Heterogeneous Deformation Research Articles

ECCI analysis of shear accommodations at grain boundaries in commercially pure alpha titanium

This study uses a novel approach that couples electron channeling contrast imaging (ECCI) with controlled removal of surface material to investigate the slip system/grain boundary interactions by assessing the geometry of slip planes and grain boundaries as a function of depth. The comparison of spacing, relative distributions, and correlations of slip bands across grain boundaries reveals surface biasing on the slip band behavior in the grain boundary region. Furthermore, using analysis of the compatibility factor m’, the Schmid factor of each slip system, and the angles ϴ between slip planes, a more comprehensive understanding of the details of the mechanisms of strain transfer and heterogeneous deformation is developed. In general, the misalignment of primary slip systems on the grain boundary planes necessitates the activation of secondary accommodating slip systems close to the grain boundary area that are better aligned (higher m’ and lower ϴ) with respect to the incoming slip system across the grain boundary. Such systems have the same Burgers vector as the primary slip systems within the respective grains, but are typically on low mobility slip planes.

Hetero-deformation induced strengthening and toughening of pure iron with inverse and multi-gradient structures

Here an inverse gradient structure (IGS) is processed by induction heat treatment on ultrafine-grained substrate, and a multi-gradient structure (MGS) is gained by surface mechanically peening treatment on IGS material. The grain size distributions of coarse-ultrafine and nanostructure-coarse-ultrafine gradient from surface to interior are identified in IGS and MGS materials, respectively. Gradient structures maintain reasonable elongation and at the same time display 3–4 times higher yield strength as compared with homogeneous coarse-grained counterpart, indicating improved strength-ductility synergy. Moreover, it is found that the fracture of gradient structures depends largely on the high internal stress caused by the heterogeneous deformation of layers.

In situ EBSD study of deformation behavior of primary α phase in a bimodal Ti-6Al-4V alloy during uniaxial tensile tests

Uniaxial tension experiments and electron backscatter diffraction were performed on a bimodal Ti-6Al-4V alloy to study in situ the deformation behavior of primary hcp-Ti (αp). It was found that the strain could be accommodated by the activation of slip systems and by grain rotations. The prismatic slip was the primary slip mode of the αp. From the analysis of kernel average misorientation and geometrically necessary dislocation, it was shown that the dislocations mainly distributed in the vicinity of grain and sub-grain boundaries, and part of the dislocations distributed around slip lines. It was the dislocation activities that led to the formation of the low angle grain boundary and its transformation to the high angle grain boundary. It's important that tracking of deformation heterogeneities with significance to performance. By analyzing the rotation angle, average rotation rate, and rotation path of grains, it was shown that grain rotation heterogeneity occurred during the deformation. From the observation of the loading direction, grain rotation paths kept with the texture evolution direction of all αp. The grains activated in the basal slip gradually rotated to the ⟨101¯1⟩ pole and enhanced the intensity of the ⟨101¯1⟩ texture. Meanwhile, the grains activated in prismatic or 1st-order pyramidal slip rotated to the ⟨101¯0⟩ pole and enhanced the intensity of the ⟨101¯0⟩ texture. Grain rotation and texture evolution are related to mechanical properties.

Heterogeneous deformation of two-dimensional materials for emerging functionalities.

Atomically thin 2D materials exhibit strong intralayer covalent bonding and weak interlayer van der Waals interactions, offering unique high in-plane strength and out-of-plane flexibility. While atom-thick nature of 2D materials may cause uncontrolled intrinsic/extrinsic deformation in multiple length scales, it also provides new opportunities for exploring coupling between heterogeneous deformations and emerging functionalities in controllable and scalable ways for electronic, optical, and optoelectronic applications. In this review, we discuss (i) the mechanical characteristics of 2D materials, (ii) uncontrolled inherent deformation and extrinsic heterogeneity present in 2D materials, (iii) experimental strategies for controlled heterogeneous deformation of 2D materials, (iv) 3D structure-induced novel functionalities via crumple/wrinkle structure or kirigami structures, and (v) heterogeneous strain-induced emerging functionalities in exciton and phase engineering. Overall, heterogeneous deformation offers unique advantages for 2D materials research by enabling spatial tunability of 2D materials' interactions with photons, electrons, and molecules in a programmable and controlled manner.

