Increase In Flow Stress Research Articles

Effects of temperature and frequency on cyclic deformation behaviours of Cu single crystals

ABSTRACT Temperature and frequency are significant extrinsic factors that influence the cyclic deformation behaviour of face-centred cubic crystal (fcc) single crystals. For Cu single crystals, an increase in temperature causes the disappearance of the plateau region in the cyclic stress–strain (CSS) curves and a significant decrease in the saturation resolved shear stress at low strain amplitudes. This is attributed to the enlargement of the space between Persistent Slip Band (PSB) ladders, resulting in a lower volume fraction of the rungs in PSBs, leading to a lower saturation shear stress. Conversely, an increase in frequency does not affect the appearance of the plateau behaviour in CSS curves, but it slightly increases the corresponding plateau stress. This is because a higher frequency increases the probability of interaction between edge dislocations, leading to an increase in the local flow stress in the rungs, ultimately resulting in an increment of the saturation stress.

Hot deformation behavior and dynamic recrystallization mechanism of Ti2ZrTa0.75 refractory complex concentrated alloy

Hot-deformation behavior and dynamic recrystallization (DRX) mechanism of Ti2ZrTa0.75 refractory complex concentrated alloy were systematically studied via uniaxial compression tests at various deformation temperatures from 800 °C to 1100 °C and different strain rates from 10−3 s−1 to 1 s−1. Stress-strain curves showed an increase in flow stress with decreasing deformation temperature and/or increasing strain rate, and there was continuous strain softening after the peak stress. Microstructure analysis based on electron backscattering diffraction (EBSD) indicated that deformation bands occurred in the hot deformation microstructure with a strain rate of 1 s−1, which is the appropriate location for DRX. Microstructure analysis revealed that DRX occurred during the hot deformation process and DRX fraction and grain size increase with increasing deformation temperature and/or decreasing strain rate. The grain boundary bulging nucleation mechanism and necklace-like structure of DRX grains demonstrate a typical discontinuous dynamic recrystallization (DDRX) process. Cumulative misorientation analysis of the initial grain interior and new grains formed without relying on the initial grain boundaries proved the occurrence of continuous dynamic recrystallization (CDRX).

Hydrogen-induced intergranular cracking of pure nickel under various strain rates and temperatures in gaseous hydrogen environment

Industrial-grade pure nickel, NI-201, was tensile tested in high-pressure hydrogen gas at various testing temperatures and strain rates. The degree of hydrogen embrittlement (HE) was strongly affected by the testing temperature and strain rate, and elevating the strain rate and lowering the temperature result in suppressed HE. While hydrogen trapped at GBs via local equilibrium dominated the HE degree in hydrogen-charged cases, hydrogen supply to GBs during the tests was required in gaseous hydrogen cases. Although hydrogen–dislocation interaction was one of candidates for the hydrogen supply, the lattice hydrogen, which can interact with dislocations, hardly existed and related behaviors such as increase in flow stress and serrated yielding were not observed in the gaseous hydrogen cases; therefore, the contribution of the hydrogen–dislocation interaction was speculated to be minor. Alternatively, we proposed an HE model considering hydrogen diffusion along grain boundaries (GBs) from the crack tip, which successfully demonstrated the experimental results.

Kink Modeling and Simulations Based on Field Theory of Multiscale Plasticity (FTMP) Part II: Implicit Kink Model and Scale-Free Treatment

We discuss not only kink formations but also the attendant primary strengthening taken place during the formation processes, by proposing anew an “Implicit” kink model based on Field Theory of Multiscale Plasticity (FTMP) only by utilizing the incompatibility tensor-incorporated Rank-1 degrees of freedom in the hardening law of the slip constitutive equation. Demonstrated is either realistically inhomogeneous kink field morphologies and successfully-raised flow stress level throughout the deformation, with the help of prescribed soft/hard structure accompanied by initial misorientations. A preliminary attempt is also made for identifying the critical roles of the empirically-found “scale-free” nature in the AE characteristics, based on the proposed model combined with the interaction field formalism in FTMP, resulting in further improved kink morphologies as well as slightly increased flow stress level.

