Strain Aging Research Articles

A new finding on the benefits of heterostructured bake-hardenable steels: Enhanced strain aging ability

In this study, we have made a novel discovery by revealing that heterostructured bake-hardenable steels possess an enhanced strain aging ability compared to their homogeneous counterparts. This improved strain aging ability is attributed to the formation and accumulation of additional geometrically necessary dislocations.

Role of δ-phase on recrystallisation behaviour of Inconel 718

The present work investigates the annealing behaviour of prior-coldworked Inconel 718 (IN718) sheets over wide reduction and temperature ranges by performing cold-rolling and isothermal annealing, followed by mechanical testing and structural characterisation. The study reveals that, with increasing annealing temperature, the prior-coldworked alloy shows non-monotonic M-Type variation with double peak-hardening at 0.3 Tm and 0.6 Tm. The study discovers that the first peak is due to the ‘static–strain–aging phenomenon’ that precedes recovery-softening. The second peak-hardening is due to precipitation-hardening, following which, the alloy softens due to recrystallisation. Supported by SEM, electron back-scattered diffraction and X-Ray diffraction results, the investigation suggests that the precipitation of fine rod-shaped δ-phase creates numerous nucleation sites at the shear bands that cause recrystallisation-burst at 0.75 Tm. However, above 0.75 Tm absence of the δ-phase activates grain-boundary migration resulting in rapid grain-coarsening.

Influence of dynamic strain aging on low-cycle fatigue behavior in nano-sized κ-carbide strengthened FeMnAlC lightweight steel

The effects of dynamic strain aging (DSA) on the cyclic response and deformation behavior of a hot rolled Fe–22Mn–8Al–0.9C–0.02Nb (wt.%) lightweight steel have been investigated during strain-controlled low-cycle fatigue (LCF) tests at the temperature range of 25–400 °C. The initial microstructure consisted of a complete γ-matrix with nano-sized κ-carbides. During LCF testing, the cyclic stress amplitude displayed a monotonous softening until fracture at 25 °C, 100 °C, and 400 °C in all the investigated strain ranges (Δεt) of 0.8–2.2 %. However, at 200 °C and 300 °C, cyclic softening was followed by pronounced cyclic hardening. Notably, hysteresis loops at the cycle of half-life exhibited serrations in the flow stress at 200 °C and 300 °C. This indicated that cyclic hardening was attributed to the DSA phenomenon, which was confirmed by a negative strain rate sensitivity at 200 °C and 300 °C. A significant reduction in LCF life was observed under the DSA regime. During LCF testing, sequential transmission electron microscopic (TEM) observation indicated that deformation led to the formation of shear bands in multiple directions. Gliding of wavy dislocations was frequently observed at 200 °C compared to that at 25 °C, which resulted from the dragging effect by carbon solute atoms.

Non-monotonic plasticity and fracture in DP1000: Stress-state, strain-rate and temperature influence

Recent studies revealed a complex, non-monotonic relationship between flow stress, and temperature and strain rate for ultra-fine-grained DP1000 steel, including unexpected drops in ductility. Consequently, a comprehensive investigation into the interrelated effects of temperature, strain rate, and stress state on the plasticity and fracture behaviour of DP1000 steel is undertaken. To this purpose, eight purpose-designed specimen geometries, including a shear, a dogbone, a central hole, and various notched samples, are tested at strain rates covering 7 orders of magnitude from the quasi-static over the intermediate to the dynamic regime and in a temperature range from -40 to 400 °C. The experimental design thereby provides 336 unique combinations of stress state, strain rate and temperature. The study reveals phenomena typically associated with dynamic strain aging (DSA), such as flow stress serrations, negative strain rate sensitivity, thermal hardening, and embrittlement. As a result, the present study provides a comprehensive overview of the plastic, damage, and fracture behaviour of DP1000 steel. It also offers unique insights into the influence of external loading conditions on DSA. Increasing strain rates and decreasing temperatures suppressed DSA and its effects. However, a novel and important finding is that the impact of stress state on DSA and, consequently, on the behaviour of DP1000 steel cannot be solely explained by stress triaxiality and Lode angle. Lower stress triaxialities and Lode angle parameters amplify the occurrence and severity of DSA manifestations. However, non-proportional stress states, high-stress gradients and relatively low maximum in-plane shear stresses are also identified as contributing factors.

