two microstructures, namely the as-built solid solution and a peak aged condition, are coupled with three hybrid architectures formed by alternating face centred cubic (matrix) and octet-truss (reinforcement) domains containing 1, 8, and 64 unit cells. Digital image correlation is utilized to map the local strain distribution during compression, and finite element analysis is utilized to map stress localization. It is shown that in the as-built condition the mechanical behavior of all the hybrid lattices was very similar to that of the single-oriented components, irrespective of the reinforcement size. In the peak-aged lattices, on the contrary, including octet-truss reinforcements of sufficiently small size resulted in an increase inmechanical stability, damage tolerance, and absorbed energy, as interphase boundaries act as obstacles to shear band propagation. It is also shown that effect of hybridization is a function of the size of the reinforcements. In particular, below a critical size, the lattice behaves as a reinforced “single-phase” structure, rather than a two-phase architecture. This work defines guidelines for the design of robust lattices manufactured by laser powder bed fusion.

Shear band evolution and mechanical behavior of cold-rolled Zr-based amorphous alloy sheets: An in-situ study

Cold rolling process can regulate the microstructure and mechanical properties of amorphous alloys, but it is still a challenging task to reveal their microscopic mechanism. Here, we designed an in-situ SEM observation device for the cold rolling process of amorphous alloy, and visually observed the formation and evolution of shear bands during single-pass and multi-pass rolling process of the Zr55Cu30Al10Ni5 amorphous alloy sheets. It is found that the evolution process of shear bands in the rolling process of amorphous alloy shows heritability, which is mainly reflected in two aspects: one is that the shear band formation pattern in the single-pass rolling process is more inclined to inherit the previous shear band formation pattern; the other is that the shear deformation is more likely to occur in the pre-existing shear bands in the multi-pass rolling process. This rule can be used to guide the controlled generation of shear bands in amorphous alloys. Moreover, we emphasized the importance of pre-existing shear band orientations and systematically investigated the mechanical behavior of the amorphous alloys with pre-existing shear bands by in-situ SEM observation. It is found that the mechanical properties of the as-rolled amorphous alloys are determined by the competition between the work-softening of the pre-existing shear band itself and the work-hardening caused by the blocking effect of the pre-existing shear bands on the shear deformation. Based on this, we enhance the tensile fracture strength and the tensile ductility of the amorphous alloy by adjusting the orientation of the pre-existing shear bands parallel to the tensile stress axis so that the pre-existing shear bands prevent the linear propagation and destruction of the new shear bands.

Investigations on shear band formation in metallic nanolayered composites

While metallic nanolayered composites exhibit ultrahigh strength, they can fail due to shear bands propagation. Shear bands are affected by many factors, such as layer thickness and stacking fault energy. There is a growing demand to simultaneously prevent shear bands while harnessing the high strength potential derived from the dense interface nanostructures. The mechanisms of shear band formation vary among different nanolayered composites, and some of these mechanisms are investigated. The importance of utilizing advanced computational tools to understand shear band formation is highlighted. This review comprehensively addresses the influencing factors of shear band formation, strategies for shear band suppression, and the underlying mechanisms of shear band formation within metallic nanolayered composites.

Enhancing tensile ductility of porous metallic glasses through shear band path design

Enhancing the toughness of metallic glasses has consistently been a significant area of research. In this study, the influence of shear band paths on the properties of porous metallic glasses is thoroughly investigated using finite element simulations. The research uncovers the preferred path selection for shear band propagation in diverse heterostructures of porous metallic glasses. The findings reveal that shear bands predominantly propagate along paths where the angles formed by the first or second adjacent pores are in closer proximity to the direction of the maximum shear stress. Leveraging an extensive comprehension of the deformation mechanism of porous metallic glasses, the study proposes a hexagonal pore structure that exhibits exceptional tensile ductility. These outcomes offer valuable insights and design strategies for the engineering of porous metallic glasses in diverse applications.

Catastrophic submarine landslides with non-shallow shear band propagation

Submarine landslides are a geohazard encountered frequently in both shallow and deep waters. Catastrophic landslides are often related to strain-softening and shear band propagation (SBP). Limit equilibrium methods cannot capture this mechanism. The existing SBP theoretical methods treat slope failure as a progressive process of SBP, however, based on the shallow SBP approximation which is only valid when the length of the initial shear band is much larger than its depth. In this paper, catastrophic propagation of a deep-seated shear band is investigated via finite element analyses to understand and quantify the SBP at different conditions. The shallow SBP approximation is tested in a parametric study in terms of the relative depth of shear band and soil properties. An empirical correction equation is introduced to account for non-shallow SBP by modifying the existing failure criteria. The empirical solution is compared well with the FE results and can be implemented to assess submarine slope stability. The influences of different strain-softening relationships and inertia effects are discussed as well. As an example, the historical Storegga Slide offshore Norway is revisited, and the critical condition for its catastrophic failure is quantified using the proposed criterion.

