Macroscopic Shear Bands Research Articles

Deformation macrolocalisation and fracture in ultrafine-grained armco iron

The banded structure formed in armco iron specimens as a result of equal-channel angular extrusion causes their strong shear localisation under quasi-static tension. Applying transmission electron microscopy technique, a fine structure of meso- and macroscopic shear bands was revealed. The character of shear band propagation was studied by calculation of the maximum in-plane shear strain through a digital image correlation procedure. It was shown two different patterns of macroband propagation: two parallel shear bands with oppositely directed shears or a cross of shear bands oriented along the conjugate direction of maximum tangential stresses. The necking was revealed to be caused by the disturbance of self-consistent propagation of deformation macrobands.

The role of shear banding on deformation texture in low stacking fault energy metals as characterized on model Ag crystals

The microstructure and texture evolution during deformation of a representative low-stacking fault energy (SFE) metal was characterized by detailed local scanning electron microscopy and transmission electron microscopy orientation measurements. High purity silver single crystals with an initial (1 1 2) [ 1 1 1 ¯ ] orientation were channel die deformed to total reductions of 88% (logarithmic strains 2.1), first developing twin-matrix layers and then compact clusters of two families of shear bands. It was shown that twinning and shear banding caused several important transitions of the deformation textures. The as-deformed shear bands exhibited large orientation spreads of up to 40° with respect to the adjacent twinned areas. Most of these misorientations occurred by rotations about the transverse direction ∥〈1 1 0〉 axis with significant further rotations about 〈1 1 2〉 poles. These two rotations explained the influence of the shear bands on the formation of Goss{1 1 0}〈0 0 1〉 and brass{1 1 0}〈1 1 2〉-S{1 2 3}〈6 3 4〉 texture components, which are clearly observed in highly deformed low-SFE metals. Symmetrically equivalent crystal lattice rotations inside narrow areas led to the formation of the positive and negative macroscopic shear bands for high deformations.

Deformation inhomogeneity in flattened copper wire

The deformation inhomogeneity in flattened wire produced by the wire flat rolling process is studied. Utilizing the combined Finite and Slab Element Method (FSEM), the effective strain fields in the flattened wire are calculated at different reductions in height and frictional conditions and compared with experiment. The calculated and experimental results exhibit that the deformation is inhomogeneous and macroscopic shear bands are appeared in the cross section of flattened wire. Using the results of the analysis, it is shown that the deformation inhomogeneity, introduced by an Inhomogeneity Factor (IF), is different on the x-axis and y-axis in the cross section of the flattened wire. By decreasing the reduction in height of wire and increasing the friction factor, the deformation inhomogeneity is increased and the effect of friction factor is more pronounced at large reduction in height. The variation in IF with the reduction in height is presented by a fourth order polynomial.

Applicability of the FSEM for analyzing wire flat rolling process

PurposeThe purpose of this paper is to present an assessment of the ability of combined finite and slab element method (FSEM) for analyzing the wire flat rolling process.Design/methodology/approachUsing the FSEM, the effective strain field of flat rolled wire is predicted for different reductions in height and frictional conditions. The validity of the method is assessed by performing the Vickers microhardness measurements on the flattened wire cross section. Also, the creation of macroscopic shear bands in cross section of the flat rolled wire is investigated and confirmed by microhardness and metallographic examinations. Moreover, the lateral spread and width of contact area are predicted by the FSEM for different reductions in height and frictional conditions.FindingsThe FSEM and microhardness results show the minimum and the maximum effective strains at the round edge and center of the flattened wire, respectively. Also, the results show the bands of maximum effective strain at cross section of the flattened wire, i.e. macroscopic shear bands.Practical implicationsThis paper can be useful in rolling industries to produce electronic parts, various springs, trolley cables, piston rings, and guide rails.Originality/valueThe paper shows the applicability of the FSEM for calculating the effective strain field and geometry of wire after wire flat rolling process.

The prediction of macroscopic shear bands in flat rolled wire using the finite and slab element method

In this paper, in order to investigate the creation of macroscopic shear bands, due to material flow tracks in the cross section of a wire after flat rolling, the combined Finite and Slab Element Method (FSEM) is used. The effective strain field is predicted for different reductions in height. The validity of the method is assessed by performing the Vickers microhardness measurements on the cross section of the flattened wire. It is found that the FSEM and microhardness results show the minimum and the maximum effective strains at the round edge and the center of the flattened wire, respectively. Also, the results indicate two bands of maximum effective strain in the cross section of the flattened wire being in consistent with the two macroscopic shear bands observed after metallographic examination.

