Influence Of Dynamic Strain Aging Research Articles

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.

Structure–Property Correlation between Friction-Welded Work and Hardened Al-4.9Mg Alloy Joints

Friction welding of aluminum alloys holds immense potential for replacing riveted joints in the structural sections of the aeronautical and automotive sectors. This research aims to investigate the effects on the microstructural and mechanical properties when AA5083 H116 joints are subjected to rotary friction welding. To evaluate the quality of the welds, optical and scanning electron microanalysis techniques were utilized, revealing the formation of sound welds without porosity. The microstructural examination revealed distinct weld zones within the weldment, including the dynamically recrystallized zone (DRZ), thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ), and base metal (BM). During the friction-welding process, grain refinement occurred, leading to the development of fine equiaxed grains in the DRZ/weld zone. Tensile testing revealed that the weldment exhibited higher strength (YS: 301 ± 6 MPa; UTS: 425 ± 7 MPa) in the BM region compared to the base metal (YS: 207 ± 5 MPa; UTS: 385 ± 9 MPa). However, the weldment demonstrated slightly lower elongation (%El: 13 ± 2) compared to the base metal (%El: 15 ± 3). The decrease in ductility observed in the weldment can be attributed to the presence of distinct weld zones within the welded sample. Also, the tensile graph of the BM showed serrations throughout the curve, which is a characteristic phenomenon known as the Portevin–Le Chatelier effect (serrated yielding) in Al-Mg alloys. This effect occurs due to the influence of dynamic strain aging on the material’s macroscopic plastic deformation. Fractography analysis showcased a wide range of dimple sizes, indicating a ductile fracture mode in the weldment. These findings contribute to understanding the microstructural and mechanical behavior of AA5083 H116 joints subjected to rotary friction welding.

Dynamic strain aging-mediated temperature dependence of ratcheting behavior in a 316LN austenitic stainless steel

Tensile tests and ratcheting tests were performed on a 316LN austenitic stainless steel at 300 °C, 400 °C, 500 °C, and 600 °C, respectively, to study the influence of dynamic strain aging (DSA) on deformation behavior and microstructural evolution. The DSA temperature range in present work was determined to be ≥ 400 °C in both uniaxial tests and ratcheting tests. Especially in the ratcheting tests, a sudden drop in ratcheting strain accumulation as well as in steady ratcheting strain rate was evident when the temperature raising from 300 ° to 400 °C. While within the DSA temperature region, the ratcheting behavior was insensitive to the temperature. Microstructural examinations revealed dislocation cells and tangles in the sample ratcheted at 300 °C (free of DSA), while complex dislocation structure in the samples ratcheted at ≥ 400 °C (with DSA) that was consisted mainly by high-density dislocation walls, planar slip bands, and Lomer-Cottrell locks. This clearly indicates a change in predominant ratcheting deformation mechanism from cross-slip at DSA-free temperature region to planar-slip at DSA region. It was further demonstrated that, within the temperature range from 400 °C to 600 °C, the DSA-induced hardening effect balanced with the thermally-activated softening effect and this balance led to temperature-insensitive ratcheting behavior as experimentally observed. Finally, the DSA-affected dislocation structure was quantitatively evaluated in the ratcheted samples, with an aim in correlating to the ratcheting behavior.

Isothermal and thermomechanical fatigue behavior of 316LN stainless Steel: Effects of strain amplitude

Effects of strain amplitude on the isothermal fatigue (IF) and thermomechanical fatigue (TMF) behaviors of 316LN stainless steel were comparatively studied. The physical mechanisms for the evolution of cyclic stress response were revealed based on microstructure characterization. The manifestations of dynamic strain aging (DSA), dependence of DSA intensity on loading modes and influence of DSA on deformation properties were discussed. The dominated damage mechanism changed at the strain amplitude of about 0.27% at which the IF and TMF life curves intersected. The maximum stress was the appropriate damage parameter for life prediction with predicted life falling into the scatter band of 1.5.

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.

