Strain Aging Phenomenon Research Articles

Dynamic Strain Aging Induced by Cyclic Deformation in Two Magnesium Alloys

Cyclic deformation for two magnesium alloys AZ91 and AM50 with different processing status has been performed under total strain amplitude control mode and at room temperature. A serrated flow can be observed in both tensile and compressive directions of the stress-strain hysteresis loop for as-extruded AZ91 and AM50 magnesium alloys. It means that the so-called dynamic strain aging occurs during cyclic deformation. In addition, the dynamic strain aging phenomenon can also be observed in two extruded magnesium alloys subjected to aging treatment as well as the AZ91 alloy subjected to solution treatment. However, the dynamic strain aging seems not to take place in the extruded AM50 alloy subjected to solution treatment because there exists no significant serrated flow behavior in either compressive or tensile direction of the stress-strain hysteresis loop. It is suggested that the occurrence of the dynamic strain aging is associated with collective behavior of many mobile dislocations as well as interactions between solute atoms and dislocations.

Microstructure and dynamic strain aging phenomena in two structural steels

Two steels with bainitic and martensitic microstructures have been investigated with respect to the changes occurring in their uniaxial tensile properties in the temperature regime where they display dynamic strain aging (DSA) behaviour. The variations in yield strength, ultimate tensile strength, ductility and strain rate sensitivity in the case of martensitic steel have been found to vary in an embrittling manner as compared with those observed in the case of the bainitic steel within the DSA temperature regime. An attempt has been made to provide an insight into the difference in manifestations of DSA on the uniaxial behaviour of the two steels.

Kinetics of static strain aging after temper rolling of low carbon steel

In this study, static strain aging behaviour of cold rolled steel strips was considered with emphasis on the distribution of residual hydrostatic stress developed during temper rolling. In order to assess residual stress distribution produced by the temper rolling, a three-dimensional model was first employed. Then, samples were rolled at a reduction of 4% under single and double pass rolling programmes and the kinetics of static strain aging phenomenon as well as the required activation energies were then evaluated using hardness and tensile tests on the deformed samples. Considering the predicted residual hydrostatic stress distribution, it was found that tensile hydrostatic stresses promote the kinetics of the static strain aging process and alter the activation energy required for the phenomenon. In addition, it was revealed that different positions of the rolled steel show different aging behaviour owing to non-uniform distribution of residual stresses after temper rolling.

Relationship between relaxation mechanisms and strain aging in an austenitic FeMnC steel

Basic relaxation experiments confirm that dislocation gliding in a V-microalloyed Fe-22 wt.% Mn-0.6 wt.% C steel is thermally activated at room temperature. This impediment to gliding is believed to be a necessary condition for the occurrence of mechanical twinning inducing the twinning induced plasticity effect in the considered family of high manganese austenitic steels. The relaxation experiments also induce a static strain aging phenomenon enhanced by applied stress and systematically trigger the dynamic strain aging mechanism and observable serrations on further tensile loading.

Influence of Strain Aging on the Microstructure-Property of an X100 Pipeline Steel

The influence of strain aging on the microstructure-property characterization of an X100 pipeline steel was investigated by means of a thermal simulation technique, microscopic analysis method and mechanical properties testing. The experimental steel was prestrained in the tension at 6 pct and then was baked at 150oC, 200oC, 250oC and 300oC for 60 minutes, respectively. The results indicated that the strength and hardeness of test steel X100 increase and the plasticity and toughness decrease with the increasing of strain aging temperature, and it can get a higher ratio of yield to tensile and a lower strain-hardening coefficient which result in lower hardening capacity of X100 pipeline steel. There are two reasons for this strain aging phenomenon. One is the solute atoms of C, N in the α-Fe which Aggregated at the dislocation zones; the other is the compounds precipitation of C, N induced by the dislocations.

<i>In Situ</i> TEM and APT Analysis on the Dislocations Associated with Solute Carbons in Strain-Aged Low Carbon Pipeline Steels

Low carbon (~0.34 at.%) pipeline steels with high strength and high toughness are widely used for the transportation of petroleum and natural gas. These pipelines usually experience UOE (bending by U press, forming by O press, Expansion) piping followed by coating process at 250°C, which usually cause strain aging phenomenon with discontinuous yielding in service. In order to understand the major cause of the strain aging in these low carbon pipeline steels, the dislocations associated with solute carbon atoms have been investigated in three different types of steels (plate, pipe, and coated pipe). In-situ TEM heating up to 250°C for 30min did not cause any change of structure in both steel plate and pipe. However, APT results revealed the segregation of carbon atoms alone dislocation lines in ferrite region of both UOE pipes and coated pipes. It is believed that this solute segregation along the dislocations is the major cause in the case of strain aging of low carbon pipeline steels.

