Strain Aging Research Articles

A new strengthening mechanism driven by disruptive shear and solute segregation during warm rolling in 1.4 GPa class 12.5 wt% Al added-FeMnC ultra-lightweight steel

Novel strengthening of Fe‒29.1Mn‒12.5Al‒1.35C‒4.95Cr steel achieved by warm rolling was investigated. The solution-treated steel consisted of a γ-matrix containing nano-sized κ-carbide ((Fe,Mn)3AlC) and elongated prior ferrite, which was transformed into FeAl-type B2 and Fe3Al-type D03 phases. The solution-treated steel exhibited poor strain hardening owing to glide softening associated with κ-carbide shearing by dislocations. However, after warm rolling with a reduction ratio of 30% at 300 °C, the yield and tensile strengths significantly increased from 917 to 1300 MPa and 1025 to 1419 MPa, respectively. The tensile test conducted at 300 °C to simulate warm rolling exhibited serrated flows, indicating dynamic strain aging (DSA). Atom probe tomography exhibited that the C atoms in κ-carbide were swept away along the slip direction by disruptive shear during rolling at 300 °C. The swept C atoms along the slip direction interacted strongly with dislocations at 300 °C, with repeated pinning and breakaway of dislocations from the C atoms. This contributed to significant strengthening owing to the formation of a solute-rich atmosphere after warm rolling. The results of the tensile tests at 300 °C indicated that the degree of strengthening was proportional to the pre-strain level. Tensile strength of 1.4 GPa can be achieved with good ductility (17% elongation) by warm rolling. This novel warm-rolling strengthening method expedites the potential application of Fe‒29.1Mn‒12.5Al‒1.35C‒4.95Cr as a 1.4 GPa class ultra-lightweight steel.

Effect of Dynamic Strain Aging on Hardness in the Heat-Affected Zone of SUS316 Steel Welds

Effect of Dynamic Strain Aging on Hardness in the Heat-Affected Zone of SUS316 Steel Welds

Hydrogen-Induced Attractive Force Between Two Partials of Edge Dislocation in Nickel

AbstractHydrogen embrittlement is an important technological problem underpinning failure of many structural elements. It has been extensively investigated in the literatures; however, several open issues remain that prevent a full understanding of this phenomenon. One important issue is the uncertain knowledge of how hydrogen atoms affect the dislocation core structure. Here, by exploring the hydrogen role on the dissociated edge dislocation, we reveal that there exists an additional attractive force between two partials due to the hydrogen atmosphere, which would reduce the equilibrium separation distance. This hydrogen-induced attractive force is quantitatively estimated by means of continuum mechanics. Furthermore, molecular statics simulations also capture the hydrogen-reduced separation distance under varying hydrogen background fractions, qualitatively verifying the theoretical prediction of attractive force. These findings at the atomistic scale will inform the hydrogen embrittlement modeling and experiments, especially on the hydrogen effect on the dislocation glide, climb, dynamics strain ageing, and so on.

An investigation on the effect of change in experimental sequence of warm laser shock peening and laser shock peening on the performance of DD6 nickel-based single-crystal superalloy

Incorporated with the benefits of dynamic strain aging (DSA) and laser shock peening (LSP), warm laser shock peening (WLSP) is able to induce a compressive residual stress (CRS) layer with high thermal stability on the near-surface of the material. It has been widely used in recent years as a new surface modification technique to prolong the fatigue life of aero-engine turbine blades. Single WLSP has limited effect on material modification, while multiple WLSP when the confining layer is quartz glass will affect the thickness and thermal stability of the CRS layer during replacement of the confining layer, absorbing layer, and cooling and heating. To address this issue, this study investigates the mechanism of the CRS response, mechanism of strengthening, mechanism of action, and change of electrochemical corrosion performance of DD6 superalloy samples to different surface treatments after double LSP, single WLSP plus single LSP and double WLSP surface treatments by changing the experimental sequence of WLSP and LSP.The experimental results show that all three surface treatments deform the sample by inducing 〈110〉 cross-slip bands on the sample surface. Also all three surface treatments strengthen the sample surface by pipeline diffusion of elements to form a dislocation network similar to the γ/γ′ substructure.Meanwhile, single WLSP plus single LSP treatments can effectively prevent the thermal relaxation of the previously induced CRS layer. The final induced CRS layer thickness, dislocation distribution and dislocation density are better than those of double LSP and double WLSP. Pipeline diffusion of elements can improve the electrochemical and thermal corrosion resistance of the single-crystal superalloy.

