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We have investigated the influence of 101 mass ppm hydrogen content on the room temperature deformation structure and mechanical behavior of an austenitic Fe30Mn6.5Al0.3C (wt.%) low-density steel by several electron microscopy techniques, such as electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). The steel exhibits a high hydrogen embrittlement resistance associated with a moderated increase in strength (yield stress increase of 10%) and ductility (increase in the elongation to fracture of ∼ 8%). Analysis of the deformation structure reveals that hydrogen influences the deformation behavior by promoting deformation mechanisms associated with inhomogeneous plasticity (hydrogen-enhanced deformation banding (HEDB)) and strain localization (hydrogen-enhanced microbanding (HEMB)). These deformation mechanisms are ascribed to hydrogen-induced effects on dislocation plasticity, resulting in macroscopic kink bands, sub-micron localized strain gradients, and localized shear at cell blocks. We find that HEMB plays a relevant role in the deformation behavior of sub-micron localized strain gradients by promoting plastic relaxation and the enhanced storage of geometrically necessary dislocations within them. These effects mitigate the activation of damage mechanisms and enhance the strain-hardening capacity, contributing to the high HE resistance of the steel, comparable to that of high HE-resistant fcc alloys and steels. [Display omitted] [ABSTRACT FROM AUTHOR]

Stabilizing non-equilibrium nanograin boundaries has been shown to enhance the strength of nanocrystalline metals. Nevertheless, a thorough comprehension of how these stable grain boundaries impact the fatigue behavior in nanocrystalline metallic layered composites with heterogeneous interfaces is currently lacking. Here, aiming to develop high-performance small components in microelectromechanical systems, we prepared nanocrystalline Ni/Ni-W layered composites that underwent various degrees of annealing treatment. Our results reveal that the Ni/Ni-W layered composites annealed at 200 °C exhibit significantly higher fatigue strength, surpassing that of the as-prepared composites by 40 % and the Pt-10 at.% Au alloy by 17 % which is one of the best candidates for the microelectromechanical systems switches at present. The enhanced fatigue strength is primarily attributed to the annealing-induced grain boundary relaxation and mechanically-induced structural relaxation. Grain boundary relaxation enhances the strength by improving GB stability, while the mechanically-induced structural relaxation results in the coarsening and expansion of triangular columnar grains towards the Ni-W layer during fatigue loading. As a result, the dispersive strain localized regions triggered by the triangular columnar grains undertook cyclic strain accumulation and weakened localized damage accumulation in Ni layers, and thus further improved the fatigue resistance. The finding of the underlying mechanisms may offer a promising approach for designing high-performance materials for microelectromechanical systems switches operating at elevated temperatures. [Display omitted] [ABSTRACT FROM AUTHOR]

A comprehensive study is undertaken to investigate the evolution of microscopic deformation under in-situ uniaxial tensile testing of the Directed Energy Deposited (DED) Ti-6Al-4V alloy. The study uncovered a synergistic influence of the (α + β) lamellar microstructural features at different length scales, leading to distinct deformation mechanisms and barrier effects. At the smallest length scale, the deformation mechanisms are lattice slipping and phase boundary sliding (PBS), with a characteristic length scale comparable to the dimensions of α platelets and colonies. The slipping and PBS encountered complex barrier effects at β interlayers. At the intermediate length scale, a mechanism of strain localization manifests as shear bands along a 45° orientation to the uniaxial loading direction. The strain localization involves the activation of multiple slipping events and exhibits a propensity to choose large unobstructed phase-boundary pathways for its realization. At the highest length scale, grain boundaries introduce another barrier effect to both slipping and strain localization through intergranular deformation compatibility. The synergistic interplay between these three mechanisms constitutes a hierarchical response, enabling the material to effectively accommodate large plastic deformation. Moreover, the synergistic interplay influences the anisotropy of the material. [Display omitted] [ABSTRACT FROM AUTHOR]

