Dynamic Strain Aging Effect Research Articles

Revealing the mechanical properties and microstructure evolutions of Fe–22Mn–0.6C–(x)Al TWIP steels via Al alloying control

Three kinds of Fe–22Mn–0.6C–(x)Al (wt%) Twinning-Induced Plasticity (TWIP) steels were designed by changing the Al content. Uniaxial, unloading-reloading and stress-relaxation tensile tests were carried out to evaluate the mechanical properties. Meanwhile, electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM) and X-ray tomography (XRT) techniques were used to investigate the microstructure evolutions. Firstly, it was found that the Al addition reduced the back stress level, suppressed twinning capability, and increased the stacking fault energy (SFE), leading to concession of work-hardening rates. Secondary, the Al addition improved the friction stress level and promoted the short-rang order (SRO) effect, which brought the transformation of dislocation slip from wavy to planar mode. Last but not least, the dynamic strain aging (DSA) effect brought the pinning of dislocations, forming high dislocation activation volume (> 100 b3). With increasing plastic strain, the deformation twins and SRO effect restricted dislocations gliding, resulting in low dislocation activation volume (< 50 b3). The present study could further illustrate the effects of SFE, SRO and DSA on the mechanical properties and microstructure evolutions of Fe–Mn–C–(Al) system TWIP steels via Al addition.

Non-propagating fatigue cracks in austenitic steels with a micro-notch: Effects of dynamic strain aging, martensitic transformation, and microstructural hardness heterogeneity

The non-propagation limit of a microstructurally small fatigue crack was investigated with respect to dynamic strain aging (DSA), martensitic transformation, and microstructural hardness heterogeneity. In this study, we selected four model alloys: Fe-19Cr-8Ni-0.05C, Fe-19Cr-8Ni-0.14C, Fe-23Mn-0.5C, and as-hot-rolled Fe-30Mn-3Si-3Al steels. Transformation-induced cyclic hardening results in the most significant improvement of the non-propagation limit, i.e., in the case of the Fe-19Cr-8Ni-0.05C steel. Within different contexts, DSA, transformation-induced crack closure, and hardness-heterogeneity-enhanced plasticity-induced crack closure could also realize superior non-propagation limits. The effects of DSA and hardness heterogeneity can be combined with the effects of transformation, which is expected to create a new venue of material design and selection in terms of the crack non-propagation limit.

Mechanical properties and tensile deformation behavior of a reduced activated ferritic-martensitic (RAFM) steel at elevated temperatures

Here the tensile behavior of a 9Cr-1.7W-0.1C reduced activated ferritic-martensitic steel over the temperature region from room temperature (RT) to 600 °C was studied. The variation of the dislocation distribution under the interplay mechanism of dislocations with solute atoms and lath boundaries was found to be the main reason for the decreasing of plasticity in the intermediate temperature zone. Temperature and strain rate were confirmed to affect the interaction intensity of this dynamic strain aging (DSA) phenomenon on the tensile deformation behavior through changing the extent of dislocation pile-up on the lath boundaries. The variation of average rate of crack propagation in the fibrous region was determined to be related to the DSA effect on the dislocation mobility.

Influence of dynamic strain aging on the mechanical properties and microstructural evolution for Alloy 800H during hot deformation

Two types of hot compression tests, namely, constant strain rate and strain rate jump, were carried out for Alloy 800 H within a wide temperature range of 450–900 °C and strain rate range of 0.001–1 s−1. Following the above tests, we studied the effect of dynamic strain aging (DSA) on the mechanical properties and microstructure features to reveal the underlying mechanism responsible for DSA. The results indicated that three kinds of serrated yielding behavior (Type A, B and C) can be observed in the stress-strain curves of the DSA regime. The DSA temperature (TDSA) is located in the temperature range of 500–750 °C at a strain rate of 0.001 s−1, in 550–790 °C at 0.01 s−1, and in 600–840 °C at 0.1 s−1. The activation energy for the appearance of negative strain rate sensitivity was calculated to be 194 kJ/mol, and pipe diffusion of substitutional solutes (i.e. Cr, Ni and Ti) is regarded as the mechanism of DSA. TEM observations further elucidated a tendency of linear microstructure within the DSA regime, which also restricts the cross-slip of dislocations and promotes the strain localization along deformation bands.

Temperature-Dependence of the Mechanical Responses for Two Types of Twinning-Induced Plasticity Steels

Tensile tests and microstructure observations were conducted for two types of twinning-induced plasticity steels, Fe-22Mn-0.6C and Fe-30Mn-3Si-3Al (wt pct), from 293 K to 443 K. With increasing temperature, Fe-22Mn-0.6C steel exhibited enhanced mechanical properties and stable twinning capability, but Fe-30Mn-3Si-3Al steel displayed a decline on its mechanical properties and twinning capability. Mechanisms for the different mechanical responses were analyzed by assessing the dynamic strain aging effect.

