Manifestations Of Dynamic Strain Aging Research Articles

Dynamic strain aging in Inconel718 additively manufactured lattices

The aim of this work is to investigate the manifestations of dynamic strain aging (DSA) on the mechanical behavior of Inconel718 body centered cubic (BCC) and face centered cubic (FCC) strut-based lattices manufactured via laser powder bed fusion. With that goal, both lattices were tested uniaxially in compression at temperatures ranging from 25 to 600 °C at 1x10-3 s−1 and strain rate jump tests were carried out at the same temperatures between 1x10-5 and 5x10-2 s−1 in order to detect the occurrence of serrated flow and to analyze the strain rate sensitivity. It was found that DSA manifests at the metamaterial level within a range of testing conditions that is highly dependent on the lattice topology. In particular, DSA is comparatively more pronounced in the FCC architecture. This study emphasizes the importance of analyzing the incidence of DSA in additively manufactured lattice structures targeted for structural applications.

On the substructure evolution and its distinct role on dynamic strain aging in a multi-component cobalt base alloy at intermediate temperatures (300 to 700 °C)

Manifestations of dynamic strain aging (DSA) in a solid-solution multi-component cobalt-base alloy was evaluated over wide temperature [25–1000 °C] and strain rate range [10−4–10−1 s−1]. Trends in serration onset strain with temperature together with kinetic analysis revealed the features of Portevin-Le Chatelier effect (PLC) and inverse-PLC. Besides, distinct dependence of strain rate sensitivity on the strain rate jump direction is observed between 10−4 and 10−3 s−1 and >550 °C. Detailed post-deformation characterization was done using transmission electron microscopy. In the initial stages (∼0.05 strain), multipoles were found to be the predominant local obstacles responsible for DSA, and strain accumulation occurs through their unzipping. The substantial presence of widely separated (∼23 nm) extended dislocations indicates the strong dominance of Suzuki segregation. Besides, a continuous evolution of heterogeneous to homogeneous slip vis-à-vis multipole density and their arrangement with temperature is observed. This study provides new insights into substructural evolution and its role on DSA mechanisms in multi-component alloys.

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.

A study on tensile flow and strain hardening behaviour of β-Titanium at elevated temperatures

In this paper, the tensile flow and work hardening behaviour of Beta titanium alloy was examined over a wide temperature range of 28 to 500°C. The work hardening of the material was modelled using different empirical relations such as Swift, Voce and Ludwigsen and the prediction accuracy of these models were compared with different statistical parameters. The hardening rate of the material was analysed with K-M analysis. The fracture behaviour of the tensile samples was studied using SEM results. The yield strength and ultimate tensile strength of the material was found to be decreased from room temperature to 200°C followed by a gradual increase with increase in temperature. Uniformly distributed serrations were observed in the entire plastic region of the flow curve in the temperature range of 200– 400°C. This serrated behaviour could be due to the manifestation of dynamic strain aging in this temperature range. The Voce hardening model was identified as a best hardening model to describe the material hardening. SEM results revealed the ductile mode of fracture at all temperature with large number of dimples in fractographs.

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.

Proposition and research progress of the third-type strain aging

The traditional knowledge of plastic flow behavior of metal, dislocation activation theory, and thermoviscoplastic constitutive model of metal needs a further perfection due to the occurrence of third-type strain aging. Third-type strain aging leads to a bell-shaped flow stress-temperature curve, that is, the flow stress first increases as temperature increases, and after a peak value is reached, it decreases with further increase of temperature. Third-type strain aging occurs at both low and high strain rates. The bell-shaped segment induced by third-type strain aging on the flow stress-temperature curve shifts to higher temperature region as strain rate increases. In order to develop a systematic understanding of the third-type strain aging, firstly, macro characteristics of third-type stain aging effect that was distinguished from static strain aging effect and Portevin-Le Chatelier dynamic strain aging effect were introduced. Then, the micro mechanism of third-type strain aging and correlation of third-type strain aging, Portevin-Le Chatelier dynamic strain aging, blue brittleness phenomenon, and internal friction were systematically concluded. Third-type strain aging, Portevin-Le Chatelier dynamic strain aging, blue brittleness phenomenon, and mechanical spectroscopy are all resulted from the interaction of mobile dislocations with diffusion atoms. Third-type strain aging, Portevin-Le Chatelier dynamic strain aging, and blue brittleness phenomenon are different manifestations of dynamic strain aging. Third-type strain aging can be considered as another mode of mechanical spectroscopy. The common constitutive models are able to capture the plastic behavior of many metals under the coupling influence of strain rate and temperature in some cases. However, these thermoviscoplastic constitutive models do not take the effect of third-type strain aging into consideration, and they cannot describe the thermoviscoplastic behavior including the third-type strain aging effect. To accurately describe the plastic behavior of metals, some constitutive models including the effect of third type strain aging were proposed. The development of the thermoviscoplastic constitutive model of metal considering the effect of third type strain aging was finally introduced.

