Dynamic Strain Aging Model Research Articles

Serrated Flow Behavior in Commercial 5019 Aluminum Alloy

Serrated flow effects are visible on a metal surface even after coating. Thus, they are undesirable to manufacturers and product users. To meet the expectations of the industry, research on the conditions for serrated flow occurrence in 5019 aluminum alloy was carried out and the results were collected in the current paper. Thus, the influence of the alloy initial microstructure due to different tempers as well as plastic deformation conditions, i.e., strain rate and temperature, on the alloy stress–strain behavior was determined. Two tempers were considered: the as-fabricated F-temper and the W-temper (i.e., quenched in water after annealing at 500 °C). The synergic influence of these tempers and their tensile test conditions on the serration behavior of the stress–strain curves, i.e., the stress drop and reloading time, were also determined and categorized. Structural and X-ray diffraction studies rationalized the stress–strain characteristics according to dynamic strain aging models with positron annihilation lifetime spectroscopy providing insight into the role of lattice defects (i.e., dislocations and vacancies). The map of the serrated flow domain allowed us to obtain the activation energy of the onset of the Portevin–Le Chatelier effect equal to 56 kJ/mol. It is close to the activation energy for the pipe diffusion mechanism, obtained by applying the model formulated originally for Type B stress serration.

A crystal plasticity model of Dynamic Strain Aging for a near-[formula omitted] Ti-alloy

Timetal-834, a near-α Ti-alloy, finds use in aeroengines due to its favorable properties. However, the alloy exhibits Dynamic Strain Aging (DSA) in the temperature range of 375-475 °C, that can cause it to fail early during service. As DSA is due to the micro-scale interaction between the diffusing solutes and mobile dislocations, a crystal plasticity model is developed in this work to capture this phenomenon. Additionally, the influence of the α (HCP) and β (BCC) laths of the transformed-β colony of this alloy on the DSA behavior is captured by using an equivalent model based on Taylor’s assumption. The effect of slip system dependent lath and colony size effect due to Burger’s Orientation Relation is also incorporated. Crystal plasticity finite element method simulations of polycrystalline domains, representative of Timetal-834, are performed for calibration and validation of the model using the available temperature dependent experimental data, and successfully capture the flow stress and DSA behavior. The strain rate dependency of DSA predicted by the model also shows an excellent agreement when compared with experimental data. Microstructural analysis of the simulations show that the orientation of grains/colonies have a strong influence on the aging time and hence the DSA response. Also, the initiation of DSA and frequency of serrated slip on the aligned systems of laths show differences due to dissimilar resistances and constraints on plastic flow. Overall, the model developed in this work can capture the microstructure dependent DSA and can be utilized for engineering of Timetal-834.

Analysis of dynamic strain aging in titanium alloys using CPFEM

Titanium alloys have been reported to exhibit Dynamic Strain Aging (DSA) at intermediate temperatures that strongly influence their workability and performance under various loading conditions. It has been experimentally well established that DSA in these alloys is due to the dislocation pinning and unpinning by the Cottrell atmosphere. In the present work, a model for DSA is developed which is an extension of Kubin–Estrin–McCormick (KEMC) model. The model takes into account the reduction in material strength due to thermal softening along with its implications on DSA. The one-dimensional DSA model developed in this work is further extended to the CPFEM framework. The results from this model show good agreement with the experimental findings for a wide range of temperatures.

