Intense Shear Bands Research Articles

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

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.

Intense Shear Band Plasticity in Metallic Glass as Revealed by a Diametral Compression Test

Intense Shear Band Plasticity in Metallic Glass as Revealed by a Diametral Compression Test

Chip formation and wear mechanisms of SiAlON and whisker-reinforced ceramics when turning Inconel 718

Inconel 718 has found use in many demanding applications due to its high temperature fatigue strength, hardness and low thermal conductivity. These material properties present a challenge to productive and high quality (surface finish) machining and promote rapid tool wear. The work presented here describes the chip formation and wear mechanisms of silicon nitride (Si3N4) based ceramic SiAlON and silicon carbide whiskers reinforced alumina (WRA) (Al2O3 + SiCw) round (RNGN) inserts, when turning solution-annealed Inconel 718 (27<HRC<30) with a 10% concentration cutting fluid. Four sets of cutting trials were conducted at a cutting speed of 250 m/min and another four at 300 m/min. The results show that using SiAlON turning inserts at a cutting speed of 300 m/min delivered the best results in terms of tool flank wear, total tool life, and work-piece surface finish. The morphology of the Inconel 718 chip, at cutting speeds above 250 m/min, presents an intense shear band localisation in the primary shear zone of the chip/tool interface, which leads to chip segmentation.

Particle Swarm Optimization and Machinability Aspects during Turning of Hardened D3 Steel

Metal cutting processes are associated with excessive forces, friction and heat generation due to continuous intensive contact between the active cutting tools and the work which degrades the machined surface quality. The hardened tool steel having excellent wear resistance property receives an extensive promotion, investigation and application in the die manufacturing industries. In this research, the machinability of hardened AISI D3 tool steel has been investigated using TiN-coated Al2[Formula: see text](C,N) ceramic tool inserts as per Taguchi’s [Formula: see text] orthogonal design of experiments is in horizontal turning under the dry condition. The experimental dataset was used for the regression model development of primary process outputs as well as mean cutting force using the response surface methodology (RSM) which was found to be significant. The parametric interaction effects on each process output were studied along with with chip morphology in detail. The cutting force as well as material removal rate was found to be significantly influenced by depth of cut, whereas machined surface quality was primarily dependent on tool feed rate. The wider with less prominent saw tooth chips got changed to narrower saw-tooth form with intensive shear bands at higher feed rates. Finally, an attempt has been made to develop an optimal parametric setting using particle swarm optimization technique to achieve minimum surface roughness and maximum material removal rate at less cutting force. The optimal parametric setting was high cutting speed (308[Formula: see text]m/min), high tool feed rate (0.08[Formula: see text]mm/rev) with medium depth of cut (0.6[Formula: see text]mm) to achieve each objective. This evolutionary particle swarm optimization technique was found to be highly accurate (within 8% error) as per validation experiment.

Processing map for controlling microstructure and unraveling various deformation mechanisms during hot working of CoCrFeMnNi high entropy alloy

In the current study, the hot deformation characteristics and workability of a CoCrFeMnNi high entropy alloy was characterized using processing maps developed on the basis of dynamic materials model in the temperature range 1023–1423 K and strain rate range 10−3 – 10s−1. The processing map delineated various deterministic domains including those of cracking processes and unstable flow, thus enabling identification of a ‘safe’ processing window for the hot working of the alloy. Accordingly, a deterministic domain in the temperature and strain rate ranges of 1223–1373 K and 10−2 – 5 × 10−1s−1, respectively, was identified to be the domain of dynamic recrystallization (DRX) with a peak efficiency of the order of ~34% at 1293 K and 3 × 10−2s−1 and these were considered to be the optimum parameters for hot deformation. The DRX grain size was dependent on the deformation temperature and strain rate, increasing with the increase in temperature and decrease in strain rate, whereas DRX volume increased with the strain rate. At still higher temperatures (1403–1423 K) and lower strain rates (10−3 – 3 × 10−3s−1), there was a sharp decrease in efficiency values from 27% to 5% thus forming a trough and the microstructure was characterized with coarse grains. In the instability regime, grain boundary cracking/sliding and localized shear bands manifested at temperatures <1223 K and strain rates <10−2s−1. The increase in strain rate resulted in an intense adiabatic shear banding along with formation of voids. At 10s−1 and temperatures >1398 K, microstructural reconstitution occurred in the shear bands leading to the formation of fine grains, presumably as a consequence of continuous recrystallization.

