Initiation Of Shear Bands Research Articles

Computer simulation of the matrix–inclusion interphase in bulk metallic glass based nanocomposites

Atomistic models for matrix–inclusion systems are generated. Analyses of the systems show that interphase layers of finite thickness appear interlinking the surface of the nanocrystalline inclusion and the embedding amorphous matrix. In a first approximation, the interphase is characterized as an amorphous structure with a density slightly reduced compared to that of the matrix. This result holds for both monatomic hard sphere systems and a Cu47.5Zr47.5Al5 alloy simulated by molecular dynamics (MD). The elastic shear and bulk modulus of the interphase are calculated by simulated deformation of the MD systems. Both moduli diminish with decreasing density but the shear modulus is more sensitive against density reduction by one order of magnitude. This result explains recent observations of shear band initiation at the amorphous–crystalline interface during plastic deformation.

Quasi phase transition model of shear bands in metallic glasses

A quasi phase transition model of shear bands in metallic glasses (MGs) is presented from the thermodynamic viewpoint. Energy changes during shear banding in a sample–machine system are analyzed following fundamental energy theorems. Three characteristic parameters, i.e. the critical initiation energy ΔGc, the shear band stability index k0, and the critical shear band length lc, are derived to elucidate the initiation and propagation of shear bands. The criteria for good plasticity in MGs with predominant thermodynamic arrest of shear bands are proposed as low ΔGc, large k0, and small lc. The model, combined with experimental results, is used to analyze some controversial phenomena of deformation behavior in MGs, such as the size effect, the effect of testing machine stiffness and the relationship between elastic modulus and plasticity. This study has important implications for a fundamental understanding of shear banding as well as deformation mechanisms in MGs and provides a theoretical basis for improving the ductility of MGs.

Microstructure and tensile properties of high-strength high-ductility Ti-based amorphous matrix composites containing ductile dendrites

In the present study, two Ti-based amorphous matrix composites containing ductile dendrites dispersed in an amorphous matrix were fabricated by a vacuum arc melting method, and deformation mechanisms related to the improvement of strength and ductility were investigated by focusing on how ductile dendrites affected the initiation and propagation of deformation bands, shear bands or twins. Ti-based amorphous matrix composites contained 70–73vol.% coarse dendrites of size 90–180μm, and had excellent tensile properties of the yield strength (1.2–1.3GPa) and elongation (8–9%). The Ta-containing composite showed strain hardening after yielding, and reached fracture without showing necking, whereas necking occurred straight after yielding without strain hardening in the Nb-containing composite. The improved tensile elongation and strain hardening behavior was explained by the homogeneous distribution of dendrites large enough to form deformation bands or twins, the role of β phases surrounding α phases to prevent the formation of twins, and deformation mechanisms such as strain-induced β to α transformation.

Deformation-Induced Grain Refinement and Amorphization in Ti-10V-2Fe-3Al Alloy

The microstructural evolution and grain refinement mechanisms of a Ti-10V-2Fe-3Al alloy, β-solution quenched and cold forged (CF) to strains of 0.1, 0.35, and 1.2 have been investigated using optical microscopy (OM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results showed that the stress-induced martensitic transformation became a predominant deformation mode in the metastable Ti-10V-2Fe-3Al alloy during cold forging. These martensites α″ repeatedly divided the original β parent phase into a large number of micron-sized blocks when the forging strain was 0.1. Shear bands were observed to traverse α″/β lamellae and resulted in a significant grain refinement of the β phase, as the forging strain increased to 0.35. The degree of grain refinement inside shear bands was higher than the outside. Nanocrystalline and amorphous structures were produced in local areas of the original β phase, when the forging strain rose to 1.2. This dramatic grain refinement in the metastable Ti-10V-2Fe-3Al alloy could be attributed to the stress-induced martensitic transformation promoting the initiation and growth of shear bands across α″/β lamellae. More dislocations were produced and accumulated inside grains to accommodate plastic deformation. The crystal structure was collapsed and an amorphous structure was formed as soon as the dislocation density was accumulated to a critical value of 1014/cm2. Moreover, some of the reverse martensitic phase transformation, α″→β, was observed to contribute to grain refinement of Ti-10V-2Fe-3Al alloy as well.