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.

Synthesis and mechanical behavior of in-situ porous Ti particle reinforced Mg-Cu-Gd-Ag bulk metallic glass matrix composite

The in-situ porous Ti particle reinforced Mg-based bulk metallic glass matrix composites have been successfully prepared via dealloying reaction in metallic melt. The microstructure, thermal properties and mechanical properties of the composites containing various volume fraction of porous Ti particles were investigated in detail. It is shown that with higher content of porous Ti particles, the glass-forming ability of the composites degrades gradually with the precipitation of fine crystalline phases among the glassy matrix. When the volume fraction of porous Ti particle is 26.3 vol%, the composite shows an fracture strength of 1125 MPa, 14.1 % higher than the monolithic glassy base alloy. Furthermore, the composite also exhibit an improved plastic strain of 0.94 %. The in-situ porous Ti particle and the submicron crystalline phase can hinder the rapid propagation of the main shear bands, causing the generation of multiple shear bands. It is found that abundant shear bands are formed around the porous Ti particle. The present synthesis strategy of in-situ porous reinforcing phase will contribute to the breakthrough in designing novel metal matrix composites.

Super ductile metallic glasses for energy-saving solid-state processing

Energy-efficient materials are key to combating the high energy costs and climate change. The manufacturing temperatures of industrially important Zr-based bulk metallic glasses (BMGs) relative to steels are low, and exist between the liquidus temperature Tl (∼850 °C) and glass transition temperature Tg (∼400 °C). However, these materials show limited plastic deformability (ductility) at room temperature (strains typically less than 3%); moreover they soften but exhibit limited ductility at high processing temperatures. Their low ductility should be improved because it impedes fatigue resistance and machinability, such as via cold (plastic) forming. In this study, chemical composition changes, which reduced Tg, resulted in remarkably ductile BMGs with extreme deformations of over 70% under compression, thereby enabling their energy-efficient processing at low temperatures. In contrast to previously reported conclusions on the high GFA and deformation-induced nanocrystallization being the precursors to ductility, formation of a low amount of meso-crystallites within the glassy material during cooling efficiently hindered the propagation of shear bands and microcracks under loading, thus increasing significantly ductility. This characteristic, in addition to optimal chemical composition, played an important role in improving the ability of BMGs to undergo solid-state processing at low temperatures and increased deformation rates.

A GIS framework for prediction of basin‐wide post‐failure geometry of confined subaqueous landslides

AbstractStability analysis of submarine and sub‐lacustrine slopes requires special attention to the strain‐softening nature of clay‐rich sediments. Utilizing the often‐used infinite slope assumption using the peak strength is inadequate for the task at hand as the well‐known limit equilibrium method may provide non‐conservative estimate of marine slopes’ stability. In this paper, a GIS based framework is developed to account for a more suitable shear band propagation slope‐failure criterion for basin‐wide stability calculations. After determining the slab failure prone areas, the framework employs analytical criterion for retrogressive failure above the slab and depth‐integrated finite element method for calculating the size of confined slides at the toe. This allows predicting the landslide's post‐failure configuration and the overall landslide coverage zone. The framework is first benchmarked to prove its competence and then applied to the well documented Zinnen Slide and the southeastern banks of the Küssnacht Basin, Lake Lucerne, Switzerland. The results of the application show good agreement with the observed basin geomorphology.

Mechanisms for the Formation of an Exceptionally Gently Inclined Basal Shear Zone of a Landslide in Glacial Sediments—The Ludoialm Case Study

The Ludoialm landslide, which is located in the municipality of Münster in Tyrol, Austria, represents a large-scale translational landslide in glacial soil sediments characterised by an exceptionally low inclined basal shear zone of only 12°. Although a temporal coincidence between meteorological events and slope displacement is obvious, the hydromechanical coupled processes responsible for the initial landslide formation and the ongoing movement characteristics have not yet been identified. This article provides a comprehensive analysis of the predisposition factors and the initial failure mechanism of this landslide from geological and geotechnical perspectives. We use a prefailure geometry of the cross section to simulate the initial slope failure process by a limit equilibrium analysis (LEA), a strength-reduction finite element method (SRFEM), and a finite element limit analysis (FELA). The shape and location of the computationally obtained basal sliding zone compare well with the geologically assumed one. Based on the computational study, it turns out that a high groundwater table probably caused by snow melting in combination with different permeabilities for the different layers is needed for the formation of the exceptionally low inclined basal shear zone. This paper presents the failure mechanism of the Ludoialm landslide and discusses the role of the shear band propagation in the process of slope destabilization.

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

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.

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.

