Strain Localization Mechanisms Research Articles

Microstructure dependent strain localization during primary hot working of TA15 titanium alloy: Behavior and mechanism

Strain localization, a cross-scale issue degrading homogeneity and formability during hot working of TA15 titanium alloy, is complicated by lamellar microstructure for hierarchical feature, spatial morphology, and orientation relationship. In this work, by taking advantage of hat-shaped specimens to maintain deformation mode, the effect of heterogeneous lamellar microstructure on strain localization is accessible and visualized by cross-section comparison enlightened by cake cutting. Moreover, the evolution behavior and mechanism are also investigated by the two-scale crystal plasticity model. The mechanism of strain localization during hot working of lamellar microstructure is re-recognized as the structural softening governed by an abrupt phase interface weakening and continuous geometrical softening. The driving force of strain localization is the ‘collapse’ of the stress gradient between constituent phases intensified by the progression of compelled uniformed deformation. The key to microstructure-dependent strain localization is the influence of lamellar orientation on germination and colony interface on intensification. Colonies with lamellar orientation deviated 45 o from the direction of maximum shear stress are endowed with the intrinsic potential of strain localization. Large-size and strongly-orientated colony interfaces intensify strain localization. These findings provide new insights into microstructure-dependent strain localization supporting a more efficient design of processing routing to control localization powerfully.

Study of localized shear failure of AMg6 alloy specimens under static and dynamic loading

Experiments on dynamic loading of specimens made of AMg6 alloy were conducted on the Hopkinson-Kolsky bar apparatus, while the static loading tests were carried out on the Testometric electromechanical testing machine. Temperature fields generated in the processes of dynamic deformation were investigated in-situ by recording temperatures with a high-speed infrared camera CEDIP Silver 450M. The temperature measured in the zone of strain localization formed in the material during dynamic loading does not confirm the traditional view of the strain localization mechanism as the one associated with thermoplastic instability. Static and dynamic tests were carried out for AMg6 alloy specimens specifically designed to study plastic strain localization using for this purpose the Testometric electromechanical testing machine and split Gopkinson-Kolsky bar, as well as the StrainMaster system for noninvasive measurement of shape and strains. Displacement and strain fields generated in specially shaped AMg6 alloy specimens under static deformation and dynamic loading were constructed. Comparison of the temperature fields experimentally obtained under dynamic loading and strain fields under static and dynamic loading with the results of numerical modeling made with account of kinetics of mesodefect accumulation in the material shows good agreement to the accuracy of ~20%. After the experiments, the surface relief of specially shaped stressed specimens was examined using an optical interferometer-profile meter New View-5010. The processing of the obtained 3D strain relief data made it possible to evaluate the scale invariant (Hurst index) and spatial scale of the area where the correlated behavior of mesodefects was observed. The data of experimental studies, the examination of the surface topography of deformed specimens, structural studies, as well as the data of numerical simulation conducted with consideration of the kinetics of mesodefect accumulation in the material suggest that one of the mechanisms of plastic strain localization in the alloy AMg6 at the examined rates of loading is associated with discontinuous processes in the defect structure of materials. However, the character of specimen failure under static and dynamic loading is due to realization of different scenarios for the behavior of defective subsystems.

Исследование локализации пластической деформации при статическом и динамическом нагружении с применением системы неинвазивного измерения деформаций StrainMaster

Static and dynamic testing of specimens specially designed for studying the localization of plastic deformation in AMg6 and D16 alloys were performed on then electromechanical Testometric machine and split Hopkinson pressure bar using the StrainMaster system for noninvasive measurement of shape and deformation. Displacement and strain fields are plotted for special-shaped specimens of AMg6 and D16 alloys subjected to static deformation and dynamic loading. Comparison between the experimentally obtained strain fields and the results of numerical simulation made with account of the kinetics of microdefect accumulation in the examined material demonstrates good agreement to the accuracy of ~20%. The performed tests and their numerical simulation with consideration for the evolution of the defect material structure confirm the concept of the strain localization mechanism associated with the processes in the system of microdefects.

