Dynamic Strain Aging Phenomenon Research Articles

Dynamic strain aging and negative strain rate sensitivity in coarse-grained Al0.3CoCrFeNi high entropy alloy under hot compression

This study investigates the deformation behavior of coarse-grained (CG) Al0.3CoCrFeNi alloy under hot compression, revealing notable strain rate sensitivity (SRS) and dynamic strain aging (DSA) phenomena. DSA is observed at temperatures between 1223 K and 1373 K, exclusively at lower strain rates (0.5 s−1 – 1 s−1), while higher strain rates (≥5 s−1) exhibit DSA absence. Non-compact {112} slip activation on dislocation forest structures is proposed as the driving force behind DSA. Negative SRS behavior at 1223 K is attributed to Ni–Al cluster-based solid solution strengthening. Activation energy calculations highlight lattice diffusion and partial dislocation constriction as contributors to hot deformation. Consistency is observed in the activation volume (V*) calculation, indicating dislocation forest formation (127–519 b3, b = Bureger's vector), diminishing to 22–46 b3 upon DSA initiation. The alloy displays delayed dislocation hardening/softening transition due to CG's impact on retarding dislocation accumulation. This behavior stems from a higher V* at yielding, prolonging the typical transition observed in fine-grained alloys with lower V*. The study elucidates the intricate interplay of microstructure, strain rate, and deformation mechanisms in CG Al0.3CoCrFeNi.

Nanoindentation responses of Fe–Cr alloys from room temperature to 600 °C

In this work, the evolution of nanomechanical properties was studied systematically as a function of temperature, chemical, and microstructural complexity of different Fe-based alloys. Experiments were performed at different temperatures (room temperature, 200 °C, 400 °C, 600 °C) using the nanoindentation technique on low activation Fe9Cr–1WVTa (Eurofer97), model Fe–9Cr–NiSiP, Fe–9Cr alloys, and pure iron samples, followed by microstructural observations. The results show varying softening and hardening effects depending on the experimental temperature, demonstrating Portevin-Le-Chatelier effect, i.e., dynamic strain aging phenomenon in model alloys. Sources of the dynamic strain aging instabilities were traced back to the interaction between dislocations and alloying elements such as interstitial carbon and substitutional chromium. The materials undergo dynamic recovery and recrystallization below the regions of high-temperature indentation depending on the pre-indentation dislocation density and the alloy composition. Our findings help in the understanding of the structure and mechanical property relationship in complex Eurofer97 alloy at high-temperatures for potential nuclear applications as structural materials.

Mechanism of high temperature mechanical properties enhancement of Ni-based superalloys repaired by laser directed energy deposition: Microstructure analysis and crystal plasticity simulation

The high-temperature tensile and fatigue performance play a crucial role in the applications of repaired Ni-based superalloys by Laser directed energy deposition (LDED). This work investigates the effect of microstructure on the high temperature (650 °C) mechanical behaviors of GH4169 superalloys repaired by LDED with the microstructure characterizations and crystal plasticity modelling. Results indicate that the solution and aging heat treatment (STA) is necessary to homogenize the distributions of precipitations. Microstructure characterizations and crystal plasticity modelling imply that the plasticity deformations occur mainly in columnar dendrites of fusion area and twin grains of substrate, which enhance the tensile strength and elongation rate of repaired area and demonstrates comparable characteristics with the casting alloy. At the same time, the existence of the Laves phase and carbide cannot eliminate the dynamic strain aging phenomenon of repaired material. Similarly, under high stress levels (850– 950 MPa), the fatigue performance of repaired material is primarily influenced by the plastic deformation and exhibits similarity to that of the substrate; however, under low stress levels (650– 750 MPa), it is primarily influenced by the presence of defects, resulting in an obviously longer fatigue life for the substrate.

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.

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.

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.