On the origin of Lüders-like deformation of NiTi shape memory alloys

Polycrystalline NiTi shape memory alloys deformed in tension tend to exhibit localization and propagation of deformation in macroscopic shear bands. The propagation of the deformation bands is characterized by a plateau-type stress–strain curve. Such behavior has been reported only for NiTi alloys and only in tension. The reason for this behavior is still unclear although different hypotheses have been proposed in the literature. This article briefly summarizes relevant experimental evidences and offers an explanation based on micromechanics modelling of pseudoelasticity of polycrystalline NiTi.

Compressive behavior of wire reinforced bulk metallic glass matrix composites

Bulk metallic glasses (BMGs) possess a unique set of mechanical properties that make them attractive structural materials. However, when loaded without constraint, BMGs fracture catastrophically due to formation of macroscopic shear bands and this behavior reduces their reliability. To address this issue, BMG matrix composites have been developed. In this investigation, neutron diffraction was used during uniaxial compressive loading to measure the internal strains in the second phases of various BMG composites reinforced with Ta, Mo, or stainless steel wires. The diffraction data were then employed to develop a finite element model that deduced the in situ constitutive behavior of each phase. It was found that the reinforcements yielded first and started transferring load to the matrix, which remained elastic during the whole experiment. While the present composites exhibited enhanced ductility, largely due to their ductile reinforcements, they yielded at applied stresses lower than those found in W reinforced composites.

Fatigue and microstructure of ultrafine-grained metals produced by severe plastic deformation

The cyclic deformation and fatigue behaviour of ultrafine-grained metals produced by the severe plastic deformation technique ECAP (equal channel angular pressing) is discussed briefly with respect to cyclic softening, macroscopic shear banding and fatigue lives. Possibilities to enhance the LCF fatigue resistance by annealing are indicated.

The microstructure and recrystallization of flow-formed oxide-dispersion-strengthened ferritic alloy: Part I. Deformation structure

Oxide-dispersion-strengthened (ODS) alloy tubes with a nominal composition of Fe-19.5 pct Cr-5.5 pct Al-0.5 pct Y2O3 have been cold flow formed to deformation levels between 72 and 92 pct. The deformation structure of the tubes has been studied using a variety of techniques including transmission electron microscopy (TEM). The deformation cells produced by flow forming are elongated in both the hoop and axial directions, especially at deformation levels above 80 pct. In this case, most deformation cells can be regarded as ribbons, lying roughly along the “rolling direction,” which for flow forming is a helix around the tube surface. No obvious particle alignment was found in the tubes. Although the initial grain size is submicron, transition bands composed of parallel-sided long deformation cells similar to those in deformed single-crystal specimens have been observed in the transverse section of the tubes and a macroscopic shear band has been observed in the longitudinal section of the 92 pct deformed tube.

Shear–band development in polycrystalline metal with strength–differential effect and plastic volume expansion

Experimental studies have shown that the flow stress of some metals is clearly influenced by superimposed hydrostatic pressure. Flow stress increases with the hydrostatic pressure, and consequently it is often observed that the flow stress is clearly larger in a uniaxial compression test than in a uniaxial tension test. This phenomenon is known as the strength–differential effect (SDE). In addition, metals undergoing uniaxial tension and compression show a permanent volume expansion (dilatancy) which is insensitive to the sign of the hydrostatic pressure. In this paper, shear–band development in polycrystalline metal with the SDE and dilatancy is studied, using a rate–dependent crystal–plasticity model with a full three–dimensional, body–centred–cubic, slip–system structure. It is postulated that the appearances of the SDE and dilatancy are consequences of non–Schmid effects existing in each slip system. In the finite–element analyses performed here, each Gaussian integration point in a finite element represents a polycrystal consisting of many of crystal grains having different orientations. As a boundary–value problem, a rectangular specimen subjected to plane–strain tension is considered. Although the finite–element geometry is chosen to be two dimensional, the constitutive model incorporates the full three–dimensional slip–system structure. As a polycrystal model to be embedded in each integration point, an extended Taylor model is employed. Thus, macroscopic manifestations of non–Schmid effects are studied. The influences of hydrostatic stress, internal friction, plastic volume expansion and strain–rate sensitivity on macroscopic shear–band formation in polycrystals are investigated. Results are directly compared with predictions from a classical Schmid–law–based crystal–plasticity theory.

