Effect Of Strain Aging Research Articles

Effect of Temperature on the Kinetics of Localized Plasticity Autowaves in Lüders Deformation

The paper analyzes the elastoplastic transition in Fe–0.025 wt. % C at a temperature of 296–503 K and strain rate of 6.67·10−6–3.33·10−3 s−1. The analysis shows that the lower yield stress increases by a power law with increasing the strain rate, and that its rate sensitivity decreases linearly with increasing the test temperature. At temperatures lower than 393 K, the rate sensitivity of the lower yield stress is normal, and at 393–503 K, it is zero. In the range 393–503 K, the kinetics of the Lüders bands is changed from steady to discrete, and the higher the strain rate, the higher the temperature of this transition. Using the available data on the dynamics of dislocations and diffusion of interstitial impurities in the test alloy, it is demonstrated that the kinetics of Lüders bands are controlled by the effect of dynamic strain aging. If the arrest time of mobile dislocations tw at barriers which are overcome via thermal activation is comparable with the precipitation time of interstitial atoms ta at these dislocations, the motion of a Lüders band is discrete, and the band represents an excitation wave of localized plasticity; its refractory period is determined by the time of dynamic strain aging. If ta >> tw, the band moves monotonically and represents a switching autowave. The results of the analysis suggest that the effect of serrated yielding at the lower temperature boundary of blue brittleness can be suppressed by increasing the strain rate. When the arrest time of dislocations tw decreases, the comparability of tw and ta is broken, and no excitation autowave is formed. The data reported in the paper can be used to develop warm rolling technologies for materials with a sharp elastoplastic transition.

Effect of Dynamic Strain Aging on Hardness in the Heat-Affected Zone of SUS316 Steel Welds

Effect of Dynamic Strain Aging on Hardness in the Heat-Affected Zone of SUS316 Steel Welds

Investigation of the Dynamic Strain Aging Effect in Austenitic Weld Metals by 3D-DIC

Austenitic stainless steels similar to type AISI 316L are widely used structural materials in current and future nuclear reactors. Careful development and characterization of these materials and their welds is needed to verify the structural integrity of large-scale multicomponent structures. Understanding the local deformation behavior in heterogeneous materials and the mechanisms involved is key to further improve the performance and reliability of the materials at the global scale and can help in developing more accurate models and design rules. The full-field 3D digital image correlation (3D-DIC) technique was used to characterize two 316L multi-pass welds, based on cylindrical uniaxial tensile tests at room temperature, 350 °C, and 450 °C. The results were compared to solution annealed 316L material. The inhomogeneous character and dynamic behavior of the 316L base and weld materials were successfully characterized using 3D-DIC data, yielding high-quality and accurate local strain calculations for geometrically challenging conditions. The difference in character of the dynamic strain aging (DSA) effect present in base and weld materials was identified, where local inhomogeneous straining in weld material resulted in discontinuous type A Portevin–Le Châtelier (PLC) bands. This technique characterized the difference between local and global material behavior, whereas standard mechanical tests could not.

DISLOCATION STRUCTURE IN A LOCALIZED DEFORMATION BAND FORMED DURING TENSION OF A NORMALIZED 09G2S STEEL SPECIMEN

A transmission electron microscopy study was carried out to examine the dislocation structure of normalized steel 09G2S that exhibits a strain aging effect. In a specimen stretched to the yield point, the maximum dislocation density is observed in the center (active zone) of the localized deformation band and decreases by about an order of magnitude at the band edges. The structure of these band regions, which are at different stages of strain hardening, is analyzed. The Burgers vector of dislocations of the primary slip system is determined; the dislocation ends form ordered flat dipole walls, leading to slip polygonization. The relationship between the specimen structure at the yield point and at the ultimate load is considered.

Introduction of a constitutive material model considering variable carbon content for cutting simulation

Carbon steels show complex strain hardening and dynamic strain aging effects under high plastic strains, high strain rates, high temperatures and high heating rates, and thus a sufficiently complex constitutive model is needed to capture this behavior. Carbon content in such steels (e.g., AISI 1018, 1045, 1075) influences strength and hardening as well. A recently-developed material model for 1045 steels was therefore modified to capture the effect of carbon content on the material response. The model can also capture time-dependent plastic response associated with interrupted austenite transformation under short (sub-second) heating times, for the simulation of machining operations.