Room temperature deformation mechanisms of a Fe–Mn–Al–C steel

This study aims to investigate the deformation mechanisms of an austenite-based duplex Fe–Mn–Al–C steel at room-temperature through uniaxial tensile tests, strain rate sensitivity and stress relaxation analysis. Through numerical analysis and microstructural studies, this investigation reveals that increasing strain leads to the formation of low-angle boundaries (LABs) at the vicinity of grain boundaries. Consequently, these LABs play a facilitating role in the stress-assisted diffusion of carbon atoms, thereby triggering the dynamic strain aging (DSA) phenomenon. The evidence for this is substantiated by the strain rate sensitivity coefficient and stress relaxation results obtained from numerical analysis during uniaxial tensile tests. As deformation progresses, the misorientation of LABs increases, influencing the strain hardening behavior. The intensified DSA contributes to enhanced work hardening, while substructure development results in softening, creating a trade-off between these contrasting effects.

On the normal and inverse Portevin–Le Chatelier behavior in hot rolled Inconel 617 superalloy

In the present study the effect of hot rolling process on the normal and inverse Portevin-Le Chatelier behavior was investigated. The hot rolling process was carried out at 950 °C, 1050 °C, and 1150 °C to capture the effect of thermomechanical process on elimination or postponing of dynamic strain aging phenomenon in Inconel 617 during subsequent deformation process. The tensile test was performed within the temperature range between 350 °C and 750 °C at a constant strain rate. The microstructure evaluations were carried out by optical microscope and scanning electron microscope. The serrated flow was observed in the stress-strain curves at specific temperatures. The type of serrations changed from type B to type C at the strain rate of 5 × 10−4 s−1 as the tensile test temperature was enhanced. The change of normal PLC effect to the inverse PLC effect was intensified when the hot rolling temperature was increased; Hence, the inverse PLC effect was observed at 750 °C in the sample which had rolled at 1150 °C. Whilst the serrated flow was not seen at 750 °C in the samples which had rolled at 950 °C and 1050 °C. The critical strain, for the initiation of the serrated flow was found to reduce with the increasing temperature of hot rolling from 950 °C to 1150 °C. After tensile test, the microstructural evaluations revealed the proven substantial role of precipitation of γ′ and the Cr-rich M23C6 carbides in pinning of the mobile dislocations, which led to the inverse PLC effect.

On the Influence of Precipitation on the Dynamic Strain Aging in Mg-2%Nd

The Mg-2%Nd alloy, thermally treated at 520°C and quenched, exhibits plastic instabilities at intermediate temperatures between 100°C and 300°C when it is compressively tested because of the dynamic strain aging phenomenon caused by the presence of neodymium atoms in solid solution. During compression tests, there is a competition between the precipitation of Mg-Nd particles and the DSA phenomenon, which depends largely on the test temperature. Thus, below 200°C, the formation of neodymium-rich precipitates is slow, and the interaction is insignificant. Under these conditions, the alloy exhibits a normal behavior of the DSA, i.e., the critical strain decreases with the increase in the test temperature. However, for test temperatures > 200°C, the formation of neodymium-rich precipitates is faster and the evolution of the critical strain with temperature is inverted because of the decrease of neodymium in solid solution.

Sensitivity to dynamic strain aging of carbon–manganese steels measured by ultrasonic fatigue testing

AbstractSome carbon–manganese (C–Mn) structural steels are known to be sensitive to aging under deformation between 100 and 350°C, phenomenon called dynamic strain aging (DSA). This paper is aimed at studying the DSA sensitivity through the thermal response of C–Mn steels during ultrasonic fatigue tests. For this purpose, self‐heating tests are conducted on two C–Mn steel grades, extensively used in the nuclear industry. Tests are monitored by infrared thermography. DSA effect clearly appears on self‐heating curves by a singular temperature rise. The DSA occurrence is correlated with results from internal friction tests, in particular with the presence of the cold work peak. The temperature of 100°C related to the start of the thermally activated DSA phenomenon is retrieved by self‐heating tests. Based on a further intrinsic dissipation computation, DSA distinctly differs from the classical dislocation glide mechanism. Furthermore, a stress threshold of irreversibility is shown to emerge.