Tensile Flow Analysis of Austenitic Type 316LN Stainless Steel: Effect of Nitrogen Content

The tensile flow analysis of austenitic type 316LN stainless steel with nitrogen level ranging from 0.07 to 0.22 wt.% is studied at the temperatures from 300 to 1123 K and at a strain rate of 3 × 10−3 s−1 using Ludwigson flow relationship. It has been observed that Ludwigson flow relationship provided an appropriate description for the σ-ep behavior and subsequent work/strain hardening response which displayed three independent temperature regions of ambient (300 K), intermediate (523-923 K) and elevated temperature (≥ 923 K). The anomalous response in terms of stress–strain data grouping together in a narrow band and appearance of peaks/plateau in Ludwigson parameters in the intermediate temperature region (523-923 K) is attributed to the manifestation of dynamic strain aging (DSA). The parameters such as strain hardening exponent (n1) and coefficient (K1), transition stress (σL) and transition strain (ɛL) increased with increasing the amount of nitrogen that in turn diminished the propensity to dynamic recovery. The applicability of Ludwigson equation permitted to predict the flow stress response and yield strength as well as ultimate tensile strength values with one-to-one correlation in comparison with the experimental data.

Serrated yielding in an advanced stainless steel Fe-25Ni-20Cr (wt%)

Understanding serrated yielding behavior resulting from dynamic strain aging (DSA) is essential for design and safety considerations. In this work, uniaxial tensile tests were carried out at temperatures ranging from 298 to 1073 K and strain rates 10−5 – 10−3 s−1 followed by microstructural examination of Fe-25Ni-20Cr (wt%) austenitic stainless steel (Alloy 709), a candidate structural material for Sodium-cooled Fast Reactors. Serrated yielding was found to occur in this alloy in two temperature regimes; low-temperature serrated flow (LT-SF) from 498 to 648 K and high-temperature serrated flow (HT-SF) from 648 to 973 K (depending on the strain rate) with activation energies of 103 ± 13 kJ/mole and 204 ± 11 kJ/mole respectively. The critical strain for the occurrence of serrations was found to increase with strain rate as an exponential function with exponent (m + β) of 0.78 ± 0.1 and 1.56 ± 0.2 for the LT-SF and HT-SF regimes respectively. Based on the activation energies and m + β values, diffusion of interstitial atoms has been suggested to be responsible for serrated flow in the LT-SF regime while Cr atom migration was inferred to be responsible for DSA in the HT-SF regime. Manifestations of DSA in the Alloy 709 were observed including peaks and/or plateaus in flow stresses along with negative strain rate sensitivity at intermediate temperatures. However, no loss in ductility was observed within DSA regime attributed to the relatively high work hardening rate and strain-hardening exponent. The samples deformed in DSA regime showed planar substructure while equiaxed subgrains formed at higher temperatures. The fraction of low angle grain boundaries after deformation exhibited a valley at intermediate temperatures believed to be another manifestation of the DSA.

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.

Observations on Dynamic Strain Aging Manifestation in Inconel 718 Superalloy

The manifestation of dynamic strain aging (DSA) in Inconel 718 is reported in this work. Analysis were performed in the material with different microstructures resulting from solution anneal and aging treatment. Tensile tests were made under secondary vacuum with temperature ranging between 200 and 950°C and strain rates of 3.2 x 10-3 to 3.2 x 10-5 s-1. Results showed the range of DSA occurrence. Analysis indicates that at lower temperatures, from approximately 200 to 450°C, serrations are controlled by the diffusion of carbon. At higher temperatures, until 800°C, DSA coincided with the occurrence of other thermally activated phenomena: dynamic precipitation, especially γ’’, and Oxidation Assisted Intergranular Cracking (OAIC). It was observed that competitive phenomena affect DSA manifestation directly due to the availability of niobium in solid solution.