Unusual grain-size effects on tensile deformation behavior of twinning-induced plasticity steel with low Mn content

In this work, the effects of grain refinement on the tensile properties, dynamic strain aging (DSA) and twinning behavior of Fe–16Mn-0.6C steel, which has relatively low manganese content, were investigated using a room-temperature monotonic tensile test, OM, XRD, EBSD and TEM. Results show that the yield strength, tensile strength and elongation of the steel increase with the decrease of grain size. Grain refinement can significantly increase the serration amplitude and local strain concentration of DSA, thereby enhancing the DSA-strengthening effect. The microstructure observations show that with the formation of plentiful twins, the twinning capacity of coarse-grained steel is exhausted prematurely at initial strains. Although the twinning behavior of fine-grained steel is retarded in the early stage of deformation, a large number of fine and dense twinning structures can be produced continuously at high strains. This unique twinning behavior and enhanced DSA in the fine-grained steel maintain the strain-hardening capacity of the steel, which delays the plastic instability and hence brings about simultaneous increases in strength and ductility.

Effect of rolling temperature on the occurrence of dynamic strain aging of Inconel 617 superalloy

Inconel 617 specimens were hot rolled at 950 °C, 1050 °C, and 1150 °C to examine the effect of thermomechanical processing on the occurrence of dynamic strain aging (DSA), followed by performing the tensile tests at temperature range of 350 °C–750 °C with the strain rates of 5 × 10−4 s−1, 5 × 10−3 s−1, and 1 × 10−2 s−1. The microstructural evolution was studied by an optical microscope, a scanning electron microscope (SEM), and differential scanning calorimetry (DSC). The stress-strain curves were analyzed in terms of serration intensity and types. The serrated flow at temperature range of 450 °C–750 °C underwent changes by the strain rate and the temperature, and the serration type changed from types A + B and B to type C. The transformation of Portevin-Le Chatelier (PLC) effect from normal to the inverse PLC effect was observed at a temperature higher than 550 °C. The activation energy was calculated and it was found that the diffusion of Cr and Mo substitutional atoms are mainly responsible for the DSA effect in hot rolled Inconel 617 superalloy. The hot rolling temperature was also found to have a significant impact on the occurrence of DSA. Moreover, it was indicated that as the hot rolling temperature increases from 950 °C to 1150 °C, the occurrence of dynamic strain aging is intensified.

Workability and dynamic restoration mechanisms of Al-6.2Zn-2.1Mg-0.15Zr alloy during medium temperature compression

Medium temperature compressions were implemented in the range of 433–593 K and 0.001–5 s−1 to reveal the workability and dynamic restoration mechanisms of quenched Al-6.2Zn-2.1Mg-0.15Zr alloy. The activation energy evolution with processing parameters was inspected through the modified kinetic equation that derived by integrally considering the strain and deformation parameters, and the results suggested activation energies decreased with the increasing strain rate and deformation temperature while less affected by the increase of strain at a specific strain rate condition. The 3D processing map was constructed to optimize the processing regions, in which two unstable domains yielded by distinct microscopic instabilities were marked whilst the safe working regions could be identified at 533–593 K, 0.001–0.1 s−1 with dissipation efficiency >20%. The microstructural evolution of the compressed specimens was examined, and the results confirmed different types of dynamic recovery (DRV) involving dislocation migration and rearrangement were dominate restoration process during the compression. Also, the microstructures of the specimens deformed at unstable domains were characterized, the results revealed fluctuating flow localization, micro cracking and particles debonding as well as dynamic strain aging were responsible for compression failure.