Effect of heating rate during solution treatment on the bendability of Al–Mg–Si alloys

The bendability of Al–Mg–Si alloy sheets is a key parameter for their application as skins of automotive body panels. In the present study, the influence of heating rate on the bendability of Al-0.6Mg-1.0Si (in wt%) alloy was investigated using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The bendability of Al–Mg–Si alloy was found to be significantly dependent on the heating rate of solution treatment. After bending, the rapidly heated specimen exhibited only slight surface undulation, while the slowly heated specimen showed severe surface necking and even micro-cracks. The severe surface necking was caused by intensive shear bands formed during bending, and the micro-cracks were the results of the interaction between the intensive shear bands and coarse second phase particles. Elongated coarse grains were more likely to produce intensive shear bands, and cube-oriented grains impeded the formation of shear bands. The schematics of recrystallization and bending processes for Al-0.6 Mg-1.0 Si alloy considering two different heating rates are proposed.

The effect of loading direction on strain localisation in wire arc additively manufactured Ti–6Al–4V

Ti–6Al–4V microstructures produced by high deposition rate Wire Arc Additive Manufacturing (WAAM) can be both heterogeneous and anisotropic. Key features of the as-built microstructures include; large columnar ß grains, an α transformation texture inherited from the β solidification texture, grain boundary (GB) α colonies, and Heat Affected Zone (HAZ) banding. The effect of this heterogeneity on the local strain distribution has been investigated using Digital Image Correlation (DIC) in samples loaded in tension; parallel (WD), perpendicular (ND) and at 45° (45ND) to the deposited layers. Full-field surface strain maps were correlated to the underlying local texture. It is shown that loading perpendicular to the columnar β grains leads to a diffuse heterogeneous deformation distribution, due to the presence of regions containing hard, and soft, α microtextures within different parent β grains. The ‘soft’ regions correlated to multi-variant α colonies that did not contain a hard α variant unfavourably orientated for basal or prismatic slip. Far more severe strain localisation was seen in 45° ND loading at ‘soft’ β grain boundaries, where single variant α GB colonies favourably orientated for slip had developed during transformation. In comparison, when loaded parallel to the columnar ß grains, the strain distribution was relatively homogeneous and the HAZ bands did not show any obvious influence on strain localisation at the deposit layer-scale. However, when using high-resolution DIC, as well as more intense shear bands being resolved at the β grain boundaries during 45° ND loading, microscale strain localisation was observed in HAZ bands below the yield point within the thin white-etching α colony layer.

Bendability enhancement of an age-hardenable aluminum alloy: Part I — relationship between microstructure, plastic deformation and fracture

In this study, the bendability of two 1 mm thick sheets of monolithic AA6016 and composite AA6016X alloys is investigated using a series of wrap-bend tests, with emphasis on understanding the relationship between microstructure, the nature of plastic deformation and fracture behavior. The composite AA6016X alloy consists of a central core of AA6016 sandwiched between 100 μm thick clad layers of AA8xxx series aluminum alloy, processed using thermomechanical roll-bonding. It is shown that the bendability of monolithic AA6016 alloy is limited due to the formation of severe surface undulations and surface cracking, which is associated with the heterogenous nature of slip that concentrates into 5°–15° misoriented coarse slip bands of very high dislocation content in the order of 1014/m2, and intense shear bands originating from surface low cusps in the form of mutually orthogonal transgranular bands. The micro-cracks initiate at the surface and propagate along these intensely sheared regions, primarily consisting of grains with near S texture component. Grain boundary decohesion occurs along boundaries that are highly misoriented with misorientations ranging from 40° up to as high as 60°, further assisting crack propagation. The composite AA6016X alloy exhibits unlimited bendability with no signs of fracture even after a maximum bend angle of 180°. The extremely high bending strains in AA6016X are accommodated in a homogeneous manner through a grain subdivision process within the AA8xxx clad layers, promoting the formation of deformation induced high angle boundaries. This homogeneous accommodation of strain within the clad layers reduces surface roughness and further enhances the bendability of the alloy.