Effects of void shape and location on the fracture and plastic deformation of Cu (crystalline) /Cu64Zr36 (amorphous) composites

Voids are probably the most widely studied type of manufacturing defects since they are often formed during material processing. In this paper, the fracture and plastic deformation of Cu/Cu64Zr36 crystalline/amorphous composites with different void locations and shapes are investigated by molecular dynamics simulations. The results show that changes of the void location and shape in the composites significantly change the dislocation motion, shear transformation zones (STZs) activation, shear band propagation, and fracture strength of the composites. The largest shear strain is always found around the void, where it is most likely to activate dislocation motion, STZs and shear band formation. A stepwise increase of the shear localization is mainly related to the nucleation and emission of dislocations, while the very rapid increase is caused by shear band formation. Moreover, the variation of dislocation density and activity of shear bands are mainly responsible for the stress fluctuations in the composites. This paper provides important information for understanding and improving the plastic and fracture behaviors of composites by changing the location and shape of void from an atomistic perspective.

A reformulated non-ordinary state-based peridynamic method for dynamic failure of ductile materials

A non-local correspondence peridynamic model is reformulated to describe the dynamic failure of ductile materials, under the framework of non-ordinary state-based peridynamics. To eliminate the zero-energy mode as well as to improve the numerical stability, the deformation gradient is updated on the bond level, which owns information including both the non-local deformation and the bond deviation. Peridynamic thermo-viscoplastic constitutive formulations are presented for the failure analyses of ductile materials. Moreover, a mixed strain-stress criterion describing the bond breaking and considering the temperature effect is presented on the bond level, in which the bonds can keep intact in calculations. Several comprehensive cases, including a notched tension test and Kalthoff-Winkler impact tests with different velocities, are investigated to verify the applicability of the proposed model for ductile fracture and impact failure. Numerical results, in terms of failure patterns, cracking switching and propagation, as well as adiabatic shear band propagation, show a reasonable agreement with corresponding experimental observations.

Deformation behavior of plasma sprayed Fe-based amorphous/ nanocrystalline coating under multi-scale tribological contact

Deformation behavior of Fe-based amorphous/ nanocrystalline coating synthesized by plasma spraying has been studied under multi-scale tribological contact. Nano- and micro-scale scratch investigation under constant and ramped loading revealed that deformation occurred mainly through plowing of coating surface by the probe. During low load (nano-scale) scratch tests, shear band generation was detected in the form of serrations in lateral force/ coefficient of friction curves under both constant (2 and 5 mN) and ramped (0–5 mN) loading conditions. Prominent shear bands around the scratch track with lesser fracture/ chipping were observed at high load (micro-scale) under ramped loading. Further, high load nanoscratch test under constant loading revealed aggravated fracture and chipping on the coating surface. Macro-scale deformation behavior studied using ball-on-disc dry sliding wear test at constant applied load (10 N) revealed that dominating wear mechanism in the coating was fatigue accompanied by abrasive and oxidative wear.

The dynamics of shear band propagation in metallic glasses

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

Mechanical Properties of Internally Hierarchical Multiphase Lattices Inspired by Precipitation Strengthening Mechanisms.

In metal metallurgy, precipitation strengthening is widely used to increase material strength by utilizing the impediment effect of the second-phase particles on dislocation movements. Inspired by this mechanism, in this paper, novel multiphase heterogeneous lattice materials are developed with enhanced mechanical properties utilizing a similar hindering effect of second-phase lattice cells on the shear band propagation. For this purpose, biphase and triphase lattice samples are fabricated using high-speed multi jet fusion (MJF) and digital light processing (DLP) additive manufacturing techniques, and a parametric study is carried out to investigate their mechanical properties. Different from the conventional random distribution, the second-phase and third-phase cells in this work are continuously distributed along the regular pattern of a larger-scale lattice to form internal hierarchical lattice structures. The results show that the triphase lattices possess balanced mechanical properties. Interestingly, this indicates that introducing a relatively weak phase also has the potential to improve the stiffness and plateau stress, which is distinct from the common mixed rule. This work is aimed at providing new references for the heterogeneous lattice design with outstanding mechanical properties through material microstructure inspiration.

Yield strength of “brittle” metallic glass

“Brittle” metallic glass (MG) usually fractures catastrophically in a shattering mode under macroscopic compression, because cleavage cracking of splitting that originates from extrinsic flaws dominates the failure of such alloys, which brings challenges for studying yield strength. Here we show that the plastic yielding behavior in a brittle Fe-based MG can be successfully activated by decreasing the sample size to micrometer scale to avoid the possible large tensile stress concentrators. The yield strength was found to be at least 33% higher than the fracture strength measured with bulk samples for the present brittle MG. The results further demonstrate that the critical stresses for shear band initiation and propagation are size-independent, while the required stress for cleavage cracking increases with decreasing sample size. The competition of thermodynamic driving forces between the two processes of shear banding and cleavage cracking hence leads to the size-induced brittle-to ductile-transition. These findings clarify the physical nature of the strength of “brittle” MG, implying the great opportunity for using high-strength brittle MGs in devices with small dimensions.