Effect Of Strain Aging Research Articles

Mechanism of Fatigue-Life Extension Due to Dynamic Strain Aging in Low-Carbon Steel at High Temperature.

An enhancement in fatigue life for ferrite-pearlite low-carbon steel (LCS) at high temperature (HT) has been discovered, where it increased from 190,873 cycles at room temperature (RT) to 10,000,000 cycles at 400 °C under the same stress conditions. To understand the mechanism behind this phenomenon, the evolution of microstructure and dislocation density during fatigue tests was comprehensively investigated. High-power X-ray diffraction (XRD) was employed to analyze the evolution of total dislocation density, while Electron Backscatter Diffraction (EBSD) and High-Resolution EBSD (HR-EBSD) were conducted to reveal the evolutions of kernel average misorientation (KAM), geometrically necessary dislocations (GND) and elastic strains. Results indicate that the enhancement was attributed to the dynamic strain aging (DSA) effect above the upper temperature limit, where serration and jerky flow disappeared but hindrance of dislocations persisted. Due to the DSA effect, periods of increase and decrease in the total dislocations were observed during HT fatigue tests, and the fraction of screw dislocations increased continuously, caused by viscous movement of the screw dislocations. Furthermore, the increased fraction of screw dislocations resulted in a lower energy configuration, reducing slip traces on sample surfaces and preventing fatigue-crack initiation.

A modified constitutive model for whole-life thermal-mechanical fatigue incorporating dynamic strain aging in 316LN stainless steel

A comprehensive analysis of experimental data is presented for 316LN stainless steel subjected to isothermal and thermal-mechanical fatigue loading conditions within the temperature range of 350–600 °C. The study aims to provide a thorough understanding of the cyclic behavior through meticulous data integration, supplementation, and the use of stress decomposition methods combined with a classical damage evolution definition. The modeling approach employed in this study is based on the classical AF-OW-Kang model, with the incorporation of the Arrhenius term in the plastic flow rate equation. Furthermore, to consider the effect of dynamic strain aging at varying temperatures, temperature-dependent terms are introduced into the equations that govern the evolution of isotropic stress and backstress, resulting in enhanced accuracy in describing cyclic hardening behavior. Additionally, a modified damage evolution equation is utilized, along with equations for isotropic stress and backstress evolution, to address cyclic softening. Simulation results confirm the effectiveness of the modified model in capturing the cyclic hardening/softening behavior of 316LN stainless steel throughout the whole-life time, under both isothermal and thermal-mechanical fatigue loading conditions.

Effect of thermal ageing on fatigue crack growth behaviour of forged alloy 617M at elevated temperatures

ABSTRACT Fatigue crack growth (FCG) studies have been conducted on compact tension specimens of alloy 617M for as-received and aged for 1000, 5000 and 10,000 h at 710°C. Effects of thermal ageing on FCG behaviour of alloy 617M forged has been characterised at 27°C, 650°C, 710°C and 750°C. Results reveal that threshold stress intensity factor ( Δ K th ) decreases as temperature increases upto 710°C and then increases at 750°C for all the ageing conditions except for 1000 h aged. The Δ K th value also decreases with an increase in ageing time till 5000 h, but at 10,000 h a reverse trend is observed. The influence of thermal ageing and temperature reflects major deviations for Paris constant (C) and exponent (m). Additionally, thermal ageing and temperature influences on FCG, the effects of dynamic strain ageing, Cr-rich carbides and γ’-phase precipitations have been discussed and corroborated with fractographic and microstructural changes.