Quantitative investigation on deformation mechanism and dynamic recrystallization during localized adiabatic shearing of single crystal copper

The adiabatic shearing of single crystals is realized by using large-sized columnar crystalline pure copper specimens prepared by directional solidification and designing hat-shaped sample to make the shearing region inside a grain under shock loading with split Hopkinson pressure bar (SHPB) in the present work. The pure copper hat-shaped specimens are loaded at strain rates of 6.5, 8.2, and 9.1 × 104s−1 respectively by SHPB, then the microstructure of sheared zone is investigated with optical microscopic (OM) as well as transmission electron microscopic (TEM) and its evolution process is quantitatively analyzed. The results showed that the flow stress increase with the increasing of strain rate, and the highest flow stress are 618.6, 460.6 and 410.3 MPa, respectively. There is no twinning at 6.5, 8.2 × 104 s−1 due to the highest flow stress cannot reach the twinning critical stress of 517.3 MPa. Although the highest flow stress reached 618.6 MPa that exceed the twinning critical stress, the force acting on the twinning dislocations and slipping dislocations could not satisfy the twinning condition Ftwin > Fslip due to the strong <001> texture of the directional solidification, so there is no twinning at 9.1 × 104 s−1. Slip is the dominant deformation mechanism at those three high strain rates. The large-sized dislocation cells at the edges of the ASB gradually transformed into elongated dislocation cells in the middle at strain rate of 6.5 × 104 s−1. There are equiaxed grains with size of 100–200 nm in the middle region of the ASB. The instantaneous grains refinement from 428 μm to 100–200 nm is the result of dynamic recrystallization of pure copper under its recrystallization temperature (542 K) caused by adiabatic shearing. The sub-grains were formed at 91 μs, and then changed from sub-grains to equiaxed recrystallized grains with size of 100–200 nm grains at 108 μs within the whole shearing time of 141 μs

Evaluation of hemodynamic significance for paraprosthetic fistula after transcatheter aortic valve implantation

Highlights. Using ECHO and MSCT data, a numerical assessment of hemodynamic effects of paraprosthetic regurgitation following transcatheter aortic valve replacement was performed. A significant increase in the fluid flow, wall and viscous shear stresses in the area of regurgitation is shown. The modeling technique described in the paper can be used prospectively in assessing the optimal treatment modality in terms of predicting the quantitative characteristics of the flow, associated with the risks of destruction of red blood cells and thrombosisAim. To make a numerical assessment of hemodynamic effects of paraprosthetic regurgitation following transcatheter aortic valve replacement based on retrospective clinical data.Methods. The study included echocardiography and multi-slice computed tomography data as input data for modeling one pulsation of a fluid similar in properties to blood. Reconstruction of the paraprosthetic fistula and the ascending aorta was performed in the Mimics medium (Materialise, Belgium). The obtained 3D models were processed in the Salome software (OPEN CASCADE SAS, France), after which they were exported to HELYX-OS (ENGYS, Great Britain) to build a finite element mesh. The flows were modeled using the OpenFOAM software package version 6 (The OpenFOAM Foundation Ltd, UK).Results. The simulation result, expressed quantitatively and qualitatively in the form of diagrams of the measured parameters – fluid flow velocities, wall and viscous shear stresses, shows a significant increase in indicators in the area of paraprosthetic regurgitation. Thus, the velocity in the affected area was 1.9–4.2 m/s, which is 3.8 higher than the average value in the entire computational area. The wall shear stress value was up to 61 Pa in the critical area, which may indicate an increased risk of thrombus formation due to the initiation of the clotting cascade through the von Willebrand factor. The value of viscous shear stress, the main component of the destruction of red blood cells in laminar flow, amounted to 20–26 Pa, which, in general, is not enough for mechanical hemolysis.Conclusion. The modeling technique described in the paper can be used prospectively in assessing the optimal treatment modality in terms of predicting the quantitative characteristics of the flow, associated with the risks of destruction of red blood cells and thrombosis.