Effect of rolling temperature on the microstructure, texture, and magnetic properties of strip-cast grain-oriented 3% Si steel

The effect of rolling temperature on the evolution of microstructure, texture, and magnetic properties of ultra-low-carbon grain-oriented silicon steel was studied in strip-casting process. Dynamic strain-aging (DSA) behavior was observed during warm rolling in the temperature range of ~ 200 to 400 °C based on the fact that majority of the inhibitor elements remained in solution during the strip-casting process. Considering the initial coarse grains with strong {100} fiber prior to rolling, the cold-rolled specimen exhibited pronounced α-fiber and weak γ-fiber texture. However, intense shear bands and high stored energy regardless of the orientation were obtained in the warm-rolled specimens at the DSA temperature, accompanied by weak α-fiber and strong γ-fiber texture. While homogenous microstructure with lower stored energy was observed in the case of high temperature, the differences in shear bands and stored energy governed by the rolling temperature were strongly related to the extent of DSA effect, which is attributed to distinct characteristic of rolling and recrystallization texture. After recrystallization annealing, fine-grained homogeneous microstructure with strong Goss and γ-fiber texture was obtained at the DSA temperature, while relatively random texture with much more α-fiber and θ-fiber components was observed in the case of high temperature. The microstructure and texture of primary annealed sheets exhibited sufficient Goss grains and favorable surrounding matrix with pronounced γ-fiber texture, which was responsible for the perfect secondary recrystallization annealing in the warm-rolled specimens at the DSA temperature. The present study suggests that texture optimization of strip-cast grain-oriented silicon steel can be achieved by warm rolling in the appropriate temperature range, with improved magnetic properties.

Mechanism of Dynamic Strain Aging in a Niobium-Stabilized Austenitic Stainless Steel

Dynamic strain aging (DSA) behavior of a niobium (Nb)-stabilized austenitic stainless steel (TP347H) was studied from room temperature (RT) to 973 K via tensile testing, transmission electron microscopy (TEM), and internal friction (IF) measurements. The DSA effect is nearly negligible from 573 K to 673 K, and it becomes significant at temperatures between 773 K and 873 K with strain rates of 3 × 10−3 s−1, 8 × 10−4 s−1, and 8 × 10−5 s−1, respectively. The results indicate that a dislocation planar slip is dominant in the strong DSA regime. The Snoek-like peak located at 625 K is highly sensitive to the diffusion of free carbon (C) atoms in solid solution. C-Nb octahedrons are formed by C chemical affinity to substitutional Nb solute atoms. Octahedron structure is very stable and captures most free C atoms and inhibits DSA at low tensile test temperatures of 573 K to 673 K. At high test temperatures in the range from 773 K to 873 K, C-Nb octahedrons break up and release free C and Nb atoms, resulting in the stronger Snoek-like peak. The interaction between C atoms and dislocations is responsible for DSA at low temperatures ranging from 573 K to 673 K. At higher temperature of 773 K to 873 K, the Cr and Nb atoms lock the dislocations, and this formation contributes to DSA.

Temperature dependence of transgranular fatigue crack resistance in interstitial-free steel and Fe-C steels with supersaturated carbon: Effects of dynamic strain aging and dynamic precipitation

Tension tests at different temperatures were conducted for two ferritic steels: Fe-0.017C with 0.017% solute carbon content and interstitial-free (IF) steels. It is verified the occurrence of dynamic strain aging (DSA) in Fe-0.017C at high temperatures. Tension tests for the Fe-0.017C, following corresponding heating treatments, demonstrated the transition process of dynamic precipitation from under-aging to over-aging. Moreover, the threshold stress intensity factor ranges (ΔKth) of a small crack for these two steels both decrease with temperature growth. However, the ΔKth of Fe-0.017C was always larger than that of IF steel owing to solid solution hardening and DSA at 293 and 333 K and roughness-induced crack closure and precipitation hardening at 433 K.