Deformation mechanisms for a new medium-Mn steel with 1.1 GPa yield strength and 50% uniform elongation

A new medium-Mn steel was designed to achieve unprecedented tensile properties, with a yield strength beyond 1.1 GPa and a uniform elongation over 50%. The tensile behavior shows a heterogeneous deformation feature, which displays a yield drop followed by a large Lüders band strain and several Portevin-Le Châtelier bands. Multiple strain hardening mechanisms for excellent tensile properties were revealed. Firstly, non-uniform martensite transformation occurs only within a localized deformation band, and initiation and propagation of every localized deformation band need only a small amount of martensite transformation, which can provide a persistent and complete transformation-induced-plasticity effect during a large strain range. Secondly, geometrically necessary dislocations induced from macroscopic strain gradient at the front of localized deformation band and microscopic strain gradient among various phases provide strong heter-deformation-induced hardening. Lastly, martensite formed by displacive shear transformation can inherently generate a high density of mobile screw dislocations, and interstitial C atoms segregated at phase boundaries and enriched in austenite play a vital role in the dislocation multiplication due to the dynamic strain aging effect, and these two effects provide a high density of mobile dislocations for strong strain hardening.

Effects of hydrostatic preload on strain hardening and strain aging of boiler tubes

Tests were conducted to identify the effects of existing defects on strain hardening and strain aging of boiler tubes. Electron backscatter diffraction (EBSD) and finite element analysis (FEA) were used to quantify the local plastic strain near the pre-existing defects. Strain hardening and strain aging effects depend on the degree of plastic deformation. Any existing defects in the boiler tubes can act as stress raisers during the room temperature hydrostatic test. For the morphology and orientations of the defects, a preload of 95 MPa which is equivalent to the hydrostatic test pressure was enough to induce local plasticity. When heated to boiler service temperature and tested to fracture, the mechanical properties of boiler tubes were affected. The test results justify a recommendation that the hydrostatic pressure to be kept near 110% of maximum allowable operating pressure (MAWP) to avoid significant alteration of mechanical properties at stress concentration points in defect-containing boiler membranes.

Effect of Mn content and strain rate on mechanical properties of high-C high-Mn austenitic steel

This research deals with tensile properties and deformation behavior of Fe-xMn-1.2C (wt.%, x = 7, 11, 15 and 19) steels. The effect of Mn content and strain rate on mechanical properties is analyzed considering deformation microstructure observation. Results show that the strength and plasticity of the test steels increase either with the increase of Mn content or strain rate. The increased Mn content enhances the stacking fault energy (SFE) of the test steel, but also aggravates the dynamic strain aging (DSA) effect, which promotes the formation of deformation twins and work hardening characteristic instead. The increased strain rate is also noted conducive for the formation of deformation twins. Consequently, the deformation strain prolongs, and desired strength and plasticity are obtained. At high strain rate of 2 × 10−2 s−1, the strength and plasticity of the 120Mn15 and 120Mn19 steels are observed approximately the same, which can be explained by the combined influences of strain rate and Mn element on the deformation twins and the DSA effect. Moreover, a moderate but stable strain hardening rate during plastic deformation is proved to be a key parameter to achieve excellent strength and plasticity in high-C high-Mn steel. The presented research provides an economical method for the selection of high-C high-Mn steel, i.e., reducing the Mn content, under different service environments.

Effect of γ' particles morphology on dynamic strain aging behavior of Udimet 500 superalloy

The Udimet 500 superalloy is a promising candidate material for power plant gas turbine blades due to its excellent mechanical properties at high temperatures. However, dynamic strain aging (DSA) behavior of the Udimet 500 superalloy during high-temperature tensile test has not been investigated so far. In the present study, the effect of γ' particles size and volume fraction on dynamic strain aging behavior of the Udimet 500 superalloy were investigated in the terms of serration frequency and magnitude. The serrated flow was observed in the stress-strain curves during plastic straining at 800 °C. The results illustrated that the γ' particles morphology has significant effect on serration frequency, serration magnitude, and fracture mechanism of the Udimet 500 superalloy. Although, the serration frequency decreased with reducing γ' particles size and increasing γ' particles volume fraction, the serration magnitude increased. The governing mechanism for dynamic strain aging of the Udimet 500 superalloy have been proposed by dislocation interactions with solute atoms and γ' particles. Furthermore, a non-zero work hardening rate is observed during Lüders elongation which was attributed to high volume fraction of γ' particles in precipitation strengthened superalloys. The effect of dynamic strain aging on fracture mechanism was also investigated. When dynamic strain aging was absent, the dimples initiated at the γ/γ' interfaces during room temperature tensile test. Dynamic strain aging inhibits dimples formation at the γ/γ' interfaces.