Grain-boundary migration induced abnormally increased flow stress under large strain in a high-temperature compressed Ti-22Al-25Nb alloy

• Impeded grain boundary (GB) slip results in visibly increased flow stress. • The GB slip mechanism was impeded by zigzag GBs. • The zigzag GB result from the GB migration. • Energy difference between <001> and <111> fiber textures leads GB migration. • DDRX is a concomitant phenomenon with increased flow stress. The flow stress of Ti-22Al-25Nb alloy abnormally increased during uniaxial compression at large strains in the B2 phase field. This phenomenon is dependent on the influence of oriented grain boundary (GB) migration on GB sliding. In the early stages of deformation, straight and smooth GBs gradually evolved facilitating GB sliding, and the <001>/<111> double fiber texture has formed. There is a stored energy difference between <001> and <111> grains due to their different Taylor factors. Under low strain rates, the energy difference between the two fibers will drive GB to migrate, making the GBs zigzag and impeding GB sliding. Thus, greater stress is required to achieve further deformation, namely, the flow stress increased. The dislocation density in the shear band near the GB rapidly increased. Although the current alloy has a high stacking fault energy (SFE), the high dislocation density is sufficient to cause discontinuous dynamic recrystallization (DDRX).

Changes in miRNA expression in patients with peripheral arterial vascular disease during moderate- and vigorous-intensity physical activity

BackgroundWalking is the preferred therapy for peripheral arterial disease in early stage. An effect of walking exercise is the increase of blood flow and fluid shear stress, leading, triggered by arteriogenesis, to the formation of collateral blood vessels. Circulating micro-RNA may act as an important information transmitter in this process. We investigated the acute effects of a single bout of 1) aerobic walking with moderate intensity; and 2) anaerobic walking with vigorous intensity on miRNA parameters related to vascular collateral formation.MethodsTen (10) patients with peripheral arterial disease with claudication (age 72 ± 7 years) participated in this two-armed, randomized-balanced cross-over study. The intervention arms were single bouts of supervised walking training at (1) vigorous intensity on a treadmill up to volitional exhaustion and (2) moderate intensity with individual selected speed for a duration of 20 min. One week of washout was maintained between the arms. During each intervention, heart rate was continuously monitored. Acute effects on circulating miRNAs and lactate concentration were determined using pre- and post-intervention measurement comparisons.ResultsVigorous-intensity walking resulted in a higher heart rate (125 ± 21 bpm) than the moderate-intensity intervention (88 ± 9 bpm) (p < 0.05). Lactate concentration was increased after vigorous-intensity walking (p = 0.005; 3.3 ± 1.2 mmol/l), but not after moderate exercising (p > 0.05; 1.7 ± 0.6 mmol/l). The circulating levels of miR-142-5p and miR-424-5p were up-regulated after moderate-intensity (p < 0.05), but not after vigorous-intensity training (p > 0.05).ConclusionModerate-intensity walking seems to be more feasible than vigorous exercises to induce changes of blood flow and endurance training-related miRNAs in patients with peripheral arterial disease. Our data thus indicates that effect mechanisms might follow an optimal rather than a maximal dose response relation. Steady state walking without the necessity to reach exhaustion seems to be better suited as stimulus.

Dynamic tensile behavior of Al0.1CoCrFeNi high entropy alloy: Experiments, microstructure and modeling over a wide range of strain rates and temperatures

High entropy alloys (HEAs) are considered as new metallic materials with great application prospects due to their excellent mechanical properties. Massive experimental studies have revealed simultaneous strength-ductility enhancements in face-centered-cubic (FCC) HEAs resulting from the plasticity mechanisms transition under much harsher conditions. However, it still lacks quantitative understanding of such extraordinary mechanical behaviors. Aiming at precisely comprehending and describing the mechanical behaviors of FCC HEAs, a physical mechanism-based constitutive model is quite desirable to be established. As such, we first systematically investigated the uniaxial tension behavior of Al0.1CoCrFeNi HEA over wide strain rate (10−3-7 × 103s−1) and temperature (77–298K) ranges. Subsequent microstructural analyses indicated that its plasticity was dominated by dislocation slip at 298K or during small strain, while deformation twins acted as an additional plasticity mechanism with an increase of flow stress at higher strain rates or cryogenic temperatures. Moreover, combining theoretical calculations and experimental verification we further determined the critical twinning stress for this HEA. On basis of these experimental analyses, some specific models were employed to quantify the evolution of dislocation density or twin volume fraction during the plastic deformation. Finally, a physical-based constitutive model was developed to accurately describe the strain rate and/or temperature-dependent behaviors of this typical FCC HEA. This work not only helps quantitatively understand effects of various micro-mechanisms on the mechanical behaviors, but also provides the numerical simulation foundation for their future applications.