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

Effect of Multistage High Temperature Thermomechanical Treatment on the Microstructure and Mechanical Properties of Austenitic Reactor Steel

The deformation microstructures formed by novel multistage high-temperature thermomechanical treatment (HTMT) and their effect on the mechanical properties of austenitic reactor steel are investigated. It is shown that HTMT with plastic deformation at the temperature decreasing in each stage (1100, 900, and 600 °C with a total strain degree of e = 2) is an effective method for refining the grain structure and increasing the strength of the reactor steel. The structural features of grains, grain boundaries and defective substructure of the steel are studied in two sections (in planes perpendicular to the transverse direction and perpendicular to the normal direction) by Scanning Electron Microscopy with Electron Back-Scatter Diffraction (SEM EBSD) and Transmission Electron Microscopy (TEM). After the multistage HTMT, a fragmented structure is formed with grains elongated along the rolling direction and flattened in the rolling plane. The average grain size decreases from 19.3 µm (for the state after solution treatment) to 1.8 µm. A high density of low-angle boundaries (up to ≈ 80%) is found inside deformed grains. An additional cold deformation (e = 0.3) after the multistage HTMT promotes mechanical twinning within fragmented grains and subgrains. The resulting structural states provide high strength properties of steel: the yield strength increases up to 910 MPa (at 20 °C) and up to 580 MPa (at 650 °C), which is 4.6 and 6.1 times higher than that in the state after solution treatment (ST), respectively. The formation of deformed substructure and the influence of dynamic strain aging at an elevated tensile temperature on the mechanical properties of the steel are discussed. Based on the results obtained, the multistage HTMT used in this study can be applied for increasing the strength of austenitic steels.

Influence of dynamic strain aging on ferritic/martensitic steel and stability analysis

Fe-9Cr-2W-based ferritic/martensitic (F/M) steels exhibit high performances in a lead-cooled reactor. However, there have been few studies that examined the dynamic strain aging (DSA) effect on Fe-9Cr-2W-based F/M steels. In this article, Fe-8.5Cr-1.5W-based F/M steel was examined. By analyzing the dynamic strain-stress curves, the sensitivity was observed. When strain rates were 10−3 s−1 and 10−4 s−1, serrated flow occurred from 300 to 650°C. The yield and tensile strength plateaued but overall decreased from 300 to 500°C. The initial microstructural analysis verified the stability of Fe-8.5Cr-1.5W-based F/M steel. These results provide scientific basis and guidance for the optimization of the preparation process of F/M.

Influence of dynamic-strain aging due to excess Mg on fatigue crack growth rate scatter in Al6061-T6 alloy

Fatigue failure results in high industrial costs, and its mechanism requires close examination; however, current methods are costly and time-consuming due to the need for a large number of test specimens. The purposes of this study are to investigate the influence of dynamic strain aging on fatigue crack growth rate (FCGR) scatter in Al 6061-T6 alloys and to present a new approach to evaluate the scatter of FCGR using a limited number of the test specimen. Rotating bending fatigue tests of 6061-T6-based Al alloys with added Zr and excess Mg were performed under constant amplitude loading using smooth specimens. The scatter behavior of FCGR is investigated by examining the fatigue crack growth on the specimen surface and fractographic observation on the fracture surface. The accounting for the interaction effects of multiple surface cracks and fractographic examination on striation formation from previous findings revealed that excess Mg promoted small scatter in Mode I fatigue crack growth. This study showed that local plastic deformation affected the FCGR scatter of microstructurally-large fatigue cracks. These findings suggest that dynamic strain aging of Mg induces stable Mode I crack growth due to pinning of dislocation movement on slip planes during the crack growth process.