Influence of Mg on tensile deformation behavior of high Mg-content Al-Mg alloys

Al-Mg based alloys are known to experience dynamic strain aging (DSA) at room temperature, which is often associated with the diffusion of mobile Mg atoms to dislocation segments temporarily arrested at obstacles (e.g., forest dislocations) upon deformation in the DSA regime. With the emergence of new promising high Mg-content (> 6 wt.%) Al-Mg alloys, the combined effects of Mg content and DSA on their tensile flow behavior need to be better understood. In the present work, the tensile deformation behavior of Al and Al-(5.3, 6.4, 7.6 and 8.7) wt.% Mg samples tested at room temperature and at strain rates ranging from 0.0001 to 0.1 s−1 was investigated, based on the Kocks-Mecking-Estrin model combined with a DSA model, the Haasen plot as well as transmission electron microscopy. Particular focuses are placed on the influence of Mg solutes on DSA and associated strain hardening effects, on dislocation accumulation and annihilation as well as on apparent activation volume. We show that the synergistic effect of the Mg-dislocation interactions and DSA effectively reduces the rate of dynamic recovery via suppressing dislocation cross-slip, leading to an enhanced strain hardening capacity for the Al-Mg alloys with a higher Mg content. The influences of DSA on the yield strength, flow stress and apparent activation volume of thermally-assisted deformation of the Al-Mg alloys are also discussed to elucidate fundamental insights into the development of new high Mg-content Al-Mg alloys with superior mechanical performance.

Experimental and numerical investigations of dynamic strain ageing behaviour in solid solution treated Inconel 718 superalloy

PurposeThe purpose of this paper is to systematically investigate the dynamic strain aging (DSA) effect in solid solution treated IN718 at different temperatures through experiments and simulations to gain an understanding of the inelastic deformation mechanisms.Design/methodology/approachIn the present work, uniaxial tensile tests have been carried out in conjunction with finite element (FE) simulations to investigate the behaviour of the solid solution treated Inconel 718 superalloy at different temperatures and strain rates. Dynamic strain aging (DSA) effects, which manifested during the tests in the form of a negative strain rate sensitivity and stress serrations, are investigated. The most significant DSA effect occurs at 500°C and at a strain rate of 10–4 s-1. In a newly proposed rate-dependent constitutive formulation, the DSA model, proposed by McCormick, Kubin and Estrin, was introduced into slip-assisted solute hardening, and an activation energy-dependent exponential flow rule was adopted.FindingsThe observed negative strain rate sensitivity and stress serrations are well predicted by a 3 D FE. The FE results indicate that the equivalent plastic strain rate distribution in the specimen gauge length is as highly inhomogeneous as in the other materials exhibiting DSA effects such as aluminium and titanium alloy. During inelastic deformation, propagating high strain rate bands can be closely correlated to the stress serrations.Originality/valueFor the DSA effect in solid solution treated IN718, the existing researching mainly focuses on the mechanical properties experiment and microstructure observation. In this study, a constitutive formulation, combined with the DSA model, has been proposed, and the mechanical behaviors, including the DSA effect, have been well predicted by a finite element model.

A Finite Element Analysis on a Localized Deformation Band Moved in Thin Specimens Made of SUS304 by Nonlocality and Dynamic Strain Aging

During tensile tests at room temperature using thin specimens made of SUS304, the propagation of X-shaped strain localization with serrations in flow stress occurs in the later stage of deformation. As a mechanism of the phenomenon, it can be considered that the necking in this material can be strengthened by strain-induced martensitic transformation (SIMT). On the other hand, the mechanism of the phenomenon can be dynamic strain aging (DSA) which cause the similar phenomenon called Portevin-Le Chatelier (PLC) band observed frequently in the aluminum alloys. That is, the mechanism has not been fully-concluded. In the past, it is reported that the propagation of strain localization bands can be generated by nonlocality of deformation in order to represent local necking and size effects of the specimen accurately. The analysis with nonlocality is quite hard to express the propagation of X-shaped strain localization with serrations. In another previous study, the model of DSA is introduced into the stress-strain, strain rate, temperature relationship for either austenite or martensite phase and that the propagation of X-shaped strain localization and serrations is confirmed. In the analysis which introduce the DSA, a use of value for the coefficient parameter n might be incorrect. Therefore, it can be said hardly that the analysis can express proper phenomena. The purpose of this study is to elucidate the mechanism of the phenomenon. The analysis considering the nonlocality with the damage or DSA solely with different value of parameter n are done by including them into the finite element method. Then, all the results including experimental ones are compared.