Studying the Localized Shear Fracture Mechanism in Alloys under Dynamic Loading

Experiments on the dynamic loading of specimens from AMg6 alloy were conducted on a split Hopkinson–Kolsky bar and with target perforation. The thermodynamics of the deformation process was studied in situ by recording temperature fields using a CEDIP Silver 450M high-speed infrared camera in order to reveal the characteristic stages of strain localization. The temperature measurements in the localization zones do not confirm the traditional concept of the strain localization mechanism caused by thermoplastic instability. Dynamic tests of specimens made of Steel 3, AMg6, and D16 alloys of a special shape designed to study the localization of plastic deformation on a split Hopkinson–Kolsky bar were conducted using the StrainMaster noninvasive strain measurement system. The displacements and strain fields in these specimens are constructed. A comparison of the experimentally measured temperature and strain fields with the results of numerical simulation with allowance for the mesodefect accumulation kinetics in the material gives a satisfactory agreement within ~20%. In the specimens after the experiments, the surface relief was analyzed using a NewView-5010 optical surface profiler with subsequent processing of the 3D strain relief data and calculation of the scale invariant (Hurst index) and the spatial scale of the region on which the correlated behavior of mesodefects is observed. The data of the experiments and analysis of the surface relief of deformed specimens, as well as the data of numerical simulation taking the mesodefects accumulation kinetics in the material into account, suggest that one of the plastic strain localization mechanisms under high-speed loading is caused by jump-wise processes in the defect structure of materials.

Enhancing the bond strength in the meta-crystal lattice of architected materials by harnessing the non-equilibrium solidification in metal additive manufacturing

Abstract Architected materials produced by metal additive manufacturing techniques offer realization of complex structural hierarchies that mimic the principles of crystal plasticity while still being ultralight-weight. However, they suffer from deep-rooted multiscale defects including microstructural heterogeneity caused by the complex thermo-mechanical transients in the melt pool. Here we manufacture meta-crystal 316L stainless steel microlattice structures for utilizing the strain localization mechanism in bulk structures akin to dislocation slip mediated plasticity. These lattices are characterized here using scanning and transmission electron microscopy, along with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) to reveal the microstructural, chemical and crystallographic orientation information at various locations in the struts and joints. The presence of inherent voids was the major drawback that would undermine their structural performance as mechanical metamaterials. Nevertheless, other defects in the form of spatially correlated dislocation networks and micro-segregated cellular substructures overcome the strength-ductility trade-off and render the bulk structures comparable to other engineering materials including conventional steels. By exploiting this intrinsic strengthening mechanism due to the rapid (i.e. non-equilibrium) solidification phenomenon, the bond (i.e. strut) strength of meta-crystals can be enhanced (or controlled) on top of employing hardening principles of metallurgy to design architected materials with tailored properties.

Strain localization mechanisms for subduction initiation at passive margins

It is widely accepted that subduction initiation at modern Earth passive margin systems critically depends on the buoyancy and strength of the oceanic lithosphere and requires failure of the load-bearing crustal and mantle layers. As such, subduction initiation upon orthogonal convergence is controlled by the age of the oceanic lithosphere and thus the strength contrast at the margin. However, it is still unclear where along the margin and how subduction initiates. In particular, rheologically-controlled mechanisms are poorly understood and require further investigation. Therefore, this combined analogue and numerical modelling study aims at exploring the effects of first order rheological and kinematic conditions on subduction initiation at passive margins. Our results highlight the sensitivity of early stages of subduction initiation to the initial rheological setup as well as evolving thermo-mechanical feedback mechanisms. Additionally, they provide more insights on the conditions responsible for the locus of subduction initiation. We infer that the locus of subduction is controlled by the rheology of both the crust and the mantle lithosphere at the margin, which is in strong correlation with the thermal age of the oceanic lithosphere. Suitable conditions for subduction initiation at passive margins correspond to an intermediate age (ca. 80 Myr) of the oceanic lithosphere. In all other cases deformation localizes within the oceanic lithosphere or affects the entire continent. We advocate the significance of crust-mantle decoupling at the passive margin for its inversion and possible evolution towards a subduction zone. Additionally, the development of a self-sustaining subduction zone will fail in the absence of weakening mechanisms taking place in the mantle lithosphere such as thermal softening (shear heating) and low temperature plasticity (Peierls mechanism). • Analogue and numerical models are used to infer favorable mechanical conditions for subduction initiation at passive margins. • Ductile continental crust is a key element for localizing deformation during the early stages of subduction initiation. • Subduction initiation is controlled by the strength of the mantle lithosphere at the margin. • Evolution towards sustainable subduction is controlled by both Peierls creep mantle rheology and shear heating.