Crystal plasticity modeling of the effects of crystal orientation and grain-to-grain interactions on DSA-induced strain localization in Al–Li alloys

We develop a crystal plasticity model to investigate the coupled actions of crystal orientation, grain neighborhood and grain-to-grain elasto-plastic interactions on dynamic strain aging (DSA) and the onset and development of associated plastic strain localization in Al–Li alloys. Considering simple model multilayered microstructures with preferred orientations representative of rolled alloys, the aim is to identify grain orientation couples that can limit dynamic strain aging induced strain localization without compromising the flow stress and strain hardening properties. To this end, a slip system-based formulation of dynamic strain aging is implemented in a crystal plasticity finite element framework. The model validity is first checked with the simulation of a tensile specimen loaded at quasi-static applied strain rate. The introduction of dynamic strain aging allows predicting complex propagation of intense plastic localization bands. We further investigate the influence of crystal orientations on early strain localization in Al–Cu–Li–Mg alloys, by performing simulations representative of the early stage of a Kahn Tear test for single crystals and layered polycrystals. Using experimentally reported crystal orientations for rolled microstructures, the simulation results show that in both single and multilayered crystals, there is a strong influence of dynamic strain aging on localization patterns, as well as a significant orientation dependence. In multilayered crystals, the nature of strain localization can be remarkably modified when stand-alone crystals of a certain orientation are coupled with other orientations: strain localization may intensify or fade away depending on the coupling with neighboring orientations.

Crystal Plasticity Modeling of the Effects of Crystal Orientation and Grain-to-Grain Interactions on DSA-Induced Strain Localization in Al-Li Alloys

We develop a crystal plasticity model to investigate the coupled actions of crystal orientation, grain neighborhood and grain-to-grain elasto-plastic interactions on dynamic strain aging and the onset and development of associated plastic strain localization in Al-Li alloys. Considering simple model multilayered microstructures with preferred orientations representative of rolled alloys, the aim is to identify grain orientation couples that can limit dynamic strain aging induced strain localization without compromising the flow stress and strain hardeningproperties. To this end, a slip system-based formulation of dynamic strain aging is implemented in a crystal plasticity finite element framework. The model validity is first checked with the simulation of a tensile specimen loaded at quasi-static applied strain rate. The introduction of dynamic strain aging allows predicting complex propagation of intense plastic localization bands. We further investigate the influence of crystal orientations on early strainlocalization in Al-Cu-Li-Mg alloys, by performing simulations representative of the early stage of a Kahn Tear test for single crystals and layered polycrystals. Using experimentally reported crystal orientations for rolled microstructures, the simulation results show that in both single and multilayered crystals, there is a strong influence of dynamic strain aging on localization patterns, as well as a significant orientation dependence. In multilayered crystals, the nature of strain localization can be remarkably modified when stand-alone crystals of a certain orientation are coupled with other orientations: strain localization may intensify or fades away depending on the coupling with neighboring orientations.

Strain controlled isothermal low cycle fatigue life, deformation and fracture characteristics of Superni 263 superalloy

The total axial strain controlled low-cycle fatigue (LCF) behaviour of a wrought nickel-base superalloy Superni 263 has been evaluated at 298 K, 673 K, and 923 K employing the strain amplitudes in the range ± 0.25% to ± 0.80%. All the tests were performed using a triangular wave form at a constant strain rate of 10−3 s−1. The Superni 263 alloy was subjected to a solutionizing treatment at 1373 K/1.5 h followed by water quenching. An ageing treatment of 1073 K/8 h was given to the solutionized samples. The microstructure of the alloy is composed of γʹ in the intragranular regions and discrete M23C6 precipitates on the grain boundaries. Few undissolved (Ti, Mo)C and TiN precipitates were also noticed in the matrix. The strain amplitude and temperature played a significant role in the evolution of macroscopic cyclic stress response and the development of deformation substructure. The alloy displayed serrated flow in the plastic portions of stress-strain hysteresis loops in the tests conducted at 673 K and 923 K, predominantly at strain amplitudes higher than ±0.40%. Several manifestations depicting the occurrence of dynamic strain ageing (DSA) were derived by analysing both the macroscopic and micro-mechanistic information generated. The comparative evaluation of monotonic and cyclic stress-strain curves was conducted and the factors responsible for rapid cyclic hardening during DSA were identified. The fatigue life decreased drastically with increasing temperature, particularly at low strain amplitudes. The strain-life data could be represented well by using Basquin and Coffin-Manson relationships. The slope in Coffin-Manson relationship revealed more negative values under the influence of DSA. The crack initiation and propagation modes at all the test conditions remained transgranular without the indications of interference from creep and oxidation effects. The deformation substructure at elevated temperatures was characterized by the occurrence of co-planar slip, stacking faults, micro twinning and high dislocation density in between the slip bands.