The electroplastic effect in titanium alloys under tension

The electroplastic effect (EPE) is a phenomenon which consists in a decrease in the strain resistance and enhancing of the plasticity of metals under the effect of the electric current of a sufficiently high density [1]. The goal of the study is to compare the deformation behavior of single-phase commercially pure tita­nium Grade 4 and two-phase VT6 alloy under tension and external heating with introduction of a pulsed current. Current of various pulse ratio and density was supplied to the grips of the tensile testing machine from a pulse generator. To estimate the relative contribution of the electroplastic effect during passage of current to the reduction of flow stresses, the materials were also exposed to external heating. The microstructure of the samples in the sample head and in the vicinity of the fracture region in the longitu­dinal section was studied using optical microscopy. The electroplastic effect in the studied materials is manifested on the tensile curve through individual jumps in the downward flow stress at a high pulse ra­tio, whereas at a low current pulse ratio a decrease in the flow stress and strain hardening and increase in the plasticity are observed. It is shown that tension of the sample under the effect of current results in a greater decrease in the flow stresses than that observed under external heating at the same temperature for both materials. This confirms the athermal nature of the pulsed current effect. The critical density of the high pulse ratio current (q - 5000) capable of providing manifestation of the electroplastic effect is two times lower for a VT6 alloy than for pure titanium Grade 4. Under the same pulsed current modes, the flow stresses for VT6 decrease more than for Grade 4. Pulsed current of high pulse ratio caused an anomalous hardening effect in a VT6 alloy, but the physical nature of this effect requires an additional study. The pulsed current modes used in the study did not lead to any structural changes noticeable under optical magnification in the samples under tension, except for the disappearance of twins and separation of the impurity particles in Grade 4 and spheroidization of grains in VT6.

Influences of grain size, temperature, and strain rate on mechanical properties of Al0.3CoCrFeNi high–entropy alloys

This study investigates the mechanical properties and deformation behaviours of Al0.3CoCrFeNi high-entropy alloys (HEA) under tension tests by using molecular dynamics (MD) simulations. The effect of different grain sizes (d), temperatures, and strain rates are examined. By varying the grain size from 4.0 nm to 20.0 nm, the shift from mechanical softening to strengthening was observed for the simulated samples. The result shows that the critical grain size of the inverse Hall – Petch and Hall – Petch regime is 12.0 nm. The average flow stress varies slightly increases from 3.02 to 3.19 GPa (5.3%) with the decrease of grain size from 20.0 to 12.0 nm in the Hall – Petch relation. While in the inverse Hall – Petch regime, the average flow stress significantly reduces from 3.19 to 2.7 GPa (15.3%) with grain size reducing from 12.0 to 4.0 nm. The migration of grain boundary (GB), and the rotation of grains are the dominant deformation mechanism in the inverse Hall – Petch regime. Meanwhile, in the Hall-Petch region, the dislocation activities are the main contribution to the deformation mechanism of polycrystalline. The GB plays as a barrier to hinder the dislocation propagation. Therefore, in the Hall - Petch regime, the fraction of GB increases as the grain size decreases, resulting in higher barrier densities for samples with small grain sizes and thereby causing the flow stress to increase. The result points out that Young's modulus of the HEA specimen increased with an increase of the average d. In addition, the result also shows that the ultimate stress, flow stress, and Young's modulus increase with rising the strain rate or decreasing the temperature.