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.

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.

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.

Strain-ageing effects on the residual low-cycle fatigue life of low-carbon steel reinforcement

Low-cycle fatigue (LCF) failures can be expected when rebars are subjected to a small number of cycles at large stress/strain amplitudes. Major seismic events are often preceded or followed by other events of smaller/larger magnitude. During each event, the residual fatigue life of the steel reinforcement may reduce, eventually leading to failure of the bars due to the cumulative damage. Strain ageing may reduce even more the residual fatigue life of steel. This paper presents the results of an experimental testing campaign conducted on samples fabricated from New Zealand Grade 300E steel. A benchmark LCF life was obtained for 12-mm steel reinforcing bars subjected to axial-strain-controlled completely reversed cyclic tests at amplitudes ranging between 0.78 and 2.75%. The reduction in fatigue life attributed to strain ageing was obtained by comparing the benchmark fatigue life with that of steel reinforcing bars precycled up to 33% and 66% of the benchmark life and artificially strain aged for an equivalent period of 1 year at 15 °C. The Coffin-Manson and Koh-Stephens models were employed to evaluate the experimental results. The study demonstrates that, depending on the strain amplitude, strain ageing can cause a reduction in total and residual fatigue life ranging from 20 to 70%. Strain ageing should not be ignored during the assessment of earthquake-damaged steel reinforcing bars.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ СКОРОСТИ ДЕФОРМАЦИИ НА СКЛОННОСТЬ УГЛЕРОДИСТОЙ СТАЛИ У8 К ДЕФОРМАЦИОННОМУ СТАРЕНИЮ

The effects of the strain rate and degree on the scale of the strain aging effect in carbon steel U8 with lamellar pearlite microstructure, which is a eutectoid mixture of ferrite plates and brittle plates of secondary cementite Fe3C have been investigated. The microstructure and properties of Russian steel U8 after quasistatic compression, dynamic compression (shock processing), and explosion processing have been investigated. A low-temperature annealing (150–500 °C) of the non-deformed steel U8 has been shown to result in a monotonous reduction of hardness. The quasistatic compression and shock processing of the carbon steel were found to result in the strain aging manifested as an increasing hardness at annealing of the deformed steel U8. The increasing of the deformation degree and strain rate was accompanied by the increasing of hardness of the annealed steel. The explosion treatment was shown not to result in a notable improvement of the mechanical properties of carbon steel as well as in a change of the morphology of cementite plates. The deformation aging effect in steel U8 after the explosion loading was absent. The strain aging effect in U8 steel after quasistatic compression and dynamic loading was suggested to originate from an increasing of the lattice dislocation density, the long-range internal stress fields from which lead to the dissociation of the cementite Fe3C plates and to increasing of the concentration of carbon atoms in the ferrite crystal lattice. The absence of the strain aging effect in the carbon steel after the explosion loading is due to a small degree of the explosion deformation that doesn't allow providing an essential increasing of the lattice dislocation density in ferrite.

Characterization of Dislocation Structure in a Nb-bearing Austenitic Stainless Steel After Low Cycle Fatigue via TEM and EBSD

In this paper, the relationship between Kernel average misorientation (KAM), geometrically necessary dislocation (GND) density and dislocation structures of Nb-bearing austenitic stainless steel under low cycle fatigue (LCF) was studied at 600°C at the total strain amplitude ranged from 0.3% to 1.0%. The results based on EBSD analysis show that the GND density in fatigue specimens gradually increases with the increase of strain amplitude. Under LCF loading, the dislocation structures are mainly planar slip bands (PSBs) and the cell structures. With the increase of strain amplitude, the number of PSBs increases with decrease in width, and the average diameter of cells also decreases. The PSBs originate due to the dynamic strain aging (DSA) effect, and DSA is more significant under high strain amplitude. The average diameter of cell structures has a specific relationship with GND density.