Effect of cellular uniformity on the necking propagation of foam injection Molded PP/HDPE blend parts

AbstractNecking propagation (NP) is an important phenomenon that is facilitated by the superductility of many materials, although necking is often considered to be a type of unstable and heterogeneous deformation under tensile loading. In this study, the NP of samples produced with injection molding of polypropylene (PP) and high‐density polyethylene (HDPE) blends with or without a blowing agent was investigated by standard tensile testing, and the foam morphology and cellular structure were characterized with scanning electron microscopy. It was found that the foam parts fabricated with the PP/HDPE blends had the ability to be superductile under a low tensile test speed, which was attributed to the necking spreading throughout the tested gauge section stably and reliably. However, the NP of the foam PP/HDPE blend samples strongly depended on the distribution of the cellular foam. To investigate the relationship between the cellular structure and NP, a design of experiment approach based on the Taguchi method was used to fabricate many samples with different foam structures. The tensile test results of these samples suggested that the cellular uniformity, which was determined based on the statistical results of the fracture surface morphology, played an important role in the NP. The results suggested that there is a close relationship between the NP and the cellular uniformity. Using nonlinear fitting, it was straightforward to obtain a model between the NP length and cellular uniformity index for PP/HDPE molded foam parts.

Size dependent hidden serration behaviors of shear banding in metallic glass thin films

Serration usually observed in metallic glass (MG) is attributed to the shear band propagation and reflects the heterogeneous deformation in a large-length scale. However, the knowledge concerned small length scale heterogeneous deformation intrinsically related to shear transformation zones (STZ) in metallic glass is inadequate, for the difficulty to probe the variable in deformation process on small spatial scales. In this study, we investigated the small length scale hidden serration upon nanoindentation on metallic glass film. The hidden serration was attributed to STZs evolution, instead of shear band propagation. More heterogeneous structure which contains more STZs with larger volume, getting more possibilities to be activated and percolated, result in small-scale serration in the slope curve of load-displacement. The hidden serration patterns established a correlation between load-displacement curve and shear banding process down to microscopic level. It offers a deep understanding of heterogeneous structures dependent plasticity in metallic glasses.

High accuracy measurement of heterogeneous deformation field using spatial-temporal subset digital image correlation

Subset-based digital image correlation (DIC) has limitations in subset size selection for heterogeneous deformation field measurement. To maintain good spatial resolution, selecting sufficiently small subsets is necessary, but the measurement resolution is reduced because of poor matching information, inducing large errors in the correlation matching procedure. Spatial-temporal subset digital image correlation (STS-DIC) incorporates the temporal continuity of displacement in the deformation analysis. This can increase the matching information of subset in the temporaldimension. Here, the advantages of STS-DIC for the measurement of heterogeneous deformation fields were investigated through simulations and experiments. The results demonstrated that the measurement resolution was improved while ensuring a good spatial resolution for heterogeneous deformation field measurement by using STS-DIC. Finally, STS-DIC was applied to calculate the displacement field near a precast crack, and the stress intensity factors were calculated accurately from high accuracy displacement fields.

Experimental Investigation on Friction-Stir-Assisted Incremental Forming with Synchronous Bonding of Aluminum Alloy and Steel Sheets

A novel incremental sheet forming process, friction-stir-assisted incremental sheet forming with synchronous bonding (FS-ISF&SB) using aluminum alloy and steel sheets was developed. The influences of stacking sequence, wall angle, ratio of rotational speed to feeding rate and step down on the forming and bonding quality, geometric profile, interface morphology and thickness distribution were investigated through a series of experiments. Experimental results demonstrate that FS-ISF&SB is specifically suitable for the fabrication laminated sheet metal parts. The part with good forming and bonding quality can be obtained with the stacking sequence as of S-A, wall angle as of 40°, rotational speed as of 3000 RPM, feeding rate as of 750 mm/min and step down as of 0.4 mm, respectively. Based on the scoring of various failure modes, five factors are listed in descending order of significance in forming and bonding quality as wall angle, step down, stacking sequence, rotational speed and ratio of rotational speed to feeding rate. A special geometric feature named as “bump structure” reveals the heterogeneous deformation. Results also indicate that the thickness distribution of the formed part does not conform to sine law, and the deviation from design model increases with the increasing of wall angle.