Effects of Static Strain Aging on Mechanical Performance of Ductile Cast Iron

EN-GJS-400-15U nodular cast iron intended to be used as load-bearing element in long-term geological disposal canisters containing spent nuclear fuel in Finland and Sweden was studied for static strain aging (SSA). Tensile test specimens manufactured from the nodular cast iron were pre-strained to 1%, 2% and 3% nominal plastic strains. The pre-strained specimens were aged at different temperatures ranging from room temperature to 400 °C for varying times. The aged specimens were tested with conventional tensile testing using constant cross-head speed of 0.016 mm/s. Additionally, four specimens were studied with digital image correlation (DIC) during the tensile testing to obtain full-field strain measurements. SSA resulted in elevated pronounced yield point in all the conditions, while the as-received material showed continuous yielding behavior. SSA reduced the elongation to fracture. DIC tests showed more localized yielding behavior in the SSA specimens. Over-aging effect was observed at 400 °C where increasing pre-strain did not increase the yield stress more. For 1-day aging time, the highest yield stress increment was found after aging at 200°C. The yield stress of the material was almost identical after aging in 100°C and 200°C.

Effect of dynamic strain ageing on flow stress and critical strain for jerky flow in Al-Mg alloys

A comprehensive approach addressing the flow behavior and the critical strain for the initiation of serrations in Al-Mg alloys is developed in the present work. The basic premise of the approach is that the solute atmosphere influences the friction as well as the strain hardening component of the flow stress. The friction effect of the solute cloud is modeled by considering the interplay between the characteristic solute migration time and the dislocation waiting time according to the cross-core diffusion mechanism. The impact on strain hardening is modeled by considering the apparent strengthening of the forest dislocations because of formation of solute aggregates near the vicinity of dislocation junctions. The apparent forest strengthening effect scales as the square root of the ratio of solute concentration in vicinity of the dislocation junctions and the bulk solute concentration. The modified constitutive model is validated against experimental flow curves obtained for strain rates varying over several orders of magnitude. It was observed that the modified constitutive model outperforms the standard constitutive model (considers only the friction effect of solute atmosphere) in predicting the flow curves in the dynamic strain aging domain. Furthermore, the modified constitutive model also accurately predicts the critical strain for the initiation of the jerky flow in both the normal and inverse regimes of the critical strain versus strain rate curve. Additional validation of the modified constitutive model is provided by dislocation character and density measurements via X-ray diffractograms, dislocation structure investigation via transmission electron microscopy along with fracture surface analysis.

Unique irradiation damage behavior and deformation mechanisms in crystalline/amorphous Ag/Cu-Zr nanolaminates

Nanolaminated materials due to the abundant interfaces that are efficient sinks for irradiation defects have received broad attention. In this work, Ag/Cu-Zr crystalline/amorphous nanolaminates (C/ANLs) with a wide modulation ratio η from 0.1 to 9.0 were prepared using sputtering magnetron and implanted to a total influence of 1×1017 ions cm-2 He ions to investigate their irradiation damage behavior and mechanical properties. The irradiated Ag/Cu-Zr C/ANLs enable an interesting distribution behavior of He bubbles that only aggregate in the crystalline Ag layers but are absent in the amorphous Cu-Zr layers and interfaces, and become increasingly obvious with reducing η. This is attributed to the synergistic effect of C/A interfaces and amorphous layers as efficient sinks for irradiation defects. The irradiation-induced hardening is found to go through a minimum at η = 3.0, owing to the transformation of strengthening mechanism from He bubbles strengthening in Ag layers as η ≤ 3.0 to free volume annihilation in Cu-Zr layers as η > 3.0. Moreover, the strain rate sensitivity (SRS) m of irradiated C/ANLs firstly decreases rapidly from positive to negative and then increases slowly with raising η, differing from the monotonic η-dependent SRS m of as-deposited C/ANLs. Relevant deformation mechanisms are elucidated based on the dislocation-bubble interactions in terms of pinning effect on dislocations bow-out and dynamic strain aging effect. Our findings could provide valuable insights for the microstructural design of C/ANLs for applications in nuclear reactors.