Correlation between relaxations and plastic deformation, and elastic model of flow in metallic glasses and glass-forming liquids

We study the similarity and correlations between relaxations and plastic deformation in metallic glasses (MGs) and MG-forming liquids. It is shown that the microscope plastic events, the initiation and formation of shear bands, and the mechanical yield in MGs where the atomic sites are topologically unstable induced by applied stress, can be treated as the glass to supercooled liquid state transition induced by external shear stress. On the other hand, the glass transition, the primary and secondary relaxations, plastic deformation and yield can be attributed to the free volume increase induced flow, and the flow can be modeled as the activated hopping between the inherent states in the potential energy landscape. We then propose an extended elastic model to describe the flow based on the energy landscape theory. That is, the flow activation energy density is linear proportional to the instantaneous elastic moduli, and the activation energy density ρE is determined to be a simple expression of ρE=1011G+111K. The model indicates that both shear and bulk moduli are critical parameters accounting for both the homogeneous and inhomogeneous flows in MGs and MG-forming liquids. The elastic model is experimentally certified. We show that the elastic perspectives offers a simple scenario for the flow in MGs and MG-forming liquids and are suggestive for understanding the glass transition, plastic deformation, and nature and characteristics of MGs

Shear band initiation induced by slip-twin boundary interactions

Strain localization is an important mode of inhomogeneous deformation controlling the ductility of a wide range of materials. In this work, the atomistic mechanism of shear band initiation in nanotwinned polycrystals is studied and is found to be governed by the interactions between dislocations and twin boundaries. A simple model is developed to illustrate the effect of twin boundary density on the initiation of shear bands.

Three-Point Bending of Heat-Treatable Aluminum Alloys: Influence of Microstructure and Texture on Bendability and Fracture Behavior

Three-point bending tests of extruded aluminum alloys showed lower bendability when the bending axis is aligned with the extrusion direction compared to the transverse direction. In the present work, three different microstructures of a commercial AA7108 aluminum alloy were studied with respect to mechanical properties, texture, constituent particles, and crack propagation. The three different microstructures were an as-cast and homogenized material, a fibrous extruded material, and a cold rolled and recrystallized material. While the mechanical properties in tension are more or less the same for the three materials, the bendability is strongly dependent on the microstructure and the global alignment of constituent particles. The as-cast and homogenized material shows poor bendability due to large grains and constituent particles on the grain boundaries, which leads to decohesion and premature failure. The response of both the fibrous and the recrystallized materials depends on the direction of the bending axis. A strong fiber texture is found to influence the bendability by initiation of shear bands. The crucial fracture mechanism, however, seems to be the global alignment of constituent particles, which is inherited from the deformation process.

Nanocrystallization in a shear band: An in situ investigation

Preferential nanocrystal formation in shear bands that occurred upon applying a tensile strain on an Al-rich metallic glass has been analyzed in situ in a transmission electron microscope as a function of time after the shear band initiation and as a function of local heating. The results indicate the presence of a transient period before nanocrystal formation sets in, as well as the necessity of thermal activation and further show that nanocrystals developed only within the shear bands. These results support models that explain nanocrystal formation in shear bands based on an increased local mobility.

The Analysis of Saw-Tooth Chip Formation in High Speed Machining through Material Micro-Hardness Measurement

The material micro-hardness is used to reflect the plastic deformation within the localized shear band and the segment of saw-tooth chip. A high strength alloy steel Aermet 100 is taken as the workpiece material. The saw-tooth chip roots are obtained by cutting operation. The material micro-harnesses along the shortly initiated localized shear band, across the shortly initiated localized shear band and fully developed localized shear band, and within the segment are examined and analyzed. The localized shear band initiation and propagation are revealed by a new proposed model. The results show the localized shear band initiates firstly at the tool tip end of the primary shear band, and consequently another localized shear nucleates at the free surface end of the primary shear band. These two localized shears propagate face to face until the whole localized shear band is formed.

Effects of microstructure on the adiabatic shearing behaviors of titanium alloy

The effects of microstructure on the adiabatic shearing behaviors in the Ti-1300 alloy were investigated by means of the split Hopkinson pressure bar (SHPB) with hat-shaped specimens. Shear bands were developed in both Ti-1300 alloys with β phase and α + β lamellar microstructures during a forced dynamic shear deformation. The microstructures have a significant influence on the adiabatic shearing behaviors of Ti-1300 alloy. The critical strain for the initiation of shear bands in α + β lamellar microstructure is less than that in β phase microstructure. The α + β lamellar microstructure experiences more extensive microscopic damage than β phase microstructure. Therefore, the α + β lamellar microstructure is more susceptive to adiabatic shear localization deformation than β phase microstructure at the same strain rate.

