Decrease In Flow Stress Research Articles

Effect of Extrusion Ratio on Mechanical Behavior and Microstructure Evolution of 7003 Aluminum Alloy at High-Speed Impact.

The extrusion ratio (ER) is one of the most important factors affecting the service performance of aluminum profiles. In this study, the influence of ER on the mechanical behavior and microstructure evolution of 7003 aluminum alloy at high-speed impact with strain rates ranging from 700 s-1 to 1100 s-1 was investigated. The studied alloy with an ER of 56 formed coarse grain rings during the heat treatment. The microstructure of the alloys with ERs of 20 and 9 is relatively uniform. The results indicate that under high-speed impact, the mechanical response behavior of the 7003-T6 alloy with different ERs is different. For the alloy with an ER of 56, strain hardening is the main mechanism of plastic deformation. In contrast, a flow stress reduction occurs at middle deformation stage for the ones with ERs of 20 and 9 due to concentrated deformation, which is more significant in the alloy with an ER of 20. Under high-speed impact, the alloy with an ER of 56 undergoes uneven plastic deformation due to the presence of coarse grain rings. The deformation is mainly borne by the region of coarse grains near the edge, and the closer to the center, the smaller the deformation. The deformation of the alloys with ERs of 20 and 9 is relatively uniform, but exhibits localized concentrated deformation in the area near the edge. The significant plastic deformation within deformation band causes a local temperature rise, resulting in a slight decrease in flow stress after the peak. These results can provide reliable data support for the application of 7003 aluminum alloy in the vehicle body crash energy absorption structure.

Effect of grain size, temperature, and tensile strain rate on mechanical response of CoCrNi medium-entropy alloys

Grain boundary (GB) strengthening of metallic materials faces limitations as grain sizes are reduced to the nanoscale, primarily due to the transition from the strengthening to the softening effects. Understanding the intrinsic mechanisms behind such pronounced transitions is essential for optimizing the mechanical properties of nanocrystalline (NC) alloys. In this work, the critical grain size responsible for this transition is determined for NC CoCrNi medium-entropy alloys using molecular dynamics simulation, and the underlying mechanisms concerning varying temperatures and strain rates are systematically investigated for the samples with the critical grain size. The transition from Hall-Petch (HP) strengthening to Inverse Hall-Petch (IHP) softening is established by decreasing the average grain sizes from 21 nm to 3 nm. The critical grain size for the current alloys is estimated to be 12 nm, and the underlying deformation mechanisms are illustrated as follows: In the IHP regime, GB-mediated deformation becomes dominant, leading to softening. Conversely, in the HP regime, dislocation slip dominates the deformation mode, contributing to strengthening. It is found that an increase in temperature from 77 K to 1100 K leads to a decrease in the average flow stress and Young's modulus. Meanwhile, the deformation mechanisms change from dislocation slip (77K–700K) to GB-mediated deformation (700 K–1100 K). Furthermore, the underlying mechanisms correlating to the critical strain rate are uncovered. It can be attributed to the shift in deformation mechanisms from dislocation slip at relatively low strain rate regimes (5 × 107 s−1-2 × 109 s−1) to dislocation drag at relatively high strain rate regimes (2 × 109 s−1-5 × 109 s−1). Our atomistic insights into the tensile response of NC CoCrNi may provide a crucial basis for designing superior properties to meet the growing demands of advanced engineering fields.

Hydrogen contribution to low temperature embrittlement during PWR exposure of an alloy 82 weld metal

The embrittlement of alloy 82 welds exposed to a simulated pressurised water reactor (PWR) environment was analysed and compared with that observed for cathodically hydrogen-precharged specimens. Similarities in mechanical behaviour, i.e. the evolution of the constitutive equations with a decrease in flow stress compared with non-precharged specimens, and in the fracture modes, with brittle zones in the outer layer, enabled us to conclude that the embrittlement observed in simulated PWR environment corresponded to a two-step hydrogen-assisted cracking mechanism. The first step corresponded to hydrogen absorption into the surface layer leading to crack initiation, and the second step to hydrogen localisation ahead of the crack tip promoting crack propagation.

