Formation Of Macroscopic Shear Bands Research Articles

Distinct avalanche dynamics detected in metallic glasses with high energy state revealing the crack-like shear banding mechanism

When a sufficiently high stress is applied to a metallic glass, causing plastic deformation, the material undergoes structural reconfiguration through dissipative slip avalanche events that release local stresses. By utilizing isothermal annealing and cold rolling techniques to tune the energy levels of metallic glasses, it has been observed that structural rejuvenation is accompanied by structural relaxation, as evidenced by distinct changes in avalanche dynamics. We present detailed statistics of the avalanche dynamics during shear band formation in energy-tuned metallic glasses, ranging from structurally relaxed to rejuvenated states. By analyzing shear band characteristics and examining scaling exponents, avalanche durations, and stress relaxation rates, we can establish a connection between the local activation of shear transformation zones and the formation of macroscopic shear bands. The statistics of avalanche duration indicate that an increase in soft zones within metallic glasses can alleviate stress release and stabilize plastic flow, as evidenced by the characteristics of shear bands. We attribute the significant transition of serrated flow, observed at different energy levels (i.e., as-cast, relaxed, and rejuvenated states) to the variations in nucleation and multiplication of shear bands that originate from local weak spots. Analysis of the distinct avalanche dynamics suggests that in lower energy level metallic glasses, the nucleation and propagation of shear bands exhibit localized crack-like behavior, while in higher energy level metallic glasses, they display diffused crack-like characteristics. Indeed, our results strongly support that the decreased avalanches observed in the high energy level metallic glasses originate from the nucleation of numerous small shear bands, which directly compete with the propagation of the main local shear band. These findings deepen our fundamental understanding of the relationship between the microscopic mechanism of slip avalanche dynamics and shear banding, providing a pathway to control the plasticity of metallic glasses.

Finite-Disorder Critical Point in the Yielding Transition of Elastoplastic Models

Upon loading, amorphous solids can exhibit brittle yielding, with the abrupt formation of macroscopic shear bands leading to fracture, or ductile yielding, with a multitude of plastic events leading to homogeneous flow. It has been recently proposed, and subsequently questioned, that the two regimes are separated by a sharp critical point, as a function of some control parameter characterizing the intrinsic disorder strength and the degree of stability of the solid. In order to resolve this issue, we have performed extensive numerical simulations of athermally driven elastoplastic models with long-range and anisotropic realistic interaction kernels in two and three dimensions. Our results provide clear evidence for a finite-disorder critical point separating brittle and ductile yielding, and we provide an estimate of the critical exponents in 2D and 3D.

Advanced Thermo-mechanical Process for Homogenous Hierarchical Microstructures in HSLA Steels

Engineering microstructures in high-strength low-alloy steels via advanced thermo-mechanical processing is a promising approach to overcome challenges around low work hardenability and toughness in ultrafine-grained mild steels. Recently, multiscale-hierarchical microstructures with ultrafine grains and two populations of precipitates decorating high-angle grain boundaries and dislocation structures were achieved by some of the current authors in a modern Ti-Mo-Nb high-strength low-alloy steel. However, the high-strain rate of 10 s−1 during single-pass plane-strain compression at 600 °C led to the formation of macroscopic shear bands. Here, we propose an optimized advanced multi-hit thermo-mechanical process for achieving homogenous hierarchical microstructures in the same steel without strain localization. This is verified via microscopy and thermo-kinetic modelling, using the software MatCalc. A typical body-centred-cubic rolling texture is achieved in contrast to previous process design. Ultrafine crystallites confined by a mixture of high-angle gain and subgrain boundaries are formed, decorated by two types of precipitates. Large FeMnC-rich cementite particles are found on grain boundaries and smaller TiNbC-rich precipitates on dislocations and subgrain boundaries. It is shown that TiNbC particles transform to a core-shell structure when subjected to direct aging. Thermo-kinetic modelling underpins experimental results concerning the detailed evolution of crystallite size, precipitate morphology and composition, enabling a through-process description of the microstructural evolution.