Effect of severe plastic deformation on the structure and properties of the Zn–1%Li– 2%Mg alloy

Through the optimization of processing parameters, including pressure, temperature, and deformation degree, a high pressure torsion (HPT) regime was identified. This regime allows for the creation of a unique microstructure in the biodegradable Zn–1%Li–2%Mg alloy, which exhibits exceptional physical and mechanical properties. Following 10 revolutions of HPT treatment (resulting in an accumulated deformation degree, γ = 571) at the temperature of 150 °C and an applied pressure of 6 GPa, the Zn–1%Li–2%Mg alloy displayed notable mechanical characteristics, including a high yield strength (~385 MPa), ultimate tensile strength (~490 MPa), and ductility (44 %) during tensile tests. To elucidate the underlying reasons for these remarkable mechanical properties, an examination of the alloy’s microstructure was conducted employing electron microscopy and X-ray phase analysis (XPA). The study revealed the formation of a distinct microstructure characterized by alternating bands of the α-phase Zn, a mixture of Zn and ~LiZn3 phases, as well as the α-phase Zn containing Mg2Zn11 particles, as a consequence of HPT treatment. Additionally, it was observed that HPT treatment induced a dynamic strain aging process, leading to the precipitation of Zn particles in the LiZn3 phase and the precipitation of Mg2Zn11 and β-LiZn4 particles in the Zn phase. These precipitated particles exhibited a nearly spherical shape. The application of the XPA method helped to confirm that the Zn phase becomes the predominant phase during HPT treatment, and microscopy data showed the formation of an ultra-fine grained (UFG) structure within this phase. A comprehensive analysis of the hardening mechanisms, based on the newly acquired microstructural insights, revealed that enhanced strength and ductility of the Zn–1%Li–2%Mg UFG alloy can be attributed primarily to the effects of dispersion, grain boundary, and heterodeformation-induced hardening, including dislocation strengthening.

Static strain aging in cold rolled stable austenitic stainless steel

The effects of cold rolling reduction ratio, initial grain size, and aging condition (temperature and time) on static strain aging (SSA) behavior were systematically investigated in stable austenitic stainless steel. The aging treatment increased the yield strength (YS) and ultimate tensile strength (UTS), and the SSA effect was more pronounced at higher reduction ratio or smaller initial grain size. The SSA effect occurred in the temperature range of 400∼600 °C, where the accumulated dislocations remained in the form of wavy boundaries. Quantifying the dislocation density clearly showed that the SSA effect was directly related to the dislocation density. The volume shrinkage and increase in thermal expansion coefficient by aging treatment suggested the correlation between SSA behavior and atomic distribution changes. 3D atom probe tomography analysis clearly exhibited the clustering and segregation of solute C atoms during aging treatment. These results suggest that the SSA effect may be attributed to the interaction between solute C atoms and dislocations. Through grain refinement and optimal aging treatment at 500 °C for 1 h, the YS and UTS were significantly increased by 319 MPa and 230 MPa, respectively, to obtain a non-magnetic stainless steel with ultrahigh YS of up to 1600 MPa.

Allelopathic effects of phenolic acid extracts on Morchella mushrooms, pathogenic fungus, and soil-dominant fungus uncover the mechanism of morel continuous cropping obstacle.

The prominent problem of continuous cropping obstacle has been frustrating the morel farming. To deepen the understanding on morel continuous cropping obstacle, the allelopathic effects of phenolic acid extracts from morel continuous cropping soils on growth and development of Morchella sextelata, M. eximia, M. importuna, pathogenic fungus Fusarium sp. and soil-dominant fungus Chaetomium sp. were investigated. These effects were expressed as response index (RI). Under actual content of phenolic acids (6.150μg/g fresh mixed continuous cropping soil), the mycelial growth of all tested morel strains was inhibited (RI < 0), while the allelopathic effect of control phenolic acids (4.252μg/g fresh mixed control soil) was between promotion and inhibition, which suggested that the phenolic acid extracts from morel continuous cropping soils may exhibit certain extent of autotoxicity for the existence of morel-specific allelochemicals. In addition, the aggravated pigmentation and reduced occurrence of sclerotium in three Morchella fungi at growth inhibitory concentrations of phenolic acids indicated the induction of morel strain aging. Meanwhile, most concentrations of phenolic acids showed stimulatory effects on sporulation of Fusarium sp. and Chaetomium sp. (RI > 0), manifesting the enrichment of soil-borne pathogenic fungi and dominance of certain fungal population in soil ecosystem. Collectively, the allelopathic effects of phenolic acid extracts play an instrumental role in morel continuous cropping obstacle. The study will be beneficial for healthy development of morel artificial cultivation.