Monotonous deformation behavior of ferritic FeCrAl alloy in the dynamic strain aging regime

Monotonous deformation behavior of a ferritic FeCrAl alloy at a temperature range from room temperature to 600 °C was studied using the uniaxial tensile tests. The results showed that dynamic strain aging (DSA) occurred at a strain rate of 3.333 × 10−4s−1 over the temperature range from 200 °C to 400 °C. Tensile tests were carried out at four different strain rates in the DSA temperature domain to investigate the combined action of temperature and strain rate on DSA. Negative strain rate sensitivity of ultimate tensile strength (UTS) was observed as the characteristic manifestation of DSA performed in FeCrAl alloy. It was also found out that, type A, A+B, B, C+D and C serrations appeared in turn in the stress-strain curves tested at a constant strain rate with the increase of temperature, and the decrease in strain rate shifted the DSA temperature range toward the lower temperature region. Statistical method was used on stress drops with DSA to investigate the character of serration. From the activation energy calculations with Mccormick's model, the solute atoms responsible for DSA were identified to be substitutional aluminum atoms.

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.

Features of Dynamic Strain Aging of Low-Carbon Steels during Severe Plastic Deformation Processing

The paper considers the features of the manifestation of dynamic strain aging (DSA) effect during severe plastic deformation processing via equal-channel angular pressing of low-carbon steel 10 and during the deformation processing via rolling of steel 20Kh. The deformation mechanisms under different regimes of deformation processing are analyzed. The temperature ranges for the manifestation of the DSA effect during the deformation by rolling of steel 20Kh and by equal-channel angular pressing of steel 10 are established. It is demonstrated that the deformation of low-carbon steels in the temperature range of DSA leads to further structure refinement and, as a consequence, to the enhancement in strength properties.

Effects of temperature and strain rate on tensile deformation behavior of superalloy UNS N10276

Tensile deformation behavior of nickel-base superalloy UNS N10276 was investigated in a wide temperature range of 200–1000°C with intervals of 50°C, employing the strain rates of 5 × 10−5, 3 × 10−4 and 3 × 10−3s−1. Various types of serrations as one of manifestations of dynamic strain aging (DSA) were observed in this alloy. Normal Portevin-Le Chatelier effect (PLE) was observed at temperatures lower than 600°C, while inverse PLE was noticed at temperatures above 600°C. The average activation energy for serrated flow was determined to be 132kJ/mol in the normal PLE regime, which was in line with migration of Mo through pipe diffusion in a Ni matrix. Negative strain rate sensitivity was observed in the DSA regime in this alloy. Yield strength increased with increasing temperature at strain rates of 5 × 10−5 and 3 × 10−4s−1 in the temperature range of 650–850°C, which was mainly related to the formation of M6C carbides at the grain boundary as well as to the enhanced DSA effect during tensile deformation. Typical intermediate temperature embrittlement (ITE) was observed in this alloy. The elongation had high values and remained almost unaffected by the temperature until 500, 600 and 650°C for 5 × 10−5s−1, 3 × 10−4s−1 and 3 × 10−3s−1, respectively. This alloy showed the minimum elongation at 700°C irrespective of strain rates employed, whereas the reduction in area (RA) exhibited the minimum at 700°C for 5 × 10−5s−1, 750°C for 3 × 10−4s−1 and 800°C for 3 × 10−3s−1, respectively. The lower limit of temperature at which intergranular brittle failure happened was reduced with decreasing strain rate. Additionally, it was found that reducing strain rate expanded the brittle temperature spectrum of the alloy.

Flow Behaviour of Modified 9Cr–1Mo Steel at Elevated Temperatures

In this investigation, the flow behaviour of modified 9Cr–1Mo steel at elevated temperatures is reported. To understand the flow behaviour of the steel, tensile tests were performed at nominal strain rate of 3 × 10−3 s−1 and temperatures in the range of 300–823 K. The yield strength and ultimate tensile strength were found to decrease with increase in temperature with a plateau in a intermediate temperature regime (523–673 K). Serrations were also observed in the tensile curve at intermediate temperatures which caused plateau/peak in tensile behaviour of material and was a typical manifestation of dynamic strain ageing. An attempt was made to represent the flow behaviour of the material using different constitutive equations viz., Hollomon, Ludwik, Swift, Ludwigson and Voce. It was observed that the Voce equation could describe the experimental flow curve at different temperatures quite well. Instantaneous work hardening rate with respect to flow stress exhibited two stages of hardening especially at relatively lower temperatures.