Mechanism of DSA effect correlating to the macroscopic PLC banding in high-Mn austenitic steel

Mechanism of dynamic strain aging (DSA) effect in high-Mn austenitic steel, and its correlation with the Portevin-Le Chatelier (PLC) banding, was elucidated using the in-situ synchrotron XRD measurement and digital image correlation (DIC) method during tensile test. The average dislocation velocity beyond the PLC band ranged from 10−2 to 10−1 nm/s, while reached to the order of 10° nm/s within the PLC band. In comparison, carbon diffusivity was significantly lower, allowing carbon atoms to diffuse only from 10−7 to10−9 nm in one second. The findings indicate that dislocations beyond the advancing PLC band were pinned by nearby carbon atoms, while dislocations within the PLC band became de-pinned from the carbons. Such localized pinning and de-pinning of dislocations occurred sequentially during deformation, appearing as the propagation of PLC bands in macroscopic scale.

Investigation on work hardening rate of Al-Zn-Mg-Cu alloy under solid solution process and thermomechanical coupled multi-step fast aging process

In this paper, the work hardening rate of Al-Zn-Mg-Cu alloy in the conditions of solid solution and thermo-mechanical coupled fast aging were investigated separately, and the main research content and results are as follows. Under the solid solution processes at different temperatures, the uniaxial tensile test curves of work hardening rate are relatively dispersed at a lower strain rate, and the work hardening rate increases with the increase of solution temperature. Then, as the strain rate increasing, the work hardening rate curves at each solution temperature gradually converge, until all curves intersect at a common intersection point. The above mechanism is attributed to the comprehensive effect of precipitates, solid solutions, and dynamic strain aging in the matrix. For the thermo-mechanical coupled aging, the work hardening rate has a great different with that of solid solution process. Consequently, the influence of pre-deformation on the work hardening rate were investigated, which concludes the curve shape type and the softening coefficient. The introduction of dislocation have some significant influences on the work hardening rate. Firstly, it makes the shape of work hardening rate curves change from shape 1 to shape 2. On the other hand, as the pre-deformation increasing, the variation law of softening coefficient shows an abnormal trend that is different from the ordinary law, which is due to the coordinated effect between dislocation density and dislocation induced precipitation. Furthermore, in order to reveal the internal mechanism in depth, the microstructure are observed by TEM (Transmission electron microscope). Finally, a targeted set of analytical models for work hardening rate were established, especially the mathematical explanation of the multicollinearity phenomenon of work hardening rate curves in the solid solution condition.

Strain rate-temperature effects and constitutive models for Q690D QT steel

This paper investigates the mechanical properties of the high-strength structural steel Q690D QT (Quenched and Tempered) over a wide temperature range and a wide strain rate range. Firstly, a series of quasi-static and dynamic tests adopting the universal testing machine and Split Hopkinson Pressure Bar were carried out to investigate the mechanical properties at strain rates from 0.00025 s−1 to 5000 s−1 and temperatures from room temperature (20 °C) to 700 °C. Stress-strain curves and dynamic yield strength increase factors were extracted and investigated. Test results show that Q690D QT exhibits different strain rate effects at different temperatures, more significant at the temperatures of 600 and 700 °C, as well as the high-temperature softening effect. Dynamic strain aging effect is identified at temperatures between 300 − 600 °C for plastic strain rates of 800 s−1 and 2300 s−1, nevertheless, this effect is observed at a higher temperature range of 500–700 °C for higher plastic strain rates of 4000 s−1 and 6400 s−1. From the analysis, the adiabatic thermosoftening effect cannot be ignored when the plastic strain rates reach 4000 s−1. Finally, an improved form of the Johnson-Cook model is proposed for capturing the adiabatic temperature effects at different temperatures and validated by the test data. A parameter dynamic temperature factor is proposed to describe the comprehensive effect of strain rate, test temperature, and adiabatic thermosoftening.