Influence of Prior Fatigue Damage on Tensile Flow Behaviour of Type 316L(N) Austenitic Stainless Steel

The present study aims to understand the influence of prior fatigue loading on the tensile flow behaviour of austenitic 316L(N) stainless steel. The steel specimens were subjected to prior fatigue damage up to various fatigue life fractions. i.e., 5, 10, 30 and 50% of the fatigue life at a strain amplitude of ±0.6% and a nominal strain rate of 3×10-3 s-1 and 300, 823 and 873 K. Subsequently the specimens were subjected to tensile deformation at the above temperature and strain rate and the tensile flow behaviour was evaluated in terms of Hollomon, Swift, Ludwigson and Voce equations. At 300 K, the flow curves of the specimens of as-received, and prior fatigue damaged were well described by the Ludwigson equation, whereas at 823 and 873 K, both Voce and Ludwigson equation could be used to describe the flow behaviour of the specimens. Increase of work hardening with the increase in prior fatigue damage at 823 and 873 K is attributed to the occurrence of dynamic strain ageing (DSA). At 300 K, a three-stage work hardening behaviour with a prominent stage II athermal hardening was observed for all the specimens, whereas at higher temperatures, only the as-received specimen exhibited the three-stage work hardening behaviour. At 823 and 873 K, a two-stage work hardening behaviour, comprising of a transient stage followed by stage III was observed for the specimens subjected to prior fatigue damage up to various fatigue life fractions. The inter relationship between the work hardening parameters of Ludwigson was rationalised in terms of the linearly inverse relationship between the work hardening rate and plastic strain rates.

On the Potential Correlation between Dynamic Strain Aging and Liquid Metal Embrittlement in T91/LBE System

In the study of the liquid metal embrittlement (LME) of the T91/lead-bismuth eutectic (LBE) system, it is observed that LME occurs in a temperature interval which is similar to the temperature range where dynamic strain aging (DSA) is observed. However, the potential correlation between DSA and LME has not yet been satisfactorily investigated. This investigation for the T91/LBE system is exactly the topic of this work. For the evaluation of DSA and LME, slow strain rate tensile tests are conducted in the temperature range between 200 °C and 450 °C with strain rates of 5×10−5 s−1 and 5×10−6 s−1 in reference and a molten oxygen-depleted LBE environment. The resulting tensile properties, as well as the fracture surfaces and lateral surfaces of the failed samples, suggest a correlation between DSA and LME in the T91/LBE system. The maximum mechanical degradation of T91 is observed in the case where the effects of both DSA and LME on material properties are found to be at maximum. However, the observation of DSA was not identified as a prerequisite for LME to take place. Therefore, these results may indicate that DSA partly contributes to the ductility minimum observed in the T91/LBE system. In addition, the results of this work show that changes in the fracture surface and lateral surface are more sensitive features to claim for the potential occurrence of LME than the changes in total elongation.

Absence of dynamic strain aging induced by precipitation in 316 austenitic stainless steel during thermal aging at 750 °C

Dynamic strain aging (DSA) has long been seen in the solution-treated austenitic stainless steels, but DSA effect is affected after long-term exposure to high temperature in the fourth-generation nuclear reactors. This study investigated the effect of precipitates on the DSA effect of 316 austenitic stainless steel after accelerated aging at 750 °C for 10, 100 and 1000 h, which is used to simulate the long-term microstructural evolution at service temperature. A gradual reduction in the DSA effect is found with an increase in the grain boundary precipitates after aging for 10 and 100 h. The frequency of the interactions between dislocations and grain boundaries decreases because moving dislocations are blocked by the precipitates formed at the grain boundaries. Meanwhile, the grain boundary precipitates occupy part of the grain boundaries, reducing the probability of dislocation emission from grain boundary. As the aging time prolongs to 1000 h, the newly-formed intragranular σ phase acts as an obstacle to the dislocation movement, resulting in an increase in waiting time (tw-AG1000). Also, Cr atoms can be drained from mobile dislocation into σ phase, leading to an increase in the aging time (ta-AG1000) required for pinning the arrested dislocations. As a result, the critical strain for the occurrence of DSA significantly increases, leading to the absence of DSA phenomenon.