Comparison of compressive deformation and fracture behaviors of Zr- and Ti-based metallic glasses with notches

Compressive tests on the Zr- and Ti-based metallic glasses with different notches were investigated to compare their shear fracture mechanism and plastic deformation abilities. It is found that the plasticity of the two metallic glasses can be improved by installing two semicircular symmetrical notches even for the Ti-based metallic glass which has nearly zero compressive plasticity. The enhanced plasticity may be ascribed to the easy initiation of shear bands (SBs) around the notches, and the consequent blocking effect of notches on the propagation of shear bands according to the large-scale stress gradient. Additionally, based on a theoretical model originated from the concept of critical steady shear displacement (CSSD), compared with the sizes of smooth regions on the fracture surface, the plasticity difference of the two different metallic glasses was analyzed quantitatively. The current findings might provide an approach to understand and estimate the difference in the plastic deformation abilities on diverse metallic glasses, as well as the ones with large-scale stress gradient.

Deformation behavior and enhanced plasticity of Ti-based metallic glasses with notches

The plasticity of Ti-based metallic glasses with different aspect ratios can be improved by introducing two semicircular notches on the edges of the samples, owing to the interactions of shear bands (SBs) under conventional compression tests. The interaction of SBs can be ascribed to the easy initiation of SBs around the notches due to the large stress gradient, and the consequent blocking effect of notches on the propagation of shear bands. Additionally, the current findings provide a new way to understand the physical nature for the plastic deformation behavior of some brittle metallic glasses and supply an effective approach to enhance the plasticity to some extent.

Deformation-induced microstructure refinement in primary α phase-containing Ti–10V–2Fe–3Al alloy

Microstructural evolution in primary α phase-containing Ti–10V–2Fe–3Al alloy subjected to cold forging under different applied strains was studied. Experimental results showed that, even at a strain of 0.1, stress-induced α″ martensites were abundantly produced within the β matrix, resulting in alternative α″/β lamellae. Shear bands initiated and grew across α″/β lamellae as the strain increased to 0.35. When the strain increased to 1.2, the volume fraction of shear bands significantly increased and the grains were almost occupied by the shear bands. Interestingly, nanocrystallines were observed inside shear bands. While in the primary α phase, slip was always the predominant plastic deformation mode and dislocations were accumulated to a high density within the strain range from 0.1 to 0.35. When the strain was up to 1.2, the dislocation density was further increased and α/β interface boundary became ill-defined. However, no grain refinement was observed in the α phase. The microstructure refinement in the β matrix could be attributed to that stress-induced α″ martensitic transformation promoted the initiation, thickening and coalescence of shear bands. The plastic deformation combined with martensitic phase transformation could provide a potential effective technique to produce nanocrystalline materials.

Plastic deformability of metallic glass by artificial macroscopic notches

Abstract This paper reports that the plasticity of Zr-based metallic glass can be improved by creating two symmetrical semi-circular notches. Unlike the experimental findings of the samples without notches, a steady shear deformation can be created by the large-scale stress gradient around the two symmetrical notches and the plasticity of metallic glass can be enhanced to a high value of ∼10% under compression tests. The improved plasticity may be due to the easy initiation of shear bands around the notches, and the consequent blocking effect of notches on the propagation of shear bands, similar to the dislocation mechanism in crystalline materials. To reveal the particular plastic deformation behavior of metallic glass, Ti3SiC2 ceramic and high-strength steel specimens with two symmetrical semi-circular notches were also conducted under compressive loadings; however, no enhancement in plasticity was found. It is suggested that creating a stress gradient is a particular strategy for designing metallic glasses in order to improve their plasticity.

Energetic criterion on the intrinsic ductility of bulk metallic glasses

We report the use of an energetic criterion to assess the deformability of glassy alloys based on the cooperative shearing model and energy landscape theory. The barrier energy density necessary for the activation of shear transformation zones and the initiation of shear bands shows a strong correlation with the ductility of bulk metallic glasses. This finding directly ties the ductility of metallic glasses to the forming ability of shear bands and offers an energetic insight into the mechanical behavior of metallic glasses.

On the differences of dynamic localisations between different types of metals

This paper presents a numerical study of inelastic localisations in metals after utilising a thermodynamically consistent framework of viscoplastic formulation implemented into a commercial finite element code. The significance of using physically-based flow stress relations, instead of empirical relations, is examined by considering three microstructure-based constitutive equations developed previously for three different crystal types of metals. The same numerical values of material constants are used with a range of loading initial temperatures and velocity impacts. Differences of dynamic localisations between three body-centred cubic metals (niobium, tantalum and vanadium), one fcc metal (OFHC copper) and one hcp metal (titanium) are illustrated and discussed through studying the initiation and propagation of necking and shear bands in a circular bar and simple tension plane strain problems, respectively. Moreover, the effect of initial temperatures and strain rates on dynamic localisations is also scrutinised. Objective results using different mesh configurations are verified as a result of including viscosity in the constitutive models.