Solute segregation to individual grain boundaries is used by design to produce strong and stable nanocrystalline metallic alloys. Grain-boundary segregation, however, is known to cause adverse embrittlement effects from a strain-localization failure mechanism that imposes significant material limitations for structural applications. Here, using atomistic simulations, it is discovered that heterogeneous Ni segregation in nanocrystalline Ni-mixed Ag alloys dramatically shuts down localized shear bands during plastic deformation, while simultaneously increasing the tensile strength. Nanocrystalline Cu-mixed Ag metals are predicted to exhibit standard homogeneous Cu segregation and a tensile strength that saturates above a solute concentration of 8 at.% due to glass-like shear localization induced by grain boundaries. By contrast, it is found that heterogeneous Ni segregation in nanocrystalline Ag-Ni alloys forms solute-rich clusters along interfaces leading to strain delocalization at high strain and continuous strengthening at high solute concentrations up to 15 at.%. This study reveals the importance of heterogeneous versus homogeneous segregation behaviors on strain localization and points to a fundamentally new strategy to design failure-resistant nanostructured materials through grain boundary segregation engineering. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Strain partitioning and localization were investigated in a high-Mn steel (17.1 wt.% Mn) during tensile testing by a correlative probing approach including in-situ synchrotron X-ray diffraction, micro- digital image correlation (μ-DIC) and electron microscopy. By combining Warren's theory with the μ-DIC analysis, we monitored the formation of planar faults (stacking faults and mechanical twins) and correlated them with the local strain partitioning behavior within the microstructure. Starting with an initial microstructure of austenite (γ) and athermally formed ε- and α'-martensite, strain accumulates preferentially near the γ/ε interfaces during tensile straining. The local microscopic von Mises strain (ε vM) maps obtained from μ-DIC probing show that these local strain gradients produce local strain peaks approximately twice as high as the imposed macroscopic engineering strain (ε), thus locally triggering formation of ε-martensite already at early yielding. The interior of the remaining austenite, without such interfacial strain peaks, remained nearly devoid of planar faults. The local strain-driven growth of the ε-domains occurs concomitantly with the α'-martensite formation. At intermediate macroscopic applied strains, austenite grain size is considerably reduced to a few nanometers and the associated γ/ε interfacial microscopic strain peaks increase in magnitude. This scenario favors twinning to emerge as a competing strain hardening mechanism at engineering strain levels from ε = 0.075 onwards. At large tensile strains, the γ → ε → α' transformation rates tend to cease making both twinning and SFs formation to operate as the main strain hardening mechanisms. The findings shed light on the transformation micro-mechanisms in multiphase Mn-TRIP steels by revealing how strain localization among the constituents can directly influence the kinetics of the competing strain hardening mechanisms. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Irreversible deformation in relation to the microstructure was investigated for a polycrystalline Ni-based superalloy (Alloy 718) from room temperature to 650 °C using high-resolution digital image correlation (HR-DIC) techniques. Interrupted tensile tests were performed under a protective atmosphere to ensure the stability of the speckle pattern to track kinematics fields from surface analyses. In-plane strain localization was captured using HR-DIC on scanning electron microscopy (SEM) images. A statistical analysis of different strain localization events in relation to the microstructural features was conducted, i.e. , intragranular slip localization, slip localization parallel to and near Σ 3-twin boundaries (Σ 3-TB), and grain boundary sliding (GBS). Alloy 718 exhibited slip localization at room temperature and 350 °C. Intense strain localization develops parallel and in the vicinity of Σ 3-TB from the onset of the microplasticity. Few intense slip stimulated-grain boundary sliding events were found due to slip localization on both grains adjacent to the grain boundary. At 650 °C, Alloy 718 experienced grain boundary sliding at the onset of the yield without particular slip localization in adjacent grains. At lower temperatures, strain localization parallel to and near Σ 3-TB was intense, and intragranular slip localization intensified with increasing macroscopic deformation. Particular microstructural configurations were found at 650 °C leading to premature damage: (i) sub-surface cavitation at grain boundaries, and (ii) grain boundary cracking due to intense shearing near a Σ 3-TB. [Display omitted] [ABSTRACT FROM AUTHOR]

This work provides a comprehensive characterization and analysis of deformation and fatigue damage mechanisms in a nickel-based superalloy during ambient temperature fatigue and points to a fundamental deformation mechanism that results in the onset of crack nucleation. Strain and slip irreversibility are investigated at the nanometer scale using high-resolution digital image correlation and high-resolution electron backscatter diffraction, highlighting distinct deformation mechanisms contributing to crack nucleation. It is observed during early fatigue cycling at relatively low applied stress, the formation of intense slip events that induce grain boundary shearing. This results in intense micro-scale strain in the neighboring grains, producing localized plasticity and stresses. Such stresses facilitate fatigue extrusion–intrusion mechanisms during subsequent cycling, resulting in preferred crack nucleation. Finally, the configurations within the microstructure that promote such deformation and damage mechanisms sequence are highlighted. [Display omitted] [ABSTRACT FROM AUTHOR]

It is challenging to describe quantitatively the effects of ceramic grain morphology on the stress and strain distributions in the cermet materials under different loading conditions. Here the dependence of the distribution of residual thermal stress (RTS) in the as-prepared cermets and its mechanical behavior under external stress on the morphology of ceramic grains were quantified and resolved by modeling and experimental studies, using WC Co cermets as a typical example. A model was proposed to visualize the strain localization and reveal the mechanisms of the initiation and propagation of micro-cracks in the microstructure of cermets under varying loads, and the calculation results were verified by experimental measurements. It was discovered that the effects of WC grain morphology on the mechanical behavior and failure mode of cermets are distinctly different under tension or compression loading conditions, whose mechanisms were disclosed based on detailed analysis of micro-scale strain responses. The results indicate that it is feasible to modulate the strain distribution by tailoring the grain shape character for adapting to external loading, hence optimizing the material mechanical behavior. [Display omitted] [ABSTRACT FROM AUTHOR]

We report on recurrent shear localization by formation of strictly alternating shear and matrix bands during equal-channel angular pressing (ECAP) of a 6000 series aluminum alloy. The strain partitioning process is documented by analyzing the deformation of a grid of indents as well as reconstructing the corresponding flow lines. Interestingly, shear strains of ∼3.6 in the shear bands considerably exceed the conventional maximum shear strain achievable in a single ECAP pass, whereas much lower strains occur in the matrix bands, maintaining on average the macroscopic deformation that is expected for ECA-pressing with a 90° die. Microstructural analysis by electron back-scatter diffraction (EBSD) and scanning transmission electron microscopy documents the different stages of microstructural evolution in shear and matrix bands and confirms the pronounced differences associated with the novel strain partitioning process. Furthermore, an EBSD-based analysis of texture evolution for billets with different orientations with respect to the initial extrusion direction demonstrates the important role that texture softening plays in triggering shear localization in two characteristic orientations as opposed to homogeneous deformation in the third orientation. Shear banding during ECAP is often interpreted in the light of failure mechanisms and cracking; the present study demonstrates that stable strain partitioning facilitates the fabrication of bulk laminated materials by ECAP. Image 1 [ABSTRACT FROM AUTHOR]