Dynamic strain aging of a commercial Al–Mg–Si–Cu alloy during equal channel angular extrusion process

In this research, the occurrence of dynamic strain aging (DSA) phenomenon during equal channel angular extrusion (ECAE) of a commercial air cooled Al–Mg–Si–Cu alloy is investigated. In order to detect the negative strain rate sensitivity behaviour associated with DSA, samples were ECAE-ed at various temperatures and strain rates. Subsequently, the extruded billets were subjected to mechanical and physical tests and it was found that at a certain ECAE temperature range there is a critical strain rate above which DSA occurs. The implication of the results to strengthening of dilute solid solutions is discussed.

Effect of rolling speed on the occurrence of strain aging during and after warm rolling of a low-carbon steel

In this study, effect of rolling speed on strain aging phenomena in warm rolling of a carbon steel has been investigated. For this purpose, by using a mathematical model and predicting temperature and strain rate fields, the possibility of occurrence of dynamic strain aging during the warm rolling was first evaluated. In the next stage, warm-rolled samples were aged up to 11 months at room temperature for studying the kinetics of static strain aging, while mechanical tests as well as microstructural evolutions have been performed to determine the effect of strain aging on material behavior. The results indicate that dynamic strain aging may not occur for the employed rolling program; however, static strain aging takes place after warm rolling leading to changes in the mechanical behavior of the warm-deformed samples. In addition, by increasing rolling speed, the aging time required to achieve the maximum hardness increases.

Investigation into dynamic strain aging behaviour in high carbon steel

In this work, the phenomenon of dynamic strain aging in a high carbon steel is studied and different initial microstructures including fine and coarse pearlite structures are considered. Tensile tests at different temperatures and strain rates are performed to evaluate the occurrence of dynamic strain aging and mechanical properties as well as to calculate apparent activation energies for onset and termination of dynamic strain aging. The results show that dynamic strain aging occurs for both microstructures while the initial microstructures alters the activation energies for appearance and termination of this phenomenon. The microstructural studies illustrate that a combination of cementite decomposition and fracture of cementite layers are responsible for occurrence of dynamic strain aging depending on applied strain rate and deformation temperature.

Defects in Ge caused by sub-amorphizing self-implantation: Formation and dissolution

High Resolution X-Ray Diffraction (HRXRD) was used to study the strain evolution of lattice defects formed in an array of B delta layers grown by Molecular Beam Epitaxy (MBE) and damaged by sub-amorphizing Ge self-implantation. The MBE structure was implanted at room temperature (RT) with 840 keV Ge at a dose of 1.5 × 10 12 Ge/cm 2. First of all, we observed a RT strain reduction of ~ 40% with respect to the strain value found in the just-implanted sample. This strain ageing phenomenon saturates in about 5 months. Then, the complete defects dissolution was monitored by in-situ HRXRD during isochronal annealings. Three others strain-recovery steps were identified, the last at T = 157 °C. Moreover, Secondary Ion Mass Spectrometry performed after the strain recovery did not detect any B diffusion till T was raised up to 840 °C, measuring in this case a B diffusion equal to the equilibrium one. The whole set of data will be discussed and compared with existing literature.

Effects of strain-ageing on New Zealand reinforcing steel bars

Modern seismic design codes, which are based on capacity design concepts, allow formation of plastic hinges in specified locations of a structure. This requires reliable estimation of strength of different components so that the desired hierarchy of strength of the structural components can be ensured to guarantee the formation of plastic hinges in the ductile elements. As strength of longitudinal reinforcing bars governs the strength of reinforced concrete members, strain-ageing, which has significant effect on the strength of reinforcing bars, should be given due consideration in capacity design. Strain-ageing can increase the yield strength of reinforcing steel bars and hence the strength of previously formed plastic hinges, thereby likely to force an unfavourable mechanism (such as strong beam-weak column leading to column hinging) to take place in subsequent earthquakes. In this paper, the strain-ageing effect of commonly used New Zealand reinforcing steel bars is experimentally investigated. Common New Zealand steel reinforcing bars are tested for different levels of pre-strain and different time intervals up to 50 days, and the results are discussed focussing on the extent of strain-ageing and its possible implications on seismic design provisions. The results indicate that designers need to use a higher flexural strength (in addition to overstrength) for the weaker member in checking the strength hierarchy in capacity design of reinforced concrete frames. Similarly, in designing retrofit measures to restore a damaged reinforced concrete member engineers need to take into account an increase of yield strength of the reinforcing steel bars employed in the member due to the strain-ageing phenomenon and the extent of increase in the yield strength depends on the level of damage.