Intrinsic and extrinsic effects on the brittle-to-ductile transition in metallic glasses

The effects of cooling rate, temperature, and applied strain rate on the tensile deformation behavior of a Cu64Zr36 metallic glass (MG) are investigated using large-scale molecular dynamics simulations. An increase in the quenching rate during sample preparation, as well as an increase of the temperature or the applied strain rate, affects the activation of shear transformation zones (STZs) and, consequently, the shear-banding processes, which ultimately causes a brittle-to-ductile transition in the deformation behavior of MGs. A quantitative interpretation for the observed enhanced ductility in MGs with an increasing quenching rate is obtained by sampling the saddle points on the potential energy surface. High quenching rates lead to lower energy barriers for activation of a local atomic rearrangement (STZ) as compared to those MGs obtained at low quenching rates. Although the glassy structure does not show significant variations with increasing temperature, the kinetic energy of the atoms increases dramatically, which allows the atoms to rearrange easily; therefore, the probability of homogeneous thermal activation of STZs increases. Finally, a large number of STZs can also be activated by deformation at high strain rates when a large amount of elastic energy is stored in the glassy matrix. Consequently, a high density of STZ events and, therefore, a more complex percolation process results in a low probability for strain localization and formation of critical shear bands. Our results provide an atomistic understanding for the strain localization mechanisms in metallic glasses and shed more light on the brittle-to-ductile transition.

Deformation fabrics and strain localization mechanisms in graphitic carbon-bearing rocks from the Ailaoshan-Red River strike-slip fault zone

Graphitic carbon-bearing rocks generally occur in low-to high-grade metamorphic units. In many brittle faults, graphitic carbon is often associated with gouge or low-grade metamorphic rocks whereas in ductile faults, graphitic carbon commonly occurs in marble, schist or gneiss. Carbonaceous material gradually transforms from an amorphous into an ordered crystalline structure by increasing thermal metamorphism. The degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock. In this contribution, based on detailed field observations, the variably deformed and metamorphosed graphitic gneisses to phyllites, located within the footwall and hangingwall unit of the Cenozoic Ailaoshan-Red River strike-slip shear zone are studied. According to lithological features and temperatures determined by Raman spectra of carbonaceous material, these graphitic rocks and deformation fabrics are divided into three types. Type I is represented by medium-grade metamorphism and strongly deformed rocks with an average temperature of 509 °C and a maximum temperature of 604 °C. Type II is affected by low-grade metamorphism and deformed rocks with an average temperature of 420 °C. Type III is affected by lower-grade metamorphism and occurs in weakly deformed/undeformed rocks with an average temperature of 350 °C. Slip-localized micro-shear zones and laterally continuous or discontinuous slip-planes constituted by graphitic carbon aggregates are developed in Types I and II. The electron back-scattered diffraction (EBSD) lattice preferred orientation (LPO) patterns of graphitic carbon grains were firstly observed in comparison with LPO patterns of quartz and switch from basal <a>, rhomb <a> to prism<a> slip systems, which indicate increasing deformation temperatures. Comparison of quartz and graphite LPOs indicates that graphite LPOs show higher temperature conditions. According to the graphitic slip-planes, micro-shear zones with grain-size reduction and mylonitic foliation constituted by graphitic carbon minerals, we also propose that the development of fine-grained amorphous carbon plays an important role in rheological weakening of the whole rock during progressive ductile shearing.