Activation energy for serrated flow in type 316L(N) austenitic stainless steel

Tensile tests were performed at a wide range of strain rates (3.16×10−5−3.16×10−3s−1) and temperatures (300–1123K) to examine serrated flow behaviour, an important manifestation of dynamic strain ageing (DSA), in type 316L(N) austenitic stainless steel. The steel exhibited distinct low and high temperature serrated flow regimes separated by a retardation bay. Based on the activation energy value of 115kJmol−1, diffusion of interstitial carbon has been suggested to be responsible for serrated flow in the low temperature regime. In the high temperature regime, an average activation energy value of 206kJmol−1 obtained using different methods indicated that diffusion of substitutional chromium atom is responsible for DSA. Disappearance of serrations at high temperatures has been ascribed to the effects associated with precipitation reaction between chromium and carbon atoms. The influence of nitrogen retarding DSA in type 316L(N) stainless steel has been discussed.

Influence of temperature on tensile flow and workhardening behaviour of modified 9Cr–1Mo steel

Tensile true stress (σ)–true plastic strain (ϵ) and workhardening behaviour of modified 9Cr–1Mo steel have been analysed using flow constitutive relationships proposed by Hollomon, Ludwik, Ludwigson, Swift and Voce for wide range of temperatures (300–1023 K) at a strain rate of 3×10−4 s−1. Relationship proposed by Voce has been found to describe stress–strain data adequately at all temperatures. Variation of instantaneous workhardening rate (θ) with stress (σ) displayed the two stage workhardening behaviour. It was characterised by a rapid decrease in the transient stage followed by gradual decrease in the stage III of workhardening. Three distinct regimes have been exhibited in the variation of workhardening parameters of constitutive relationships and workhardening rate with temperature. The decrease in flow stress and workhardening parameters with increasing temperature indicated the dominance of dynamic recovery at high temperatures. At intermediate temperature (523–723 K) plateau has been observed and considered to be one of the manifestation of dynamic strain aging (DSA). Tensile properties of the steel have been predicted well from the parameters of voce constitutive relationship.

Influence of Temperature and Strain Rate on Tensile Deformation and Fracture Behavior of P92 Ferritic Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10−5 to 1.26 × 10−3 s−1 over a temperature range of 300 K to 923 K (27 °C to 650 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of P92 ferritic steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The fracture mode remained transgranular. The steel exhibited serrated flow, an important manifestation of dynamic strain aging, along with anomalous variations in tensile properties in terms of peaks in flow stress/strength and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. At high temperatures, the rapid decrease in flow stress/strength values and work hardening rate, and increase in ductility with increase in temperature and decrease in strain rate, indicated the dominance of dynamic recovery.

Dynamic strain aging behavior of modified 9Cr–1Mo and reduced activation ferritic martensitic steels under low cycle fatigue

Influence of temperature and strain rate on low cycle fatigue (LCF) behavior of modified 9Cr–1Mo ferritic martensitic steel and 1.4W–0.06Ta reduced activation ferritic martensitic (RAFM) steel in normalized and tempered conditions was studied. Total strain controlled LCF tests between 300 and 873K on modified 9Cr–1Mo steel and RAFM steel and at various strain rates on modified 9Cr–1Mo steel were performed at total strain amplitude of±0.6%. Both the steels showed continuous cyclic softening at all temperatures. Whereas manifestations of dynamic strain aging (DSA) were observed in both the steels which decreased fatigue life at intermediate temperatures, at higher temperatures, oxidation played a crucial role in decreasing fatigue life.

Influence of dynamic strain aging on the deformation behavior during ratcheting of a 316LN stainless steel

Ratcheting behavior of 316LN austenitic stainless steel (SS) was investigated at 823K as a function of mean stress (σm) and stress amplitude (σa). Test results obtained under different combinations of σm–σa were analyzed in terms of total accumulated strain and cyclic life in order to understand the manifestations of dynamic strain aging (DSA) during ratcheting. The results obtained under specific combinations of σm and σa were correlated with the surface relief, crack density distribution, fracture behavior and substructural changes. Deformation behavior was found to vary with the loading conditions. DSA increased the fatigue life of 316LN SS.