Microstructure and Mechanical properties of Cu-7wt.% Al Alloy produced by Equal Channel Angular pressing with different routes

In this paper the effect of Equal Channel Angular Pressing (ECAP) on microstructures, mechanical properties of Cu-7wt.% Al alloy has been investigated. After ECAP process the tensile yield strength, ultimate tensile strength, and microhardness of the sample are increased than before ECAP. With Cu-Al alloy with very low SFE, material being a shear fracture (SF) occurred during ECAP due to the formation of macroscopic shear bands. The tensile test at room temperature shows that the strength increases with an increase in the number of passes, but the elongation to failure remains little changed following a large decrease after the first pass and reasonably constant after increasing the passes. This study shows that SPD has the potential to significantly improve the mechanical properties of annealed Cu-Al alloys.

Mechanism of macroscopic shear band formation in plane strain compressed fine-grained aluminium

The microstructure and microtexture evolutions during shear banding of very fine grained commercially pure aluminium (AA1050) have been characterized to elucidate the mechanisms of band formation and propagation across layered microstructures and the resulting texture evolution. Samples were pre-deformed in ECAP to 6 passes via route C, then machined and further compressed in a channel-die to form two sets of copper-type macroscopic shear bands (MSBs). The local changes in microstructure and crystallographic orientations were characterized by SEM equipped with a high-resolution EBSD facility. The ECAP-processing led to the formation of a fine-grained structure composed of complementarily oriented layers parallel to the transverse direction and inclined ~20° to the extension direction (ED). During the secondary straining in the channel-die, the layers exhibited two rotation tendencies: (i) a global rotation of the layers towards the compression plane, and (ii) a deflection of layers within narrow areas. The latter increased the layers’ inclination with respect to ED and led to kink-type bands, which are the precursors of MSBs. The mechanism of MSB formation is strictly crystallographic since in all the grains of the sheared zone, the crystal lattice rotated such that one of the {111} slip planes became nearly parallel to the shear plane and the 〈011〉 direction became parallel to the direction of maximum shear. This strain-induced crystal lattice rotation led to the formation of specific MSB components that facilitated slip propagation across the grain boundaries.

Shear bands formation after a strain path change for AA1050 alloy pre-deformed by ECAP and subsequently plane strain compressed

Crystal lattice rotations induced by shear bands developed in an AA1050 aluminium alloy have been examined in order to investigate the influence of the finegrained structure on the slip propagation across the grain boundaries and the resulting texture evolution. Samples of the AA1050 alloy were pre-deformed in ECAP up to 6 passes via route C, then machined and further compressed in a channel-die up to ~25% at room temperature. The microstructure and texture were characterized by SEM equipped with a high resolution EBSD facility.The ECAP-processing leads to the formation of a fine grained structure. The grains were grouped into nearly complementarily oriented layers. During the secondary straining in the channel-die, the layers of fine grains, initially situated almost parallel to the compression plane, undergo deflection within some narrow areas. This is the beginning stage of the macroscopic shear band (MSB) formation. In all the deformed grains examined (within MSB) a strong tendency for strain-induced re-orientation could be observed. The SEM orientation mapping shows how the layers of flattened grains are incorporated into the MSB area, and what kinds of mechanisms are responsible for the strain accommodation at the macro-scale. Finally, a crystallographic description of the mechanism of MSB formation in AA1050 aluminium alloy is proposed based on the local lattice re-orientations due to localized kinking.