The Effect of Dynamic Strain Aging on Strengthening of AA5754 Aluminum Alloys: Theoretical and Numerical Approaches

The Effect of Dynamic Strain Aging on Strengthening of AA5754 Aluminum Alloys: Theoretical and Numerical Approaches

Theoretical and numerical modeling of the effect of damage and dynamic strain aging on the plastic response of C45 steel alloys

A constitutive model for C45 steel alloys is proposed in this work by integrating the effect of damage and a specific phenomenon, so-called dynamic strain aging. For damage modeling, an energy-based isotropic damage model is implemented within a frame of continuum damage mechanics. The total stress is decomposed into athermal and thermal elements. The former includes the additional term for dynamic strain aging. This term is conceptually inspired by the probabilistic nature of dynamic strain aging, and its derivation is micromechanics-based. Both athermal and thermal components are defined as a function of temperature, equivalent plastic strain, and equivalent plastic strain rate because the occurrence and characteristics of dynamic strain aging are dependent on these factors. A finite element solution for the developed model is addressed additionally to further investigate the characteristics of plastic-damage behaviors and dynamic strain aging. The numerical results are compared to the experiments and theoretical predictions for its validation. The modified model developed in this work has largely reduced the number of fitting parameters compared to the previous model originally developed by the authors in 2019. Nevertheless, predictions from the proposed model still capture the experimental data accurately.

Temperature dependence of tensile mechanical properties and work hardening behavior in direct laser deposited austenitic stainless steel 316L

Austenitic stainless steel 316 L is widely used in various industries due to its good mechanical properties at elevated temperatures (up to 600 °C), excellent corrosion resistance, and relatively low cost. However, there is lack on investigations of high-temperature properties of direct laser deposited (DLD) 316 L alloy. In the present investigation, tensile tests were performed at temperatures ranging from 20 to 1000 °C to study the effect of temperature on mechanical properties and work hardening behaviour. It was found that DLD-processed 316 L alloy has minor anisotropy of mechanical properties and work hardening behaviour at elevated temperatures higher than 200 °C. A gradual decrease in yield stress accompanied by a slight altering of ductility in the temperature range between 200 °C and 700 °C is caused by the dynamic strain ageing effect. At higher temperatures, a rapid decrease in mechanical properties is observed. Strength properties of the DLD alloy are lower in comparison with selective laser melted alloy but higher than those of wrought alloy up to 900 °C. Stress-strain curves of the material has shown stage-by-stage work hardening behaviour, depending on the test temperature. Dilatometric tests revealed a linear increase in the coefficient of thermal expansion (CTE) during heating up to 670 °C for the longitudinal sample and up to 770 °C for the transverse one. Then a non-linear growth of the CTE was observed due to diffusion-controlled transformation of the delta-ferrite to austenite.

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.

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.

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.

Effect of strain aging on mechanical properties and engineering critical assessment of X80 girth weld metal

Brittle fracture of girth weld metal in high steel grade oil and gas pipelines is the main failure mode of this kind of facilities, which may be related to the strain aging embrittlement of weld metal. To investigate the influence of strain aging on embrittlement mechanism and safety in service, the mechanical properties and engineering critical assessment of X80 girth weld metal was studied. The results demonstrate that strain aging process leads to the increase in dislocation density and low angle grain boundary. After 3 % and 5 % strain aging process, the yield strength increased from the original 565 ± 9 MPa to 675 ± 6 MPa and 754 ± 4 MPa, respectively, and the crack tip opening displacement decreased from 0.22 ± 0.03 mm to 0.11 ± 0.02 mm and 0.06 ± 0.01 mm, respectively. The deterioration of fracture toughness was attributed to the decrease in plastic deformation ability of weld metal, stress concentration at M-A constituent and Cottrell atmosphere. The change of fracture mechanism caused by strain aging increases the risk of brittle fracture of girth weld, which can be assessed by engineering critical assessment.

Induction of deformation twinning by dynamic strain aging effect: Rate-temperature coupling effect and constitutive modeling