Interaction of the Portevin–Le Chatelier phenomenon with ductile fracture of a thin aluminum CT specimen: experiments and simulations

An attempt is made here to capture numerically slant ductile fracture and its early slant strain precursors via combining a dynamic strain aging (DSA) model with ductile damage models. In recent experimental studies it has been shown that in an AA2XXX alloy strain localization in slant bands preceded the onset of damage, originating slant fracture ahead of a notch. Here tensile tests are performed at different strain rates revealing some negative strain rate sensitivity which is an indication of DSA effect for AA2198-T8. A McCormick-type DSA model in conjunction with a Rousselier damage model, a reduced polycrystalline plasticity model and a Coulomb fracture criterion for slip systems have been used. Full 3D finite element simulations using this model and typical parameters for aluminum alloys capture the early strain localization in slanted bands, their intermittent activity and the final slant fracture. Prior simulation results without the DSA model and others using the von Mises plasticity or the GTN model did not capture the early slant strain localization thereby suggesting that DSA may well be the physical origin of the early slant strain localization and final slant fracture phenomena in this alloy.

Role of Self-Organization of Dislocations in the Onset and Kinetics of Macroscopic Plastic Instability

The present paper examines two aspects of the problem of critical conditions of jerky flow in alloys, or the Portevin–Le Chatelier (PLC) effect. Recent development of dynamic strain aging (DSA) models proved their capacity to qualitatively reproduce complex non-monotonic behavior of the critical strain, providing that the parameters of theory are allowed to depend on strain. Experimental measurements of such strain dependences have been realized for the first time and used to revise the predictions of the critical strain and stress relaxation kinetics upon abrupt strain-rate changes. On the other hand, it is usually omitted from consideration that the PLC stress serrations can last very short time in comparison with the characteristic time of stress transients. The development of stress drops was studied with the aid of the acoustic emission (AE) technique. It is shown that such macroscopic instabilities are caused by clustering of AE events which otherwise occur all the time, including the periods of smooth plastic flow. The role of synchronization of dislocation avalanches in the development of abrupt stress serrations and its relationship with the predictions of the local DSA models is discussed.

Dynamic strain aging of twinning-induced plasticity (TWIP) steel in tensile testing and deep drawing

The dynamic strain aging (DSA) of metallic materials due to solute atom diffusion to mobile dislocations induce deformation instability with load fluctuations and deformation localizations, hence reducing their sheet formability. In this paper, DSA behaviors of twinning induced plasticity (TWIP) steel with and without Al during tensile testing and deep drawing are investigated in terms of strain localization and the Portevin-Le Chatelier (PLC) band. A theoretical DSA model with internal variables of dislocation density and twin volume fraction is presented for an estimation of strain localization and strain hardening behavior of TWIP steels. The simulation results of the load history and PLC bands during tensile testing and deep drawing are in good agreement with the experimental values. A serration behavior is observed in high-Mn TWIP steels and its tensile residual stress is higher than that in the Al-added TWIP steels, which results in a deformation crack or delayed fracture of deep drawn specimens.

Descriptors of temporal signatures of serrations in a low alloy steel

Abstract The serrated flow behaviour in a low alloy steel has been analysed with new metrics that captures the signatures of its typical temporal characteristics. It has been found that the temporal manifestation of serrated flow could be decomposed into a slow and fast kinetics framework. The relationship of tensile ductility with the evolving temporal characteristic of in-homogenous flow in the temperature–strain rate regime displaying PLC phenomenon has been explored. The evolutionary behaviour of these descriptors along the flow curve has been related to the unit processes of PLC phenomenon within the framework of the unified model of dynamic strain aging proposed by Beukel and Kocks.