Effect of natural and artificial ageing on steel mechanical properties and fracture toughness

The operating conditions of welded structures of shipbuilding steels, including operation at northern latitudes, determine high requirements for their quality. Materials used for such structures should guarantee stable mechanical properties, good processability during hull fabrication and serviceability at subzero temperatures. Strain aging is due to the thermodynamic non-equilibrium of steel structure in its initial state and gradual transition to the equilibrium state provided the diffusion mobility of interstitial atoms is sufficient. In unfavorable conditions, this can lead to the degradation of properties during processing (cold straightening, bending, welding), operation or long-term storage. The paper studies the probability of natural and artificial ageing processes in steels of different chemical compositions due to bulk diffusion and carbon dislocation core diffusion (dislocation pipe diffusion). The effect of strain ageing on mechanical properties and the CTOD parameter value has been examined.

Study on Static Strain Aging Kinetics of High-Carbon Steel Wires and Its Impact on High-Strength Steel Cords

Under quasi-static loading, an irregular failure mode of high-strength thin carbon steel cords were observed after low-temperature thermal aging. Character and kinetics of damage in such wire ropes highly depend on the plastic elongation of the steel wires, which is significantly modified by the strain aging effect. In this paper, the static strain aging effect on heavily drawn high-carbon steel wires and their cords is experimentally studied in the 80–200 °C temperature range. The kinetics of the aging process is studied in detail. Experimental data are fit by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetic model. The temperature dependence of the static strain aging process is given by means of the Arrhenius equation. The associated JMAK exponents, the apparent activation energy and the pre-exponential constant are determined. Quantitative analysis of the affected strength and strain parameters is given, and based on this, the macroscopic failure mechanism is fundamentally explained.

Effect of Strain Aging on the Rate Dependence of the Strain Resistance and the Cold Resistance of Low-Carbon Steels with Ferritic–Pearlitic and Bainitic Structures

Effect of Strain Aging on the Rate Dependence of the Strain Resistance and the Cold Resistance of Low-Carbon Steels with Ferritic–Pearlitic and Bainitic Structures

Deformation mechanism and microstructure evolution of medium-Mn AHSS under various loading conditions

Third-generation advanced high-strength steels possess an exceptional combination of high strength and ductility owing to their high work-hardening rate during deformation. In this study, the mechanical properties and microstructure/texture evolution of medium-Mn steel under various loading conditions (uniaxial tension, in-plane torsion, and equibiaxial tension) were investigated. Interrupted electron back-scattered diffraction and X-ray diffraction characterizations were carried out to track the microstructure and texture evolution of individual phases. Digital image correlation was employed for the in-situ measurement of formation and expansion of deformation bands, i.e., the Lüders and the Portevin-Le Châtelier (PLC) bands. The dynamic strain aging effect in terms of the formation of PLC bands and stress serrations was significantly affected by the loading conditions and stress level. Multiple types of stress serrations were observed in all tested specimens at different deformation stages. The texture evolution of the steel under various deformation modes was further tracked. Ferrite and fresh martensite exhibited the plain texture evolution like most body-centered cubic metals, whereas austenite showed a more complex texture evolution. The martensitic transformation notably decreased the RD||<001> intensity of the austenite; austenite debris was rotated with ferrite grains, while some intact austenite grains self-rotated to a stable orientation, which is unfavorable for martensitic transformation. The obvious intragranular orientation gradient observed in the austenite intact grains implies that considerable deformation occurred in the austenite. Finally, the experimental results indicate that equibiaxial tension primarily promoted the martensitic transformation, and the classical Olson–Cohen model was calibrated to capture the volume fraction evolution of austenite under various stress states.

Dynamic Plasticity Model for Rapidly Heated 1045 Steel Up to 1000 °C.

The National Institute of Standards and Technology (NIST) developed an experimental technique to measure the dynamic flow stress of metals under rapid heating to study their time-dependent plastic response when heating times are short enough to interrupt or bypass thermally driven microstructural evolution. Such conditions may exist as chips are formed in the machining process. Measurements of American Iron and Steel Institute1045 steel behavior up to 1000 °C showed complex thermal softening due to dynamic strain aging effects and the diffusion-limited austenite transformation process beginning at the A1 temperature (712 °C). This paper proposes a constitutive model to capture the flow stress and hardening evolution of 1045 steel under rapidly heated conditions for simulating metal cutting. The model combines the Preston-TonksWallace plasticity model, which uses five parameters to capture complex rate- and temperature-sensitive strain hardening, with a dual-ratesensitivity model to capture the response of rapidly heated 1045 steel. Finally, a strain-rate-dependent Gaussian function is introduced to capture dynamic strain aging effects, which act over a narrow range of temperatures that change with strain rate. The proposed model is compared to existing plasticity models for 1045 steel over the range of data available and at a representative machining condition.