The Role of Mesoscopic Deformation Heterogeneities in Plastic Flow and Recrystallization of a Magnesium Sheet Alloy

Unconventional rolling schemes characterized by high reductions and a low number of passes were employed to deform a new magnesium sheet alloy to large strains in order to obtain a heterogeneous deformation microstructure with large local variations in defect density. Microstructure heterogeneity was present in the form of large elongated grains, twinning, and shear banding. The rolled material was further deformed in tension at room temperature along different sheet directions to study the effect of such microstructure on plastic flow and planar anisotropy. Other rolled samples were subjected to annealing treatments before tensile testing to investigate the mechanisms of recrystallization in conjunction with deformation heterogeneity. Recrystallization chains were seen to form by stages of progressive subgrain rotation and subgrain growth giving rise to new grain boundaries capable of long-range migration during annealing. Unlike the as-rolled condition where planar anisotropy of the yield strength and maximum elongation was evident due to several reasons discussed in the study, the annealed condition with its homogeneous equiaxed grain structure and weak textures demonstrated an almost isotropic plastic flow behavior.

First-principles study of phase stability and temperature-dependent mechanical properties of (Cr, M)23C6 (M = Fe, Mo) phases

Although Cr-rich (Cr, M)23C6 (M = Fe, Mo) carbides as ineluctable precipitates dramatically affect the mechanical performances of advanced materials such as refractory steels and superalloys, their high-temperature material properties are difficult to be determined in experiments. In the present work, the first-principles method is applied to comprehensively investigate the phase stability and thermodynamic and temperature-dependent mechanical properties of Cr23-xMxC6 (M = Fe, Mo; x = 0–23). The results indicate that Cr23-xFexC6 presents thermodynamic phase stability under a broad occupying concentration of Fe (i.e., x = 1, 3, 9, 12, 13, 14, 15). Amongst which, the Cr8Fe15C6 with 4a, 8c and 48 h sites co-occupied by Fe is the most stable. Whereas Cr21Mo2C6 is the only stable phase among Cr23-xMoxC6. The mechanical properties of (Cr, M)23C6 are found to exhibit a strong doping composition-position dependence and a negative temperature dependence. Doping a low concentration of M leads to an increase of the elasticity of Cr23C6. The occupation of 4a or 8c sites alone plays an effective role in enhancing mechanical properties. The present work provides necessary information for understanding the heterogeneous deformation process and the fatigue crack initiation mechanism for alloys with the precipitate of M23C6 inclusions.

On how asymmetric stimulated rock volume in shales may impact casing integrity

AbstractMicroseismic data and production logs in our study area have confirmed asymmetric developments of the stimulation rock volume in shales with respect to the wellbore, while severe casing deformation problems have been reported frequently in this area. Here, we propose a systematic methodology to investigate the possibility of casing failure due to strong shear stresses induced by asymmetric stimulated zones. A mechanical earth modeling (MEM) is initially performed to determine the in situ stress field in the target layer before fracturing by incorporating the existing geological features, logging data, and rock anisotropy. Then, we provide a computationally cheap and efficient estimation for stimulated rock volume of each stage by considering the possible overlaps in adjacent stages based on the clustered microseismic clouds. Using this approach, a reservoir‐scale 3D coupled model tied to a more detailed near‐wellbore part incorporating the casing string and the cement sheath is established to simulate the development of stimulation zones, stress redistribution, and their impacts on casing deformation as each stage fracturing treatment chronologically goes on. Our numerical results indicate that continuous redistribution and re‐orientation of stress field near the borehole are tracked during pumping the treatment which reveals formation of some pockets of tensile stresses along and around the wellbore. Asymmetric stimulations are observed to generate strong shear stress on the suspended casing. These shear forces result in deflection and S‐shape deformations accompanied with cross‐sectional ovality. Some regions receive repeating treatments, which results in intensifying formation stress heterogeneity and worsen casing deformation severity. The calculation results are compared with measurement of multi‐finger imaging tool (MIT) to validate the accuracy. Our analysis has indicated that simply increasing the flexural strength by increasing thickness of casing cannot radically mitigate casing deformation problems. This paper presents a novel workflow for a coupled modeling of casing deformation during hydraulic fracturing operations, while current modeling efforts assume symmetric bi‐wing fracture geometries.