Hysteresis and dichotomous mechanics in cyclic crushing failure of confined freshwater columnar ice

Cyclic crushing experiments with a haversine velocity waveform were performed on passively confined, freshwater columnar ice specimens for a variety of velocities and frequencies. The aim of the experiments was to study the ice deformation and failure behavior in crushing when loaded at a predefined displacement pattern closely resembling the frequency lock-in regime of ice-induced vibrations. The focus of the experiments was on the development of load and ice deformation behavior at the grain and ice specimen scales during each cycle. To this end, the deformation and failure of the ice were observed with crossed-polarized light to highlight the microstructure in-situ during cyclic crushing. It was shown that there are dichotomous mechanical behaviors of the damaged and confined ice during a single crushing cycle: brittle at high velocity and non-brittle at low velocity. At low velocity, ice fracture was interrupted and stress relaxation occurred until the predefined velocity began increasing in the cycle. The stress relaxation in the load was accompanied by stress-optic effects in the ice. It was found that a load peak-velocity hysteresis developed in each crushing cycle: peak loads following the non-brittle behavior were temporarily higher than the peak loads of the brittle behavior. The temporary load peak enhancement tended to increase with increasing duration of stress relaxation, i.e. the peak enhancement tended to increase with decreasing velocity and frequency. Negligible peak enhancement and stress relaxation duration were observed for the highest frequency and mean velocity tested of 2 Hz and 10 mm s−1, respectively. For tests with a minimum velocity of 1 mm s−1, no stress relaxation was observed in the load measurement. Preliminary results from deviating from the haversine velocity waveform by increasing the minimum velocity showed that the stress relaxation duration decreases, but the non-brittle peak load does not decrease. It is speculated that ice anelastic ice behavior could account for the rapid stress relaxation at low velocity. It is unclear what causes the hysteresis, although it is speculated that dynamic strain aging might play a role. The change in ice behavior during the experiments demonstrates a mechanism which develops rapidly and might therefore incite the development of the frequency lock-in regime of ice-induced vibrations of vertically-sided structures.

Dynamic and static strain aging of steel during high-speed wire drawing

This article presents the analytical studies results on the effect of high-speed drawing technological parameters on the temperature and deformation conditions of the wire. The temperature increase of the wire during drawing in a monolithic die is the sum of the strain heating and heating caused by sliding friction on the “die–metal” surface. In the course of the research, it was found that the strain heating of the wire does not exceed 50 °C. An increase in the drawing speed from 10 to 40 m/s leads to an increase in deformation heating by 2–4 °C. The heating of the wire due to contact sliding friction, depending on the technological parameters of drawing, can reach 320 °C, while the proportion of metal heated to a temperature above 150 °C does not exceed 5%. The degree of influence of the friction coefficient on the heating of the wire is shown. The average strain rates under the conditions of modern high-speed drawing reach 7000 s‒1. The research results demonstrated that the maximum strain rate during drawing in monolithic dies can exceed twice the average strain rate. Increasing the drawing speed from 10 to 40 m/s leads to an increase in the strain rate up to 5 times. In the course of the research it was established that there are absence of conditions for the occurrence of dynamic strain aging due to impurity atoms of carbon, nitrogen and oxygen was established. At the same time the temperature-rate parameters of high-speed wire drawing creates conditions for the beginning of dynamic deformation aging of steel in the presence of hydrogen atoms. Therefore, during heat treatment and pickling, it is necessary to exclude the hydrogenation of steel. It has been established that in order to exclude static deformation aging of steel during drawing, it is necessary to prevent heating of the wire above 180‒200 °C.

