Strain Aging Research Articles

A Novel Two-Stage Heat Treatment with Medium-Temperature Aging Influence on Microstructure, Al3(Sc, Zr) Nanoprecipitation, and Application Properties, Enhancing Selective Laser Melting of Al-Mg-Sc-Zr Alloy.

Al–Mg–Sc–Zr alloy fabricated through selective laser melting (SLM) is an additive manufacturing alloy with promising industrial potential. In this study, as-printed specimens were subjected to either single-stage or two-stage heat treatment processes to investigate the effect of temperature from room temperature to high temperature on the specimens’ tensile and fatigue properties to establish a reliable reference for aerospace applications. The tensile test results indicated that the heat treatment contributed to determine the properties of the nanoprecipitate Al3(Sc, Zr) with a strengthening phase, improving tensile strength. Moreover, the dynamics strain aging (DSA) effect vanished as temperature increased. It is noteworthy that the nanoprecipitation was distributed at the boundary of the melting pool after single-stage heat treatment with the highest tensile properties in all tests. In addition, the microstructure observed after the two-stage heat treatment indicated a melting pool interface decomposition, and the nanoprecipitation was homogeneously scattered over the Al matrix, increasing strength and further delaying fatigue crack transmission. Those features build a high-fatigue-resistance foundation. TEM analysis also confirmed the promotion of Sc thermal diffusion and an Al3(Sc, Zr) precipitation transformation mechanism under two-stage heat treatment, corresponding to aforementioned inferences. The SLM Al–Mg–Sc–Zr alloy with two-stage heat treatment brings about balance between tensile properties and fatigue resistance, providing new insight into additive manufacturing with Al alloys.

Investigation of Static Strain Aging Behaviour of AISI 304 Austenitic Stainless Steel

In this study, the change in the material's mechanical properties by the static strain ageing process after the pre-deformation applied to AISI 304 austenitic stainless steel was investigated and analysed. The samples were first applied to 5% pre-deformation and then annealed in the oven at 150°C, 250°C, 400°C and 550°C in separate groups for 30 and 60 minutes. The data obtained by performing hardness and tensile tests after static strain ageing were compared, and the interaction between test parameters and these parameters was investigated by performing Pareto analysis. As a result of the study, the highest tensile strength and hardness values were observed in the samples applied static strain ageing for 60 minutes at 150°C. The yield and tensile strengths of the as-received material increased by 10% with the ageing process. It was determined that these values decreased from 400°C. In addition, as a result of Pareto analysis, it was determined that the temperature value is the most critical parameter affecting the results.

Temperature-dependent cyclic plastic deformation of U75VG rail steel: Experiments and simulations

To reveal the temperature-dependent cyclic plastic deformation of U75VG steel, the monotonic tension, strain- and stress-controlled cyclic deformation experiments are carried out at different temperatures. The results show that the U75VG rail steel shows the cyclic softening, cyclic hardening and cyclic softening at 278 K, 573 K and 873 K. Moreover, the U75VG rail steel exhibits obvious ratcheting behavior at differ-ent temperatures, and the ratcheting strain depends on the applied mean stress and stress amplitude. The ratcheting strain is restrained at 573 K due to the influence of dynamic strain aging, while it rapidly increases at 873 K due to increasing the viscos-ity at high temperatures. A temperature-dependent cyclic plastic model is constructed to consider the influence of dynamic strain aging on the cyclic plastic deformation. By comparing the simulation and experimental results, the prediction ability of the proposed model for U75VG rail steel under different temperatures is verified.

Comparison of the Internal Fatigue Crack Initiation and Propagation Behavior of a Quenched and Tempered Steel with and without a Thermomechanical Treatment

Previous studies have shown that a thermomechanical treatment (TMT) consisting of cyclic plastic deformation in the temperature range of dynamic strain aging can increase the fatigue limit of quenched and tempered steels by strengthening the microstructure around non-metallic inclusions. This study considers the influence of a TMT on the shape, size and position of crack-initiating inclusions as well as on the internal crack propagation behavior. For this, high cycle fatigue tests on specimens with and without TMT were performed at room temperature at a constant stress amplitude. The TMT increased the average lifetime by about 40%, while there was no effect of the TMT on the form or size of critical inclusions. Surprisingly, no correlation between inclusion size and lifetime could be found for both specimen types. There is also no correlation between inclusion depth and lifetime, which means that the crack propagation stage covers only a small portion of the overall lifetime. The average depth of critical inclusions is considerably higher for TMT specimens indicating that the strengthening effect of the TMT is more pronounced for near-surface inclusions. Fisheye fracture surfaces around the critical inclusions could be found on all tested specimens. With increasing fisheye size, a transition from a smooth to a rather rough and wavy fracture surface could be observed for both specimen types.