The effect of hydrous mineral content on competitive strain localization mechanisms in felsic granulites

This study investigates the grain-scale mechanisms that facilitated strain localization in the lower crust within the East Athabasca Mylonite Triangle of northern Saskatchewan, Canada. There, two distinct lithotectonic units, the Upper Deck and Northwestern Domain, are juxtaposed across a km-scale shear zone that has been previously interpreted to record thrust and/or normal-sense displacement. Our study reports new quartz c-axis opening angle, Ti-in-biotite and Ti-in-quartz geothermometry and amphibole-plagioclase geothermobarometry within syn-kinematic phases indicating that both units were deformed and metamorphosed at upper amphibolite to granulite facies (~725 °C and ~0.8 GPa). While sharing similar conditions, the dominant mechanisms of deformation across the two units are strongly linked to modal proportion of syn-kinematic hydrated mineral phases. Detailed microstructural and crystallographic preferred orientation (CPO) analyses reveals that specimens poor in hydrated minerals (<10%) deformed mainly by dislocation creep and dynamic recrystallization allowing it to develop a strong quartz CPO, while in specimens rich in hydrated minerals (>10%) pressure solution and (re)precipitation processes enhanced phase mixing and promoted grain boundary sliding resulting in weakly developed or absent quartz CPO. These results indicate a strong mineralogical and fluid control on deformation and, therefore, which strain softening process (es) may dominate in lower crustal shear zones.

Shear instability and considerably localized melting in quasi-static compression

It is a big challenge to identify whether or not a high temperature rise at high strain rate is a prerequisite for shear instability in crystalline materials. In contrast to the prevailing report of shear instability at high strain rate loading, the shear band (SB) was generated at quasi-static loading in a martensite steel. This gives strong evidence that the high temperature rise is not essential as the quasi-static loading is not able to generate enough heat to initiate the shear instability. Different from the conventional narrow SB at high strain rate loading, a large patch of SB was firstly observed. Considerably localized melting occurred due to the abrupt fracture of SB. SB displays different TEM microstructures at melting and no-melting places. This indicates the significance of distinguishing the microstructure formed during shear instability from the one affected by the fracture of SB. The result suggests that the strain localization mechanism in this material is attributed to the microstructural change instead of the thermal softening. The findings provide a new insight into understanding the formation mechanism of SB from the perspective of microstructural evolution in crystalline materials.

Nondislocational Mechanisms of Strain Localization in Nickel Nanocrystals During Deformation by High-Pressure Torsion in Bridgman Anvils

Using the methods of transmission electron microscopy, new aspects of formation of reorientation nanobands with partial involvement of nondislocational deformation mechanisms are investigated in nickel nanocrystals under the conditions of its severe plastic deformation by high-pressure torsion in Bridgman anvils: local reversible (FCC→BCC→FCC) transformations of the martensitic type and quasi-viscous mass transfer by the flows of nonequilibrium point defects in the fields of high local pressure gradients. The features of disclinational structure and elastically stressed state at the nanoband propagation front are studied. A theoretical analysis of the rate of plastic deformation via the mechanisms of quasi-viscous mass transfer by the flows of nonequilibrium point defects is performed. A possibility of simultaneous realization of the martensitic and quasi-viscous deformation modes at the front of nanoband propagation is established. An analysis of the conditions and mechanisms where these modes are involved is performed as a function of the type of point defects (vacancies and interstitial atoms), deformation temperature, and peculiarities of disclination structure and elastically stressed state.

Nanodipoles of Partial Disclinations and Strain Localization Mechanism of Nanostructured Materials in the Elastic Region

In the course of severe plastic deformation of nickel in the Bridgman anvils, a phenomenon of strain localization is observed in the region of elastic distortions, which is accompanied by the formation of reorientation nanobands with an elastic crystal lattice curvature of hundreds of deg/μm and nanodipoles of partial disclinations as the defects of elastically deformed medium and carriers of elastic shears and rotations at the front of nanoband propagation. A theoretical analysis is performed of the elastically stressed state and energy of these defects, including the case of their transformation into more complex ensembles of interconnected disclinations. Using the results obtained and the data of an MD investigation of dynamic vortex defects formed in the region of elastic distortions, the mechanisms of strain localization in this region in the stages of nucleation and propagation of these nanobands are proposed.

