Individual Shear Bands Research Articles

A systematic and precise study of deformed Zr52.5Cu17.9Ni14.6Al10Ti5 BMGs for notable shear bands towards better global plasticity

Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit 105) bulk metallic glasses with varying deformation strains (1.6, 4.5, 6.6, 8.7, 10.8, and 12.5 %) were subjected to compressive deformation. The evolution of shear bands was systematically and mathematically conducted to investigate the impacts of largest plastic strains. It was observed that the average length of shear steps per unit area of surface increased with increasing plastic strain. The protruded areas exhibited non-uniform heights, which appeared to gradually extend as the plastic strain was increased. Mathematical formulation was employed to calculate the contribution of individual shear bands to the total strain, aiming to explore the microstructure and mechanical features of BMGs. This approach provided insights into the relationship between atomic-scale structure, local dynamics, and macroscopic mechanical behavior. Moreover, the contribution of shear bands to normal strain along with the assessment of congruent results for normal strain originating from shear bands were evaluated for attaining better global plasticity.

Deformation-Induced Martensitic Transformation in Laser Cladded 304 Stainless Steel Coatings.

There are only a few cost-effective solutions for coating applications in combined mechanical loading and corrosive environments. Stainless steel AISI 304 has the potential to fill this niche, showing excellent corrosion resistance while utilizing the deformation-induced phase transformation from γ-austenite to α’-martensite, which results in an increase in strength. However, it is not known whether this can occur in laser cladded material. Therefore, laser cladded AISI 304 coatings in as-cladded condition and after heat treatment at 1100 °C for 60 min were investigated before and after bending deformation, by means of light microscopy, energy-dispersive X-ray spectroscopy and electron backscatter diffraction. It was shown that due to the dendritic microstructure accompanied by an inhomogeneous distribution of the main alloying elements (Cr and Ni), no deformation-induced phase transformation occurred in the as-cladded coating. The applied approach with subsequent solution heat treatment at 1100 °C for 60 min resulted in a homogeneous γ-austenite microstructure, so that a deformation-induced martensitic transformation (DIMT) could occur in the coatings. However, the volume fraction of martensite that had been formed locally at individual shear bands was rather low, which can be possibly attributed to the high Ni content of the feedstock, stabilizing the γ-austenite microstructure. This study shows the possibility of exploiting the DIMT mechanism in heat-treated laser-cladded AISI 304 coatings.

Synergistic effects of microstructural inhomogeneity and phase-transformation-induced plasticity for ductility improvements in metallic glass composites

Recent experiments show some successes in ductility improvements in bulk metallic glass (BMG) composite with metastable crystalline second-phase particles that are capable of phase transformation induced plasticity (TRIP), e.g., austenite to martensite upon straining. The underlying mechanisms are yet to be quantified for a few reasons. First, ductility enhanced by TRIP mechanism in crystalline materials is often credited to the delayed necking due to the enhanced work hardening rate, while BMG composites with such metastable second phases never fail by necking. Second, load partitioning amongst austenite and martensite phases and BMG matrix can be probed by in situ diffraction techniques, but its consequence on preventing or delaying the transition from shear bands to failure has not been revealed. In this work, synergistic effects of microstructural inhomogeneity and TRIP mechanism are quantitatively investigated from micromechanical finite element simulations, using the free-volume theory that is capable of explicitly modeling individual shear bands in the BMG matrix and a strain-driven TRIP model that can be calibrated from available neutron diffraction measurements. For metastable second phases to improve the BMG composite ductility, our parametric studies have found that the strength of the BMG matrix has to be in between the strengths of soft austenite and hard martensite phases, thus leading to the effectively confinement of shear bands near the second phase, the reduction of maximum shear band strain, and therefore the improvement of tensile ductility. Our micromechanical simulations also allow us to identify the roles played by geometric factors such percolation and core-shell or inverse core-shell distributions.

Mapping Shear Bands in Metallic Glasses: From Atomic Structure to Bulk Dynamics.