Occurrence of dynamic strain aging in intercritically annealed low carbon high aluminum medium manganese steels

Medium manganese steel grades heat treated with the quench and partition (Q&P) treatment have shown promising results of strength and formability, but the number of studies on high aluminum medium manganese Q&P steels still remains rather limited. Consequently, this study investigates the mechanical properties, behavior and microstructures of a low carbon-high aluminum medium manganese (3%) steel after intercritical annealing and subsequent Q&P heat treatments with varying heat treatment parameters. Samples that were intercritically annealed above 760 °C showed an ultimate tensile strength (UTS) of around 1500 MPa with uniform elongation values of approximately 10%. On the other hand, the samples annealed at 740 °C showed somewhat lower UTS values (in the range of 1000 … 1200 MPa) but much higher uniform and total elongations, i.e., considerably improved formability. However, the stress-strain curves of the samples annealed at 740 °C showed rather severe serrations that can be connected to the dynamic strain aging (DSA) phenomena, which limits the usability of the steel especially in applications where good surface quality of the deformed (sheet) is required. Therefore, the main focus of this article is on the samples annealed at temperatures close to A1, where DSA serrations tend to appear. The evidence gathered from these investigations leads to the conjecture that the DSA serrations are due to the free carbon and nitrogen originating from the dissolution of M(C,N) and M2(C,N) type carbides, which were formed during the intercritical annealing at temperatures producing less than 50% of the austenite phase. Other reasons for the occurrence of DSA in the current test materials can be the scarcity of martensite and the morphology of the retained austenite.

Thermo-viscoplastic behavior of DP800 steel at quasi-static, intermediate, high and ultra-high strain rates

Using the shear-compression specimen (SCS), thermo-viscoplastic behavior of DP800 steel has been investigated systematically over a wide range of strain rates, including quasi-static (0.001–1 s−1), intermediate (10–100 s−1), high (2000–8000 s−1) and ultra-high (16,000–50,000 s−1) regions. Temperatures between 293 and 1073 K were considered. Experimental results show: (1) DP800 steel exhibits non-linear strain rate sensitivity. For tests at strain rates lower than 4000 s−1, the flow stress increases slightly. However, for tests at strain rates above 8000 s−1, a significant flow stress upturn exists; (2) dynamic strain aging (DSA) occurs within the temperature range of 473–873 K and strain rates below 100 s−1; (3) both strain rate sensitivity and temperature sensitivity of DP800 steel are influenced by DSA, and bell-shaped sensitivity curves form; (4) with increasing strain rates, peaks of the bell-shaped temperature sensitivity curves move to higher temperature regions, and this law is equally valid for the effect of temperature on the strain rate sensitivity curves; (5) strain hardening behavior of DP800 steel is dominated by a competition between strain rate hardening, DSA hardening and thermal softening. With increasing strain rate, single peak or double peaks are observed in average strain hardening rate-strain rate curves. According to experiments, a semi-physical constitutive relation has been established, taking into account both the viscous drag effect and the DSA phenomenon. The established constitutive model can predict flow stress curves of DP800 steel accurately with an average error of 7.12%. Both the enhanced strain rate sensitivity due to viscous drag and the bell-shaped flow stress curves caused by DSA are captured correctly.

Thermo-viscoplastic behavior and constitutive relations for 304 austenitic stainless steel over a wide range of strain rates covering quasi-static, medium, high and very high regimes

Using the recently developed single shear specimen, deformation behavior of 304 austenitic stainless steel is studied systematically over a wide range of strain rates, covering quasi-static (0.001-0.1 s−1), medium (1-10 s−1), high (3000-6000 s−1) and very high (13000-39000 s−1) regimes, and temperatures between 77 and 1073 K were considered. The experimental flow stress curves show that: (1) the flow stress increases slightly at strain rates below 6000 s−1; concerning strain rates above 13000 s−1, however, an obvious flow stress upturn exists; (2) dynamic strain aging (DSA) occurs within the temperature range of 673 to 873 K and strain rates lower than 10 s−1; (3) both temperature sensitivity and strain rate sensitivity of the material are affected by DSA, and bell-shaped sensitivity curves form; (4) with increasing strain rates, peaks of the bell-shaped temperature sensitivity curves move to higher temperature regimes, and the evolution law is equally valid for the influence of temperature on the strain rate sensitivity curves. According to experiments, eight constitutive relations, including three phenomenological, four physical-based, and an artificial neural network, are established, including viscous drag effect at very high strain rates but excluding DSA phenomenon. Prediction capacities of the established constitutive models are checked by testing condition jump tests at randomly chosen strain rates and temperatures. Description capacities of the constitutive models in terms of strain hardening rate, temperature sensitivity and strain rate sensitivity are also analyzed.