Influence of Dynamic Strain Aging on Ratcheting Deformation Behavior of SA333 Gr-6 Steel

Influence of Dynamic Strain Aging on Ratcheting Deformation Behavior of SA333 Gr-6 Steel

Characterization of strain rate sensitivity of 7075 aluminum alloy at different solution temperatures by novel kinetic models

The strain rate sensitivity (SRS) of Al-Zn-Mg-Cu aluminum alloy under the effect of dynamic strain aging (DSA) was investigated in this paper by tensile tests at room temperature after different solution treatments. The experimental results show that for the samples after solution treatments, with the increase of strain rate, the flow stress fluctuates in waveform. The dynamic precipitation and the redistribution of solutes around dislocations due to saturation of solute atoms under DSA were described by interaction between solutes and dislocations in different strain rate ranges. The competition mechanism between DSA stress and dynamic precipitation stress was then put forward and discussed in detail. Finally, a set of kinetics models combined DSA and dynamic precipitation were firstly established, in which the concentration variation of solute atoms in matrix during DSA was taken into account to analyze the coordination between solutes and dislocations, and the mechanism of the dynamic precipitation.

A New Strategy for Restraining Dynamic Strain Aging in GH4169 Alloy During Tensile Deformation at High Temperature

A New Strategy for Restraining Dynamic Strain Aging in GH4169 Alloy During Tensile Deformation at High Temperature

Suppression of serrated flow in medium Mn steel under pulsed electric current

Based on the electroplasticity effect and dynamic strain aging interpretations, serrated flow under the influence of electroplasticity in Fe-0.23C-9.85Mn-2.15Al (wt.%) medium-Mn steel was investigated. Tensile tests under different electropulsing parameters were performed. When the pulsed current increased from 0 A to 100 A, the critical strain of serrated flows increased from 10.6% to 24.6% and the number of serrated flows was decreased from 35 to 0. The suppression effect is the best at the current parameter of 100 A-100 Hz-100 μs. With the decrease of dislocation waiting time and the increase of critical strain, the continuity of plastic deformation stage can be extended by the action of electron wind force. Meanwhile, the dislocation density that affects the plastic instability due to the interaction of mobile dislocations with obstacles is greatly reduced, which plays a dominant role in suppressing Portevin-Le Chatelier effect. Through the experimental results of pulsed current suppression of Portevin-Le Chatelier effect in medium Mn steel, the mathematical model of current density and dislocation waiting time is established. It is found that the current density is proportionate to the reduction of dislocation waiting time.

Hot deformation behavior and microstructure evolution of non-equimolar Ti2ZrHfV0.5Ta0.2 refractory high-entropy alloy

This work aims to elucidate the hot deformation behavior and microstructure evolution of a non-equimolar TiZrHf-rich refractory high-entropy alloy (RHEA) with a composition of Ti2ZrHfV0.5Ta0.2 (at. %). The as-cast Ti2ZrHfV0.5Ta0.2 alloy composed solely of a body-centered cubic phase showed superior tensile mechanical properties with a yield strength of 865 MPa and a total elongation of 22.5%. The uniaxial compression tests were performed at temperatures ranging from 900 to 1100 °C and strain rates varying from 10−3 to 10−1 s−1. The relation among the flow stress, deformation temperature, and strain rate was represented by the Arrhenius-type constitutive equations. The calculations show a comparatively low apparent activation energy of 144–119 kJ/mol over the entire range of strains, revealing a noticeable contradiction between the concentration of group IV elements (Ti, Zr, Hf) and the high-temperature strength. The excessive addition of Group IV elements tends to deteriorate the high-temperature strength due to the relatively low melting points and self-diffusion activation energies of Group IV elements. The misorientation analysis based on electron back-scattered diffraction indicated that the dynamic recrystallization mechanism depends strongly on the thermo-mechanical processing conditions. A low deformation temperature and high strain rate results in an abnormal discontinuous dynamic recrystallization mechanism. Additionally, the dominant role played by the edge dislocation in RHEAs during deformation is supposed to be the reason for the abnormal discontinuous dynamic recrystallization mechanism. A series of stress drops were observed under low deformation temperatures and high strain rates. Based on an interrupted hot-compression experiment and a strain aging experiment, the stress drop can be attributed to the unlocking of dislocations pinned by the Cottrell atmosphere or short-range ordering.