Static strain aging and dislocation–impurity interactions in irradiated mild steel

Interactions between dislocations and interstitial impurity atoms lead to strain aging phenomenon in ferritic steels that are affected by the defects produced during neutron radiation exposure. We present here results on static strain aging in a silicon-killed mild steel before and after neutron irradiation. It is noted that the degree of strain aging (as measured by the yield point following restraining) decreased with increasing neutron dose resulting in essentially non-aging type at the highest dose (∼10 19 n/cm 2). The strain aging kinetics were investigated using data at various aging temperatures and were found to be unaffected by the neutron radiation exposure. These experimental results are compared to those observed in dry hydrogen treated (partially denitrided) samples and are correlated with models on Cottrell locking.

Dynamic strain ageing of P91 grade steels of varied silicon content

Plate materials of modified 9Cr–1Mo steel (P91) containing four levels of silicon (Si) exhibited dynamic strain ageing (DSA) phenomenon characterized by gradually reduced failure strain and serrations in the engineering stress–strain diagrams under tensile loading within a temperature range of ambient to 400 °C. Since parameters including dislocation density ( ρ), activation energy ( Q) and work-hardening index ( n) can play important roles in developing a mechanistic understanding of DSA, efforts have been made to evaluate them. A maximum value of ρ was calculated at 400 °C due to the formation of critical sized precipitates near grain boundaries, thus impairing the dislocation motion. The average value of Q ranged between 64 and 80 kJ/mole depending on Si content. The magnitude of n was gradually enhanced with increasing temperature up to 400 °C, followed by significant drop at 550 °C. Except for the alloy with 2.0 wt.% Si, the other tested materials exhibited a combination of intergranular cracking and cleavage failures at the primary fracture surface.

SUS316L ステンレス鋼の溶接熱影響部における動的ひずみ時効の発現に及ぼす温度とひずみ速度の影響

The dynamic strain aging phenomenon is believed to be one of the most important mechanisms that harden the weld-heat-affected zone of low-carbon austenitic stainless steels. Hence, we investigated the effects of the test temperature and strain rate during tensile tests on the dynamic strain aging of type 316L stainless steel (SS) in this study. The results obtained are as follows; (1) Two types of stress-strain curves were obtained with serrations caused by the dynamic strain aging in the tensile tests at different test temperatures and strain rates, one with a high-frequency serration and the other with a low-frequency serration. (2) The area where the serration was observed was determined by two lines, showing that the temperature dependence of the strain rate was in the form of e=A exp {-Q/(RT)}, where A: constant, Q: apparent activation energy (150 kJ/mol or 300 kJ/mol), R: gas constant, T: absolute temperature. (3) The increase in the micro-Vickers hardness of type 316L SS after being strained to certain strains showed maximum values determined by both the test temperature and the strain rate. The largest increase in the hardness was obtained at the combination of the lowest temperature and the lowest strain rate among the test conditions.

High Temperature Deformation of a Structural Steel

The mechanical behavior of a fire resistant steel was studied by means of tensile tests performed in the temperature interval from 25 to 600oC, at strain rates from 10-4 to 10-2 s-1, aiming at analyzing the role of alloying elements in the dynamic strain aging (DSA) phenomena occurring in this steel. Typical features of DSA were observed: serrated flow (the Portevin-LeChatelier – PLC effect), the presence of a maximum and a minimum in ultimate tensile strength and elongation versus temperature curves, respectively and a plateau in yield strength versus temperature curves. Apparent activation energies were calculated based on the onset of PLC effect and the maximum in ultimate tensile strength. Results suggest that changes in mechanical properties associated with DSA in the fire resistant steel are related to the dynamic interaction of C-substitutional dipoles and dislocations.

Effect of temperature on tensile properties of near-α alloy Timetal 834

Tensile properties of titanium alloys are known to be strongly dependent on their microstructures and test conditions. Equiaxed α + β microstructures offer high tensile strength and ductility, high resistance to fatigue crack nucleation and better LCF resistance as compared to the acicular microstructure. On the other hand, acicular microstructures exhibit better creep and fatigue crack propagation resistance and high fracture toughness. In general the flow stress decreases with rise in test temperature, however, under certain test conditions of temperature and strain rate, the flow stress shows a rising trend due to occurrence of the phenomenon of dynamic strain ageing (DSA). DSA was observed in the present investigation in various heat-treated conditions over a wide range of temperature from 623 to 823 K. In addition, a detailed account of the tensile behaviour of the alloy Timetal 834 both in the α + β and β treated conditions, over a wide range of temperature from 300 to 923 K has been discussed.