A deep understanding of the mechanisms controlling shear banding is of fundamental importance for improving the mechanical properties of metallic glasses. Atomistic simulations highlight the importance of nanoscale stresses and strains for shear banding, but corresponding experimental proofs are scarce due to limited characterization techniques. Here, by using precession nanodiffraction mapping in the transmission electron microscope, the atomic density and strain distribution of an individual shear band is quantitatively mapped at 2nm resolution. We demonstrate that shear bands exhibit density alternation from the atomic scale to the submicron scale and complex strain fields exist, causing shear band segmentation and deflection. The atomic scale density alternation reveals the autocatalytic generation of shear transformation zones, while the density alternation at submicron scale results from the progressive propagation of shear band front and extends to the surrounding matrix, forming oval highly strained regions with density consistently higher (∼0.2%) than the encapsulated shear band segments. Through combination with molecular dynamic simulations, a complete picture for shear band formation and propagation is established.

Impact of cryogenic cycling on tracer diffusion in plastically deformed Pd40 Ni40 P20 bulk metallic glass

The impact of thermo-mechanical treatment, in particular of room temperature high-pressure torsion (HPT), cold rolling and cryogenic thermo-cycling on shear band diffusion in a model Pd40Ni40P20 bulk metallic glass is investigated. Tracer diffusion measurements reveal the existence of fast short-circuit diffusion in deformed glasses and the diffusion enhancement was found to depend on the processing mode. The HPT processing imposes an extremely large shear deformation under quasi-hydrostatic conditions whereas cold rolling induces individual well-separated shear bands, though shear band diffusion is found to be faster after cold rolling. The cryogenic thermo-cycling enhances the shear band diffusion further and a conceptual explanation for the processes leading to kinetic rejuvenation is proposed.

Geometry of folded and boudinaged pegmatite veins emplaced within a strike-slip shear zone: A case study from the Caledonian orogen, northern Scotland

The deformation of sheet-like igneous intrusions in multiple orientations can give detailed insights into the kinematics of ductile flow in shear zones. Using the Torrisdale Vein Complex in the northern Scottish Caledonides as a case study, we examine the relationships between folded and boudinaged pegmatitic intrusions deformed by strike-slip dominated shear. Within the shear zone, intrusions that trend clockwise of the adjacent regional foliation were commonly (75%) folded, whereas >80% of pegmatites at anticlockwise angles were asymmetrically boudinaged, giving dextral senses of shear parallel to the dominant and gently-plunging mineral lineations. Pegmatite fold trains at high angles to foliation (Sn) display the greatest % shortening, marked by steeply plunging ‘Z’ shaped folds when viewed down plunge. Pegmatites typically form Class 1C folds and were therefore more competent than their host gneisses at the time of deformation, i.e. they were not in the magmatic state. Such fold geometries are consistent with flattened parallel folds and suggest that up to 0.5 homogenous flattening was superimposed on the folds. Fold styles also indicate viscosity contrasts between the pegmatites and host gneisses of between 50 and 250 at the time of folding. Fold hinges are dispersed about an arc within the steeply-dipping foliation, suggesting a component of hinge rotation towards the mineral lineation during non-coaxial deformation. Folded pegmatites may also display boudinage on their limbs indicating that fold limbs had rotated into the extensional field during progressive deformation. Pegmatite boudin trains are typically developed <45° anticlockwise of Sn and display the greatest % extension when trending sub-parallel to the shear zone foliation. Within such trains, individual shearband boudins are more anticlockwise than the overall train and show right-stepping relative to neighbouring bodies. More equant boudins have rotated less than elongate boudins (aspect ratios >3) and preserve the largest anticlockwise angles with the overall boudin train. Conversely, domino boudins are clockwise of overall boudin trains and have undergone clockwise rotation marked by left-stepping of adjacent boudins. Domino boudins display flanking folds which progressively open as the deformable boudin rotates towards the foliation and margins of the boudin lozenges locally become clockwise of flow. This ‘unfolding’ of deflected foliation, together with local reversals in the sense of shear around rotating boudins, is consistent with a clockwise vorticity associated with bulk dextral shear. These observations collectively indicate that bulk dextral shear with an additional component of pure shear operated across the shear zone and provides further insights into the kinematics of mid-crustal deformation.