Mechanism of Electropulsing Treatment Technology for Flow Stress of Metal Material: A Review

Residual stress is caused by non–uniform deformation caused by non–uniform force, heat and composition, which is of great significance in engineering applications. It is assumed that the residual stress is always the upper limit of the elastic limit, so the reduction of the flow stress will reduce the residual elastic stress. It is particularly important to control the flow stress in metal materials. Compared with traditional methods, the use of electropulsing treatment (EPT) technology stands out due to its energy–efficient, highly effective, straightforward and pollution–free characteristics. However, there are different opinions about the mechanism of reducing flow stress through EPT due to the conflation of the effects from pulsed currents. Herein, a clear correlation is identified between induced stress levels and the application of pulsed electrical current. It was found that the decrease in flow stress is positively correlated with the current density and the duration of electrical contact and current action time. We first systematically and comprehensively summarize the influence mechanisms of EPT on dislocations, phase, textures and recrystallization. An analysis of Joule heating, electron wind effect, and thermal–induced stress within metal frameworks under the influence of pulsed currents was conducted. And the distribution of electric, thermal and stress fields under EPT are discussed in detail based on a finite element simulation (FES). Finally, some new insights into the issues and challenges of flow stress drops caused by EPT are proposed, which is critically important for advancing related mechanism research and the revision of theories and models.

Elucidating the dual effect of vanadium microalloying on hot deformation behavior of high nitrogen martensitic stainless steel

Precipitates always have a complex effect on the hot work hardening and recrystallization softening of materials. In this work, the dual effect of V-microalloying on hot deformation behavior and microstructure evolution of high nitrogen martensitic stainless steel (HNMSS) was investigated by hot compression experiments, modeling study and microstructure characterization. The results showed that V-microalloying enhanced the flow stresses and activation energy of 0.2V steel due to the fine grain strengthening effect and the blocking effect of dynamic precipitation on dislocation movement and grain boundary migration. On the other hand, the recrystallization model and deformed microstructure indicated the more intense dynamic recrystallization (DRX) softening effect in 0.2V steel, inducing the faster decrease of flow stress after the peak stress. V-microalloying promoted the dynamic precipitation of HNMSS during hot deformation, which reduced the size and increased the number of precipitates in 0.2V steel. The fine precipitates hindered dislocation movement, promoted dislocation accumulation and entanglement, formed a particle deformation zone (PDZ) with high dislocation density and large orientation gradient, and then induced the occurrence of particle-stimulated nucleation (PSN). In addition, the dislocation cells evolved from dislocation entanglement were also favorable nucleation sites for DRX.

Maintain sort order of grain boundary to investigate the deformation mechanism of CoCuFeNiPd high–entropy alloys

Molecular dynamics simulations of tension tests on CoCuFeNiPd high–entropy alloys (HEAs) are performed to study the effects of grain sizes (d), temperatures, and strain rates on the mechanical properties. After a series of tension simulations with different grain sizes, the Hall – Petch (H–P) and inverse Hall – Petch relationships are discovered. Hall–Petch breakdown is observed at grain size (d) of 10 nm. In the inverse Hall – Petch region, the average flow stress (σ(ε)) decreases from 1.97 to 1.76 (GPa) when grain size is decreased from 10 nm to 4 nm. On the contrary, the average flow stress increased from 1.90 to 1.97 (GPa) as d reduced from 18 nm to 10 nm. The microstructural evolution of the CoCuFeNiPd HEAs model exhibits that the primary deformation process in the H–P relation (d > 10 nm) is dislocation slips. The grain boundaries (GB) prevent dislocation expansion in the H– P relation, causing mechanical strengthening. Meanwhile, the main deformation mechanisms for the inverse H–P (d < 10 nm) are grain rotation and GB migration, decreasing flow stress. In addition, Young's modulus of the HEAs model increases with grain size (d). The yield strength, flow stress, and Young's modulus of HEAs specimens tend to reduce at high temperatures. Additionally, the number dislocation length reduces at high temperatures due to the amorphization phenomenon. The results also showed that Young's modulus and yield strength increase when strain rates rise.