Microstructure Features and Mechanical Properties of Modified Low-Activation Austenitic Steel in the Temperature Range of 20 to 750 °C

A new low-activation austenitic steel with a modified composition and high austenite stability is proposed. The features of its microstructure after solution treatment (ST) and cold rolling (CR) are studied. The mechanical properties and features of the fracture behavior of this steel under tensile tests in the temperature range of 20–750 °C are discussed. After ST, an austenitic structure with stacking faults and dispersed carbide particles of the MC and M23C6 types is observed in the steel. After CR, the grains are refined, and the average grain size decreases from 41.4 µm (after ST) to 33.9 µm. High-density microtwin packets form in the material, and the dislocation density increases relative to that after ST. As the test temperature increases from 20 to 750 °C, the yield strength of the steel decreases by approximately two times, from ≈300 to 150 MPa (for ST) and from ≈700 to 370 MPa (for CR). In the studied temperature range, the steel demonstrates up to 2.6 times higher values of elongation to failure, ≈40–80% (for ST) and ≈13–27% (for CR), compared to steels of similar compositions and lower manganese content. Mechanical twinning contributes to the high steel ductility up to 300 °C. Signs of discontinuous flow in the tensile curves after ST in the temperature range of 500–600 °C and a decrease in the elongation to failure in the close temperature range indicate dynamic strain aging (DSA). Steel fracture after tension at all test temperatures mainly occurs via a ductile dimple transcrystalline mechanism with elements of ductile intercrystalline fracture. It is shown that cracks nucleate on clusters of dispersed second-phase particles. The mechanisms of plastic deformation, fracture, and strengthening of the proposed modified low-activation austenitic steel are discussed.

Ratcheting prediction of stainless steel 304 and 316L samples undergoing asymmetric loading cycles at elevated temperatures incorporating dynamic strain aging phenomenon

The present study intends to assess the plastic strain accumulation in austenitic stainless steel 304 and 316 L samples undergoing uniaxial loading cycles at elevated temperatures incorporating the dynamic strain aging (DSA) phenomenon. The DSA function was adapted into the dynamic recovery term of the Ahmadzadeh-Varvani (A-V) kinematic hardening rule to address the accumulated plastic strain (ratcheting strain) of steel samples at elevated temperature ranges where materials show a negative strain rate sensitivity. The range of temperature at which stainless steel samples manifested DSA event was evidenced between 573 K and 900 K. At this temperature range, the interaction of solute atoms with dislocations resulted in an elevation in materials strength. The evolution of yield strength at various temperatures was calibrated with a multi-variable function developed earlier by the Voyiadjis-Song-Rusinek (VSR) model. The higher strength due to the interaction of solute atoms and dislocations, suppressed the ratcheting magnitude and rate of steel samples at the DSA domain. Applied loads beyond the elastic limit resulted in the yield surface translation, in the deviatoric stress space, governed by the A-V hardening rule. The DSA phenomenon promoted the yield surface and the backstress evolution for stainless steel samples at the operating temperature of 673 K. The predicted ratcheting curves within the DSA domain were largely associated with the operating temperature, accumulated plastic strain, and asymmetric loading cycles.