Structural and microstructural influence on deformation and fracture of dual-phase steels

The objective of this work is to demonstrate that the mechanical response of multiphase materials is fundamentally different in an imposed deformation field that is homogeneous, versus in an imposed deformation field that is heterogeneous, at a length-scale greater than the microstructural length-scale. To this end, we focus on two dual-phase steels with significantly different nominal chemical composition and microstructure. The mechanical response of the steels is characterized by in-situ SEM tensile tests of flat dog-bone and single-edge notch specimens. The experimental results show that the dog-bone specimens of the two steels exhibit very similar mechanical response but the mechanical response of their single-edge notch specimens differs significantly. This is in contrast to any classical analysis that will predict the same mechanical response in the presence of a notch for two materials that give the same mechanical response under uniaxial tension. The high resolution in-situ tests coupled with microstructure-based digital image correlation and finite element analysis are then used to elucidate how the interlacing of imposed heterogeneous deformation field and material microstructure affects the mechanical response of these steels. Our results clearly highlight that a mechanistic analysis of multiphase materials under imposed heterogeneous deformation field must involve explicit consideration of the length-scales associated with the material microstructure.

The τ-plot, a multicomponent 1-D pole figure plot, to quantify the heterogeneity of plastic deformation

An approach is presented that allows multi-scale characterisations of heterogeneous deformation in crystalline materials by employing a range of characterisation techniques including: electron backscatter diffraction, digital image correlation and neutron diffraction powder measurements. The approach will be used to obtain critical information about the variations in parameters that characterise the deformed state in different crystallographic orientation texture components of a sample in a statistically significant way. These parameters include lattice strains, texture evolution, peak broadening, dislocation density, planar faults, phase changes and surface strain. This approach allows verification of models of plastic deformation to provide a more detailed view of plastic deformation heterogeneity at multiple length scales than obtained by other characterisation approaches. The approach demonstrated here is applied to two stainless steel alloys; an alloy that exhibits phase transformation during deformation and an alloy that remains the same phase all through deformation process.

Heterogeneous sub-simple deformation in the Gol-e-Gohar shear zone (Zagros, SW Iran): insights from microstructural and crystal fabric analyses

Petrofabrics and microstructural characteristics of the quartz-rich mylonites were investigated in the NW–SE-trending Gol-e-Gohar shear zone with length of ca. 25 km within the Zagros Transpression Zone, SW Iran. The semi-quantitative vorticity analysis using quartz c-axis-fabric (Rxz/β method) and rigid porphyroclast geometry (Rigid Grain Net method) reveals that mean kinematic vorticity number (Wm) increases towards the Baghat Thrust Fault displaying the occurrence of spatial and temporal variations of flow during progressive deformation. The Wm values show pure shear-dominated flow progressively was changed to a simple shear-dominated flow toward to the late stages of ductile deformation, implying that the accelerating flow path was accompanied with progressive cooling and strain localization at the lower parts of the zone. The measured vertical thinning/dip-parallel elongation suggests that thrust emplacement and metamorphic rock extrusion likely occurred by a combination of simple shear with a pure shear component of deformation (i.e., general shear). The estimated deformation temperatures based on the quartz c-axis fabric patterns and their opening angles highlight greenschist-to-amphibolite-facies conditions.