Dislocation density-based modeling of the yield drop phenomenon in nickel-based single crystal superalloy

The phenomenon of yield drop, characterized by a decrease in flow stress after initial yield, has been observed in various nickel-based superalloys. Despite numerous proposed physical mechanisms, there is still a lack of a meso-mechanism-based constitutive model to explain this phenomenon. In this study, the tensile behavior of a nickel-based single crystal superalloy (DDX), was investigated at different strain rates and a temperature of 900 °C. It was observed that the yield drop phenomenon in DDX became more pronounced with increasing strain rate. To predict the yield drop phenomenon during tensile processing, an improved strength law based on continuum dislocation density theory was considered in the crystal plasticity framework. The proposed constitutive model was implemented using nonlinear iteration and incorporated into a finite element analysis software. The simulation results exhibited a good agreement between the experimental data and the stress–strain curve in the vicinity of the yield drop region, affirming the predictive aptitude of the proposed model in elucidating the yield drop phenomenon at various strain rates.

Investigation of thermal and mechanical effects during electrically-assisted compression of CoCrFeNiW0.5 high entropy alloy

Electrically-assisted manufacturing (EAM) has many advantages than ambient temperature manufacturing and thermoforming for the building of hard-to-deform alloys. The EAM technique of High entropy alloys (HEAs), a newer alloy category, is an issue worth of research, especially the deformation mechanism in the process of EAM. In the paper, the electrically-assisted compressive mechanical behavior of CoCrFeNiW0.5 HEA composed of face centered cubic phase and μ phase precipitate was studied under current density and strain rate region of 0–40 A/mm2 and 0.0005 s−1-0.1 s−1, respectively. At high current density and strain rate, the reduction of yield stress is remarkable. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques were utilized to analyze the effect of current density on the flow stress behaviors and microstructure evolution under plastic deformation. The results show that the introduced electric current enhances the movement and annihilation of dislocations as well as dynamic recovery, resulting in the continuous decrease of flow stress. Additionally, we can clearly observed that the dislocation-free ring or some very low dislocation density regions around the μ phase precipitates, that's because μ phase precipitate can lead to severe lattice distortion and thus increase the electron scattering, and then the local high temperature appeared around the defect regions due to local Joule heating effect, which is recognized as the principal reason leading to the significant decrease of mechanical properties during electrically-assisted compression.

On the consequences of intrinsic and extrinsic size effects on the mechanical response of nanoporous Au

In this study, the consequence of intrinsic and extrinsic size effects on mechanical responses of nanoporous gold is investigated via microcompression testing. By varying the micropillar diameter (D) between 1 µm and 20 µm and the ligament size (L), 50 nm and 350 nm, a critical ratio (α = D/L = 20) was found, above which the test structure can be considered a representative volume element, resulting in identical mechanical response and uniform deformation. Below that value, both flow stress and elastic modulus decrease with decreasing pillar diameter, as evidenced for a measurement series with a fixed ligament size of 350 nm where the flow stress decreased by more than 50% (from approximately 5 to 2.5 MPa) and the elastic modulus was reduced from approximately 0.5 GPa to almost zero. Stochastic behavior along with non-uniform deformation and failure is observed for α < 10, suggesting that the size of the load-bearing units in this material is about 10 times the corresponding ligament size.

Effects of intermediate annealing time on texture evolution during multi-pass cold-shear-straining of an Al–Mg–Si–Cu alloy

This study examines the effects of different intermediate annealing times during multi-pass cold-torsion tests on the microstructure in alloys of aluminum, magnesium, silicon, and copper (Al–Mg–Si–Cu). The mean flow stress, when the intermediate annealing time is shorter, increases until sufficient internal strain accumulates and then decreases rapidly. In contrast, the mean flow stress decreases continuously when the IA time is longer. X-ray line profile analysis revealed a rapid decrease in mean flow stress because of the high stored internal energy represented by the dislocation density. In addition, a short annealing time allows the B component to form in the initial stage of multi-pass torsion, and a random texture distribution occurs as the multi-pass torsion stage progresses. In contrast, a long annealing time can dissipate the texture distribution in a low-pass stage (low strain) and decreases the overall deformation resistance, despite the much stronger texture intensity in a high-strain area. Finally, the B texture component is consumed by the A component in the initial stages of the multi-pass cold torsion test. This is attributed to the different orientation relationships of the shear plane in the A and B components, which caused deviations in dislocation density.