Cottrell Cosegregations of Carbon and Hydrogen: Characteristics and Role in the Strain Aging and Embrittlement of Steels

Cottrell Cosegregations of Carbon and Hydrogen: Characteristics and Role in the Strain Aging and Embrittlement of Steels

On the Characteristics and Role of Cottrell Co-Segregations of Carbon and Hydrogen in Strain Aging and Embrittlement of a Number of Steels

On the Characteristics and Role of Cottrell Co-Segregations of Carbon and Hydrogen in Strain Aging and Embrittlement of a Number of Steels

Study on the Flow Behavior of 5052 Aluminum Alloy over a Wide Strain-Rate Range with a Constitutive Model Based on the Arrhenius Model Extension

The formability at room temperature and low speed limits the application of aluminum alloy, while high strain rates positively improve the formability of materials. The constitutive behaviors of materials under high strain rates or impact loadings are significantly different from those under quasi-static conditions, while few constitutive models consider the effect of the mobile dislocation and forest dislocation evolution on the dynamic strain aging (DSA) over a wide strain-rate range. The 5052 aluminum alloy, of which the primary source of strain-hardening is dislocation–dislocation interaction, is widely used in manufacturing automotive covering parts and is considered one of the most promising alloys. Therefore, this study conducts uniaxial tensile tests on AA5052-O under conditions of temperatures ranging from 293 K to 473 K and strain rates ranging from 0.001 s−1 to 3000 s−1, and compares the stress–strain relationships of AA5052-O under different conditions to illustrate the constitutive relationship affected by the dislocation evolution over a wide strain-rate range. The Arrhenius model based on the thermal activation mechanism is modified and extended by considering the effects of dynamic strain aging (DSA), drag stress, and the evolution of mobile dislocation and forest dislocation. Thus, a new physics-based constitutive model for AA5052-O is proposed, which can well reflect the change in strain-rate sensitivity with the strain rate increasing. The mobile dislocation density and total dislocation density are predicted with a modified Kubin–Estrin (KE) model, and the influences of variable mobile dislocation on DSA and dislocation drag are discussed as well. In order to verify the reliability of the new constitutive model, the dislocation densities of the specimens before and after deformation are obtained with TEM and XRD, which are in good agreement with the predicted values. This study also compares the newly proposed model with classic constitutive models using multiple statistical evaluation methods, which shows that the new physics-based constitutive model has not only more clear physical meanings for its parameters but also has a higher prediction accuracy.

The effect of Mn content on a novel Al–Mg–Si-Sc-Zr alloy produced by laser powder bed fusion: The microstructure and mechanical behavior

Scandium is very expensive and limits the application potential of Sc containing alloys. A novel high-strength Al–Mg–Mn-Sc-Zr alloy was designed for reducing Sc. Multiple strengthening mechanisms were utilized for designing this alloy, especially for the solid solution strengthening which benefits from ultrahigh cooling. With the addition of Mn and Si, the strength of our alloy reaches comparable level of similar alloys while the Sc content is 40–60 % less than them. The effect of Mn addition on microstructure and mechanical properties were carefully studied. Near full-dense samples with relative density >99.88 % were obtained with the help of overlapping prediction model. The continuous Mg2Si network was observed with Si introduction and broken up with increasing Mn content. According to the tensile experiment results and corresponding calculations, it was considered that the thermal stability and the effect of precipitate strengthening was slightly deteriorated. However, obvious improvement of the tensile strength was observed with increasing Mn addition, and the ductility maintains in a certain range. After aging at 280 °C for 4 h, the sample of 1.4 wt% Mn exhibits the highest ultimate strength of 497.08 ± 7.22 MPa, and possesses a considerable ductility of 8.01 ± 0.34 %. The Portevin-Le Chatelier (PLC) effect was observed in stress-strain curves, which reflects a dynamic strain aging of mobile dislocations and solute atoms. After aging, the stress amplitude and waiting time of serration flow decrease obviously, because of the introduction of bypassing precipitates and reduction of diffusion time. Compared with other similar alloys, our alloy exhibited comparable yield strength and elongation, but considerable advantages in material costs resulting from lower Sc content. It will offer not only an idea for alloy design utilizing the ultra-high solid solubility of elements, but also a feasible way to further expand application field of high-strength Al-based alloys through reducing the material costs.