Bendability enhancement of an age-hardenable aluminum alloy: Part II — multiscale numerical modeling of shear banding and fracture

In this study, the sequence of microstructural events leading to failure and the relationship between crystallographic texture, shear band development, micro-crack initiation and propagation during wrap-bending of monolithic AA6016 and composite AA6016X sheet alloys are studied using crystal plasticity based finite element methods (CPFEM). The numerically predicted results for texture evolution, shear bands development and fracture behavior after wrap bending show good agreement to the corresponding experimental observations. It is shown that failure during bending of AA6016 is controlled by the development of intense shear bands that emanate from surface low cusps along the outer tensile edge and provide a minimum energy path for micro-cracks to propagate, promoting a predominant transgranular failure. Upon intersection with another shear band, the advancing crack tip alternate from a less critical localization condition to a more critical one, as it requires lesser energy for the creation of new fracture surfaces while sustaining the imposed plastic deformation. Grains with Cube or near Cube texture are rather resistant to shear banding and crack propagation whereas the contrary is true for grains with near S and near Goss orientations. It is also shown that the ductile clad layers within the composite AA6016X alloy act as an efficient barrier against the development and propagation of shear bands within the less ductile inner core, thereby significantly enhancing the bendability of the alloy. Through a systematic study, it is further shown that the bendability of AA6016 alloy can be improved significantly through proper engineering of the microstructure.

Elucidating how correlated operation of shear transformation zones leads to shear localization and fracture in metallic glasses: Tensile tests on Cu[sbnd]Zr based metallic-glass microwires, molecular dynamics simulations, and modelling

The plastic deformation of metallic glasses (MG) is well-known to occur via shear transformation zones (STZs) on the scale of atomic clusters, yet fracture of MG takes place via shear bands of the micron scale. So far, understanding on how the operation of STZs leads to shear localization and fracture remains limited. In this work, tensile tests on Cu/Zr-based MG micro-wires show that both the first-yield and fracture stress exhibit the Weibull distribution, and fractography reveals that shear localization in the form of intense shear bands leads to shear fracture. Molecular dynamics (MD) simulations show that shear bands form via the correlated emergence and operation of discrete STZs close to one another. To describe how the stochastic yet correlated occurrence of the discrete STZs leads to shear localization, a model is constructed to relate the probability of the successive operation of discrete STZs, to their nucleation density. The model predicts that, if nucleation density of the STZs grows along the strain path, as prior shear events triggers the emergence of new STZs, then successive occurrence of discrete shear events speeds up rapidly to an asymptotic state which is exactly the condition of shear localization and shear banding. Furthermore, the MD results suggest an exponential growth law for the occurrence of the STZs along the strain path, which also gives predictions in good agreement with the experimental Weibull distributions of the first-yield stress of Cu/Zr-based MGs.

In situ investigation of strain heterogeneity and microstructural shear bands in rolled Magnesium AZ31