This study focuses on the rate-temperature dependence analysis and constitutive modeling of the DSA-induced deformation twinning, which is a new plastic deformation mechanism discovered in several low-stacking-fault-energy metallic materials. Two types of deformation twinning, namely primary deformation twinning and DSA-induced deformation twinning, were noticed during the plastic deformation of the commercially pure titanium (CP–Ti). The strain, temperature, and strain rate ranges of the DSA-induced deformation twinning, as well as its rate-temperature coupling dependence, were systematically analyzed. The DSA-induced deformation twinning becomes more pronounced with the increasing strain rate and thereby plays a more significant role in enhancing the strain hardening rate than the primary deformation twinning. According to the dynamic Hall-Petch effect of the deformation twinning, a thermo-viscoplastic constitutive model based on the microstructural evolution and the thermal activation theory was developed. The volume fraction of the deformation twins during plastic deformation was expressed as a function of strain, temperature, and strain rate. The developed model was shown to be able to describe the S-shaped true stress vs. true strain curves over the wide ranges of temperature (77–998 K) and strain rate (0.001–8000/s). Finally, considering the interaction between the DSA and deformation twinning, the authors proposed the synergistic strengthening & toughening effect of DSA and deformation twinning, which was believed as a promising mechanism to achieve the simultaneous strengthening and toughing of the metallic materials.

Damage-coupled unified constitutive modeling of 316LN stainless steel including dynamic strain aging under various tension dwell time: A macroscopic phenomenological study

A damage-coupled unified constitutive model is developed for 316LN stainless steel based on the framework of the Abdel-Karim and Ohno model. In the modified model, a kinematic hardening coefficient related to accumulated plastic strain is introduced in the linear hardening term, and a damage coefficient is incorporated in the static recovery term. Meanwhile, the parameters of isotropic hardening and kinematic hardening are associated with the maximum plastic strain rate and plastic strain memory to describe the effects of dynamic strain aging and plastic strain memory. Additionally, the kinematic hardening coefficients and static recovery coefficients correlate with dwell time to simulate stress relaxation throughout the whole-life time. The comparison between the simulation and experimental results indicates the validity of the modified model under the conditions of low-cycle fatigue and creep-fatigue interaction. After identifying the material parameters using a combination of classic parameter determination methods and optimization algorithms, the cyclic stress response and hysteresis loops can be accurately simulated throughout the whole-life time.

Defect-induced inhomogeneous atomic environments in complex concentrated alloys

The multiple elements in equal ratios in complex concentrated alloys (CCAs) favor the formation of local atomic environments like segregation or chemical ordering observed in recent experiments. Such special local atomic environments may pin dislocations on the slip plane, resulting in a higher flow stress for the escape of dislocations and hence a dynamic strain aging (DSA) effect in the form of stress drop. Here, Monte Carlo molecular dynamics (MC/MD) simulations are used to elucidate the atomic atmospheres around dislocations and stacking faults in the NiCoCr alloy system and quantitatively determine the stress drop effect. Increased chemical ordering and segregation around stacking faults and dislocation cores are observed, but detailed analysis shows that atomic segregation around stacking faults (Suzuki atmospheres) contributes the most to stress drop, compared with segregation around dislocation cores (Cottrell atmospheres) and enhanced ordering (Fisher) effects. The MD simulated stress drops are in good agreement with theoretically predicted and experimental values. This work fills an important gap in the understanding of the DSA effect in H/MEAs.

A robust phenomenological modeling framework based on cross-slip propensity factor for capturing the effect of dynamic strain aging on work hardening behavior of an Al-Mg alloy

In the present work, a new modeling framework for analyzing flow behavior of Al-Mg alloys is proposed by considering both direct and indirect effect of solutes on flow stress. While direct effect addresses the increased slip resistance due to dynamic strain aging (DSA), the indirect effect represents the role of solutes on cross-slip probability. This affects the dislocation annihilation propensity and consequently the work hardening behavior. The dislocation density evolution equation is suitably modified by including a factor based on activation volume and DSA-induced strengthening (σdsa) to account for the effect of solutes on dislocation annihilation propensity and develop a new phenomenological modeling framework for predicting flow curves spanning over a wide range of strain rate (0.00002–0.2 s−1). The proposed modeling framework is shown to be performing better than the existing models which consider only the direct effect of solutes on flow stress. Experimental validation of the proposed concept is shown via dislocation density measurements using X-ray line profile analysis, dislocation substructure analysis via transmission electron microscopy and fractography studies. The new modeling framework is also extended to predict flow curves of several Al-Mg alloys with varying Mg concentration to establish the robustness of the proposed concept.