Strain rate response and rheological characteristics of ZrCuNiAl bulk metallic glass in supercooled liquid region

In this study, the plastic deformation behavior of Zr55.7Cu22.4Ni7.2Al14.7 bulk metallic glass (BMG) in the supercooled liquid region (SLR) was studied by high temperature compression experiments at various strain rates. The results show that the high temperature plastic deformation behavior of the BMG in SLR is highly sensitive to the strain rate. With the decrease in strain rate, the steady-state flow stress and peak stress decreases, and the viscosity increases. Meanwhile the stress overshoot gradually disappears, the deformation of the BMG specimen changes from non-Newtonian flow to near-Newtonian flow, and there is a critical strain rate of 5 × 10−4 s−1. The surface deformation profile changes from an "S" shaped truncated cone to a true truncated cone. The plastic deformation of BMGs at room temperature is completed by the initiation and expansion of the shear bands, while the high temperature plastic deformation is caused by the generation and annihilation of free volume.

Microstructure and Mechanical Properties of Biomedical Ti-Zr-Nb-Ta-Sn High-Entropy Alloys

Ti(50-x)Zr38NbxTa8Sn4 high-entropy alloys with x = 0, 10, and 20 at.% were produced by vacuum arc melting in a high-purity argon atmosphere. The initial microstructures consisted of equiaxial bcc grains with sizes of 115 ± 30 µm, 250 ± 60 µm, and 280 ± 70 µm for the Ti30Nb20, Ti40Nb10, and Ti50Nb0 alloys, respectively. The Ti30Nb20 and Ti40Nb10 alloys showed untypical mechanical behavior with a short strain-hardening stage followed by a gradual decrease in flow stress after reaching the yield point. Although these two alloys had some inclination toward macroscopic strain localization, their tensile elongation was similar to that obtained in the Ti50Nb0 alloy, which had a more extended stage of uniform deformation. The differences were associated with distinct microstructures observed after deformation to fracture. The formation of dislocation bands and the activation of cross-slip at the microscale, as well as the appearance of kink bands at the mesoscale, can result in plastic instability. In contrast, a lamellar-like microstructure with parallel dislocation bands, such as the one observed in the Ti50Nb0 alloy, can ensure a more stable mechanical behavior. The developed alloys (Ti30Nb20 and Ti40Nb10) have properties that make them highly attractive for biomedical application due to a combination of very high yield strengths (1090 and 930 MPa, respectively), low Young’s moduli (~78 and ~69 GPa, respectively), reasonable ductility, and excellent biocompatibility.

Investigation of initial metallurgical factors on the dynamic impact response and adiabatic shear bands formation of the 6061 Al alloy

The effect of multi-pass friction stir processing and initial base metal condition on the dynamic impact response of AA6061-T6 alloy at high strain rates was investigated in this study. The Split-Hopkinson Pressure Bar (SHPB) was used in dynamic experiments with strain rates ranging from 1600 to 4200/s. The results revealed that the base metal alloy's (BM) dynamic flow stress was comparable to all the friction stir processed AA6061-T6 (FSP'd) specimens as the strain rate and true strain increased. The general increase in stress response levels as strain rates and true strain increases can be attributed to strain hardening. The higher values of strain rate sensitivity in multi-pass FSP'd specimens are an indication that they are more ductile in comparison to the BM condition. The optical and electron backscattered diffraction (EBSD) analysis showed that adiabatic shear bands were formed by heterogeneous deformation. The EBSD analysis revealed that the FSP'd specimens showed more abnormal grain growth and dynamic recrystallized sites than BM. The substantial decrease in dynamic flow stress, and high strain rate sensitive values of multi-pass FSP'd specimens compared to BM for all the investigated strain rates attributed to softening effect by dynamic recrystallization (DRX), and the emergence of abnormal grain growth during friction stir process.