Hydrogen repartitioning and the related embrittlement behavior were characterized by studying Al–Zn–Mg–Cu aluminum alloys with different intermetallic particle contents. Using high-resolution X-ray tomography and related microstructural tracking techniques, hydrogen-induced quasi-cleavage cracks and the related strain localization were observed regardless of the content of the intermetallic particles. The area of quasi-cleavage cracks on the fracture surface increased and the strain localization became more intense with a decrease in the content of intermetallic particles, thereby revealing that trapped hydrogen at intermetallic particles increases the resistance to hydrogen embrittlement. In addition, a quantitative assessment of the hydrogen repartitioning taking into account vacancy production and dislocation multiplication during deformation, was applied to characterize the hydrogen embrittlement behavior. Because of the thermal equilibrium among various hydrogen trap sites, internal hydrogen atoms are mainly repartitioned to vacancies and precipitates in the strain localization region during deformation because of their high trap site densities and high hydrogen trap binding energies. Since the concentration of hydrogen trapped at dislocations is extremely limited, it can be assumed that hydrogen repartitioned to precipitates induces decohesion of precipitates along specific crystallographic planes, where quasi-cleavage cracking may originate. Image 1 [ABSTRACT FROM AUTHOR]

Abstract High-resolution digital image correlation (HR-DIC) techniques have become essential in material mechanics to assess strain measurements at the scale of the elementary mechanisms responsible of the deformation in polycrystalline materials. The purpose of this study is to demonstrate the use of laser scanning confocal microscopy (LSCM) coupled with DIC techniques to deepen knowledge on the deformation process of a polycrystalline nickel-based superalloy at room temperature. The LSCM technique is capable of detecting both in-plane and out-of-plane strain localization within slip bands at the sub-grain level. The LSCM observations are consistent with previous in-situ scanning electron microscopy (SEM) studies: The onset of crystal plasticity occurs primarily near Σ3 twin boundaries with macroscopic loading in the elastic domain (macroscopic stress as low as 80% of the 0.2% offset yield strength (Y.S. 0.2%)). This intense irreversible strain localization occurs with either a high Schmid factor (μ > 0.43) or a significant elastic modulus difference between the pair of twins (ΔΕ > 100 GPa). In the plastic deformation domain, transgranular slip activity following slip systems with the highest Schmid factor is mostly responsible for the deformation at the grain level, thus leading to strain percolation. The simultaneous in-plane and out-of-plane deformation assessment via the HR-LSCM-DIC technique was found to be essential for the identification of active slip systems. Finally, the HR-LSCM-DIC technique enabled the quantification of the glide amplitude involved in the three-dimensional shearing process at the grain level that solely in-plane measurements cannot provide. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

We have investigated the influence of the deformation temperature from 25 °C (RT) to -196 °C on the dislocation structures associated with strain localization phenomena in an austenitic Fe-30Mn-6.5Al-0.3C (wt.%) low-density steel by electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and bright-field transmitted forescattered electron imaging ((BF) t-FSEI) techniques. The characteristics of the dislocation structures were evaluated on the main texture components, i.e. <111>//tensile axis, <112>//tensile axis, and <001>//tensile axis directions. The inhomogeneous character of the plastic behavior is promoted upon cryogenic deformation due to the formation of dislocation structures associated with strain localization, namely microbands (MBs) and deformation bands (DBs). ECCI analysis of the dislocation structure reveals that the deformation temperature has a strong influence on the thermal-assisted dislocation processes controlling the dislocation configurations and MB formation mechanisms. The MB nucleation mechanism evolves from a cross-slip-assisted mechanism at RT deformation conditions to a slip band-assisted mechanism at cryogenic deformation temperatures. This effect has a profound effect on the grain orientation dependence of the MB structure but not on its crystallographic alignment. On the other hand, cryogenic deformation temperatures (-196 °C) enhance the material's mechanical strength and ductility due to the activation of deformation twinning, which is associated with the reduction of the stacking fault energy. We find that MBs have a small contribution to strain-hardening and ductility due to the small mechanical resistance of these dislocation structures against the advance of deformation twins and dense dislocation layers, and the comparatively small plastic strain accommodated by them, respectively. These findings provide new insights into the microband-induced plasticity (MBIP) effect. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract One of the most confounding controversies in the ductile fracture community is the large discrepancy between predicted and experimentally observed strain-to-failure values during shear-dominant loading. Currently proposed solutions focus on better accounting for how the deviatoric stress state influences void growth or on measuring strain at the microscale rather than the macroscale. While these approaches are useful, they do not address a significant aspect of the problem: the only rupture micromechanisms that are generally considered are void nucleation, growth, and coalescence (for tensile-dominated loading), and shear-localization and void coalescence (for shear-dominated loading). Current phenomenological models have thus focused on predicting the competition between these mechanisms based on the stress state and the strain-hardening capacity of the material. However, in the present study, we demonstrate that there are at least five other failure mechanisms. Because these have long been ignored, little is known about how all seven mechanisms interact with one another or the factors that control their competition. These questions are addressed by characterizing the fracture process in three high-purity face-centered cubic (FCC) metals of medium-to-high stacking fault energy: copper, nickel, and aluminum. These data demonstrate that, for a given stress state and material, several mechanisms frequently work together in a sequential manner to cause fracture. The selection of a failure mechanism is significantly affected by the plasticity-induced microstructural evolution that occurs before tearing begins, which can create or eliminate sites for void nucleation. At the macroscale, failure mechanisms that do not involve cracking or pore growth were observed to facilitate subsequent void growth and coalescence processes. While the focus of this study is on damage accumulation in pure metals, these results are also applicable to understanding failure in engineering alloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Hydrogen-induced dislocation motion is characterized in terms of the microscopic strain distribution in Al Zn Mg Cu aluminum alloys. Hydrogen-induced strain localization was visualized in 3D using X-ray tomography and related microstructural tracking techniques. The strain localization was observed as a form of obliquely aligned shear bands. The strain localization becomes more intense with an increase in holding time at each loading step, indicating that more internal hydrogen is partitioned to the strain localization regions with holding time. In addition, the concentration of hydrostatic strain is observed in the strain localization region. Numerous nano voids were generated after deformation and were determined from the precise interpretation of the measured hydrostatic tension. Direct observation of the nano voids was then successfully performed by employing high-angle annular dark-field (i.e., HAADF) scanning transmission electron microscopy imaging and imaging-type computed tomography (CT) techniques. It is assumed that nano voids can serve a dual role as a fracture origin site and a hydrogen trap site. However, no evidence for hydrogen embrittlement originating from nano voids was observed. Instead, it can be assumed that the most hydrogen was partitioned to nano voids in strain localization regions during deformation due to its high density. A hydrogen embrittlement model was proposed based on these findings, where in-situ hydrogen repartitioning, which is necessary for hydrogen embrittlement to occur, is considered. Graphical abstract (a) Hydrogen induced strain localization during deformation (b) Hydrostatic tension concentration due to the formation of hydrogen induced vacancies and nano voids in the strain localization region (c) Identification of nano voids in the strain localization region during deformation. Image 1 [ABSTRACT FROM AUTHOR]