Gradient structure formation in the process of severe plastic deformation of carbon steels under the conditions of the effect of dynamic strain aging

The formation of a gradient structure during the severe plastic deformation (SPD) by high-pressure torsion (HPT) of low- and medium-carbon steels under the influence of dynamic strain aging (DSA) is studied. Deformation mechanisms under various regimes of deformation processing are analyzed. The temperature ranges of the DSA effect during the ECAP processing of steel 10 and the fact of the formation of a gradient structure during the HPT processing under these conditions are established. It is shown that the deformation of carbon steels in the DSA temperature range enables controlling the structure of the deformed state.

Investigation on Fracture Behavior of Grade 91 Steel at 300-550 °C

The fracture toughness (J1c) of modified 9Cr-1Mo (P91 or Grade 91) steel was evaluated in the temperature range 300-550 °C. The yield strength and ultimate tensile strength values were observed to decrease with test temperatures. The ductility (% uniform elongation and post-necking elongation) was minimum in the temperature range 350-400 °C. From J–R curves established at 300, 350, 400, 450, 500, and 550 °C, fracture toughness (J1c) and tearing modulus were determined at different loading rates of 0.01, 0.05 and 0.1 mm/s. For all the loading rates, J1c values decreased in the temperature range 300-450 °C, followed by an increase in fracture toughness up to 550 °C. These trends in tensile and fracture properties can be attributed to the influence of dynamic strain aging.

Effect of dynamic strain aging and precipitation on the hot deformation behavior of 253MA heat-resistant alloy

In this research, we performed an experimental investigation into the influence of dynamic strain aging (DSA) and precipitation behavior on the plastic deformation and microstructure evolution in alloy 253MA considering its service temperature. The serrated flow behavior, stress anomaly behavior and negative strain rate sensitivity are considered as the manifestation of DSA within a certain temperature range (TDSA), which can be classified as low (550–700 °C), intermediate (600–750 °C) and high temperatures (700–790 °C) at 0.001 s−1, 0.01 s−1 and 0.1 s−1, respectively. The activation energy of DSA was determined to be 242 kJ/mol, indicating that the diffusion of substitutional solutes, such as Cr and Ni, is mainly responsible for the DSA phenomenon. The characteristic of dislocation substructures is prone to change from linear structure to cellularization structure with a rising deformation temperature in the DSA regime. The stress anomaly behavior can be attributed to the rapid dissolution of σ phase. The initiation of dynamic recrystallization should contribute to the disappearance of DSA by suppressing the dynamical solute atom–dislocation interaction.

Influence of dynamic strain aging on the mechanical properties and microstructural evolution for Alloy 800H during hot deformation

Two types of hot compression tests, namely, constant strain rate and strain rate jump, were carried out for Alloy 800 H within a wide temperature range of 450–900 °C and strain rate range of 0.001–1 s−1. Following the above tests, we studied the effect of dynamic strain aging (DSA) on the mechanical properties and microstructure features to reveal the underlying mechanism responsible for DSA. The results indicated that three kinds of serrated yielding behavior (Type A, B and C) can be observed in the stress-strain curves of the DSA regime. The DSA temperature (TDSA) is located in the temperature range of 500–750 °C at a strain rate of 0.001 s−1, in 550–790 °C at 0.01 s−1, and in 600–840 °C at 0.1 s−1. The activation energy for the appearance of negative strain rate sensitivity was calculated to be 194 kJ/mol, and pipe diffusion of substitutional solutes (i.e. Cr, Ni and Ti) is regarded as the mechanism of DSA. TEM observations further elucidated a tendency of linear microstructure within the DSA regime, which also restricts the cross-slip of dislocations and promotes the strain localization along deformation bands.