Effect of Cr content in temperature-dependent mechanical properties and strain hardening of a twinning-induced plasticity steel

This study aims at assessing and analyzing the effect on the mechanical behavior at temperatures between 25 °C and 350 °C of the Cr additions (5 % and 10 %) to a TWIP steel (Fe–22Mn-0.6C). We conducted tensile tests and characterized the microstructure after deformation by light microscopy, X-ray diffraction (XRD) and FEG-SEM-EBSD. Different temperature-dependent strengthening mechanisms were determined. Mechanical twinning, Dynamic Strain Aging (DSA), and slip bands were the predominant mechanisms, with a combination of them depending on the stacking fault energy (SFE). The SFE decreases with the chromium content and increases with temperature, influencing the percentage of mechanical twinning generated during plastic deformation and consequently, affecting the strain hardening rate (SHR). There exists a direct relationship between strain hardening rate with temperature of deformation and mechanical twinning. DSA was observed and its effect on strain hardening was analyzed in detail.

Improving the Mechanical Properties of Low‐Carbon Steel by In Situ Strain Aging

A novel continuous process that utilizes the concept of strain aging not only in the automotive industry but also in structural materials as a whole is developed. The principle behind achieving in situ strain aging is as follows. 1) Dislocations are generated through compressive deformation during continuous pressing. 2) The processing conditions at the strain aging temperature create a favorable environment for the diffusion of interstitial atoms, leading to the formation of Cottrell atmospheres. The in situ strain aging‐processed low‐carbon steel demonstrates a significant increase in strength compared to the unprocessed sample (increased yield strength by ≈37.6 MPa), which can be attributed to the strain aging effect, as well as the combined effects of grain refinement and pre‐existing dislocations. Additionally, the generation of dislocations during compressive deformation suppresses void nucleation during pre‐strain, preventing a loss of elongation (reduced uniform elongation by ≈1.5%). The in situ strain aging‐processed low‐carbon steel exhibits a superior strength–elongation combination compared to both the unprocessed low‐carbon steel and strain aging‐simulated counterparts obtained through tensile deformation.

Deformation and damage mechanisms during clockwise diamond and counter clockwise diamond thermomechanical fatigue in Timetal 834 alloy

In the present investigation, the thermomechanical fatigue (TMF) behavior of Timetal 834 alloy was studied under clockwise diamond (CWD) and counter clockwise diamond (CCD) loading conditions. These tests were conducted within 723 K ↔ 873 K temperature range at the strain amplitudes of Δεm/2 = ±0.6% and ±1.0%. The cyclic softening behavior observed under CWD-TMF condition at both strain amplitudes, is attributed to shearing of coherent Ti3Al precipitates. Contrarily, the alloy exhibits both cyclic hardening and softening responses under the CCD-TMF conditions. This additional hardening response is attributed to the occurrence of dislocation–dislocation interactions coupled with dynamic strain aging. The present study reveals that both phase angle and strain amplitude affect the TMF life. The alloy exhibited relatively lower fatigue life under CCD-TMF condition than under CWD-TMF condition, at both strain amplitudes. Transmission electron microscopy analyses of the post-deformed specimens reveal that the dislocation arrangements are similar in both loading conditions at Δεm/2 = ±0.6%, while being distinctively different at Δεm/2 = ±1.0%. At higher strain amplitude, the fatigue life is significantly reduced under CCD-TMF condition. Fractographic analyses indicate the involvement of oxidation damage during both the TMF loading conditions. However, its effect is more pronounced under CCD-TMF than under CWD-TMF condition.

A quantitative insight into strain hardening behavior of typical Hadfield steel under dynamic load

In this paper, the critical threshold associated to the activating of strain hardening capacity with respect to typical Hadfield steel Mn13Cr2 was explored quantitatively utilizing a Separated Hopkinson Press Bar (SHPB). The strain hardening behavior of the Mn13Cr2 under dynamic load was investigated, systematically, and the strain hardening mechanisms involved in different plastic deformation conditions were illuminated. Results indicated the overall plastic deformation process under different dynamic loads could be divided into three stages, including the (LHS), the (HHS) as well as the parabolic hardening stage (PHS). The critical transformation strength (CTS) from LHS to the HHS showed a trend of increasing first and then decreasing with the shock pressure increasing from 0.1 MPa to 0.8 MPa, and the maximum hardening effect with respect to the studied Mn13Cr2 steel could be activated at dynamic load threshold of 0.6 MPa with max CTS of ∼493.6 MPa. Dislocation-dominated strengthening mechanism and C–Mn atomic dipoles strengthening mechanism induced by dynamic strain aging were responsible for the excellent strain hardening capability when the impact load was increased above 0.6 MPa.