On the Relation of Microstructure and Texture Evolution in an Austenitic Fe-28Mn-0.28C TWIP Steel During Cold Rolling

In the present study, microstructure and texture evolution in an austenitic Fe-28 wt pct Mn-0.28 wt pct C TWIP steel in the range between 10 and 80 pct reduction by cold rolling were systematically analyzed. The formation of the observed microstructural features occurred in three different stages: I (10 to 20 pct)—mainly slip lines, grain elongation, and formation of few twin-matrix lamellae; II (30 to 50 pct)—severe increase of the volume fraction of twins, alignment of twins with the rolling plane, and formation of microshear bands; and III (60 to 80 pct)—further alignment of twins, evolution of a herring bone structure, and macroshear bands. In contrast to most f.c.c. metals, the transition from Copper- to Brass-type texture occurred at low strain levels (30 pct). This behavior is attributed to the early formation of deformation twins in the material and can be related to the SFE of this high manganese steel. At higher reduction levels, microscopic (≥40 pct) and macroscopic shear band formation (≥60 pct) contributed to the increase of randomly oriented grains, mainly at the expense of the Brass component. Furthermore, the formation of the Goss component and of the 〈111〉//ND fiber (γ) is attributed to severe twin formation.

Microstructure and microtexture evolution during strain path changes of an initially stable Cu single crystal

The microstructure and microtexture evolution in a deformed Goss oriented crystal were characterized after a sample rotation and consequent change in strain path, over a range of scales by optical microscopy, high resolution scanning electron microscopy equipped with field emission gun and electron packscattered diffraction facilities and transmission electron microscopy orientation mapping. High purity copper single crystals with initial Goss{1 1 0}〈0 0 1〉 orientation were channel-die compressed 59% to develop a homogeneous structure composed of two sets of symmetrical primary microbands. New samples with ND rotated orientations of Goss{1 1 0}〈0 0 1〉, brass{1 1 0}〈1 1 2〉, M{1 1 0}〈1 1 1〉 and H{1 1 0}〈0 0 1〉, were then cut out and further compressed in channel-die by a few per cent. The change in flow stress could be correlated with the change in dislocation substructure and microtexture, particularly along shear bands initiated by the strain path change. In the H{1 1 0}〈0 1 1〉 and M{1 1 0}〈1 1 1〉 orientations, the flow stress increased by Taylor factor hardening then decreased by intense macroscopic shear band (MSB) formation. In the softer brass orientation and in the absence of Taylor factor hardening, more diffuse MSB formation occurred. The local rotations in the band were used to deduce the possible local slip systems initiated during the strain path change.

Microstructural evolution and mechanical properties of Cu–Al alloys subjected to equal channel angular pressing

Ultrafine-grained (UFG) or nanocrystalline (NC) Cu–Al alloys were prepared using equal-channel angular pressing (ECAP) to investigate the influence of stacking fault energy (SFE) on the microstructural evolution during deformation and the corresponding mechanical properties. The grain refinement mechanism was gradually transformed from dislocation subdivision to twin fragmentation by tailoring the SFE of alloys. Meanwhile, homogeneous microstructures and nanoscale grains were readily achieved in the low-SFE Cu–Al alloys and the equilibrium grain size was decreased by lowering the SFE. Moreover, in the Cu–Al alloy with extremely low SFE, shear fracture occurred during ECAP at strain levels higher than two due to the formation of macroscopic shear bands. In addition, the normalized deformation conditions at large strain were qualitatively discussed. More significantly, the strength and uniform elongation were simultaneously improved by lowering the SFE. This simultaneity results from the formation of profuse deformation twins and microscale shear bands, and their extensive intersections.

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.

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.

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.

Dynamic void collapse and material failure mechanisms in metallic crystals

The micromechanical failure mechanisms of compression-induced high strain-rate void collapse in monocrystalline copper are studied. A theoretical and a computational constitutive model are introduced to study the material failure mechanisms of shear-strain localization and cleavage fracture in monocrystalline fcc structures. An explicit dynamic finite-element algorithm is introduced for the integration of the numerically stiff micromechanical visco-plastic constitutive relations. The combined effects of strain hardening, strain-rate history, lattice rotation, and thermal softening on void collapse are investigated, and criteria for the formation of macroscopic shear-bands and tensile cracks formation are developed. The computed results are compared with experimental results and observations, and it is shown that the material failure mode is governed by the competition between the strengthening and the softening mechanisms of the crystalline structure. Shear bands may form, if the thermal and geometrical softening mechanisms of the crystal surmount the dynamic strain hardening and the strain-rate hardening of the crystal. An increase in the strain hardening and the strain-rate hardening of the crystal retards the formation of shear bands. Material failure, in this case, is due to the coupled effects of large stress concentrations and physically limiting values of plastic strain-rates, temperatures, resolved shear-stresses, and dislocation velocities near the tip of the collapsed void. These physically limiting values result in the homogeneous generation of dislocations and the local unloading of the stresses from compression to tension at the void tip. This opening mode of stress, at the void tip, in combination with the limiting values, can result in tensile cracks occurring normal to the compression stress axis.