In polycrystalline metallic materials, quantitative and statistical assessment of the plasticity in relation to the microstructure is necessary to understand the deformation processes during mechanical loading. Plastic deformation often localizes into physical slip bands at the sub-grain scale. Detrimental microstructural configurations that result in the formation and evolution of slip bands during loading require advanced strain mapping techniques for the identification of these atomically sharp discontinuities. A new discontinuity-tolerant DIC method, Heaviside-DIC, has been developed to account for discontinuities in the displacement field. Displacement fields have been measured at the scale of the physical slip bands over large areas in nickel-based superalloys by high resolution scanning electron microscopy digital image correlation (SEM DIC). However, conventional DIC methods cannot quantitatively measure plastic localization in the presence of discontinuous kinematic fields such as those produced by slip bands. The Heaviside-DIC technique can autonomously detect discontinuities, providing information about their location, inclination, and identify slip systems (in combination with orientation mapping). Using Heaviside-DIC, discontinuities are physically evaluated as sharp shear-localization events, allowing for the quantitative measure of strain amplitude nearby the discontinuities. Measurements using the new Heaviside-DIC technique are compared to conventional DIC methods for identical materials and imaging conditions. [ABSTRACT FROM AUTHOR]

The microstructural configurations that favor early strain localization and fatigue crack initiation at intermediate and high temperature (400 °C–650 °C) have been investigated using novel experimental techniques, including high resolution digital image correlation and transmission scanning electron microscopy. Cyclic fatigue experiments in the high and low cycle fatigue regimes have been performed on a René 88DT polycrystalline nickel-base superalloy at temperatures up to 650 °C and compared to previous fatigue results obtained from tests in the very high cycle fatigue regime. Competing failure modes are observed along with an inversion in the temperature fatigue life dependence of fatigue strength from the low to high cycle fatigue regime. Oxidation-assisted processes are dominant at high applied stresses while cyclic plastic localization and accumulation govern fracture at low applied stresses. In addition, a second competing mode exists in the high and very high cycle fatigue regime from non-metallic inclusions as compared to internal intrinsic initiation sites. The grain-scale features that exhibit strain localization and crack initiation were investigated in detail. Transmission electron microscopy (TEM), transmission scanning electron microscopy (TSEM) and electron channeling contrast imaging have been conducted on samples removed from targeted regions with microstructural configurations that favor crack initiation to characterize the associated dislocation sub-structure and its evolution with temperature. Plasticity is observed to be less localized during cyclic loading at high temperature compared to room temperature. The microstructural features that drive initiation across the temperature range investigated are: twin-parent grains pairs that are at the upper end of the size distribution, are oriented for near maximum elastic modulus mismatch, and have high stresses along planes parallel to the twin boundaries. [ABSTRACT FROM AUTHOR]

Understanding the micromechanical consequences of transformation-induced plasticity (TRIP)-assisted steels is challenging, due to the presence of multiple phase constituents and difficulties in spatially and temporally resolving the transformation of sub-micron scale retained austenite. Local stress or strain partitioning among constituents is considered important in determining the stability of retained austenite, which often competes with other effects such as composition or crystal orientation. In this work, we study the neighborhood effects on micromechanical behaviors of multi-phase TRIP steel using a ferrite-containing quenching and partitioning steel as a model system by employing in situ SEM/EBSD and in situ synchrotron X-ray diffraction tensile tests, phase-specific nanoindentation, and atom probe tomography. Our results reveal that retained austenite, bainite, and tempered martensite clusters behave similarly due to underlying strengthening mechanisms, resulting in the early transformation of retained austenite surrounded by bainite and tempered martensite matrices. Conversely, in large ferrite grains, the intragranular strain distribution develops heterogeneously, which retards the transformation of some embedded retained austenite grains. The in situ microstructure-based strain mapping also demonstrates that the local strain increment of the retained austenite inside ferrite grains decreases as the deformation level increases, leading to a non-linear strain partitioning behavior at high strain levels. Another neighborhood-induced effect is observed in ferrite grains. When ferrite grains are neighbored by hard fresh martensite, their lateral contraction is inhibited, causing a plane strain tension path locally in ferrite, even when the globally applied strain path is uniaxial tension. [Display omitted] [ABSTRACT FROM AUTHOR]