Dynamic strain aging and twin formation during warm deformation of a novel medium-entropy lightweight steel

The present study deals with the extraordinary twin formation during compressive deformation of a Fe-37.55Mn-8.5Al-0.6C (wt.%) low-density steel at warm temperature regime (200–600 °C). Despite the conventional opinion regarding the restrictive effect of increasing temperature on the occurrence of deformation twinning, not only the twin boundary fraction did not decrease, but also twinning was encouraged at 400 °C and even 600 °C. This was discussed relying on the occurrence of dynamic strain aging phenomenon, which caused a bump in corresponding hardening rate curves. The medium entropy of the alloy, which made short-range diffusion of solute atoms feasible and facilitated local pinning of the mobile dislocations, provided a proper condition for the occurrence of high temperature strain ageing. Finally, the unbalanced pinning of trailing and leading dislocations influenced on their velocity and caused widening the stacking faults which in-turn could significantly assist deformation twinning at such high deformation temperatures.

Effect of solute clusters on plastic instability in magnesium alloys

The interaction between solute atoms and matrix dislocations during plastic deformation of materials strongly influences the flow behavior and can lead to plastic instability at the macroscopic scale. In this study, Lüders band formation and dynamic strain aging phenomena were investigated in a Mg–1%Mn-1%Nd (wt.%) alloy, deformed in uniaxial tension at ambient temperature and 150 °C. The investigated alloy was studied in the initial as-extruded state and after a subsequent heat treatment at 275 °C for 120 min, which allowed for a variation of solute concentration in the microstructure, and thus for a different type/level of solute – dislocation interaction. Microstructure investigations at near-atomic resolution using three dimensional atom probe tomography were carried out in order to visualize the spatial distribution of alloying elements and reveal potential solute clustering trends in connection with the occurrence of plastic instability. Identified nanoscale solute clusters were analyzed on the basis of their size distribution, chemical composition, and statistical significance. Results demonstrate that the number, as well as the types of solute clusters, play an essential role in triggering plastic instability by interacting with deformation carriers at the atomic scale.

Effect of grain size on dynamic strain aging behavior of C-bearing high Mn twinning-induced plasticity steel

In this work, the effects of grain size on the tensile properties, serrated flow, Portevin-Le Châtelier (PLC) band evolution, dislocation density, and microstructure evolution of Fe–22Mn-0.6C steel were investigated using digital image correlation (DIC), X-ray diffraction (XRD), transmission electron microscope (TEM) and monotonic tensile tests. Results show that the yield strength, tensile strength, and work hardening value of Fe–22Mn-0.6C steel increase whereas its elongation decreases with decreasing grain size. The critical strain of serration decreases whereas the amplitude of serration increases with decreasing grain size. Moreover, the plateau of serrations is larger and that the strain concentration within PLC bands is more severe in fine-grained steel than in coarse-grained steel. The dynamic strain aging (DSA) behavior of the steel is enhanced by grain refinement. The speed of PLC band movement decreases faster in fine-grained steel than in coarse-grained steel, which results in a decrease in plasticity. XRD and TEM analyses show that the high dislocation density and unique twin substructure may be the main reasons for the strong DSA phenomenon observed in fine-grained steel.