Kinking in niobium

Single crystals of niobium with a longitudinal axis near to [111] were strained in tension at 78 K and at the yield point rapid localised plastic deformation occurred accompanied by a large drop in load. Surface examination indicated that kinking had occurred and the crystallographic axes had rotated by ~ 25° relative to the matrix crystal. The plane of the 'sharp' kink/matrix interface bisected the angle between the slip traces in the kink and those in the matrix. The kink had the form of a macroscopic shear band. In some cases, but not in all, there was an associated coarse slip band. Macroscopic crystallographic traces of the coarse slip band indicated that the slip system appeared to be (211)[-111] but, in fact, the band comprised intense aggregates of fine slip on {101} planes. A dislocation dynamics model of kinking is presented that accounts for the observed upper yield stress, yield drop, and the observed rotations of the kinked crystal and its interface with the matrix. Finally the effect of adiabatic heating of the kink and its effect on the stability of deformation is considered.

Post-critical plastic deformation in incrementally nonlinear materials

The formation of multiple macroscopic shear bands is investigated as a mechanism of advanced plastic flow of polycrystalline metals. The overall deformation pattern and material characteristics are determined beyond the critical instant of ellipticity loss, without the need of introducing an internal length scale. This novel approach to the modelling of post-critical plastic deformation is based on the concept of a representative nonuniform solution in a homogeneous material. The indeterminacy of a post-critical representative solution is removed by eliminating unstable solution paths with the help of the energy criterion of path instability. It is shown that the use of micromechanically based, incrementally nonlinear corner theories of time-independent plasticity leads then to gradual concentration of post-critical plastic deformation. The volume fraction occupied by shear bands is found to have initially a well-defined, finite value insensitive to the mesh size in finite element calculations. Further deformation depends qualitatively on details of the constitutive law. In certain cases, the volume fraction of active bands decreases rapidly to zero, leading to material instability of dynamic type. However, for physically hardening materials with the yield-vertex effect, the localization volume typically remains finite over a considerable deformation range. At later stages of the plane strain simulation, differently aligned secondary bands are formed in a series of bifurcations.

Comparison of macroscopic mechanical properties and shear band characteristics established by acoustic emission measurements in Fe85-xCoxB15 amorphous alloys ribbons

A complex statistical and spectral analysis of acoustic emission during tensile deformation of the Fe 85-x Co x B 15 (x=19. 21, 30, 40, 50) metallic glasses is performed aiming at clarification of the effect of topological order on inhomogeneous plastic flow and shear resistance. A correlation is established between the critical shear stress for the catastrophic shear bands and quantitative characteristics of acoustic emission pulses, corresponding to local shear bands., This correlation is understandable within the framework of existing in the literature polycluster model of amorphous solids, which considers some suitable intercluster boundaries as main regions for nucleation and spreading of plastic shears.

Development of plastic strain heterogeneities on cold-rolled aluminium alloys during in situ shear tests inside an SEM

Shear tests inside an SEM were performed to study plastic localisation phenomena on cold rolled aluminium alloys 1200, 3004 and 5182. Strain distribution was obtained by image analysis of micro-grids deposited on the sample surface prior to the tests. Macroscopic shear bands initiate from the samples ends and close to the grips after the load maximum. Their spreading through the whole sample occurs by nucleation and propagation of microscopic shear bands, and depends on the alloy and on the direction of shear. The link between the different scales are discussed, from the dislocation microstructures to the macroscopic scale. The global boundary conditions govern the distribution of spatial heterogeneities, but the local hardening behaviour controls the localisation sharpness.

Strain localisation in solution heat treated Al–Zn–Mg alloys

Two Al–Zn–Mg alloys with recrystallised and fibrous grain morphologies are studied with regards to the microstructure after solution heat treatment, cold water quenching and immediate room temperature deformation. It was found that the dislocation movement was localised in narrow slip bands cutting through the dislocation tangles. This observation is related to dynamic strain ageing and to macroscopic shear bands frequently observed in these alloys.