Experimental study by pyrometry of Portevin–Le Châtelier plastic instabilities—Type A to type B transition

The dynamic strain aging phenomenon which occurs in some materials under certain temperature and strain rate conditions can cause a localization of the plastic strain in the form of Portevin–Le Châtelier (PLC) bands. According to the strain rate and the temperature, the spatiotemporal pattern of these bands changes and three main types of band are identifiable (types A–C). The increment of plastic strain associated with each band causes an increment in temperature which can be measured by infrared pyrometry. This experimental technique was used in this paper to study the PLC phenomenon in an aluminium–copper alloy at ambient temperature and to visualize the strain localization patterns. Some characteristics of these bands are measured: bandwidth, slope angle, increment of plastic strain in the band, time between two band formations, time of band formation, propagation velocity of the bands, band spacing… The evolution of these characteristics according to the strain enables us to suggest an explanation of the type A–type B transition.

Static Strain Aging in Cold Rolled Metastable Austenitic Stainless Steels

Transformation of austenite to martensite during cold rolling operations is widely used to strengthen metastable austenitic stainless steel grades. Static strain aging (SSA) phenomena at low temperature, typically between 200°C and 400°C, can be used for additional increase in yield strength due to the presence of α’-martensite in the cold rolled metastable austenitic stainless steels. Indeed, SSA in austenitic stainless steel affects mainly in α’-martensite. The SSA response of three industrial stainless steel grades was investigated in order to understand the aspects of the aging phenomena at low temperature in metastable austenitic stainless steels. In this study, the optimization of, both, deformation and time-temperature parameters of the static aging treatment permitted an increase in yield strength up to 300 MPa while maintaining an acceptable total elongation in a commercial 301LN steel grade. Deformed metastable austenitic steels containing the “body-centered” α’-martensite are strengthened by the diffusion of interstitial solute atoms during aging at low temperature. Therefore, the carbon redistribution during aging at low temperature is explained in terms of the microstructural changes in austenite and martensite.

A Physically Based Model for Bake-Hardening Steels and Dent Resistance

Arcelor produces « Bake-Hardening » steels for automotive outer panels, which present the advantages of a remarkable drawability combined with a significant hardening after stamping and paint baking by the car maker. This hardening enables to increase the dent resistance of those automotive parts. In order to give easy design criterion and support the development for new “bake-hardening” steels, a physically-based model for Bake-Hardening steels has been developed. It is suitable to predict: -the physical phenomenon of strain ageing based on Cottrell atmospheres formation. A detailed description of the strain ageing kinetics is given based on a generalized form of the Harper model taking into account the diffusion of carbon atoms in the stress field of a dislocation, progressive carbon depletion in the matrix and saturation of the available dislocation sites. -the plastic instabilities propagation during tensile testing according to Piobert–Lüders phenomenon using the finite element method. A local mechanical behaviour is introduced whose shape schematically describes the local dislocation behaviour. The effect of the grain size on the velocity of the Lüders’ band front is especially enlightened. -the effect of ageing process on dent resistance. To do so, a physical extension of the former approach to more complex loading paths is proposed.

Measurement of sensitivity to dynamic strain aging in C–Mn steels by internal friction experiments

The dynamic strain aging (DSA) phenomenon is present in C–Mn steels and associated welds commonly used in many industrial applications. This phenomenon, which appears in metals containing solute atoms interacting with dislocations, induces an increase in flow stress and ultimate tensile strength (UTS) as well as a decrease in ductility. The DSA phenomenon was studied by tensile, strain rate sensitivity and internal friction tests between 20°C and 350°C in two C–Mn steels with very different sensitivities to DSA (a fully killed A42 steel and a semikilled A48 steel). The internal friction technique, which allows one to evaluate the balance between carbon and nitrogen atoms free in the lattice and carbon and nitrogen atoms interacting with mobile dislocations, appears to give results well correlated with those obtained by tensile (and strain rate sensitivity) tests. The ductility loss or the UTS increase is shown to be proportional to Snoek peak height. It is concluded that, even in industrial materials, a single internal friction test can give a good evaluation of DSA sensitivity.