Temperature and frequency influence on the cyclic deformation behavior of EN-GJS-600 (ASTM 80-55-06) ductile cast iron at 0.005 and 5 Hz

In the present paper, the physically based fatigue lifetime calculation approach “PhyBaLSIT” was modified for isothermal low cycle fatigue of ductile cast iron EN-GJS-600 in the temperature range from ambient temperature up to 400 °C at a frequency of 0.005 Hz. One strain controlled temperature increase test is sufficient to determine the temperature for onset and most pronounced influence of dynamic strain aging, i.e. the reduction of dislocation mobility due to diffusing interstitial atoms. The comparison to prior results at 5 Hz and to results at 47 and 92 Hz reveals the significant influence of deformation rate on the temperature range of dynamic strain aging. With the results at 0.005, 47 and 92 Hz presented in this paper and at 5 Hz previously published, sound knowledge how elevated temperatures and deformation rate affect dynamic strain aging, cyclic deformation behavior and fatigue lifetime in the LCF- and HCF-regime is generated.

Manifestations of dynamic strain aging under low and high cycle fatigue in a type 316LN stainless steel

Influence of Dynamic strain aging (DSA) under low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated by conducting LCF and HCF tests on specimens over a wide range of temperature from 573 to 973 K. DSA was found to be highly pronounced in the temperature range of 823–873 K. DSA was seen to have contrasting implications under LCF and HCF deformation. The cyclic hardening owing to DSA caused an increase in the cyclic stress response under LCF, leading to decrease in cyclic life. On the other hand, the DSA-induced strengthening suppressed the crack initiation phase under HCF where the applied stress remains fixed, leading to an increase in the cyclic life.

A constitutive model accounting for strain ageing effects on work-hardening. Application to a C–Mn steel

One of the most successful models for describing the Portevin–Le Chatelier effect in engineering applications is the Kubin–Estrin–McCormick model (KEMC). In the present work, the influence of dynamic strain ageing on dynamic recovery due to dislocation annihilation is introduced in order to improve the KEMC model. This modification accounts for additional strain hardening rate due to limited dislocation annihilation by the diffusion of solute atoms and dislocation pinning at low strain rate and/or high temperature. The parameters associated with this novel formulation are identified based on tensile tests for a C–Mn steel at seven temperatures ranging from 20 °C to 350 °C. The validity of the model and the improvement compared to existing models are tested using 2D and 3D finite element simulations of the Portevin–Le Chatelier effect in tension.

Implications of dynamic strain aging under LCF-HCF interactions in a type 316LN stainless steel

Influence of dynamic strain aging (DSA) under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated by conducting HCF tests on specimens subjected to prior LCF cycling over a wide range of temperature from 573 to 973K. DSA was found to be pronounced at 823–873K depending on the magnitude of the stress employed under HCF cycling. DSA was seen to have contrasting implications under LCF and HCF deformation resulting in an anomalous fatigue behavior in terms of remnant HCF life under LCF-HCF interaction. LCF-HCF interaction was found to be pronounced at intermediate levels of prior LCF exposure, where the remnant HCF life is dictated by competitive damage mechanism resulting from the influence of DSA under LCF as well as HCF. Detailed fracture surface examination revealed that extensive hardening associated with DSA leads to an extended zone of faceted appearance with river markings (Stage-I crack) under HCF cycling (with or without LCF exposure). This reduces the crack growth rate, delaying the transition of crack from Stage-I to Stage-II, thereby leading to an extension of life in such cases. On the other hand, a highly striated fracture surface indicating a quick transition in crack from Stage-I to Stage-II, was observed for loading conditions with minimal or no influence of DSA, thus leading to lower life compared to the previous case.