Influence of severe plastic deformation on dynamic strain aging of ultrafine grained Al–Mg alloys

This investigation addressed the influence of severe plastic deformation (SPD) on dynamic strain aging (DSA) of ultrafine grained (UFG) Al–Mg alloys with different Mg content. Confined channel die pressing (CCDP) carried out at room temperature was used for SPD. Microcharacterization by TEM revealed a remarkable grain refinement and retarded dynamic recovery with increasing Mg content and plastic strain during SPD. Mechanical tests with jumping and constant strain rates demonstrated a complicated deformation behaviors of the UFG Al–Mg alloys: (i) the critical strain εc for initiation of serrated flow increased considerably with increasing strain and Mg content contrary to the behavior of the coarse grained and non-deformed counterparts; (ii) the instantaneous stress response (Δσi) and the instantaneous strain rate sensitivity (mi) during rate jumps were always positive and increased monotonically with CCDP pass and Mg content, however, they exhibited a distinctive asymmetry with respect to the strain rate jump direction, i.e. the values for strain rate towards down were about one order of magnitude larger than those for rate towards up and increased with progressing CCDP as well as with increasing Mg content; (iii) the steady state strain rate sensitivity ms was negative and decreased firstly with progressing CCDP up to a certain strain and then increased again. This mechanical behavior of UFG Al–Mg alloys is discussed on the basis of recently developed DSA models by relating the microstructure evolution of Al–Mg alloys during SPD to the influence of SPD on DSA.

Investigations on the Plastic Instability in an HCP Mg-Li Alloy

Plastic instability is reported in hexagonal close-packed (HCP) LA41 magnesium alloy during tensile tests. Serration amplitude associated with plastic instability is measured to increase with increasing strain and decrease with increasing strain rate. The model of dynamic strain aging (DSA) is proposed to be the controlling mechanism. Moreover, it is reported that annealing could reduce the flow instability, which has potential practical applications.

Finite element implementation of a kinetic model of dynamic strain aging in aluminum–magnesium alloys

AbstractSoare and Curtin (Acta Mater. 2008; 56:4091–4101, 4046–4061) have recently developed a model of dynamic strain aging in solute‐strengthened alloys. Their constitutive law describes time‐dependent solute strengthening using rate equations that can be calibrated using atomistic simulations. In this paper, their material model is incorporated into a continuum finite element simulation, with a view to completing a multi‐scale method for predicting the formability of solute‐strengthened alloys. The Soare–Curtin model is first re‐formulated as a state‐variable constitutive law, which is suitable for finite element computations. An efficient numerical procedure is then developed to track the strength distribution of aging mobile and forest dislocations in the solid during deformation. The method is tested by simulating the behavior of a 3D aluminum–magnesium alloy tensile specimen subjected to uniaxial loading at constant nominal strain rate. The model predicts the influence of strain rate on the steady‐state flow stress of Al–Mg alloys, but no Portevin–Le Châtelier bands or serrated flow were observed in any of our simulations, and the influence of strain rate on tensile ductility is not predicted correctly. The reasons for this behavior and possible resolutions are discussed. Copyright © 2010 John Wiley & Sons, Ltd.

Portevin-Le Chatelier effect of LA41 magnesium alloys

Uni-axial tensile deformation of LA41 magnesium alloy has been carried out and the Portevin-Le Chatelier (PLC) effect, also known as serrated flow or plastic instability, is observed. This kind of alloy exhibits negative strain rate sensitivity (SRS) at room temperature, that is, SRS is negative at the strain rate range from 3.33 × 10−4 to 6.66 × 10−3 s−1 at ambient temperature. Both ultimate stress (σb) and 0.2% proof stress (σ0.2) decrease when strain rate (\(\dot \varepsilon \)) increases, whilst critical strain (ɛc) of serrated flow is found to rise with enhancing \(\dot \varepsilon \). A new explanation for this unusual phenomenon is presented. The model of dynamic strain aging (DSA) is established through diffusion of solute atoms to mobile dislocations, which are temporarily arrested at obstacles. Such interaction renders the movement of mobile dislocations more difficult so as to necessitate the required force to overcome the obstacles.

A model for prediction of flow behavior and temperature distribution during warm rolling of a low carbon steel

A two-dimensional finite element analysis is coupled with dynamic recovery and dynamic strain aging models to achieve metal flow during warm deformation of a low carbon steel. In order to assess metal behavior, compression experiments are utilized under warm deformation conditions and the results are used in the model and temperature and velocity fields are predicted with regard to the effects of dynamic recovery and dynamic strain aging. In addition, warm rolling tests are performed to verify the modelling results. Comparison between the predicted and measured roll forces shows reliability of the employed model.