Investigation of strain redistribution mechanism in α titanium by image-based crystal plasticity analysis

Mechanisms of strain localization and localized activation of slip systems in α titanium were investigated using a crystal plasticity finite element (CPFE) method. A microscopic image of polycrystalline α titanium was obtained by electron back scatter diffraction (EBSD), and the data was converted from the microscopic image into the geometric model for the CPFE analysis. The uniaxial tensile deformation of the model was numerically reproduced by the CPFE method employing a dislocation density based constitutive equation. The results showed that the strain distribution corresponds well with that obtained by the experiment when the ratio of critical resolved shear stress (CRSS) employed in the numerical simulation is basal:prismatic ⟨a⟩:1st-pyramidal ⟨a⟩:1st-pyramidal ⟨c + a⟩:2nd-pyramidal ⟨c + a⟩ = 1.0:1.0:1.3:2.0:2.0. Next, numerical simulations were performed by changing the ratio of CRSS among the slip systems but keeping all other conditions the same as those of the above uniaxial tensile analysis. The results showed that strain redistribution typically occurs between hard and soft regions with high and low CRSSs for the primary slip systems; this redistribution resulted in a localized higher strain and activation of slip systems. However, localized activation of slip systems was observed even in slip systems with higher CRSS; the mechanism could be explained by the strain redistribution in the tensile direction.

On the Appearance of Scaling in the Time Dependence of the Rate of Shear Processes in a Metallic Glass

A possible explanation of the square scaling observed in the time dependence of the velocity of shear motions in a metallic glass has been proposed. Numerous experiments demonstrating this scaling in the time variation range by four orders of magnitude and in the shear band velocity range by nine orders of magnitude indicate its universality and the possibility of separating mechanisms of strain localization in metallic glasses.

Spontaneous generation of ductile shear zones by thermal softening: Localization criterion, 1D to 3D modelling and application to the lithosphere

The generation of ductile shear zones is essential for the formation of tectonic plate boundaries, such as subduction or strike-slip zones. However, the primary mechanism of ductile strain localization is still contentious. We study here the spontaneous generation of ductile shear zones by thermal softening using thermo-mechanical numerical simulations for linear and power-law viscous flow in one-dimension (1D), 2D and 3D. All models are velocity-driven. The 1D model exhibits bulk simple shear whereas the 2D and 3D models exhibit bulk pure shear. The initial conditions include a small temperature perturbation in otherwise homogeneous material. We use a series of 1D simulations to determine a new analytical formula which predicts the temperature evolution inside the shear zone. This temperature prediction requires knowledge of only the boundary velocity, flow law and thermal parameters, but no a priori information about the shear zone itself, such as thickness, stress and strain rate. The prediction is valid for 1D, 2D and 3D shear zones in bulk pure and simple shear. The results show that shear heating dominates over conductive cooling if the relative temperature increase is >50°C. The temperature variation induced by the shear zone is nearly one order of magnitude wider than the corresponding finite strain variation so that no significant temperature variation occurs between shear zone and wall rock. Applying typical flow laws for lithospheric rocks shows that shear zone generation by thermal softening occurs for typical plate tectonic velocities of few cm.yr−1 or strain rates between 10−16 and 10−14 s−1. Shear stresses larger than 200 MPa can already cause strain localization. The results indicate that thermal softening is a feasible mechanism for spontaneous ductile shear zone generation in the lithosphere and may be one of the primary mechanisms of lithospheric strain localization.

A numerical study of plastic strain localization and fracture across multiple spatial scales in materials with metal-matrix composite coatings

The mechanisms of stress concentration and plastic strain localization operating in an aluminum substrate with an aluminum-matrix composite coating subjected to uniaxial loading are investigated. The plane strain deformation of the coated material across micro-, meso- and macroscales is simulated numerically by the finite-difference method. The constitutive models used in the study include the elastic-brittle and isotropic elastic-plastic responses of ceramic particles and metal matrixes/substrates, respectively. The experimentally observed microstructure of the coated material is explicitly taken into account in the calculations. The formation of local regions experiencing bulk tension under compression of the composite material is simulated in order to control crack initiation and growth in ceramic particles, with the irregular geometry of matrix-particle and coating-substrate interfaces being a major factor of stress concentration. The influence of the coating thickness, distance between ceramic particles, mechanical properties of the constituents, and matrix-particle interfacial strength on the macroscopic strength of the composite and coated materials are studied.