Mechanical properties for a series of Zr-based bulk metallic glasses

It is an intriguing and urgent goal to quantitatively or semi-quantitatively unveil the correlation between the properties and the microstructure for metallic glasses. The relationship between the microstructure and mechanical property of three Zr-based bulk metallic glasses (BMGs) were investigated. The results show that three BMGs have similar XRD patterns, crystallization behavior and comparable relaxation enthalpy. The precipitated phases of three fully crystallized BMGs are Zr6Al2Ni-type and CuZr2-type phases. The content of CuZr2-type phase increases but that of Zr6Al2Ni-type phase decreases with increasing Zr content. A remarkable discrepancy in plastic flow behavior in compression and nanoindentation tests originates from the different contents of CuZr2-type cluster named as the geometrically unfavorable motifs (GUMs). The plastic flow stress and strain decrease while the plasticity increases with increasing fraction of CuZr2-type cluster. The fitting parameters from cumulative probability statistics are fingerprints for investigating the plastic flow and plasticity of the BMGs, and would be a valuable clue for developing the BMGs with good plasticity. The mechanism of correlation between the GUMs and the plasticity was also discussed. This work would provide a favorable route for discovering the quantitatively correlation between the GUMs and the plasticity in the BMGs.

Plastic deformation behavior of Ti45Nb in ultrasonic vibration-assisted compression

Ti45nb is a commonly used titanium alloy with high strength for aviation. However, it is mainly cold formed under conventional conditions and cracks often occur when deformation is large. Ultrasonic vibration-assisted compression test of Ti45Nb titanium alloy was carried out on a universal testing machine equipped with an ultrasonic vibration system. The influence of ultrasonic vibration on the deformation behavior of Ti45Nb was investigated. The results showed that ultrasonic vibration can significantly reduce the flow stress in the deformation process, and the reduction amplitude increases with the continuation of the compression process. The decrease in flow stress is caused by the stress superposition and acoustic softening effect. Compared to the conventional compression test, the grain in the center of the ultrasonic vibration-assisted compressed sample is finer, and the thickness of the lamellar substructure in the shear band is thinner. Therefore, ultrasonic vibration has a promoting effect on the deformation of Ti45Nb alloy.

The microstructure evolution and deformation mechanism in a casting AM80 magnesium alloy under ultra-high strain rate loading

Ultra-high strain rate impact tests were conducted by Split-Hopkinson pressure bar to investigate the microstructure evolution and impact deformation mechanism of a solution treated casting AM80 Mg alloy at 25, 150 and 250 °C with a strain rate of 5000 s −1 . The microcrack and dynamic recrystallization (DRX) preferentially nucleate at grain boundary (GB) and twin boundary (TB), especially at the intersections between GBs and TBs, and then propagate along twin direction. In contrast, the adiabatic shear bands preferentially occur at high-density twined regions. At 25 °C, the dominated deformation mechanisms are basal slip and twinning. As deformation temperature increases to 150 and 250 °C, the deformation gradually shifts to be dominated by a coordinated mechanism among non-basal slip, twinning and DRX. The flow stress behavior and deformation mechanism indicate that the degree of decrease in flow stress with temperature is associated with the change of deformation mode.

Effect of Functionalization of the Polycaprolactone Film Surface on the Mechanical and Biological Properties of the Film Itself.

The lack of suitable functional groups for cell adhesion on the surface of Polycaprolactone (PCL) is one of the main limitations in order to use PCL for biomedical applications. The aim of this research is to modify the PCL film surface using arginine, via an aminolysis reaction. In this regard, after PCL films formation by casting method, they were immersed in arginine solutions of various concentration at room temperature or then heated to 40 °C and in the presence of isopropanol or without it. To assess the structure of the modified surface, its wettability, and mechanical properties, methods of measuring the contact angle and the strip tensile test were used, and to compare the degree of attachment and the rate of cell proliferation, the method of fluorescent staining of cultured cells was used. The change in protein synthesis by cells on the modified surface was assessed using Western blotting. The results obtained show that the treatment of PCL films with an aqueous solution of arginine at room temperature for 1 day increases the hydrophilicity of the surface. Wherein surface modification led to a two-fold decrease of mechanical strength and flow stress, but elongation increase by about 30% for PCL films after modification in 0.5 M aqueous arginine solution at room temperature. Moreover, cell attachment and proliferation, as well as collagen synthesis, were significantly enhanced after arginine modification. The proposed simple and effective method for modifying PCL films with arginine significantly expands the possibilities for developing biocompatible scaffolds for tissue engineering.