A comprehensive characterization of microstructural strain accommodation is performed on rolled Magnesium AZ31 tensioned in the rolling direction (RD). Scanning microscopic digital image correlation (SMDIC) is applied over macroscopic areas, strain-mapping ∼106 grains in situ at their natural scale. Further, for rigorous volumetric interpretation, two samples have been tested in RD-tension to monitor both lateral material planes (perpendicular to transverse and normal directions of the plate) with SMDIC. Very different strain localization patterns are observed on the two faces. Each pattern family is isolated by logical filtering for a quantitative analysis of their shearing orientations, allowing volumetrically-compatible and surface-mediated structures to be distinguished. It is found that the most intense strain localization takes place on microstructural bands in a rhombic pattern that are inherited from the rolling process. These are found to activate at the onset of the elastic-plastic transition and rapidly incur the heaviest strains, primarily through basal slip due to the weaker texture of the band grains. The deformation of the bulk matrix surrounding these bands exhibits localization patterns consistent with the predominant activity of basal and prism slip. The two-face measurement provides a unique handle on the degree of surface effects and shows them to be significant in this case of intense plastic anisotropy and directional shear banding. It is directly shown that the activity of the microstructural bands suppresses the r-ratio, the primary macroscopic measure of anisotropy in these plates, as suspected in the literature. The importance of accounting for the spatial-coordination of orientations in these highly-anisotropic polycrystals is clearly presented.

Effect of rolling temperature on the microstructure, texture, and magnetic properties of strip-cast grain-oriented 3% Si steel

The effect of rolling temperature on the evolution of microstructure, texture, and magnetic properties of ultra-low-carbon grain-oriented silicon steel was studied in strip-casting process. Dynamic strain-aging (DSA) behavior was observed during warm rolling in the temperature range of ~ 200 to 400 °C based on the fact that majority of the inhibitor elements remained in solution during the strip-casting process. Considering the initial coarse grains with strong {100} fiber prior to rolling, the cold-rolled specimen exhibited pronounced α-fiber and weak γ-fiber texture. However, intense shear bands and high stored energy regardless of the orientation were obtained in the warm-rolled specimens at the DSA temperature, accompanied by weak α-fiber and strong γ-fiber texture. While homogenous microstructure with lower stored energy was observed in the case of high temperature, the differences in shear bands and stored energy governed by the rolling temperature were strongly related to the extent of DSA effect, which is attributed to distinct characteristic of rolling and recrystallization texture. After recrystallization annealing, fine-grained homogeneous microstructure with strong Goss and γ-fiber texture was obtained at the DSA temperature, while relatively random texture with much more α-fiber and θ-fiber components was observed in the case of high temperature. The microstructure and texture of primary annealed sheets exhibited sufficient Goss grains and favorable surrounding matrix with pronounced γ-fiber texture, which was responsible for the perfect secondary recrystallization annealing in the warm-rolled specimens at the DSA temperature. The present study suggests that texture optimization of strip-cast grain-oriented silicon steel can be achieved by warm rolling in the appropriate temperature range, with improved magnetic properties.

Influence of ECAP Processing on Fatigue Crack Nucleation on ZK60A Magnesium Alloy

Low-cycle fatigue tests were conducted on ZK60A magnesium alloy that were processed by equal-channel angular pressing (ECAP) method. The ECAP processing was carried out on a hot-extruded ZK60A alloy at a die temperature of 473K. Total pass number was four and the processing route was Route Bc. One of the ECAPed sample was annealed at 573K to induce grain coarsening. The low-cycle fatigue tests were conducted under a constant plastic strain amplitude of εpl=2×10-3, in air at room temperature. A hysteresis loop shape during the low-cycle fatigue tests depended on the sample: a tension-compression asymmetric shape was recorded in the as-drawn specimen having the tensile axis parallel to the extruction direction, while a symetric shape was seen in case of the tensile axis perpendicular to the extrusion direction. Deformation twinning during fatigue was most probably attribute to the asymmetric hysteresis loop shape. Because the asymmetric hysteresis loop was absent in the ECAPed specimen, it is possible that grain refinement suppressed the deformation twinning in the fatigue test. In the as-drawin specimen having the tensile axis parallel to the extrusion direction, a small crack was observed near a deformation twin. By contrast, a fatigue crack in the as-ECAPed specimen was generated along a shear band that was inclined to the tensile axis at 45°. After the ECAPed specimen was annealed at 573K, an intense shear banding and a subsequent crack nucleation were moderated. Accordingly, it can be said that the low-temperature annealing on the ECAPed ZK60A magnesium alloy reduced the fatigue crack nucleation sites.