Effect of Thermo-Mechanically Activated Precipitation on the Hot Deformation Behavior of High Strength Aluminum Alloy AA7075

The present study investigates the effect of two different microstructural conditions on the hot deformation behavior of precipitation-hardenable AA7075 by compression tests ranging from 200 °C to 350 °C and strain rates from 0.1 s−1 to 10 s−1. The first condition is solution heat-treated and quenched in water, whereas the second condition is achieved by subsequent artificial aging and stabilization for 24 h at the respective intended deformation temperature. Both conditions indicate an increase in flow stress with increasing strain rate and decreasing deformation temperature. Moreover, with increasing deformation temperature and decreasing strain rate, the flow behavior gradually changes as dynamic recrystallization becomes the dominant factor for the flow curve appearance. At the same deformation temperature, higher flow stresses are obtained for the as-quenched condition due to the dynamic precipitation and growth of very small precipitates (r &lt; 20 nm) during hot deformation. For the deformation temperature of 200 °C and the strain rate of 10 s−1, higher peak stresses of 110 MPa are obtained for the as-quenched condition. This is confirmed by the transmission electron microscopy investigations, which show the formation of very fine precipitates for the as-quenched condition, while coarse precipitates can be found in the stabilized microstructure. Despite this observation, the work hardening analysis reveals lower strain-hardening rates for the as-quenched condition and higher critical stresses for the onset of dynamic recrystallization compared to the thermally stabilized microstructure.

Shear-induced amorphization in nanocrystalline NiTi micropillars under large plastic deformation

Shear-induced amorphization is a phenomenon observed in polycrystalline NiTi shape memory alloys under severe plastic deformation, but the underlying mechanism of such a microstructural change remains elusive. To study the isothermal large-strain plastic deformation behavior and the associated mechanisms, uniaxial compression is performed on nanocrystalline NiTi cuboidal micropillars with initial grain sizes of 35 and 110 nm. It is found that the micropillars demonstrate high deformability with plastic strains up to 110% via shear-induced amorphization. It is shown that plastic strain localization in shear bands of the 35 nm-grain-size sample prompts notable amorphization up to an area fraction of 90% and crystal refinement down to 5 nm. High-resolution transmission electron microscope and molecular dynamics simulations reveal that the shear-induced amorphization starts from the local martensite phase near grain boundaries through accumulation of crystalline defects. The amorphization can lead to a reduction in the crystal size and an increase in the plastic flow stress. The large plastic deformation of the nanocrystalline NiTi micropillars via shear-induced amorphization is enabled by the suppression of crack nucleation through dynamic recovery of nanovoids. Our work provides new perspectives for producing crystalline-amorphous nanostructures in shape memory alloys at small scale.

High strain rate superplasticity and secondary strain hardening of Al-Mg-Sc-Zr alloy produced by friction stir processing

There have been many studies on superplasticity of Al-Mg-Sc-Zr alloy with middle magnesium content in friction stir processing (FSP), while few studies on superplasticity of FSPed Al-Mg-Sc-Zr alloy with low-magnesium were researched. In this work, the microstructure of Al-3Mg-0.1Sc-0.1Zr alloy with the grain size of ~1.6 µm and high-angle grain boundaries dominated was obtained by FSP. The high-temperature tensile test was carried out, which showed that Al-3Mg-0.1Sc-0.1Zr alloy had good superplasticity in the range of 400 ℃ ~ 500 ℃ and 3 × 10−3 s−1 ~ 3 × 10−1 s−1, and the optimal elongation reached 1850% at 475 ℃ and 10−2 s−1, showing a high strain rate superplasticity. The FSPed alloy had a significant secondary strain hardening ability with increasing flow stress in the failure fracture stage at higher temperature or relatively low strain rate, showing “H” type (continuous hardening) and “C” type + secondary hardening type (initial hardening then softening + secondary hardening) curves, which was mainly attributed to the concurrent grain growth at higher strain during high-temperature tension. By studying the microstructure evolution and constitutive equation, it was found that the main mechanism of superplastic deformation was grain boundary sliding accommodated by grain boundary diffusion, the coordination mechanism was lattice diffusion. Al3(Sc, Zr) particles hindered the coarsening of grains obviously with good thermal stability, which were conducive to the realization of excellent superplastic properties.

The acute effects of passive heating on endothelial function, muscle microvascular oxygen delivery, and expression of serum HSP90α.