Prediction of Flow Behavior during Warm Working

In this work, the effects of dynamic strain aging and dynamic recovery on metal flow during warm working are studied. Compression experiments are utilized to assess the flow behavior of a low carbon steel under warm deformation conditions. Then, a two dimensional finite element routine is coupled with dynamic recovery and dynamic strain aging models. In this way, the temperature and the velocity fields are predicted during warm working operations with regard to the effects of dynamic recovery and dynamic strain aging. Warm rolling tests are performed in order to verify the modelling results. Comparison between the predicted and measured roll forces shows reliability of the employed model.

Ductility and strain rate sensitivity of Zircaloy-4 nuclear fuel claddings

The circumferential mechanical properties of nuclear fuel claddings for Canada deuterium uranium (CANDU) power reactors were examined and the constitutive equation which can predict the temperature dependence of ductility and strain rate sensitivity was developed. The loss of ductility associated with dynamic strain aging was observed between 250°C and 400°C. The elongation minimum results from the concentration of deformation in the necked region in the temperature range of the flow stress plateau. Oxygen atoms actually strengthen the alloy. However, the strengthening of the alloy due to dynamic strain aging decreases the strain rate sensitivity in the temperature range of the flow stress plateau. The decrease in the strain rate sensitivity results in a low ductility. Above the temperature range of the flow stress plateau the elongation of the alloy increases rapidly with temperature. The prediction based on a dynamic strain aging model is in good agreement with the temperature dependence of the circumferential ductility of Zircaloy-4 nuclear fuel claddings.

Amplitude Equation for a Dynamic Strain Aging Model: Beyond Linear Stability Analysis of Serrated Flow in Metallic Alloys

ABSTRACTA method for construction of “processing windows” to avoid negative strain rate sensitivity and associated serrated flow in some aluminum alloys is described. The method is based on the amplitude Ginzburg-Landau (GL) equations and analysis of bifurcation diagrams. The mathematical technique developed in the present work was applied to a specific aluminum alloy, Al-0.4%Mg-0.2%Si considered earlier in the literature [1-3], and yielded good results in terms of predicting the negative strain rate sensitivity regions in the “strain rate “temperature” parameter space. Using the GL-analysis it was demonstrated that even though the instability area is located in the region of intermediate strain rates, a qualitative difference exists between the areas of (relatively) fast and (relatively) slow strain rates. In the first case the dynamic behavior of the system is supercritical, in the second case it is subcritical. The second case is highly undesirable because it causes a sudden onset of stable stress serrations that are difficult to suppress, while in the first case the development of instability is gradual and, consequently, more easily controllable.

Special features of dynamic strain aging in high-speed tension of steel 20

The article investigates the influence of the strain rate and of the temperature on the mechanical properties of steel 20. It suggests a model of dynamic strain aging (DSA) at high strain rates based on the interaction of thermally activated dislocations with mobile atoms of the admixture.

A personal computer based system to evaluate J-integral by a single specimen unloading compliance method—part II: Results on A533B class I steel and corona-5

A PC-based single specimen unloading technique was described in Part I of the paper. In this paper the applicability of the technique is demonstrated using two different materials: a nuclear grade pressure vessel material, A533B Class I steel and an α- β titanium alloy, CORONA-5. Experimental parameters investigated in the case of steel include the effect of test temperature, loading rate and neutron irradiation. Results revealed the effect of dynamic strain aging (DSA) as a drop in the critical crack initiation fracture toughness ( J q ); the minimum in J q shifts to higher temperatures at higher test speeds as predicted from DSA models. In CORONA-5, the effects of microstructure and heat treatment for two different sizes of equiaxed α morphology in a β-matrix heat treated to different yield strengths were investigated. It is found that the observed higher values of J q and tearing modulus result from a greater tortuosity of the crack path and ease of crack blunting. The largest increase in J q occurred in fine equiaxed α microstructure aged to a nominal yield strength of 896 MPa. Results reveal potential application of the low cost technique for determining fracture toughness of engineering materials.