Kinematics, deformation partitioning and late Variscan magmatism in the Agly massif, Eastern Pyrenees, France

In order to constrain the finite deformation pattern of the Variscan basement of the Agly massif, a detailed structural analysis over the whole Agly massif was performed. Our investigation combined geological mapping, reappraisal of published and unpublished data completed with our own structural work. Results are provided in the form of new tectonic maps and series of regional cross-sections through the Agly massif. At variance from previous studies, we identified three deformation fabrics named D1, D2 and D3. The D1 deformation is only relictual and characterized by a broadly northwest-southeast striking and eastward dipping foliation without any clear mineral and stretching lineation direction. D1 might be attributed to thickening of the Variscan crust in a possible orogenic plateau edge position. The D2 deformation is a heterogeneous non-coaxial deformation, affecting the whole massif, that produced a shallowly dipping S2 foliation, and an anastomosed network of C2 shear zones that accommodated vertical thinning and N20 directed extension. D2 is coeval with LP-HT metamorphism and plutonism at ca. 315–295 Ma. D2 corresponds to the extensional collapse of the partially molten orogenic crust in a global dextral strike-slip at the scale of the whole Variscan belt. The D2 fabrics are folded and steepened along a D3 east-west trending corridor, called Tournefort Deformation Zone (TDZ), where the Saint-Arnac and Tournefort intrusives and surrounding rocks share the same NE-SW to E-W subvertical S3 foliation. Along the D3 corridor, the asymmetrical schistosity pattern and kinematic criteria suggest a D3 dextral kinematics. The D3 deformation is a record of E-W striking dextral shearing that facilitated and localized the ascent and emplacement of the diorite and granitic sheet-shaped plutons. D3 outlasted D2 and turned compressional-dominated in response to the closure of the Ibero-Armorican arc in a transpressional regime. The progressive switch from D2 thinning to D3 transpression is attributed to the lessening of gravitational forces at an advanced stage of extensional collapse that became overcome by ongoing compressional tectonic forces at the southern edge of the Variscan orogenic plateau.

Microscopic strain localisation in WAAM Ti-6Al-4V during uniaxial tensile loading

Wire Arc-Based Additive manufacturing is a high deposition rate process suitable for building large-scale aerospace components. However, the larger heat source can cause greater microstructural heterogeneity and, in particular, a coarse columnar ß grain structure. The effect of the subsequent related transformation microstructure heterogeneity on the mechanical behaviour is investigated, in both standard WAAM materials and samples subjected to inter-pass rolling, which leads to substantial ß grain refinement and texture randomisation. Full-field strain maps were produced by digital image correlation, using tensile samples loaded in different orientations. When loaded normal to the columnar grain structure, it is shown that the coarse ß grains lead to a highly heterogeneous deformation distribution, which is linked the presence of dominant hard and soft α variants in texture colonies within each parent ß grain. ß grain refinement through the application of inter-pass rolling was found to be very effective at homogenising the strain localisation for all test orientations.

Numerical and experimental study of a 5754-aluminum alloy processed by heterogeneous repetitive corrugation and straightening

Repetitive corrugation and straightening (RCS) is a process that induces cyclic plastic deformation on sheet geometries, reporting promising improvements in mechanical properties in metals and alloys. Alternative die geometries, as well as the effect of the process over strain-hardenable alloys, have not been widely studied. In this work, sheets of the aluminum 5754 alloy were processed by RCS, using a novel die design to induce heterogeneous repetitive plastic deformation at room temperature. Numerical 3D Finite Element Analysis (FEA) was performed using the new die; the changes in the homogeneity of deformation, ductility and mechanical strength due to the process were studied. The processed samples were mechanically characterized by means of microhardness and tensile tests. Global and local XRD peak broadening were measured for determining the microstrain evolution during the process. After the initial heterogeneous RCS pass, a decrement in ductility was observed, but the increase in yield strength was significant. The microhardness mapping showed a heterogeneous distribution of the deformation with good agreement with the numerical simulation. XRD peaks width enlarged with the heterogeneous RCS passes in concordance with the mechanical results.