Shear band formation in plane strain compression

Shear band formation during plane strain compression of single crystals and polycrystals of an Al-3 wt% Cu alloy was studied. X-ray and electron diffraction, optical microscopy, and electron (TEM/STEM) microscopy were used to document the structure and micromechanisms of the localization process. Complementary finite element studies were performed using the measured single crystals' single slip system strain hardening data. The experimental observations and the computed deformation response are in very close agreement, and indicate that localization, through macroscopic shear band formation, occurs in continuously strain hardening, damage-free material. Shear band formation was preceeded by the development of very coarse slip which, like the shear bands, propagated across entire grains and, in single crystals, across the entire crystal. The computations and experiments showed that geometrical softening, caused by nonuniform lattice reorientation, is an important micromechanical influence on the localization process in both single crystals and polycrystals. Also, the propagation of shear bands across grain boundaries in polycrystals was looked at experimentally and computationally. Particular attention was paid to the crystallography of shear band transmission through grain boundaries.

Shear band formation in cold rolled Cu-6% Al single crystals

This paper concerns a study of the nature of shear bands in Cu-Al single crystals. The experimental results show that during the rolling deformation two families of shear bands form independently of the crystal orientation. However, initial orientation of the crystal was found to influence both the onset of band formation and orientation of the first family of bands with respect to the rolling direction. It appears from the TEM observations that shear bands result from correlated multiple coarse slip events. The results show that formation of macroscopic shear bands on an apparently nonrational plane results from the instability of pre-existing structures and the occurrence of a sequence of coarse slip events plus accommodating localized plastic flow.

Effect of volume fraction and shape of sulfide inclusions on through-thickness ductility and impact energy of high-strength 4340 plate steels

The effect of volume fraction and shape of sulfide inclusions on the tensile ductility and impact energy of high-strength AISI 4340 plate steels in the transverse and through-thickness testing directions was investigated for four tensile-strength levels of 930, 1210, 1410, and 1960 MPa (135, 175, 205, and 285 ksi). The volume fraction of sulfide inclusions was changed by varying the sulfur content from 0.002 to 0.022 pct. The shape of the sulfide inclusions was changed from lenticular MnS to spheroidal RE2O2S (RExSy) by rare-earth treatment. Axisymmetric tensile ductility and charpy impact energy [22 °C (72 °F)] decreased much more rapidly in the through-thickness testing direction than in the transverse testing direction when the volume fraction of sulfide inclusions was increased. Changing the shape of the sulfide inclusions from lenticular MnS to spheroidal RE2O2S (RExSy) by rare-earth treatment increased axisymmetric tensile ductility and impact energy in the through-thickness testing direction for tensile-strength levels at or below 1410 MPa (205 ksi), but not at 1960 MPa (285 ksi). Impact energy was also improved in the transverse testing direction but only for tensile strength levels at or below 1210 MPa (175 ksi). Changes in the volume fraction or shape of the sulfide inclusions had little or no effect on transition temperature. Plane-strain tensile ductility was much less affected than either axisymmetric tensile ductility or impact energy by changes in inclusion morphology or in testing direction because of the formation of macroscopic shear bands inclined at about 45 deg to the tensile axis. As a result impact energy correlated better with axisymmetric tensile ductility than with plane-strain tensile ductility. The results are discussed in terms of various models involving the formation of voids at inclusion sites and their growth and eventual coalescence by localized shear during plastic straining.