Macroscopic Shear Banding in an Aluminum-Based Alloy During Equal-Channel Angular Pressing

ABSTRACTEqual-channel angular pressing is now recognized as an effective technique for fabricating materials with ultrafine structures. It is generally considered that the strain is homogeneously distributed in the materials after ECA pressing. The present investigation shows, however, that severe macroscopic shear band, which extends from bottom to top surface and slants to the longitudinal axis of specimen at an angle of about 45°, develops at regular intervals, with a high shear strain accommodation of about 3.7 within the band. Different families of macroscopic shear bands may cut across each other, and the total volume fraction of macroscopic shear bands may well surpass 50%.

Analysis of flat rolling of superconducting silver/ceramic composites

The flat rolling process from wire to tape is presumably the most crucial link in the chain of mechanical processes leading from loose powder and silver tubes to the final superconducting Ag/BSCCO tape. In order to improve the critical current density of the superconducting filaments, one must process these to the highest possible density without at the same time introducing failures as large cracks and macroscopic shear bands. In order to analyse and optimise the process, the interaction between the involved materials and their very different mechanical properties must be taken into account. The silver is considered an incompressible von Mises material, hardening as a function of equivalent strain. The powder on the other hand is a compressible friction material implying pressure dependent “flow stress”. In this paper particular attention is paid to stresses and strains and defects in the deformation zone. The evolution of density in the powder cores is analysed by the finite element method.

Process of Nanocrystallization and Partial Amorphization by Cold Rolling in TiNi

TiNi alloy is vulnerable to solid-state crystalline-amorphous transformation by many means, such as particle irradiation, shot peening, compression and cold rolling. In the present study a process of structural refinement and partial amorphization by cold rolling was studied on Ti-50.2 at%Ni. Crystal structures and microstructures were characterized by X-ray diffractometry, optical microscopy and electron microscopy. Differential scanning calorimetry revealed that the introduction of high density of dislocations and resultant structural refinement by cold rolling suppressed martensitic transformation. In the transversal section of cold rolled samples, microscopic and macroscopic shear band formation was observed after 40-50% rolling. Transmission electron microscopy observations revealed that amorphous bands exist in the sample with rolling reduction over 40%. The amorphous bands form about 35-45 degrees with respect to the rolling direction. This suggests that a part of the shear bands becomes amorphous.

Flow stability of binary Al–Li alloys

The constitutive stress–strain behavior of extruded Al–1 wt.% Li and Al–2 wt.% Li has been characterized in the temperature range 573–823 K between strain rates of 5×10 −4 and 1 s −1. These results together with dynamic materials modeling (DMM) were utilized to establish strain rate–strain-temperature processing maps. Stable regions, defined on the basis of two mechanistic stability criteria, 0< m<1 and δ log(m)/δ log( ε ̄ ̇ )<0 , and two thermodynamic criteria, s>1 and δ log(s)/δ log( ε ̄ ̇ )<0 , were found to be associated with dynamic recovery (DRC) at low temperatures and dynamic recrystallization (DRX) at higher temperatures. Unstable flow was observed at both low temperatures (573–623 K)/high strain rates (10 −1 s −1) and at high temperatures (773–823 K)/low strain rates (10 −3–5×10 −4 s −1), these instabilities being associated, respectively, with macroscopic shear band formation and dynamic grain growth.

Post-critical deformation pattern in plane strain plastic flow with yield-surface vertex effect

This work is concerned with the formation of multiple macroscopic shear bands viewed as a mechanism of large plastic deformation of polycrystalline metals. The plastic deformation pattern in a time-independent material with a yield-surface vertex effect is investigated numerically in plane strain beyond the critical instant of ellipticity loss under quasi-static loading. The energy criterion of path instability applied to a family of post-critical solutions eliminates unstable paths and enables the overall deformation pattern to be determined, although the solutions remain locally indeterminate due to the absence of an internal length scale. In particular, the volume fraction of incipient shear bands is found to have a well-defined value irrespective of the mesh size in finite element calculations. As an apparently novel qualitative result, the formation of coarse, differently aligned secondary bands is observed at later stages of simulation.