Effect of strain and strain rate on the development of deformation heterogeneity during tensile deformation of a solution annealed 304 LN austenitic stainless steel: An EBSD study

Evolution of the microstructure and geometrically necessary dislocation (GND) structure was studied during tensile deformation of a solution annealed 304 LN austenitic stainless steel. Microstructures of the steel at varying engineering strains and strain rates (i.e., 1x10-4 s-1, 1x10-3 s-1 and 1x10-2 s-1) were analyzed using electron back scatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) at ambient temperature. EBSD was used to quantify the evolution of the GND structure and the martensite formed during tensile straining to reveal the deformation mechanisms in the presence of microstructural heterogeneities. The average GND density and the amount of deformation induced martensite increased with increasing plastic strain at a faster rate than expected, following a concave up pattern that accelerated GND formation as strain increased. ECCI was used to examine the dislocation storage arrangements with plastic strain. Various strain rates were imposed on the steel specimens and the results show that the average GND density increased only slightly with increasing engineering strain rate. This substantiates the small decrease in strain rate sensitivity of the steel that was observed as the plastic strain increased. Therefore, the steel is tolerant to a change in the strain rate during forming. This study provides an understanding for the way in which the plastic deformation behavior of the steel is influenced by the evolution of GND density in the presence of microstructural heterogeneities, and by deformation induced martensitic transformations.

Integrating X-Ray Computed Tomography With Chemical Imaging to Quantify Mineral Re-crystallization From Granulite to Eclogite Metamorphism in the Western Italian Alps (Sesia-Lanzo Zone)

Metamorphic transformations and fabric evolution are the consequence of thermo-dynamic processes, lasting from thousands to millions of years. Relative mineral percentages, their grain size distribution, grain orientation, and grain boundary geometries are first-order parameters for dynamic modeling of metamorphic processes. To quantify these parameters, we propose a multidisciplinary approach integrating X-ray computed microtomography (µ-CT) with X-ray chemical mapping obtained from an Electron MicroProbe Analyzer (EMPA). We used a metapelitic granulite sample collected from the Alpine HP-LT metamorphic rocks of the Mt. Mucrone (Eclogitic Micaschists Complex, Sesia-Lanzo Zone, Western Alps, Italy). The heterogeneous Alpine deformation and metamorphism allowed the preservation of pre-Alpine structural and mineralogical features developed under granulite-facies conditions. The inferred granulitic mineral association is Grt + Bt + Sil + Pl + Qtz ± Ilm ± Kfs ± Wm. The subsequent pervasive static eclogite-facies re-equilibration occurred during the Alpine evolution. The inferred alpine mineral association is Wm + Omp ± Ky + Qtz+ Grt though local differences may occur, strongly controlled by chemistry of microdomains. X-ray µ-CT data extracted from on centimeter-sized samples have been analyzed to quantify the volumetric percentage and shape preferred orientation for each mineral phase. By combining tomographic phase separation with chemical variation and microstructures (i.e., different grain-size classes for the same phase and morphology of different pre-Alpine microdomains) the pre-Alpine mineralogical phases from the Alpine overprint have been distinguished and quantified. Moreover, the sample preserves 100% of the pre-Alpine granulite fabric, which surprisingly corresponds to less than 22% of the corresponding pre-Alpine metamorphic assemblages, while the Alpine eclogitic static assemblage corresponds to 78% though no new fabric is developed. This contribution demonstrates that the combined use of EMPA X-ray chemical mapping with the X-ray µ-CT shape analysis permits a dynamic approach to constrain the chemistry of the mineral phases linked to the development of metamorphic-related static and dynamic fabrics.

Analysis of Plastic Deformation Instabilities at Elevated Temperatures in Hot-Rolled Medium-Mn Steel.

The study addressed the microstructure and mechanical properties of hot-rolled advanced high-strength medium manganese steel. Some of the curves that were obtained in static tensile tests at deformation temperatures of 20–200 °C showed the occurrence of the heterogeneous plastic deformation phenomenon, called the Portevin-Le Chatelier (PLC) effect. The deformation temperature significantly influenced a serration character. The correlations between the deformation temperature, serration range, microstructural features, and fracture behavior were investigated. The curves showed no Lüders elongation as a result of the thermomechanical processing applied. The serrated flow phenomenon was observed at 60 and 140 °C. The serration type was different and the most enhanced at 140 °C, where the PLC effect was present in both uniform and post-uniform elongation ranges. The disappearance of serrations at 200 °C was related to the increased diffusion intensity.