Effect of natural aging time on tensile and fatigue anisotropy of extruded 7075 Al alloy

In this study, 7075 Al alloy was prepared by solution treatment + natural aging. The anisotropic behavior of 7075 Al alloy in extrusion direction (ED), 45° direction (45°) and radial direction (RD) was studied by quasi-static tensile test and strain controlled low cycle fatigue (LCF) test. The specimens of ED, 45° and RD were 0°, 45° and 90° to the extrusion direction of the bar, respectively. The mechanical properties of the specimens in three directions showed strong anisotropy. The yield strength (YS) and ultimate tensile strength (UTS) of ED specimens were better than 45° and RD specimens, but 45° specimens had better plasticity. With the extension of aging time, the YS and UTS increase significantly. In the quasi-static tensile process, the specimens would have dynamic strain aging (DSA), and obvious serrated flow behavior could be observed on the curve. For the fatigue performance of specimens in three directions, when the strain amplitude was low, the fatigue life of ED and 45° specimens was high. When the strain amplitude was high, the fatigue life of RD specimens was high. The experimental results showed that after cyclic loading, the second phase size of solution treatment + cyclic loading specimens was larger than T6 (480 °C × 3 h + 130 °C × 24 h). Therefore, the fatigue fracture mode was multi-source fracture at low strain amplitude.

Role of thermally-stable deformation twins on the high-temperature mechanical response of an austenitic stainless steel

In the present study, a two-step thermo-mechanical processing consisting of cold work and heat treatment steps was performed to increase the operating temperature of 316 austenitic stainless steels. A hierarchical microstructure of thermally-stable, nano twin bands was achieved forming into bundles in elongated grains. The mechanical response of the samples with this microstructure was evaluated through uniaxial tension tests at temperatures ranging from 20 °C to 500 °C and compared with those from the fully annealed samples. The results demonstrate that such hierarchical microstructure leads to a significant increase in the elevated temperature yield strengths due to the presence of nano-twin boundaries and resulting decrease in dislocation mean free path and increase in dislocation storage capacity. In fact, the yield strength ratio of the twinned and annealed samples increases with increasing temperature up to 500 °C, indicating the effectiveness of pre-existing thermally-stable twin boundaries as the strengthening source at temperatures as high as 0.46 homologous temperature. The hierarchical microstructure also led to irregular serrations through dynamic strain aging in the stress-strain response at 500 °C, which is attributed to the bi-modal microstructural length-scales present in the structure affecting the diffusion distances during dynamic strain aging. This structure also increases the tensile strength, and without a total loss in ductility, even though the flow stress of the twinned samples surpasses the tensile strength of the annealed samples, especially at elevated temperatures. Total hardening rate is consistently higher in the twinned samples as compared to the annealed samples, indicating the positive role of nano-twin boundaries in the dislocation storage capacity at elevated temperatures. Overall, the present study clearly demonstrate the positive role of thermally stable nano-twins on the elevated temperature mechanical response of austenitic stainless steels.

Use of jump-tests for the characterization of the viscoplastic behavior of pharmaceutical powders during compaction

Strain rate sensitivity (SRS) corresponds to the influence of the compaction kinematic on the powder behavior during compaction and on the final tablet properties. Several physical phenomena can explain the SRS like viscoelasticity or viscoplasticity. This work focuses on the case of viscoplasticity, especially by choosing products that are known to present little viscoelasticity. First, monotonic compression at different speed were performed and demonstrated that viscoplasticity was not constant during the whole compaction. Then, Jump tests showed the existence of transient phenomena during speed changed that are similar to phenomena described in the literature as dynamic strain aging. Introduction of long dwell-times during jump tests also indicated that all the products presented a stress relaxation even at slow speed. This was confirmed on long relaxation experiments (10h). As a consequence, for viscoplasticity, even compactions performed at very slow speed must be considered as dynamic.