Microstructure and texture evolution in Cu–Ni–Si–Mg alloy processed by secondary cold rolling

In this work,experiments were conducted to evaluate the evolution of microstructure and texture in a commercial Cu–2.77Ni–0.65Si–0.18Mg (wt.%) alloy during processing by secondary cold rolling at room temperature. The influence of rolling reduction was investigated by electron backscatter diffraction. The results show that with the increase of secondary rolling reduction after formation of the solid solution, both the strength and elongation anisotropy of the Cu–Ni–Si–Mg alloy show an initial decrease followed by an increase. The anisotropy is weakest when the alloy is in its intermediate reduction state. Detailed measurements demonstrate that the anisotropy was largely affected by Brass and S texture. The shear bands contribute to second phase precipitation and lead to a decrease in the anisotropy degree. In addition, the Brass texture gradually increased by intense microscopic shear band formation.

Effect of Sn addition on the microstructure and deformation behavior of Mg-3Al alloy

Mg alloys generally suffer from their poor formability at low temperatures due to their strong basal texture and a lack of adequate deformation systems. In the present study, a small amount of Sn was added instead of Zn to Mg-3Al alloy to modify its deformation behavior and improve the stretch formability. Microstructural examinations of the deformed Mg-3Al-1Sn (AT31) alloy by electron backscatter diffraction and transmission electron microscopy show that prismatic <a> slip is quite active during deformation, resulting in much lower r-values and planar anisotropy than the counterpart Mg-3Al-1Zn (AZ31) alloy. Polycrystal plasticity simulation based on visco-plasticity self-consistent (VPSC) model also shows that prismatic <a> slip is the dominant deformation mode in AT31 alloy besides basal <a> slip. As a consequence, AT31 alloy shows a much higher stretch formability than AZ31 alloy. On the other hand, AZ31 alloy shows the development of intense shear bands during stretch forming, and these shear bands act as crack propagating paths, limiting the stretch formability of AZ31 alloy.

Failure in granular media from an energy viewpoint

The present paper examines failure in discrete granular media with respect to its mode and nature based on energetic considerations through a comprehensive discrete element modelling computational analysis. The mode of failure refers to whether deformations will localize into an intense shear band or be diffuse. More subtly, a given failure mode can be effective with a burst of kinetic energy, or non-effective with a more controlled release of kinetic energy, depending on the control parameter. Physical quantities based on energy considerations with elementary decompositions of externally applied energy into elastic, plastic and kinetic contributions at the particle level are computed for a number of granular assemblies under a variety of failure scenarios. With the picture of a particle system experiencing macroscopic deformations according to underlying micro- and meso-mechanisms, it is concluded that computed grain-energy components depend on the microstructural detail that emerges as a function of loading program and control parameter. The relationship between elastic unloading outside a shear band and plastic dissipation inside it under both strain and stress control modes determines the genesis of shear banding in terms of plastic work dissipation minimization. Most interestingly, it is found that the energy signature inside the shear band in a dense granular packing is germane to energy characteristics of diffuse failure in a loose assembly, suggesting some similarities in microscopic failure processes between shear banding and diffuse failure.

Analysis of the Deformation and Damage Mechanisms of Pearlitic Steel by EBSD and “in-situ” SEM Tensile Tests

The processes governing the deformation and damage of C70 pearlitic steel were investigated in nanometer and micrometer scales using electron backscatter diffraction technique and “in-situ” scanning electron microscope tensile testing. The ferrite behavior was identified by “in-situ” x-ray tensile tests. Investigations were carried out on annealed microstructure with two interlamellar spacings of Sp = 170 and Sp = 230 nm. It is shown that pearlite yielding is controlled by the deformation mechanisms occurring in ferrite. Deformation and damage mechanisms were proposed. At low strain, pearlite deforms homogeneously with low misorientation (<5°) inside the pearlite colonies and elongates the cementite plates. At high strain, pearlite deforms heterogeneously in intense localized shear bands inside the more favorably oriented pearlite colonies. Misorientation reaches values up to 15°. Cementite deforms by an offset of lamella along the shear bands. The nucleation of these shear bands occurs at strain level of E 11 = 7% for coarse pearlite and at a higher value for fine pearlite. Damage occurs by brittle fracture of the elongated cementite lamellae parallel to the tensile axis and which are developed by shear micro-cracks along the slip bands. The plastic-induced damage is thus delayed by the fine pearlite structure.