A computational study of 3D virtual instantiations of microtextured Ti-6Al-4V with varying initial slip system strengths is presented. Electron backscatter diffraction (EBSD) scans of a rolled and heat-treated mill annealed plate were used in order to determine the approximate geometric morphology of both the grain structure and the microtextured regions. Data from the EBSD experiments were used to calculate representative orientation distribution functions (ODFs) and grain size distributions for the α (HCP) crystallographic phase. Laguerre tessellations were employed to create idealized geometric representations of the microstructure and microtextured regions, while orientations were sampled from the experimentally derived ODFs. A highly parallelized crystal plasticity finite element framework was used to model the deformation response of single phase polycrystals under uniaxial tension, with attention paid to the intragrain slip system activity. Simulations were conducted with changes in the orientations within microtextured regions, as well as with various sets of initial slip system strengths to reflect differences in reported values in literature. Results were compared to a strength-to-stiffness parameter designed to predict succession of yield as a function of orientation. Presented are slip activity trends as a function of microstructure and initial slip system strengths, as well as results concerning the development of long-range localization of plasticity as a function of the microstructure. Predictions are compared to slip system activity measured by scanning electron microscope based digital image correlation. [ABSTRACT FROM AUTHOR]

Incipient slip localization in the vicinity of hundreds of grain boundary triple junctions (TJs) in a lightly deformed nickel-base superalloy IN718 is studied using a combination of three-dimensional (3D) crystal plasticity finite element (CPFE) modeling, high resolution digital image correlation (HR-DIC) and 3D electron back-scatter diffraction tomography (3D EBSD). A 3D reconstruction method enables identification of thousands of TJs and correspondence of any observed slip bands with their originating TJ lines below the specimen surface. We present a large-scale CPFE model of the experimental 3D microstructure composed of high-fidelity representation of the TJ lines and the boundaries and interiors of the parent grains and use it to calculate the local micromechanical response and slip activity of all TJs at the onset of macroscopic yielding. Statistical analysis of the calculated quantities reveal TJs develop larger stress concentration and grain-average re-orientation than grain interiors and grain boundaries, however no substantial differences in cumulative slip were found among these microstructural regions. We find that TJs with observed slip bands generate lower grain-average re-orientation, fewer active slip systems, and more localized slip on a single system than those without. The distinctions in the reorientation and slip activity are stronger in TJs that experience more intense slip. [Display omitted] [ABSTRACT FROM AUTHOR]

High resolution scanning electron microscope digital image correlation (SEM DIC) was performed in situ during uniaxial loading on Ti-6Al-4V rolled titanium plate to determine the dependence of strain localization on microstructure and microtexture. Individual grains with preferred orientation for basal slip exhibited plastic localization along basal planes before macroscopic yielding. With additional strain, but still below macroscopic yielding, pyramidal and prismatic plastic activity was observed as slip bands transmitting across many grains and entire microtextured regions (MTRs). The localization of long range plastic strain occurred within MTRs that allowed for slip transmission across grains with low angle boundaries. The rolled titanium plate material having strong [ 0001 ] and [ 10 1 ¯ 0 ] texture components showed pyramidal and prismatic type slip extending across entire MTRs at strains well below macroscopic yielding. These strain localization processes occur much earlier in straining and over lengthscales much longer than observed with conventional slip offset imaging. The implications for properties are discussed. [ABSTRACT FROM AUTHOR]

Damage development during tensile straining of a cast AlSi12Ni alloy has been characterized by in situ SEM imaging and synchrotron X-ray microtomography. Two main damage mechanisms consisting of intermetallic particle rupture and decohesion of Al-Si interfaces were revealed. Particle rupture takes place already from the beginning of the deformation, while decohesion of Al-Si interfaces is more characteristic at later stages. It is shown that damage evolution in the alloy is closely related to the underlying geometry of the intermetallic phase, its clustered structure playing a double role in controlling strain localization in the matrix as well as the rupture sequence of the intermetallic phase itself. The latter shows that small clusters break-off from the complex intermetallic network before large clusters, in good agreement with their load carrying capacity determined by the local volume fraction of the intermetallic phase. Finite element modeling of the alloy's tensile behavior evidences that deformation bands formed in the alloy with complex structure are similar to those developed in a model particle reinforced metal-matrix composite, where the particles correspond to the clustered regions of the intermetallic phase. [ABSTRACT FROM AUTHOR]

A phenomenological dislocation-based approach is proposed to account for the necking phenomenon during tensile deformation of metals and alloys. The critical strain corresponding to the onset of tensile instability is predicted in a simple explicit form based on the Kocks-Mecking dislocation kinetics approach. The model strongly suggests that uniform elongation is controlled primarily by the rate of dislocation recovery. The role of the stain rate sensitivity in stabilizing uniform plastic flow is also elucidated. Model predictions are found to be in excellent agreement with experimental data obtained for ultrafine grained 316L steel produced by severe plastic deformation. The approach presented provides general ques for designing materials with enhanced ductility, including ultrafine grained and bulk nanostructured metals and alloys. The proposed recipe is based on microstructural control of the rate of dynamic recovery of dislocation. [ABSTRACT FROM AUTHOR]