Effects of microstructure on the dynamic strain aging of ferriticpearlitic steels at high strain rates

This paper presents an experimental study of the effects of dynamic strain aging on the mechanical behavior of selected high carbon and chromium-manganese steels in dynamic loading condition. In ferritic-pearlitic steels, the dynamic strain aging is typically caused by carbon, nitrogen, and possibly some other small solute atoms. Therefore, the thermomechanical treatments affect strongly how strong the dynamic strain aging effect is and at what temperature and strain rate regions the maximum effect is observed. In this work, we present results of the high temperature dynamic compression tests carried out for two different ferritic-pearlitic steels, 16MnCr5 and C60, that were heat treated to produce different microstructure variants of these standard alloys. The microstructures were analyzed using electron microscopy, and the materials were tested with the Split Hopkinson Pressure Bar device at three different strain rates at temperatures ranging from room temperature up to 680 °C to study the effect of the heat treatments and the resulting microstructures on the dynamic behavior of the steels and the dynamic strain aging effect. The results indicate that for both steels, a coarse grain structure has the strongest dynamic strain aging sensitivity at small plastic strains. However, at higher strains, all microstructures show similar strain aging sensitivities.

Fatigue behavior of Zircaloy-2 under asymmetric loading at 400°C

Nuclear core components such as cladding tubes of Zircaloys often exhibit progressive extension with number of cycles under asymmetric cyclic loading with tensile mean stress and undergo premature failure against the life stipulated under symmetric loading. The accumulation of plastic strain depends on the applied mean stress, stress amplitude, stress rate and service temperature. In the present investigation accumulation of plastic strain in the Zircaloy-2 and its fatigue life is studied under different combinations of the mean stress from 45 to 70 MPa, stress amplitude from 95 to 120 MPa and stress rate from 30 to 750 MPa/s, at 400°C. While there is decrease in fatigue life with increase in the mean stress and stress amplitude, fatigue life is increased with rise in stress rate from 100 MPa/s and above. The increase in fatigue life above the stress rate of 150 MPa/s, at 750 MPa/s, is due to decrease in the effective cumulative plastic strain. Fatigue life is increased also below the stress rate of 100 MPa/s, at stress rate 30 MPa/s, but due to the effect of dynamic strain aging in this alloy at 400°C. There is also significant effect of the test variables on fracture behavior and the deformed microstructure.

Investigation of fatigue performance improvement in SiCw/Al composites with different modified shot peening treatments by considering surface mechanical properties

In order to investigate the fatigue performance improvement of SiCw/Al composite under different shot peening (SP) treatments, conventional SP, warm peening (WP), stress peening (STP) and compound peening (CP) were carried out on SiCw/Al composite specimens. The fatigue test, residual stress measurement and full width at half maximum (FWHM) from X-ray diffraction measurement were conducted on different SP specimens. The fatigue strength of SiCw/Al composite specimen increases by 16%, 37%, 21% and 42% via SP, WP, STP and CP respectively. The surface mechanical properties such as quasi-static surface compressive yield strength (Re(c),s) and cyclic surface yield strength (Rcycle,s) were calculated via analyzing residual stress relaxation during cyclic loading. The results show that the decrement of Re(c),s and Rcycle,s caused by Bauschinger effect during shots' impact is compensated by dynamic strain aging effects in WP and CP. The degree of fatigue strength improvement is more obviously in WP and CP specimens, which is due to the improvement of stability of residual stresses caused by the dynamic strain aging effects in elevated temperature condition.

Dynamic strain aging in DP steels at forming relevant strain rates and temperatures

Mechanical testing of dual phase (DP) steels at low strain rates (10−3s−1) have shown that they are susceptible to dynamic strain aging (DSA) between 100°C–400°C. During industrial forming processes at intermediate strain rates (1–102s−1), the local temperatures may rise to the DSA range due to deformation heating which may disturb the exceptional formability of these steels. In this study, two grades of DP steel (DP590 and DP800) are tested at thermomechanical conditions relevant to forming and the effects of DSA on the formability are established. Test results show that the DSA controls the deformation between 200°C–300°C through serrations in the stress-strain curves of both grades. With increasing strain rates (up to 1s−1) and temperatures, DSA intensifies and results in severe drops in uniform and total ductility with negative strain rate sensitivity, indicating poor formability at these conditions. A detailed analysis of the serrations coupled with dislocation density measurements by x-ray analysis suggests that the serrations can be linked to a periodic microstructural feature.

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.

Effect of dynamic strain aging on the deformation behavior and microstructure of Cu-15Ni-8Sn alloy

The compression deformation behavior of the Cu-15Ni-8Sn alloy prepared by powder metallurgy was investigated under a temperature range from 300 to 573 K and a strain rate range of 5 × 10−5 ∼1 × 10−2 s−1. The results showed that in a certain range of strain rates and temperatures, the stress-strain curve exhibited obvious serrated flows which were induced by dynamic strain aging (DSA). The activation energy calculations indicated that the DSA was mainly attributed to the interaction of Sn solute atoms with dislocations. In the microstructure of the alloy pre-deformed in the region of DSA, Sn atoms were more likely to aggregated along the slip bands and therefore resulted in the dynamic precipitation of Sn-rich phase at lower temperature than that of natural aging alloy without pre-deformation.