On the flow and mechanical behavior of Al matrix composite reinforced by nickel based (90% Ni–10% Cr) wires during equal channel angular pressing

Equal channel angular pressing (ECAP) is currently being used for its ability to produce ultra-fine grain microstructures in metallic alloys with potential industrial application. In this work a novel research was carried out to study the deformation behavior of ECAP Al matrix composite reinforced by longitudinal nickel based (90% Ni–10% Cr) wires. In addition, the composite provides an opportunity to characterize the flow pattern of ECAP in route A. Although the macrostructural flow pattern of the deformed composite and that of CP Al show some similarities, their microstructural flow patterns are different. Two ductile-phase deformation concept and strain hardening diagrams were used to analyze the wire matrix interactions. As a result of the interaction of the matrix with the wires during deformation, the matrix turbulence appeared in the microstructure among the wires with adjacent intensive shear bands. The results show that the presence of high strengths wires in the Al matrix not only improves the homogeneity of strain throughout the specimen even in the dead zone, but also enhances grain refinement within the matrix. The mechanical properties investigation reveals a substantial improvement of UTS and elongation of the ECAP composite specimen due to a finer microstructure along with the interaction of the wires and the matrix.

Strain localization and damage development during bending of Al–Mg alloy sheets

► The grain scale strain distribution during bending was studied in AA5754 CC sheets. ► DIC based strain mapping from SEM images of in situ bent samples was applied. ► Tendency for strain localization due to the presence of particle stringers was found. ► Facilitated matrix shearing and decreased local fracture strength were observed. ► Maximum Mises strain of 2.0 was calculated within dominant shear bands. The mechanism triggering failure during deformation in Al–Mg alloys often includes localization of the plastic flow into narrow and intense transgranular shear bands propagating through the microstructure with little evidence of damage prior to the final fracture event. The cracks initiate in the sheared zones and propagate by conventional ductile mechanism of fracture, including nucleation of voids at second-phase particles, followed by their growth and ultimate coalescence. In an attempt to fully understand the mechanism of damage in continuous cast (CC) AA5754 aluminium alloy sheet, a methodology for characterization of the microscopic fracture strain distribution during bending was adopted in this work. A batch of digital images representing the deformation history of the samples bent during in situ V-bending tests performed in a scanning electron microscope (SEM) was recorded and later used as an input to a digital image correlation system (DIC) for strain calculations. Local strain maps of the tensile through-thickness cross-section of the bent sheets were built. The results clearly reveal development of spatial inhomogeneity of the strain at microscopic level. The strain concentration inside the formed intensive shear bands, which were the predecessors of the subsequent crack propagation, was found to be considerably larger than the macro-strains typically suggested by the forming limit diagrams for aluminium sheet materials. The presented results are consistent with previously published results on the general forming characteristics of continuous cast AA5754 aluminium alloy sheet materials.

Numerical prediction of thermomechanical field localization in orthogonal cutting

Using a complete 2D adaptive numerical methodology together with fully coupled 3D advanced thermo-elasto-viscoplastic-damage constitutive equations; this paper focusses on the spatial localization of the main thermomechanical fields under dynamic loading conditions with high strain rates, high temperature and large inelastic strains. First, the advanced fully coupled and time dependent (i.e. thermo-elasto-viscoplastic) constitutive equations accounting for mixed nonlinear isotropic and kinematic hardening and ductile isotropic damage are presented. The associated numerical aspects are then briefly discussed in the framework of fully adaptive 2D finite element analysis. The thermomechanical fields’ localization is examined through the typical example of orthogonal cutting. Attention is paid to the localization of the main thermomechanical fields inside intensive shear bands as well as to the macroscopic crack paths through the chip thickness leading to the chip segmentation.