Effect of dynamic strain aging on cyclic deformation in piping material SA333 Gr-6 steel

In piping structures, the low cycle fatigue loading arises from thermally induced strain cycles associated with start-up and shut-downs and fluctuation in loading conditions. SA333 Gr-6 steel is prone to dynamic strain aging (DSA) and the parameters that govern the DSA are temperature and strain rate. There are limited studies on DSA in the investigated steel. In the present investigation, fatigue experiments were conducted at a fixed strain amplitude of ±0.5 % with the variation of temperatures (RT to 400 °C) and strain rates (1 × 10−4 s−1 to 1 × 10−2 s−1). The results reveal that the cyclic deformation behavior depends on temperatures and strain rates. The steel shows cyclic hardening characteristics at elevated temperatures which indicates the occurrence of the DSA phenomenon. The negative strain rate sensitivity and temperature dependency of stress amplitude are the manifestations of DSA. The present study shows the deleterious effect of DSA on fatigue life. The DSA temperature regime found in the temperature 200–250 °C for 1 × 10−4 s−1 strain rate, 250–300 °C for 1 × 10−3 s−1 strain rate, and 250–350 °C for 1 × 10−2 s−1 strain rate. The DSA regime gets shifted to a higher temperature with the increase in strain rate. Transmission electron microscopy investigations reveal that the fatigue failed specimen at the DSA regime shows severe dislocation activities and dislocation tangles, whereas minor dislocation activities and cell structures were observed at the non-DSA regime. The dislocation density calculated from the X-ray diffraction analysis was higher at DSA compared to the non-DSA regime.

Constitutive modeling of flow stress and work hardening behavior while considering dynamic strain aging

The phenomenon of dynamic strain aging (DSA) occurs in many alloy systems and inevitably leads to variations in the flow stress and work hardening behavior. In this study, a modified constitutive relationship that takes into account DSA is proposed for modeling the flow stress and work hardening behavior of alloys. We theoretically show that the additional strengthening caused by DSA depends on the strain and strain rate at a given temperature. The contribution of DSA on flow stress and work hardening rate is clearly predicted by our model, which can account for as much as 9% and 14%, respectively. The modeling results agree well with those of experiments performed on an AlMg alloy that exhibits the DSA phenomenon. Finally, the intrinsic mechanism of DSA-induced strengthening on flow stress and work hardening rate is discussed. It is proposed to be that when the imposed strain or strain rate changes, the binding energy between the diffusing solute atmosphere at the dislocation core and the dislocations also changes, thus affecting the flow stress and work hardening rate during the deformation process. This study quantifies the influence of DSA on flow stress and work hardening rate during deformation, which can provide insight into the understanding of mechanical behavior of materials with DSA.

Mechanical response of a metastable austenitic stainless steel under different deformation modes

The deformation behaviour of AISI 304L stainless steel was examined under uniaxial tension and shear deformation modes in the temperature range of 0–500°C. A linear relationship between the tensile and shear yield stresses was observed over the whole temperature range, which was in a good agreement with the predictions of the von-Mises criterion. The relationship between the ultimate shear and tensile strength values, however, showed three distinct temperature-dependent regimes. In the temperature ranges corresponding to the transformation-induced plasticity effect and the dynamic strain aging phenomenon, a deviation from the expected behaviour was observed, which was related to the dependency of these phenomena on the loading mode.

Theoretical Study for Dynamic Strain Aging in Niobium: Effect of Temperature and Strain Rate on the Flow Stress

A constitutive model for niobium with the effect of dynamic strain aging is proposed. The crystal structure of metals hugely influences the dynamic strain aging phenomenon and causes considerable alterations in the material’s macroscopic mechanical responses. Dynamic strain aging needs to be accounted for in a constitutive model to obtain accurate predictions of material’s thermo-mechanical behaviors during deformation. The proposed constitutive model attempts to describe the material’s flow stress responses during deformation by separating the flow stress contributions into the athermal component, thermal component, and dynamic strain aging component. Two different mathematical equations are proposed to model the dynamic strain aging component. The proposed model attempts to describe the mechanical response of niobium for a wide range of strain rates: from quasi-static loading ( $$\dot{\varepsilon } = 0.001\,{\text{s}}^{ - 1}$$ ) to dynamic loading ( $$\dot{\varepsilon } = 3300\, {\text{s}}^{ - 1}$$ ) across the temperature ranges 77 K–800 K.