An analytical model of multi-stress drops triggered by localized microcrack damage in brittle rocks during progressive failure

Stress drops in stress–strain constitutive curves of intact brittle rocks under high confining pressure have great significance for evaluating the earthquake mechanism and the safety of deep underground engineering. Microcrack growth in intact rock strongly influences the stress drops. However, the theoretical model of microcrack growth-dependent multi-stress drops rarely is proposed in stress–strain curves of intact rocks. In this study, a constitutive model depending on the damage variable relating to microcrack growth and strain increment is proposed to explain the multi-stress drops in stress–strain curves including strain hardening and softening phases of intact rocks. This model is formulated by combining the wing crack growth model, the suggested relationship between axial strain and wing crack growth, and the stepping function of damage relating to axial strain. This stepping function of damage relating to axial strain approximately is used to simulate the developing process of the small individual shear bands caused by the local microcrack accumulation and coalescence. The effects of parameters in the suggested stepping function of damage on the stress–strain curves containing stress drops are discussed. The theoretical model qualitatively explains the experimental phenomena of multi-stress drops in the stress–strain curves, which provides an important implication for evaluating the earthquake mechanism and the safety of deep underground engineering.

An analytical micro-macro model of stress drops during brittle creep in rocks

Stress drops during long-term creep in brittle rocks have a great significance for evaluating earthquake mechanisms. Microcracks initiation, growth, nucleation, and coalescence play an essential role in stress drop of rocks. However, the micro-macro relationship between stress drops and time-dependent microcracks development in intact brittle rocks is rarely proposed during long-term creep. In this study, a Damage Rate- and State- Fracture Model (DRSFM) is proposed to explain the mechanism of creep stress drops in intact brittle rocks. This DRSFM is formulated by combining the wing crack growth model, the subcritical cracking law, the relationship between axial and shear mechanical behaviors, the suggested crack-strain relationship and the step function of time-dependent initial damage. This suggested crack-strain relationship is established by linking the micro-macro damage definitions in brittle solids. The step function of time-dependent initial damage qualitatively describes the damage variables caused by the presence of shear bands from microcracks accumulation and coalescence under different time in brittle rocks during creep. A step initial damage variable is corresponding to the presence of an individual shear band, which causes a drop of shear stress in rocks. In the scale of earth crusts, this DRSFM provides an implication for the evolution of earthquake mechanisms. The smaller step initial damage variable may be corresponding to a smaller tremor or earthquake, and the larger step initial damage variable may be corresponding to a larger tremor or earthquake. The time variation explains the time difference between the two earthquakes. The rationality of this DRSFM is approximately verified by comparing the published theoretical and experimental results. Furthermore, the relationship between the friction in faults and the shear stress in intact rocks is explained in detail, which provides a certain help for the understanding of the conclusion that the onset of stress drop caused by shear fracture is similar to the onset of stress drop caused by frictional stick-slip.

In situ study of the shear band features of a CuZr-based bulk metallic glass composite

To understand the shear banding evolution and the effect of individual shear band on deformation mechanisms of bulk metallic glass composites (BMGCs), tensile tests of CuZrAlNb BMGCs have been in situ studied. It is found that the type and sequence of shear bands are independent of volume fraction of crystalline phase. All the types of shear bands can be considered as shear bands controlled by stress affected zones (SAZs) at crystal/matrix interface and those far away from SAZs. Based on the analysis of elastic energy dissipation, the shear bands far away from SAZs play a dominant role in the deformation stability. For current BMGCs, the propagation of shear bands far away from SAZs can be delayed by reducing the average elastic energy dissipation. High Young's modulus and volume fraction of crystalline phase are effective for decreasing the average elastic energy stored in shear bands. It is expected that the obtained results can offer better understanding of deformation mechanism in bulk metallic glass composites, which should be beneficial to design bulk metallic glass composites with improved mechanical performances.