Influence of microstructure evolution on hot ductility behavior of austenitic Fe–Mn–Al–C lightweight steels during hot tensile deformation

Four alloys based on the Fe–30Mn–(8.5–12)Al–(1.0–1.3)C (wt%) system were prepared to investigate the effects of microstructure evolution and κ-carbide precipitation behavior on hot ductility behavior of austenitic lightweight steels. Hot tension tests were carried out at temperatures of 500–1230 °C using a Gleeble simulator. At high temperatures above 1000 °C, dynamic recrystallization occurred in all alloys, leading to high tensile ductility. At temperatures of 700–900 °C, the ductility decreased in all alloys due to the intragranular precipitation of κ-carbide, with increases in the amounts of Al and C contents then leading to a greater loss of ductility due to the formation of coarse intergranular κ-carbides. The addition of Cr and Mo suppressed the precipitation of κ-carbide, reducing the extent of ductility loss. At 500 °C, the ductility was recovered due to a reduction of inter-/intragranular κ-carbide precipitation and the development of slip bands caused by planar gliding of dislocations through κ-carbide shearing. The spacing among slip bands then became coarse with an increase in the Al and C contents, resulting from the coarsening of κ-carbide. Meanwhile, dynamic strain aging (DSA) behavior was observed in all alloys deformed at 500 °C. This occurred because the hot tensile tests were carried out under a high strain rate condition; therefore, the mobility of the dislocations was fast and thus solute atoms pinned the dislocations despite deformation at a high temperature. With a coarsening of κ-carbide, the extent of serration was reduced, resulted from the fact that the content of solute C decreased due to the greater precipitation of κ-carbide; i.e., the amounts of solute C atoms to cause the DSA behavior were reduced.

Cyclic deformation and dynamically induced short-range ordering in small particles reinforced Al composite

Cyclic deformation of an Al-Cu-Mg composite strengthened by small non-shareable particles was firstly investigated through strain controlled low-cycle fatigue (LCF) tests. In comparison with the reported Al-Cu-Mg composites, the in-situ TiB2/Al-Cu-Mg composite presented higher fatigue life at all the tested strain amplitudes. The composite exhibited cyclic hardening response, which was contributed by non-shareable particles, dislocations and dynamically deformation-induced short-range orderings in the Al matrix. Considering plastic strain evolution, the cyclic hardening model was established and well fitted with experimental results. When cyclic loading was controlled at higher strain amplitudes, the serrated flow occurred on stress-strain hysteresis loops and then it disappeared after the first few cycles related to the occurrence of dynamic strain aging. Onset of the stress serration on stress-strain hysteresis loop was greatly dependent on the strain amplitude rather than cumulative plastic strain. Finally, the correlative cyclic hardening model of small particles reinforced Al composite was analytically discussed based on microstructure evolution and cyclic stress-strain responses.

Investigation of the Dynamic Strain Aging Effect in Austenitic Weld Metals by 3D-DIC

Austenitic stainless steels similar to type AISI 316L are widely used structural materials in current and future nuclear reactors. Careful development and characterization of these materials and their welds is needed to verify the structural integrity of large-scale multicomponent structures. Understanding the local deformation behavior in heterogeneous materials and the mechanisms involved is key to further improve the performance and reliability of the materials at the global scale and can help in developing more accurate models and design rules. The full-field 3D digital image correlation (3D-DIC) technique was used to characterize two 316L multi-pass welds, based on cylindrical uniaxial tensile tests at room temperature, 350 °C, and 450 °C. The results were compared to solution annealed 316L material. The inhomogeneous character and dynamic behavior of the 316L base and weld materials were successfully characterized using 3D-DIC data, yielding high-quality and accurate local strain calculations for geometrically challenging conditions. The difference in character of the dynamic strain aging (DSA) effect present in base and weld materials was identified, where local inhomogeneous straining in weld material resulted in discontinuous type A Portevin–Le Châtelier (PLC) bands. This technique characterized the difference between local and global material behavior, whereas standard mechanical tests could not.