Effect of particulate/matrix debonding on the formation of adiabatic shear bands

We use the cohesive zone failure model to simulate debonding and failure in high strain-rate plane strain deformations of a heat conducting particulate composite comprised of initially circular metallic particulates immersed in a metallic matrix, with the goal of delineating the effect of these failures on the initiation and propagation of adiabatic shear bands (ASBs). Failure is assumed to ensue at an interface between two elements when a predefined combination of the normal and the tangential tractions on that interface reaches a critical value. We postulate that the critical value of the traction in the cohesive zone failure model decreases affinely with an increase in the temperature. Both particulate and matrix materials are assumed to be isotropic, heat conducting, and to obey the von Mises yield criterion with the flow stress depending upon the effective strain, the effective strain rate, and the temperature according to the Johnson–Cook relation. The coupled transient thermomechanical problem is analyzed by the finite element method by using 3-node triangular elements and the finite calculus technique to prevent volumetric locking. It is found that the critical strength of the bond between the particulate and the matrix significantly influences the loss of strength of the entire specimen. The time of initiation of an ASB is influenced by the time when debonding ensues which depends upon the values of the critical traction and the mode-mixity parameter in the cohesive zone failure criterion.

Multiresolution continuum modeling of micro-void assisted dynamic adiabatic shear band propagation

A thermal–mechanical multiresolution continuum theory is applied within a finite element framework to model the initiation and propagation of dynamic shear bands in a steel alloy. The shear instability and subsequent stress collapse, which are responsible for dynamic adiabatic shear band propagation, are captured by including the effects of shear driven microvoid damage in a single constitutive model. The shear band width during propagation is controlled via a combination of thermal conductance and an embedded evolving length scale parameter present in the multiresolution continuum formulation. In particular, as the material reaches a shear instability and begins to soften, the dominant length scale parameter (and hence shear band width) transitions from the alloy grain size to the spacing between micro-voids. Emphasis is placed on modeling stress collapse due to micro-void damage while simultaneously capturing the appropriate scale of inhomogeneous deformation. The goal is to assist in the microscale optimization of alloys which are susceptible to shear band failure.

Initiation and evolution of shear bands in bulk metallic glass under tension—An in situ scanning electron microscopy observation

The initiation and evolution of shear bands in Zr64.13Cu15.75Ni10.12Al10bulk metallic glass tensile samples has been investigated in situ by scanning electron microscopy. The initial shear band originates from the highest stressed area, and does not propagate during further tension, which is attributed to the weakening of the stress field in front of the shear band tip, possibly caused by atomic rearrangement and local temperature rise. As a result, multiple shear bands occur in sequence with gradually increased length and offset. This result is due to the fact that the stress in front of the tip of the initial shear band does not concentrate again during further tension above the shear yield strength. Numerical analysis was carried out to investigate the stress distribution under tension, suggesting that the maximum pressure-dependent shear stress criterion overestimates the yield strength, while the shear plane criterion describes the conditions for the formation of shear bands well.

Synthesis and characteristics of 80 vol.% tungsten (W) fibre/Zr based metallic glass composite

Based on full understanding of wettability between Zr based bulk metallic glass (BMG) matrix and tungsten (W) fibre, and liquid/solid interfacial atomic interaction, the high density and high strength 80 vol.% W fibre/Zr based BMG composite, with optimum interfacial layer, was successfully developed by strictly controlling the infiltration and solidification process. Its density was 17 g/cm 3. The average value of tensile fracture strength was 1685 MPa. Fracture mode represented instant rupture which was vertical to the axial direction. The average value of compressive fracture strength and strain were 2550 MPa and 23% respectively. Its fracture mode was complex splitting along loading direction. The initiation of shear bands occurred when the compressive specimen reached plastic strength. Shear bands' continuous expanding, branching, propagation and build up responded the improvement of plasticity under the continuous loading. Shear bands distributed on the fracture surface in two ways. One was consistent with the maximum shear force direction and at 45° angle with the loading direction. The other was nearly horizontal and vertical to the loading direction. The characteristics of plastic deformation and fracture were also investigated and discussed in detail.