Tunable tensile ductility of metallic glasses with partially rejuvenated amorphous structures

We report that the tensile ductility of metallic glass (MGs) is tunable by introducing gradient rejuvenated amorphous structures (GRASs) using large-scale atomistic simulations. The results reveal that the ductile GRASs promote the formation and propagation of new shear bands in the interior unrejuvenated region by suppressing the catastrophic propagation of individual shear bands across the GRASs, thus resulting in a more dispersed plastic shearing throughout the sample. It is also demonstrated that increasing both the volume fraction and degree of structural disordering of GRASs can improve the tensile ductility of MGs and lead to a brittle-to-ductile transition of the deformation mode, although at the expense of some strength. Moreover, the critical volume fraction of GRASs required for switching the transition is found to depend on the specific degree of structural disordering. The observed structural state-dependent transition of the deformation mode is further understood from a mechanical perspective by considering the competition between the macroscopic yield strength and the critical stress of the material required for shear delocalization, based on which a criterion is developed to predict the critical transition boundary in MGs with GRASs across a wide range of structural states. The findings provide a detailed atomistic understanding of the relationship between the structural state and mechanical properties in MGs with partially rejuvenated amorphous structures, which may offer useful insights for designing and processing MGs with a sought-after combination of ductility and strength.

Dynamic Plasticity and Failure of Microscale Glass: Rate-Dependent Ductile-Brittle-Ductile Transition.

Glass has been recently envisioned as a stronger and more robust alternative to silicon in microelectromechanical system applications, including high-frequency resonators and switches. Identifying the dynamic mechanical properties of microscale glass is thus vital for understanding their ability to withstand shocks and vibrations in such demanding applications. However, despite nearly half a century of research, the micromechanical properties of glass and amorphous materials in general are primarily limited to quasi-static strain rates below ∼0.1/s. Here, we report the in situ high-strain-rate experiments of fused silica micropillars inside a scanning electron microscope at strain rates up to 1335/s. A remarkable ductile-brittle-ductile failure mode transition was observed at increasing strain rates from 0.0008 to 1335/s as the deformation flow transitions between homogeneous-serrated-homogeneous regimes. Detailed surface topography investigation of the tested micropillars revealed that at the intermediate strain rate (<∼6/s) serrated flow regime, the load drops are caused by the sequential propagation of individual shear bands. Further,analytical calculations andfinite element simulations suggest that the atomistic mechanism responsible for the homogeneous stress-strain curves at very high strain rates (>∼64/s) can be attributed to the simultaneous nucleation of multiple shear bands along with dissipative deformation heating. This unique rate-dependent deformation behavior of the glass micropillars highlights the importance and need of extending such microscale high-strain-rate studies to other amorphous materials such as metallic glasses and amorphous metals and alloys. Such investigations can provide critical insights about the damage tolerance and crashworthiness of these materials for real-life applications.

Mechanism of shear banding during cold rolling of a bulk metallic glass

Shear band formation within the roll bite is investigated for the cold-rolled Zr52.5Ti5Cu18Ni14.5Al10 bulk metallic glass. The results indicate that shear bands originate on the surface of the specimen in contact with the rolls and propagate approximately at 45° towards the opposite surface. This is accompanied by the rotation of the shear offsets that depends on their position along the arc of contact. The rotation is the medium that transmits the increasing pressure exerted by the rotating rolls to a propagating shear band, while the specimen progressively enters in the roll bite. This mechanism implies that the strain at opposite sides of a band changes sign from compressive to tensile, as indeed experimentally observed by strain analysis across an individual shear band.