Using a combination of mechanical testing, scanning electron microscopy, and a unified crystal plasticity framework for discrete intragranular shear localization, we investigate intense, localized slip bands on prismatic planes and { 10 1 ¯ 2 } 〈 1 ¯ 011 〉 tensile twins, and their transmission across the grain boundaries (GBs) in commercially pure titanium. The analyses show that the orientation and curvature of the GB influence the local stress fields at the GBs, and consequently, the slip/twin transmission across the boundary. In addition to host grain properties, neighboring grain properties, such as active slip systems and instances of heterogeneity like slip bands and twins, heavily affect the deformation mechanisms in each grain. Finally, the applicability of geometric factors to predict the transmission in the experimentally observed co-located pairs is discussed. The local stress field calculated by the discrete slip and twin band model is shown to be capable of determining whether a transmission has occurred in an observed co-located pair, and also the direction of the transmission. [Display omitted] [ABSTRACT FROM AUTHOR]

Multi-phase titanium (Ti) alloys exhibit highly heterogeneous plastic deformation patterns. Understanding the micro-mechanisms of plastic deformation causing strain heterogeneity is thus of fundamental importance for enhanced manufacturability and in-service performance of Ti alloys. In this study, the critical microstructural factors responsible for microscopic strain localization are systematically investigated in an (α+β) Ti-Al-V-Fe-Si-O alloy by integrating the microstructural insights obtained from in situ microstructure-based digital image correlation (μ-DIC), in situ synchrotron X-ray diffraction (SXRD), and statistical analyses of strain localization incidents. The uniaxial tensile tests along the transverse direction reveal strain localization (i) across favorably oriented adjacent α grains (multi-grain strain localization) and (ii) along grain/phase boundaries (boundary strain localization), both of which suggest the strong influence of local crystallographic orientation on strain heterogeneity. The boundaries shared by soft and hard α grains are observed to be highly susceptible to strain localization, although ductile damage mechanisms are mostly delayed until the later stages of necking. While the majority of the (α+β) microstructure can co-deform to high strain levels without significant ductile damage evolution, micro-void nucleation and growth eventually initiate at the α/β phase interfaces, with work-hardening of the β phase. The insights provided through these in situ experiments highlight the importance of local texture in the strain localization and damage in (α+β) Ti alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

Different types of grain-boundary (GB) solute segregation in nanocrystalline alloys have been described in the literature as being either homogeneous or heterogeneous, which differently impacts their mechanical properties. In heterogeneous segregation, solute atoms tend to cluster along certain GB areas, while other GB regions remain solute-free. This phenomenon is evidenced with the segregation of Ni solute in nanocrystalline FCC Ag. Yet the physical origin for this segregation behavior and its existence in non-FCC alloys are not fully determined. Here, hybrid Monte-Carlo and molecular dynamics simulations were used to study the segregation of Ni solutes (4 at.%) into nanocrystalline FCC Ag, BCC Nb and HCP Zr metals at their solubility limit temperature and the same homologous temperature (0.405·T m), respectively. A range of segregation configurations was found: Fully heterogeneous segregation in Ag 96 Ni 4 at 500 K, homogeneous segregation with second-phase precipitates in Nb 96 Ni 4 at 1110 K, homogeneous segregation with dispersion of small-scale Ni clusters in Nb 96 Ni 4 at 1564 K and Zr 96 Ni 4 at 464 K, and fully homogeneous segregation with amorphous intergranular films in Zr 96 Ni 4 at 1118 K. We rationalize these changes by quantifying the Ni solute interactions at GBs. Furthermore, significant variations in mechanical behavior and associated plastic deformation mechanisms are shown for each alloy due to their different segregation behaviors. It is found that strain-localized shear banding is the most significant at interfaces with homogeneous solute segregation containing second-phase precipitates and absent with segregation leading to amorphous intergranular films. These findings underscore the importance of solute interactions in profoundly altering segregation and mechanical behavior in stable nanocrystalline alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

High strength metastable β titanium alloys are widely used in the aerospace industry for their superior mechanical properties, including an outstanding strength-to-weight ratio. Although prior studies have suggested a major influence of the elastic anisotropies of α and β phases on deformation, the understanding of the onset and the development of plasticity at the β grain scale is still limited. Strain localization was presently investigated in various Ti alloys with different microstructures. In-situ optical microscopy tensile tests were carried out in combination with digital image correlation and electron back-scattered diffraction in order to relate the local deformation behavior with the microstructure. The low directional modulus along the [001] direction of β phase was observed to generate high incompatibility stresses, which governs the onset of slip activity and strain localization. However, it was found to emerge differently depending on the presence of α phase. In α phase free microstructures, high incompatibility stresses across β grain boundaries triggers the onset of { 112 } 〈 111 〉 slip on the side exhibiting the highest stiffness. In α+β microstructures, β grains with a [001] direction aligned with the loading direction were found to experience an early onset of plastic deformation and subsequent strain localization. A mechanistic understanding was obtained through the simulation of micromechanical fields in α and β phases. The behavior of polycrystalline aggregates with explicit modeling of hierarchical dual phase microstructure inherent to β grains was studied within an anisotropic elasticity framework using fast-Fourier transforms-based simulations. An early onset of plastic deformation was found to occur in α phase embedded in [001] oriented β phase as compared to other β orientations due to stress partitioning driven by the stiffness difference. The effects of the Burgers orientation relationships, the β phase fraction and the selected set of single crystal elastic constants on stress heterogeneity were also examined and discussed. [Display omitted] [ABSTRACT FROM AUTHOR]