Correlation between strain rate sensitivity and characteristics of Portevin-LeChátelier bands in a twinning-induced plasticity steel

The effects of strain rates on the deformation behaviors of a twinning-induced plasticity (TWIP) steel (Fe23Mn0.6C) have been investigated using uniaxial continuous tensile tests at different strain rates and strain-rate jump tests, coupled with digital image correlation (DIC) method. A positive instantaneous strain rate sensitivity (ISRS), a negative strain rate sensitivity of work hardening (SRSW) and a negative overall strain rate sensitivity (SRS) have been revealed in the present TWIP steel. The critical strain for appearance of Portevin-LeChátelier (PLC) bands increases while the height of serrated flow behaviors decreases with increasing strain rate, which indicates that the dynamic strain aging (DSA) effect is suppressed at higher strain rates. The strain concentration in PLC bands is found to be more severe at higher strain rates. Moreover, the appearance/disappearance of PLC band in strain-rate jump tests is found to be highly dependent on the deformation and microstructure evolution histories. Thus, the negative SRSW can be attributed to the higher strain concentration in the PLC bands and the restriction of DSA at higher strain rates, which are related to the suppression of deformation twins at higher strain rates.

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.

Thermal-mechanical behaviors of dual-phase steel sheet under warm-forming conditions

This paper examined the thermal-mechanical behaviors of the given dual-phase (DP) steel sheet at warm-forming temperatures (20–190°C). The dynamic strain aging effect of the given material was observed via thermal-uniaxial tensile tests and the temperature-dependent flow stress function was then established. Moreover, the biaxial tensile tests at series of elevated temperatures were also carried out and the corresponding experimental yield loci were then determined by utilizing the concept of plastic work contour. For the convenience of characterizing the thermal-mechanical yielding behaviors of the given material, a temperature-dependent Yld2000-2d yield criterion was proposed in this work, of which the exponent and anisotropic parameters were replaced by the nonlinear fitting functions expressed in terms of temperature. Afterwards, the proposed model, together with other existed constitutive models, were programmed as user subroutines VUMAT in ABAQUS with ‘NICE’ explicit integration algorithms. The cylindrical deep drawing tests under warm forming conditions and corresponding FEM simulations were then conducted to assess the precision of the proposed model.

Temperature dependent cyclic deformation and fatigue life of EN-GJS-600 (ASTM 80-55-06) ductile cast iron

The ductile cast iron EN-GJS-600 (ASTM 80-55-06) provides several beneficial properties for a wide spectrum of applications. Its near-net shaping capabilities combined with beneficial physical and mechanical properties even at elevated temperatures result in a wide use for components of internal combustion engines. Under service conditions, fatigue loading at different temperatures occurs. In this context, the present paper focuses on the isothermal high cycle fatigue behavior of ductile cast iron EN-GJS-600 in the temperature range from ambient temperature up to 400°C at a frequency of 5Hz. At elevated temperatures, the consideration of dynamic strain aging effects on cyclic deformation and lifetime behavior is of major importance. It was proved that only one temperature increase test is necessary to determine the temperature range of pronounced dynamic strain aging effects. The physically based fatigue lifetime calculation approach “PhyBaLSIT” (SIT=strain increase test) was applied and validated for total strain controlled fatigue tests. To receive maximum information with minimum effort, both test procedures were combined enabling calculation of fatigue life in the temperature range from ambient temperature up to 400°C based on only thirteen fatigue experiments.

Tensile properties and temperature-dependent yield strength prediction of GH4033 wrought superalloy

The tensile properties of superalloy GH4033 have been evaluated at temperatures ranging from room temperature to 1000°C. Fracture surfaces and precipitation were observed using a field-emission scanning electron microscope (FE-SEM). The alloy mainly consisted of γ’ precipitate particles homogeneously dispersed in the γ matrix interior. The effects of dynamic strain aging and precipitation on the strength were verified. A temperature-dependent yield strength model was developed to describe the temperature and precipitation effects on the alloy's yield behaviour. The model is able to consider the effect of precipitation strengthening on the yield strength. The yield behaviour of the precipitation-strengthened superalloy was demonstrated to be adequately predictable over a wide range of temperatures. Note that this model reflects the quantitative relationship between the yield strength of the precipitation-strengthened superalloy and the temperature, the elastic modulus, the specific heat capacity at constant pressure, Poisson's ratio, the precipitate particle size and the volume fraction of the particles.