Comparing the deformation-induced martensitic transformation with the athermal martensitic transformation in Fe-Cr-Ni alloys

The microstructure of martensite formed athermally or via deformation in Fe-Cr-Ni alloys with different austenite (γ) stability has been investigated using microscopy. Two different types of microstructures, viz. blocky and banded structure, are observed after athermal and deformation-induced martensitic transformation (AMT and DIMT, respectively). The blocky structure form during AMT or DIMT if the stability of γ is low. In both cases, there is a significant chemical driving force for the transformation from γ to α′-martensite (α′), and if it is not hindered by e.g. planar defects it can grow uninhibited into a blocky morphology without the necessity to nucleate new crystallographic variants to accommodate the transformation strains. On the other hand, the banded structure is due to the formation of ε-martensite (ε) during AMT, or the wider concept shear bands in the case of DIMT. The shear bands, and in particular ε, lower the nucleation barrier for α′ that forms within individual shear bands if the stability of γ is low. Neighbouring α′ units predominantly have a twin-related orientation relationship to accommodate the transformation strains. With increasing γ stability during DIMT, variant selection becomes pronounced with preferred formation of variants favorable oriented with respect to the applied stress/strain field. The formation of α′ at individual shear bands is also rare, since no ε is present and instead α′ forms at the intersection of shear bands for more stable γ. In conclusion, AMT and DIMT for low γ stability lead to similar microstructures, whereas the DIMT microstructure for high γ stability is distinct.

Dilatancy induced ductile-brittle transition of shear band in metallic glasses.

Dilatancy-generated structural disordering, an inherent feature of metallic glasses (MGs), has been widely accepted as the physical mechanism for the primary origin and structural evolution of shear banding, as well as the resultant shear failure. However, it remains a great challenge to determine, to what degree of dilatation, a shear banding will evolve into a runaway shear failure. In this work, using in situ acoustic emission monitoring, we probe the dilatancy evolution at the different stages of individual shear band in MGs that underwent severely plastic deformation by the controlled cutting technology. A scaling law is revealed that the dilatancy in a shear band is linearly related to its evolution degree. A transition from ductile-to-brittle shear bands is observed, where the formers dominate stable serrated flow, and the latter lead to a runaway instability (catastrophe failure) of serrated flow. To uncover the underlying mechanics, we develop a theoretical model of shear-band evolution dynamics taking into account an atomic-scale deformation process. Our theoretical results agree with the experimental observations, and demonstrate that the atomic-scale volume expansion arises from an intrinsic shear-band evolution dynamics. Importantly, the onset of the ductile-brittle transition of shear banding is controlled by a critical dilatation.

Experimental studies of shear bands in Zr-Cu metallic glass

Shear bands are the key feature that control deformation in bulk metallic glasses (BMGs). This study provides a comprehensive analysis of plastic deformation of a Zr-Cu-based BMG at room temperature. Experiments were conducted to observe the evolution of shear bands in the material. It was shown that shear bands formed discretely in the material which allows for material deformation to occur across it. Additionally, individual shear bands were characterised to obtain a better understanding of shear-band-induced plasticity. Assessment of mechanical properties, such as hardness and elastic modulus, indicate the deformed regions of the material were weaker than undeformed regions. No compositional or structural changes were found in shear-band of the studied BMG suggesting generation of local free volume in the deformed region.

Strain Distribution Across an Individual Shear Band in Real and Simulated Metallic Glasses.

Because of the fast dynamics of shear band formation and propagation along with the small size and transient character of the shear transformation zones (STZs), the elementary units of plasticity in metallic glasses, the description of the nanoscale mechanism of shear banding often relies on molecular dynamics (MD) simulations. However, the unrealistic parameters used in the simulations related to time constraints may raise questions about whether quantitative comparison between results from experimental and computational analyses is possible. Here, we have experimentally analyzed the strain field arising across an individual shear band by nanobeam X-ray diffraction and compared the results with the strain characterizing a shear band generated by MD simulations. Despite their largely different spatiotemporal scales, the characteristic features of real and simulated shear bands are strikingly similar: the magnitude of the strain across the shear band is discontinuous in both cases and the direction of the principal strain axes exhibits the same antisymmetric profile. This behavior can be explained by considering the mechanism of STZ activation and percolation at the nanoscale, indicating that the nanoscale effects of shear banding are not limited to the area within the band but they extend well into the surrounding elastic matrix. These findings not only demonstrate the reliability of MD simulations for explaining (also quantitatively) experimental observations of shear banding but also suggest that designed experiments can be used the other way around to verify numerical predictions of the atomic rearrangements occurring within a band.