Damage during cycling loading of polycrystalline metallic alloys involves localized plastic straining at the scale of individual grains. To better understand damage accumulation processes and to build models for material behavior there is a need for quantitative assessment of the heterogeneous strain fields at the grain and even more microscopic scales. In the present study, a digital image correlation (DIC) approach has been developed to measure the strains at the grain level and at finer scales where plastic strain localization is manifested as physical slip bands. Strain fields have been measured in situ and ex situ on a René 88DT polycrystalline nickel-based superalloy to assess the grain-scale deformation processes during monotonic straining in tension and compression. DIC analysis and transmission electron microscopy demonstrate that slip occurs in a highly localized manner. The highest localized strains developed in slip bands that formed on {1 1 1} planes parallel to, and slightly offset from, annealing twins. Enhanced local straining below yield was observed during compression loading. The degree of strain concentration caused by slip bands impinging on grain boundaries was also analyzed. The results are compared to predictions of plasticity models. [ABSTRACT FROM AUTHOR]

The local strain measurement method using nanometer-scaled micro grids printed on the surface of a specimen by an electron lithography technique (the micro-grid method) has been established. Microscopic deformation behavior of the ferrite–bainite steels with different bainite volume fraction, 16% and 40% of bainite, was evaluated. Strain localization in the ferrite phase adjacent to the ferrite/bainite boundary was clearly observed and visualized. Highly strained regions expanded toward the inner region of the ferrite phase and connected each other with an increase of macroscopic strain. The existence of hard bainite phase plays an important role for inducing strain localization in the ferrite phase by plastic constraint in the boundary parallel to the tensile direction. In order to obtain further understanding of microscopic deformation behavior, finite element analysis using the representative volume element, which is expressed by the axisymmetric unit cell containing a hard phase surrounded by a soft phase matrix, was conducted. It was found that the macroscopic stress–strain behavior of ferrite–bainite steels was well simulated by the unit cell models. Strain concentration in the ferrite phase was highly enhanced for the ferrite-40% bainite steel, and this imposed higher internal stress in the bainite phase, resulting in higher strain hardening rate in the early stage of the deformation. However, smaller ferrite volume fraction of ferrite-40% bainite steel induced bainite plastic deformation in order to fulfill the macroscopic strain of the steel. Accordingly, strain hardening capacity of the ferrite-40% bainite steel was reduced to a significant degree, resulting in a smaller uniform elongation than the ferrite-16% bainite steel. [ABSTRACT FROM AUTHOR]

By means of surface mechanical rolling treatment (SMRT), a gradient nanostructured (GNS) surface layer was formed on AISI 316L stainless steel. The mean grain size is ∼30 nm in the topmost surface layer and increases with depth. Tension–compression fatigue measurements were performed on the SMRT sample under the stress-controlled mode. In comparison with the coarse-grained sample, the fatigue strength of the SMRT sample is significantly enhanced in both the low- and high- cycling fatigue regimes. Meanwhile, the fatigue ratio is evidently elevated with an increasing tensile strength in the SMRT sample. The initiation and growth of cracks, the cyclic deformation behaviors, as well as effects of strength and residual stresses, have been investigated to clarify the fatigue mechanism of the SMRT sample. The results emphasized the GNS surface layer enhances the fatigue property by suppressing the initiation of cracks and accommodating a remarkable cyclic plastic strain amplitude. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution in situ synchrotron X-ray laminography combined with digital volume correlation (DVC) is used to measure the damage and plastic strain fields ahead of a notch introduced within a 2198 Al–Cu–Li alloy sheet. Synchrotron laminography is a technique specifically developed for three-dimensional (3-D) imaging of laterally extended sheet specimens with micrometre resolution. DVC is carried out using the 3-D image contrast caused by iron-rich intermetallic particles present in the alloy. The alloy is recrystallized and tested in T8 artificial ageing condition involving relatively low work hardening. Inclined strain localization bands are shown to develop at ∼800μm from the notch prior to the onset of damage. Damage in this region results mainly from the nucleation of voids on micrometric intermetallic particles and occurs at a very late stage of deformation, followed by very limited void growth. The accumulation of strain in the slanted localization band is found to be a steady process, whereas the crack propagation is a sudden process. Standard 3-D FE calculations using either von Mises plasticity or Gurson’s model do not capture the plastic localization process. [Copyright &y& Elsevier]

The microstructural characteristics of the microband (MB) structure in an austenitic Fe-30Mn-6.5Al-0.3C (wt.%) low-density steel tensile deformed at -196 °C were investigated by combined electron channeling contrast imaging and electron backscatter diffraction. The nucleation mechanisms and grain-orientation dependence of the most relevant microstructural features of the MB structure, namely, morphology, internal dislocation configuration, and alignment were quantitatively analyzed on the main texture components, i.e. <111>//tensile axis and <001>//tensile axis directions. The present study shows several unrevealed microstructural characteristics of MBs in austenitic Fe-Mn-Al-C low-density steels deformed at cryogenic temperatures. The analysis of the interplay between the activated slip systems and the microstructural characteristics of the MB structure provides further insight into the nucleation mechanisms and the boundary alignment. Several in-grain and grain boundary-assisted MB nucleation mechanisms were identified. At low strain levels (ε = 0.3), MB formation is controlled by planar slip localization phenomena acting on closely spaced slip bands. The MB structure is localized in the grain interiors of areas containing intense plastic localization. At high strain levels (ε = 0.6), the MB structure only develops in grains that are not favorable oriented for deformation twinning, i.e. grains oriented close to <001>//TA directions. At this deformation stage, the deformation behavior becomes highly inhomogeneous due to the activation of an in-grain macroscopic localization phenomenon associated with moderated lattice rotations (∼ 3–5°). It leads to the formation of a complex deformation structure formed by macroscopic deformation bands containing arrays of crystallographic MBs. The analysis of the MB alignment reveals several interesting features such as the formation of non-crystallographic MBs aligned along (3 -1 5) and (3 5 -1) planes. Their formation can be explained in terms of the lattice rotations accommodated by the active slip systems predicted by Winther's model. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The creep fatigue behavior of Sn–Ag/Cu solder joints was investigated in this study. Low-cycle creep fatigue tests were conducted on an in situ tensile stage, the deformation morphologies were observed by scanning electron microscopy and the microstructure of the solder was characterized using an electron back-scattered diffraction system. The results show that the creep fatigue process can be divided into three stages: a strain hardening stage, a steady deformation stage and an accelerating fracture stage. During the initial few cycles the strain increases rapidly because the solder is soft. After strain hardening of the solder becomes saturated, the strain increases approximately linearly with increasing cycles, strain concentration occurs in the solder close to the solder/Cu6Sn5 interface and initial microcracks are generated. When the microcracks evolve into long cracks, the failure accelerates and the specimens fracture at the interface shortly thereafter. The creep fatigue strain is contributed by plastic deformation and creep of the solder. Grain subdivision occurs in the solder when the plastic strain reaches a threshold, and then grain rotation takes place in the newly formed grains to accommodate further straining. Dislocation climb is the major creep mechanism, and grain boundary sliding can also be promoted after subdivision. [Copyright &y& Elsevier]