Portevin\u2013Le Chatelier effect: modeling the deformation bands and stress-strain curves

In the Portevin–Le Chatelier (PLC) effect sample plastic deformation takes place via localized bands. We present a model to account for band dynamics and the variability the bands exhibit. The approach is tuned to account for strain hardening and the strain-rate dependence for the case of so-called type A (propagating) bands. The main experimental features of the fluctuations are a reduction with strain and increase with the strain rate which is reproduced by a model of plastic deformation with Dynamic Strain Aging, including disorder as a key parameter. Extensions are discussed as are the short-comings in reproducing detailed avalanche statistics.

Effects of increasing the strain rate on mechanical twinning and dynamic strain aging in Fe-12.5Mn-1.1C and Fe–24Mn-0.45C–2Al austenitic steels

Understanding the relationship between mechanical twinning and dynamic strain aging (DSA) in austenitic steels is necessary for steel development. DSA can be manipulated by changing the strain rate during tension tests, or by alloying austenitic steels with aluminum. In the current study, 12.5%Mn-1.1%C austenitic steel and 24%Mn-0.45%C–2%Al austenitic steel were investigated under different strain rates. Increases in the strain rate resulted in an enhanced resistance of the steels to the plastic deformation only during early stages of strain (at 3% strain) because of mechanical twinning promotion. On one hand, at 25% strain under low strain rate deformation, the Al-alloyed steel showed a tendency to form more twin plates than the Al-free steel as indicated by the transmission electron microscopy. On the other hand, at 25% strain under a high strain rate deformation, the Al-free steel showed an enhanced twin plates propagation due to a stronger self-segregation/depletion of the interstitial atoms (the carbon concentration ranged between 0 and 11 at.%) as revealed by the 3D atom probe tomography. Furthermore, the Al-alloyed steel showed a reduced twin volume fraction with a strain rate increase due to shifting of the stacking fault energy to higher values.

A parallel discrete dislocation dynamics/kinetic Monte Carlo method to study non-conservative plastic processes

Non-conservative processes play a fundamental role in plasticity and are behind important macroscopic phenomena such as creep, dynamic strain aging, loop raft formation, etc. In the most general case, vacancy-induced dislocation climb is the operating unit mechanism. While dislocation/vacancy interactions have been modeled in the literature using a variety of methods, the approaches developed rely on continuum descriptions of both the vacancy population and its fluxes. However, there are numerous situations in physics where point defect populations display heterogeneous concentrations and/or non-smooth kinetics. Here, a kinetic Monte Carlo (kMC) approach for modeling vacancy transport in response to arbitrary stress fields is used. Vacancies are treated as point particles and are coupled to the dislocation substructure representing a deformed material via an advection term defined by the local stress gradients. The stress fields and the dislocation substructure are evolved using a discrete dislocation dynamics (DDD) module. To extend the coupled model to the treatment of large systems, we have implemented it in the massively-parallel DDD code ParaDiS. To avoid numerical incompatibilities associated with merging deterministic (DDD) and stochastic (kMC) integration algorithms, we cast the entire elasto-plastic-diffusive problem within a single stochastic framework, taking advantage of a parallel kMC algorithm to evolve the system as a single event-driven process. The large-scale implementation enables the study of the evolution of a variety of dislocation-defect scenarios governed by non-conservative transport kinetics. After carrying out an exhaustive numerical and computational analysis of our parallel algorithm, we show results that emphasize situations where inhomogeneous vacancy dynamics are of relevance, and compare discrete kinetics to continuum solutions for several cases.

Temperature and stress dependent twinning behavior in a fully austenitic medium-Mn steel

In this study in situ neutron diffraction investigations of a fully austenitic medium-Mn steel enabled key insights into the temperature dependence of the deformation response with a focus on twinning. In situ tensile loading at multiple temperatures enabled the calculation of the temperature and strain dependence of the effective stacking fault energy (SFE), which was compared with theoretical SFE calculations and ex situ tensile tests. These comparisons revealed that the γ-austenite/ϵ-martensite interfacial energy plays a critical role in determining the boundary between twinning and transformation induced plasticity. The interfacial energy, which also exhibits temperature dependence, was found to be lower than the conventionally accepted range for medium-Mn steels deforming via twinning. Dynamic strain aging (DSA) was also found to impact the deformation response. DSA contributes to increasing the separation of partial dislocations, which in turn lowers the effective SFE and was observed as fluctuations in the measured SFE when DSA was active. The additional extrinsic contributions to the SFE from DSA, which operates over a limited range of temperatures and strain rates, has not previously been accounted for.