Passive heating has been a therapeutic tool used to elevate core temperature and induce increases in cardiac output, blood flow, and shear stress. We aimed to determine the effects of a single bout of passive heating on endothelial function and serum heat shock protein 90α (HSP90α) levels in young, healthy subjects. 8 healthy subjects were recruited to participate in one bout of whole-body passive heating via immersion in a 40°C hot tub to maintain a 1°C increase in rectal temperature for 60min. Twenty-four hours after heating, shear-rate corrected endothelium-dependent dilation increased (pre: 0.004±0.002%SRAUC; post: 0.006±0.003%SRAUC; p=0.034) but serum [HSP90α] was not changed (pre: 36.7±10.3ng/mL; post: 40.6±15.9ng/mL; p=0.39). Neither resting muscle O2 utilization (pre: 0.17±0.11mL O2 min-1 (100g)-1; post: 0.14±0.09mL O2 min-1 (100g)-1); p=0.28) nor mean arterial pressure (pre: 74±11mmHg; post: 73±11mmHg; p=0.79) were influenced by the heating intervention. Finally, time to peak after cuff release was significantly delayed for % O2 sat (TTPpre=39±8.9s and TTPpost=43.5±8.2s; p=0.007) and deoxy-[heme] (TTPpre=41.3±18.1s and TTPpost=51.4±16.3s; p=0.018), with no effect on oxy-[heme] (p=0.19) and total-[heme] (p=0.41). One bout of passive heating improved endothelium-dependent dilation 24h later in young, healthy subjects. This data suggests that passive heat treatments may provide a simple intervention for improving vascular health.

Simultaneous Twinning and Microband-Induced Plasticity of a Compositionally Complex Alloy with Interstitial Carbon at Cryogenic Temperatures

Intermediate to low stacking fault energy (SFE) high entropy alloys (HEA) have shown an excellent combination of strength and ductility as a result of deformation twinning and martensite transformation. However, even in the absence of these mechanisms HEA can show a good strength-ductility combination, as is the case with non-equiatomic (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1. The room temperature mechanical behavior of this alloy has been associated with Taylor lattice and microband formation. The current research focuses on tensile cryogenic deformation of this alloy and investigates if these features and/or alternate mechanisms like deformation twinning are obtained. Surprisingly, it is not one or the other but both deformation twinning and microband formation that are observed during cryogenic deformation. The activation of both deformation mechanisms is a combination that is not often reported as the former is generally associated with intermediate to low SFE alloys and the latter with intermediate to high SFE alloys. The activation of twinning in (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1 is attributed to the high yield stress-temperature variation, as a result of solid solution strengthening being far greater than in other commonly researched compositionally complex alloys. A ductility retention down to 4 K was observed, while simultaneously showing a significant increase in flow stress. Despite the intermediate to high SFE deformation behavior, (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1 exhibits excellent cryogenic strength-ductility combination.

On the fatigue ductile-brittle transition of EA4T steel for high-speed railway axle

EA4T is a widely used high-speed railway axle steel with BCC crystal structure. It has the characteristics of ductile-brittle transition under single impact load, but whether it has the ductile-brittle transition under cyclic load has not been investigated. For this reason, the rotating bending fatigue tests (stress ratio of R = −1) for EA4T steel were carried out at RT, −40 °C, −80 °C and −120 °C, respectively. For all temperatures tested, except −120 °C, the fatigue strength increases with the drop of temperature. When the temperature is −120 °C, the fatigue strength at lower stress level was higher than that at −80 °C, whereas, the fatigue strength at higher stress level was lower than that at −80 °C. Based on the above, the mechanism of low temperature strengthening of fatigue performance was explained. When the temperature was higher than TFDBT (fatigue ductile-brittle transition temperature), lower temperature leads to increased flow stress, and then, the higher fatigue performance. When the temperature was lower than TFDBT, the longitudinal tensile stress near the crack-tip increased and the location of the maximum normal stress (σyymax) was closer to the crack tip with the drop of temperature, resulting in σyy⩾σf was satisfied at the characteristic distance, and then, cleavage fracture occurred. In addition, the FATT50 (Fracture Appearance Transition Temperature) of EA4T was −35.46 °C determined by impact test, which was higher than the TFDBT. Because the strain rate under impact load is higher, which results in higher yield stress and strain hardening exponent under impact load.