Effects of temperature and strain rate on tensile properties and dynamic strain aging behaviour of LPBF Hastelloy X

Effects of temperature and strain rate on tensile properties and dynamic strain aging behaviour of LPBF Hastelloy X

Impact of dynamic strain aging on multiaxial low cycle fatigue behavior of nickel-based alloy 690 at 350 °C

The multiaxial low cycle fatigue behavior of nickel-based alloy 690 at 350 °C was investigated. The multi-slip planar dislocation configuration under non-proportional loading paths is attributed to short-range order (SRO) and dynamic strain aging (DSA). The effect of multiaxial loading paths on DSA is quantitatively evaluated based on average stress drop, which is dominated by axial strain amplitude and weakened by the cross-slip strength. Finally, the preferential oxidation along the slip bands accelerates the nucleation and propagation of microcracks, and the multi-slip cracking for non-proportional loading paths enhances the oxidation damage and further reduces fatigue life.

Strain ageing embrittlement behaviour of X80 self-shielded flux-cored girth weld metal

ABSTRACT Strain ageing embrittlement behaviour of X80 self-shielded flux-cored arc welding (FCAW) girth weld metal at different strain ageing conditions (1% prestrain, and aged at 150°C/100°C for 1 –6 h, and 80°C for 1–30 days) was studied. Microstructure of FCAW girth weld generally consisted of coarse granular bainite and lath bainite. After strain ageing, toughness notably decreased, and higher ageing temperature or longer ageing induced lower toughness, until ageing is fully yielded. At initial stage of strain ageing, more dimples were found accompanying with cleavage facet. With the progress of strain ageing, the embrittlement was more obvious, while larger cleavage facet and less dimples were found. Meanwhile, the critical size for cleavage fracture initiation site became smaller. Martensite–austenite constituent (1–2 μm) was found to initiate cleavage fracture when partially strain aged, while submicron inclusions could initiate cleavage facet when fully strain aged.

Study on root strain concentration of girth weld joint with variable wall thickness for X80 pipe

In this study, considering the particularity of variable wall thickness joints, a strain concentration characterization method for variable wall thickness joint is proposed. The finite element simulation method is used to establish a finite element model consistent with the physical joint, and calculate the root strain concentration under different misalignment, different weld strength matching, different pipes strength collocation and different heat affected zone (HAZ) softening conditions. Based on the aforementioned analysis, it is found that the misalignment, the weld strength matching, the pipes strength configuration and HAZ softening all affect the strain concentration at the root of the joint. In particular, the misalignment is easy to cause geometric structure mutation of root, which is the main factor leading to strain concentration for root of weld. The larger misalignment, the lower strength matching of the weld, and the higher pipes strength can increase the strain concentration at the root. However, a large root strain concentration can induce strain aging embrittlement or coupling with welding defects to cause material damage, providing conditions for root crack initiation and propagation.

Deformation behaviour and notch sensitivity of a super duplex stainless steel at different strain rates and temperatures