Mechanical behaviors and constitutive model of structural steel influenced by strain aging

The effects of static strain aging on Q355B and Q460C low-alloy structural steel specimens were investigated to determine the differences and similarities of the strain aging effects, and to obtain more test data on strain aging effects on structural steel materials. The specimens made of the two low-alloy structural steel materials were pre-strained up to different strain limits and then kept at room temperature for different aging times, up to one year. At selected aging times, the specimens were reloaded until failure. The stress–strain response and mechanical parameters of both steels changed significantly. The pre-strain had a greater effect than the aging time. A possible correlation was inferred between the macro-mechanical properties and changes in the microstructure of the low-alloy structural steel. The parameters of the multi-linear and nonlinear constitutive models of low-alloy structural steel influenced by static strain aging were updated according to the test data. Additionally, a revised multi-linear constitutive model was established, which presents the nonlinear material property of the steel influenced by strain aging and makes the numerical simulation more efficient. Based on the test data, the constitutive models can describe the stress–strain relationship of low-alloy structural steel under monotonic loading with different levels of strain aging.

Accurate finite element modeling of the mechanical behavior of SS304 across the peak dynamic strain aging temperatures

ABSTRACT Dynamic strain ageing or DSA is exhibited by ferrous and aluminium alloys typically at elevated temperatures and low strain rates. This effect, in association with viscoplasticity, produces a complex material behaviour which is manifested through positive strain rate sensitivity of the flow stress in certain pockets of temperatures and strain rates and negative strain rate sensitivity in others. The conventional material models of DSA either lack the ability to accurately capture the DSA effect or to incorporate the effect of the temperature variation. The objective of this work was to propose a constitutive model which can do both. The desired material model was proposed by performing necessary augmentations to a conventional material model. The suitable conventional model for this purpose was identified through rigorous comparison of the existing models. The proposed constitutive model could accurately capture the DSA behaviour of austenitic stainless steel 304 for different strain rates over a range of temperatures across the peak DSA temperatures.

Static recovery of A5083 aluminum alloy after a small deformation through various measuring approaches

Static recovery was confirmed to be the dominant softening mechanism during annealing for the studied A5083 aluminum alloy. The kinetics of static recovery, described based on the activation parameters (activation energy and volume of static recovery) and the static softening fraction, were mainly studied through two thermomechanical tests, namely, double-pass compression and stress relaxation tests. A new approach was proposed to measure the static softening fraction in the stress relaxation test. In general, a higher temperature or strain rate accelerates static recovery, while interestingly, the effect of pre-strain on static recovery is opposite in cases with and without external stress, owing to the enhanced static recovery by external stress during annealing. In addition, microhardness tests and electron backscatter diffraction (EBSD) characterization were also conducted to verify the accuracy of the results. The grain average misorientation approach based on EBSD characterization was confirmed to be effective in distinguishing and quantifying static recovery. It is noteworthy that the special stress hardening phenomenon occurring at 400 °C and 0.1/s is caused by strain aging, showing the complexity of the material behaviors after deformation of the studied aluminum alloy.

Calf Strain in Athletes.

Calf strain is a common condition. In high-performance athletes, calf strain contributes to a substantial absence from competition. Player age and history of a calf strain or other leg injury are the strongest risk factors for calf strain injury and reinjury. Although the diagnosis is often clinical, magnetic resonance imaging and ultrasound are valuable to confirm the location of the strain and the grade of injury. Nonoperative treatment is effective for most calf strain injuries. Operative management, although rarely indicated, may be appropriate for severe cases with grade-III rupture or complications. Further investigation is necessary to elucidate the benefits of blood flow restriction therapy, deep water running, lower-body positive pressure therapy, platelet-rich plasma, and stem cell therapy for calf strain rehabilitation.