Constraining kinematic and temporal evolution of a normal-sense shear zone: Insights into the Simplon Shear Zone (Western Alps)

The exhumation of the Lepontine Dome in the Central Alps was mainly driven by extensional shear zones at its borders. The Simplon Shear Zone (SSZ), formed as a consequence of east-west lateral extrusion perpendicular to north-south convergence between Adria and Europa plates, has been the leading structure in the exhumation of the western sector of the Lepontine Dome where the deepest rocks of Central Alps are nowadays exposed. We present here a multidisciplinary study of the SSZ combining fieldwork, microstructural analyses, vorticity estimates, quartz c-axis fabric analysis, quartz paleopiezometry and 40Ar/39Ar geochronology. The SSZ evolved from epidote-amphibolite to greenschist facies and then brittle conditions during shearing. A decrease of simple shear component from 88% to 37% towards the top of the shear zone is observed, with mylonites displaying ages within the 12-8 Ma time interval. Differential stress (59–78 MPa) and strain rate (10−11-10−12 s−1) estimates are in agreement with values obtained for crustal-scale low-angle normal faults developed at medium to shallow crustal levels.Our multiscale and multidisciplinary approach points out that the SSZ experienced a complex evolution, with shear strain heterogeneously distributed across the shear zone in the frame of a decrease of the simple shear component and increase of the differential flow stress toward the top of the shear zone.

Electroplastic Effects on the Mechanical Responses and Deformation Mechanisms of AZ31 Mg Foils.

Electrical-assisted (EA) forming technology is a promising technology to improve the formability of hard-deformable materials, such as Mg alloys. Herein, EA micro tensile tests and various microstructure characterizations were conducted to study the electroplastic effect (EPE) and size effect on the mechanical responses, deformation mechanisms, and fracture characteristics of AZ31 Mg foils. With the assistance of electric currents, the ductility of the foils was significantly improved, the size effects caused by grain size and sample thickness were weakened, and the sigmoidal shape of the flow stress curves during the early deformation stage became less obvious. The EBSD characterization results showed that the shape change of the flow stress curves was due to the EPE suppressing the activation of extension twinning at the early deformation stage, especially for the coarse grain samples. The suppression of extension twinning resulted in a quick increase in flow stress due to the dislocation-dominant work hardening, and the increased flow stress eventually promoted extensive deformation twins at large deformation. Thus, as the sample strained to 10% tensile deformation, the EA-tested samples showed a larger volume fraction of deformation twins than the non-EA samples. The reference orientation deviation analysis verified that the deformation twins in the EA samples were formed in the large deformation stage. Combined with the fractography, the EPE also improved the ductility by suppressing the expansion of cleavage surfaces.

Molecular dynamics study on the strengthening mechanisms of Cr–Fe–Co–Ni high-entropy alloys based on the generalized stacking fault energy

The correlation between the generalized stacking fault energy and the strengthening mechanisms of Cr–Fe–Co–Ni high-entropy alloys during uniaxial tensile deformation is investigated using molecular dynamics simulations. An increase in the Cr and Fe content decreases the generalized stacking fault energy, while an opposite trend is observed for the content of Co and Ni. A linear correlation is identified between the unstable stacking fault energy and the yield stress. Alloys with a lower Fe content and higher Co and Ni contents show a higher unstable stacking fault energy, and hence higher yield stress. A high density of Lomer–Cottrell locks or deformation twins leads to higher flow stress. The deformation twins play a more important role in enhancing the flow stress as compared to sessile dislocations. Additionally, high unstable stacking fault energy is required to stabilize the defects formed and contribute to increasing flow stress. For a low unstable stacking fault energy, the plastic deformation progresses at lower stress and a bidirectional defect transformation is observed. The results agree with recent experiments and serve as guidelines for how the composition can be used to enhance the yield strength and the efficiency of strengthening mechanisms in Cr–Fe–Co–Ni high-entropy alloys.