The study of mechanical and microstructural aspects of localized shear fracture in metals under dynamic loading

The purpose of this work is to provide theoretical framework and experimental supporting evidence for a crucial role of structural transitions in the ensemble of defects at the meso-level (microshears and microcracks), as one of the mechanisms of plastic strain localization in metals under high-rate loading.The investigation of the sample response to dynamic loading was carried out on the split Hopkinson pressure bar and in a series of target penetration tests. To identify the characteristic stages of strain localization, the thermodynamics of the deformation process was investigated "in-situ" by recording the temperature fields using a high-speed infrared camera CEDIP Silver 450M.The temperature measured in the localization zone does not confirm the generally accepted concept of the strain localization mechanism as the mechanism governed by a thermoplastic instability. The samples stored after the experiment were subjected to microstructural analysis, using an optical interferometer-profilometer and a scanning electron microscope. The structural analysis revealed a correlated behavior of the ensemble of defects, which can be classified as a structural transition providing strain localization.The data of experimental studies, the examination of the structure of deformed samples, as well as the data of numerical modeling taking into account the kinetics of accumulation of microdefects in the material suggest that one of the mechanisms of plastic strain localization at high loading rates is associated with the jump-like processes in the defect structure of a material.

Исследование локализованного пластического сдвига при динамическом нагружении с применением стержня Гопкинсона–Кольского

The mechanisms of plastic shear localization under dynamic loading were investigated experimentally. The dynamic load tests of the specimens were performed in the split Hopkinson pressure bar. To study the localized shear under high-rate loading, we used two types of specimens of special shape, which were designed to conduct experimental studies of plastic strain localization under dynamic loading. The study was conducted on the specimens made of aluminum alloy AMg6. The surface of the specimens was investigated in real time mode using a CEDIP Silver 450M high-speed infrared camera. The obtained distribution of the temperature field at different times allowed us to judge the development of the process of plastic strain localization. The infrared studies, structural analysis, and numerical modeling of the experiments, taking into account the evolution of the defective subsystem of the material, lend support to the view that the mechanism of strain localization is not associated with thermal softening.

Image-based crystal plasticity analysis of strain localization mechanism in polycrystalline &lt;i&gt;α&lt;/i&gt;Ti

Image-based crystal plasticity analysis of strain localization mechanism in polycrystalline &lt;i&gt;α&lt;/i&gt;Ti

Numerical Simulation and Experimental Study of Plastic Strain Localization under the Dynamic Loading of Specimens in Conditions Close to a Pure Shear

Mechanisms of plastic strain localization under the dynamic loading of specially shaped specimens made of the AMg6 aluminum alloy and intended for tests under conditions close to a pure shear on a split Hopkinson–Kolsky pressure bar are studied theoretically and experimentally. The mechanisms of plastic flow instability are related to collective effects in the ensemble of microdefects in spatially localized regions visualized in situ using a CEDIP Silver 450M high-speed infrared camera. The calculation corresponding to the experimental loading scheme is implemented using wide range constitutive equations reflecting the dependence of structural relaxation mechanisms—manifestation of the collective behavior of microdefects—on the evolution of the localized flow shear instability. Microstructure analysis of deformed specimens included the study of the spatial relief (porosity) scaling by the data of a NewView-5010 microscope-interferometer in regions of plastic strain localization. An increase in the structural scaling exponent (Hurst exponent) reflected the degree of the multiscale correlated behavior of defects and the porosity induced by them in regions of localized plasticity. Infrared scanning of the strain localization region and numerical simulation followed by an estimate of the defect structure corroborated the hypothesis that effects of thermal softening do not play a decisive role in the process of plastic shear localization in the tested material under the considered loading regimes. A new, one of the possible ones, mechanism of plastic strain localization under dynamic loading is justified. It is caused by the multiscale collective behavior of mesodefects—structure-scaling transitions—and establishes the stadiality of the localized shear evolution.