Size effect on nonlinear unloading behavior and Bauschinger effect of Ni-based superalloy ultrathin sheet

• Cyclic deformation behavior of superalloy foil is represented by cyclic shear test. • Nonlinear unloading behavior at microscale becomes evident. • Bauschinger effect are affected by both geometric and grain size effect. • Size effect on instantaneous and permanent softening is revealed. • Work hardening stagnation is related to dislocation evolution and size effect. Superalloy ultrathin sheet is widely utilized in thin-walled components of aero-engine because of its excellent comprehensive performance. Nevertheless, it is hard to precisely predict the springback phenomenon during the plastic deformation of superalloy ultrathin sheet due to the size effect. To explore size effect on nonlinear unloading behavior and Bauschinger effect of Ni-based superalloy sheets, the uniaxial tensile, cyclic loading and unloading, and cyclic shearing tests were conducted on superalloy ultrathin sheets with diverse grain sizes and thicknesses of 0.2 and 0.25 mm. The experimental results demonstrated that the increasing grain size is accompanied with the decrease of flow stress of superalloy ultrathin sheet and the change of density of forest dislocation and moving dislocation. The dislocation annihilation rate of superalloy ultrathin sheet increases with decreasing t / d ratio, and the nonlinear unloading behavior becomes more obvious. Meanwhile, the degradation of elastic modulus becomes more evident with the increasing grain size. Driven by back stress, Bauschinger effect of superalloy ultrathin sheet becomes more apparent with larger t / d ratio. In addition, the fine-grained superalloy ultrathin sheet shows obvious earlier re-yielding and large instantaneous softening rates and the variation of t / d ratio also affect the permanent softening and work-hardening stagnation of superalloy ultrathin sheet.

Experimental Investigation on the Effect of Electrically Assisted Rapid Heating in Bulk Metal Forming

Many industries utilize hot metal forming to reduce forming forces and material flow issues. In this study, electrically assisted (EA) rapid heating is used in direct comparison to conventional heating methods. Three materials were investigated: AISI 4340, Al-7075-T6, and Ti-6Al-4V. Samples were heated at an approximate rate of 100 °C/s before applying compressive loads, simulating forging or other bulk deformation processes. Afterwards, samples were prepared for further microstructure evaluation. It was observed that high-rate temperature increase resulted in some developments in the microstructure and phases, as seen in AISI 4330, for example. Additionally, the effect of electricity was noticed to have a significant impact on flow stresses, as observed in many of the compression tests, immediately following the rapid heating process. Experiments and images from optical microscopy show that samples experienced drastic and immediate decrease in flow stress at all temperatures when using the EA rapid heating method versus the conventional method with seemingly no change to the material microstructure. This may be explained by the lack of phase changes at lower temperatures combined with the effect of electricity. Further investigation into the changes in micro-porosity, intermetallic phases, and changes in precipitates are suggested to supplement the current understanding of the drastic stress drop caused by passing electrical currents through metals.

The interrupted hot compression of an as-forged titanium matrix composite: flow softening behavior and deformation mechanism

In this paper, the deformation behavior and microstructure evolution of 2.5vol.% (TiBw + TiCp)-containing high-temperature titanium matrix composite (TMCs) was studied by the interrupted compression tests. The flow stress–strain curves of reloading of TMCs showed the flow softening behavior. The decrease of flow stress was associated with increasing holding time and deformation temperature or decreasing strain rate. The static softening mechanism was associated with the DRV, DRX and consume of dislocations during interrupted holding (SRV) reflected by the Q, n and microstructure. The fraction of αp phase decreased with increased holding time and deformation temperature or decreased strain rate. Due to the transformation of αp phase to β phase, the high-temperature deformation ability of the TMCs will be improved a lot, which is consistent with the decreased flow stress. For the reinforcements and αp phases, they played restricted role in the growth of α/β colonies. However, the decrease of αp phase made the grown up of α/β colonies in low strain rate. The deformation mechanism of the composites was cDRX caused by the transformation of LABs into HABs by progressive lattice rotation of α phase in (α+β) region and dDRX attributed to the nucleation and growth process of β grains in β region. Due to the addition of reinforcements, the weaken microtexture of composite can be obtained. After the interrupted compression, the optimal deformation parameter has been found for the TMCs, which can give guidance for the subsequent hot-working.