For the successful development of a structure, understanding of mechanical behaviour of the selected material is an important issue to prevent damage or accidents. Mechanics of the material failure depends on its chemical composition, heat treatment, grain size, strain rate, type of loadings etc. Apart from material properties, the performance of the structure is also affected by its geometry, crack/notch/hole/groove and manufacturing processes. In construction of offshore, marine, power plant and oil-gas industry structures, super duplex stainless steel is one of the suitable materials. Therefore, the objective of the paper is to study the deformation behaviour and notch sensitivity of a super duplex stainless steel, SDSS 2507 at different strain rates (0.0001–0.1s−1) and temperatures (25–200 °C) under tensile, compressive and flexural (three-point bending) loads. Heating rate and soaking time are 8 °C/min and 15 min respectively. Experiments are conducted on electromechanical universal testing machine using suitable fixtures. The steel is found positive sensitive to the strain rate and temperature, where the phenomenon of dynamic strain ageing (DSA) is observed during thermal effects. Fractography analysis is correlated with the typical stress-strain curves. Heat treatment (heating rate 12 °C/min and soaking time 1–3 h) enhances the tensile strength of the steel upto 18% at 400–600 °C for strain rate 0.001s−1, whereas at 700 °C, the strength decreases by 31% and the energy dissipation decreases by 88% due to its brittle behaviour. Notch profiles (C, U and V) on the flat tensile specimens of the steel decreases the energy dissipation by nearly 67–74% at 0.001s−1. Ductile-brittle transition temperature exists between 600℃-700℃ in tensile tests. Compression tests are conducted for varying geometries of cylindrical specimens. Flexural behaviour of the steel is investigated for 120–160 mm span lengths and flat-transverse orientations. Existing material models (Cowper-Symonds and Johnson-Cook) are evaluated.

Activity Assay of Amylase and Xylanase Is Available for Quantitative Assessment of Strain Aging in Cultivated Culinary-Medicinal Morchella Mushrooms (Ascomycotina)

Strain aging has been mainly contributing to the "uncertainty" of Morchella farming. The situation calls for urgent quantitative assessment of strain aging in cultivated Morchella mushrooms. In this paper, systemic senescence of the productive strains of M. eximia, M. importuna, and M. sextelata was achieved through successive subculturing to provide subcultures with different degree of aging for further studies. Then the quantitative assessment of morel strain aging was conducted by activity assay of amylase and xylanase using dinitrosalicylic acid (DNS) method. The results suggested that both activity of amylase and xylanase decreased along with the rise of subculture times. Meanwhile, the correlation between xylanase activity and time of subculturing in the tested morel strains was higher than that of amylase assay. Consequently, assay of amylase and xylanase activity by DNS method can be used in the quantitative assessment of morel strain aging, and assay of xylanase activity is the better alternative. The work will improve the settlement of "uncertainty" in the morel industry and thus be beneficial for stable development of morel farming.

Assessing Differences in the Quality Properties and Ultrastructure of Eggshell as Affected by Chicken Strain and Flock Age During Incubation Period

Three hundred hatching eggs from two layer breeder flocks (Hy-Line Brown and hy-Line W-36; 150 each) were obtained at three different periods of laying cycle (early, middle and late stages). The hatching eggs were randomly taken to assess the impact of chicken strain, age, and hatching time on some eggshell measurements and structures. Results indicate that egg weight, shell percentage and shell thickness were significantly affected by strain, breeder age, and incubation time. Increasing rate of calcium absorption and utilization by the embryos during the embryonic stage clearly reflects the decrease occurred in eggshell strength. The differences of eggshell breaking strength during incubation time periods were 0.41 and 0.55 kg/cm2, the differences due to the incubation time may be due to the higher demand for calcium utilization in the late incubation period (18 days), observed on the10th day of incubation. Brown eggs recorded significant higher calcium percentage in eggshell compared to the white ones. The brown breeder hens recorded significant higher total pores per egg compared to the white egg. Concerning eggshell ultrastructure, total thickness of the brown eggshell was significantly higher than those of the white eggshell by about 3.7%. All parameters, except for total thickness, were significantly affected by the interaction between strain and breeder age. Finally, we conclude that some traits including egg weight, shell percentage, shell thickness, calcium percentages in eggshell, and total pores per egg were significantly affected by strain, breeder age, and incubation time. Eggshell ultrastructure of brown egg was better compared to the white ones.

Effect of Viral Strain and Host Age on Clinical Disease and Viral Replication in Immunocompetent Mouse Models of Chikungunya Encephalomyelitis

Effect of Viral Strain and Host Age on Clinical Disease and Viral Replication in Immunocompetent Mouse Models of Chikungunya Encephalomyelitis