Abstract: A Ni-base superalloy single crystal with γ/γ′ microstructure was tested in tension at room temperature along the 〈100〉, 〈110〉 and 〈111〉 directions and the evolution of the deformation bands was studied in these samples. It is demonstrated that the observation of the wavy deformation traces are associated with changes in the width of γ-channels and the falling off of the γ′-particles from the surface. The paths of the deformation bands were found to depend on the loading orientation and to be influenced by the probability of particle shearing as well as the relative orientation of the dendrites. At large strains where significant γ′-particle shearing occurs, the classical straight and thin slip bands along the {111} traces were observed. In this paper, the evolution of deformation bands are discussed in terms of plastic localization at microscopic and macroscopic levels and correlated with the non-Schmid effect and strain hardening rate in single crystal superalloys. [Copyright &y& Elsevier]

The process of crack initiation has been investigated in three widely used titanium alloys with different microstructures and loading conditions. Using low-cycle fatigue tests, a unique crack nucleation mechanism involving strain localization at (0001) twist boundaries has been identified. In order to constitute a potential crack initiation site, the twist boundary must experience a high resolved shear stress and a high normal stress. Crack initiation at these boundaries is most frequently associated with twist angles spanning the 10° - 20° range. Deformation prior to crack initiation at these rare microstructural configurations has been characterized using transmission electron microscopy and high-resolution digital image correlation across large fields of view. The (0001) twist boundaries are preferential locations for early and intense strain localization. Prior to crack nucleation, deformation proceeds via shear along such boundaries where no β layer at the interface was evidenced. The presently discussed crack formation mechanism is believed to be of broad relevance as it is not significantly influenced by microstructural parameters such as the α grain size, the degree of microtexture, the β phase fraction or the surrounding microstructure as well as α and β compositions. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] Multi-phase metallic materials exhibit significant levels of strain partitioning and localization when plastically deformed. Connecting these microstructural processes to macroscopic limits of uniform deformation, e.g., plastic instability and fracture, can reveal guidelines for damage-resistant microstructure design. This connection, however, is hindered due to the absence of strain partitioning and localization data from high strain levels, near and beyond necking. A parallel gap in our current understanding of the plasticity of multi-phase materials is regarding the spatial variations in strain path development at the micro-scale. Microscopic digital image correlation, the most powerful experimental method to provide such insights, is typically limited to low strain levels (< ~15%) due to crystal-plasticity- or damage-induced surface topography evolution. Here, we address these challenges by developing a practical method that relies on serial in-situ scanning electron microscope (SEM) mechanical tests of samples pre-strained to different levels. Applying this method to a dual-phase steel, we observe that ferrite and martensite on average exhibit a linear strain partitioning trend throughout the deformation. However, the highly-deformed ferrite regions (i.e., the top 4% most strained ferritic subsets) exhibit an exponential increase in the level of strain they accommodate, as well as sequential activation of the strain localization processes in different microscopic strain paths. Martensitic constituents play an important role in the strain localization processes and the resulting microscopic strain paths. [ABSTRACT FROM AUTHOR]

The equiaxed microstructure of Ti-6Al-4V contains nominally 10µm to 15µm α -phase grains. Depending on processing, these fine grains may aggregate into large, millimeter-scale regions of similar crystallographic orientation. These so-called microtextured regions are detrimental to quasi-static and fatigue properties. Their presence may facilitate the formation of long-range strain localization—bands of plastic strain that traverse grain boundaries and terminate at the boundaries of the microtextured region which compromises strength and ductility—and can cause early fatigue crack nucleation. Furthermore, the low angle boundaries within microtextured regions offer little resistance to crack growth, and corresponding increases in crack growth rates have been observed in these regions. Despite significant research into the effects of microtextured regions on macroscopic properties, there is still a lack of rigorous definition of what constitutes a microtextured region. To this end, we present a computational study in which deformation simulations are performed to study the effect of the intensity and character of microtexture on the localization of plastic strain. Mean orientation and intensity of microtexture are parameterized in a suite of simulations. Simulations indicate that strain localization is facilitated both by the presence of some amount of orientation spread within a microtextured region, and further by the intensity of microtexture in neighboring regions. Simulations are further discussed with respect to high resolution experimental measurements of intragrain strain in a microtextured Ti-6Al-4V specimen, which exhibit similar mechanical trends. [ABSTRACT FROM AUTHOR]