Localized crystallization in shear bands of a metallic glass.

Stress-induced viscous flow is the characteristic of atomic movements during plastic deformation of metallic glasses in the absence of substantial temperature increase, which suggests that stress state plays an important role in mechanically induced crystallization in a metallic glass. However, it is poorly understood. Here, we report on the stress-induced localized crystallization in individual shear bands of Zr60Al15Ni25 metallic glass subjected to cold rolling. We find that crystallization in individual shear bands preferentially occurs in the regions neighboring the amorphous matrix, where the materials are subjected to compressive stresses demonstrated by our finite element simulations. Our results provide direct evidence that the mechanically induced crystallization kinetics is closely related with the stress state. The crystallization kinetics under compressive and tensile stresses are interpreted within the frameworks of potential energy landscape and classical nucleation theory, which reduces the role of stress state in mechanically induced crystallization in a metallic glass.

Sub-micron strain analysis of local stick-slip motion of individual shear bands in a bulk metallic glass

Nanodot deposition on a side surface of a rectangular sample and digital image correlation are used to quantify the in-plane strain fields associated with the propagation of a shear band in a PdNiP bulk metallic glass, induced by rolling. Within the resolution of the method related to an average inter-dot distance of 100 nm, deformation is found to be highly localized at the shear bands, while alternating areas with a size of 100–400 nm with opposite local shear strains are found. This phenomenon substantiates a local stick-slip nature of shear band propagation during the metallic glass deformation, even during rolling.

A Microstructural Investigation of Athermal and Deformation-induced Martensite in Fe-Cr-Ni Alloys

The microstructure of athermal and deformation-induced martensite in four high-purity Fe-Cr-Ni alloys has been investigated. The investigation is conducted using light optical microscopy, electron backscatter diffraction and electron channeling contrast imaging. It is found that ɛ-martensite, annealing twins and grain boundaries are preferred nucleation sites for athermal α’- martensite. Furthermore, when ɛ-martensite forms before α’-martensite during quenching it acts as nucleation sites as well as growth obstacles for narrow α’-martensite units producing a characteristic microstructure with sharp angular coupling, which is distinct from the packet-type α’-martensite that forms when athermal α’-martensite is able to grow without any hindrance from prior ɛ-martensite formation. It is also found that the transformation strain imposed by the martensitic transformation can be relieved by either autocatalytic martensitic transformation or generation of planar defects, i.e. shear bands. Finally the nucleation sites of α’-martensite induced by deformation seem to be mainly at intersections of shear bands and individual shear bands.

Density changes in shear bands of a metallic glass determined by correlative analytical transmission electron microscopy

Density changes between sheared zones and their surrounding amorphous matrix as a result of plastic deformation in a cold-rolled metallic glass (melt-spun Al88Y7Fe5) were determined using high-angle annular dark-field (HAADF) detector intensities supplemented by electron-energy loss spectroscopy (EELS), energy-dispersive X-ray (EDX) and nano-beam diffraction analyses. Sheared zones or shear bands were observed as regions of bright or dark contrast arising from a higher or lower density relative to the matrix. Moreover, abrupt contrast changes from bright to dark and vice versa were found within individual shear bands. We associate the decrease in density mainly with an enhanced free volume in the shear bands and the increase in density with concomitant changes of the mass. This interpretation is further supported by changes in the zero loss and Plasmon signal originating from such sites. The limits of this new approach are discussed.