Strain Aging Research Articles

Tensile Behavior of Alloy 690 in the Dynamic Strain Aging Regime

Tensile Behavior of Alloy 690 in the Dynamic Strain Aging Regime

Isothermal and thermomechanical fatigue behavior of 316LN stainless Steel: Effects of strain amplitude

Effects of strain amplitude on the isothermal fatigue (IF) and thermomechanical fatigue (TMF) behaviors of 316LN stainless steel were comparatively studied. The physical mechanisms for the evolution of cyclic stress response were revealed based on microstructure characterization. The manifestations of dynamic strain aging (DSA), dependence of DSA intensity on loading modes and influence of DSA on deformation properties were discussed. The dominated damage mechanism changed at the strain amplitude of about 0.27% at which the IF and TMF life curves intersected. The maximum stress was the appropriate damage parameter for life prediction with predicted life falling into the scatter band of 1.5.

Yield strength anomaly evaluation of W-free Co-Ni-Al-based superalloys during high temperature tensile tests

High temperature tensile behaviors of two W-free Co-Ni-Al-based γ'-strengthened superalloys at 773 K ~ 1073 K were investigated. The tensile strength and elongation of two superalloys decrease with increasing temperature. The fracture mechanism changes from dimple fracture to brittle fracture. A detailed observation of the fracture shows that the intermediate temperature embrittlement caused by decohesion of slip plane plays a significant role in reducing high temperature performance. Both superalloys reach to peak yield strength at 923 K. Occurance of yield strength anomaly (YSA) is derived from the synergistic effects of dislocation locking strengthening and dynamic strain aging strengthening. Dislocation cross-slipping hinders the dislocation migration. Dislocations' unpinning to the moving solutes increases the yield strength at 723 K ~ 973 K. A rapid weakening of grain boundaries strength decreases the yield strength at 1073 K.

Dynamic Strain Aging of Body-Centered Cubic Metals by a Snoek-Type Mechanism

Dynamic Strain Aging of Body-Centered Cubic Metals by a Snoek-Type Mechanism

Third-type of Strain Aging of GH4720Li Over a Wide Range of Temperatures and Strain Rates

In this study, the working environment of the GH4720Li alloy was simulated using a dynamic compression test with a strain rate range of 0.01–4000 s-1 and a temperature range of 20–1000 °C, and its dynamic mechanical properties under operating conditions were studied. Based on the signifi cant volume of the test data, it was observed that there was an abnormal stress peak in the GH4720Li alloy owing to third strain aging. The third strain aging behavior is an irregular bell-shaped stress curve. In addition, the effects of the temperature and strain rate on the bell-shaped stress curve were studied. With an increase in the strain rate, the bell-shaped stress curve shifted to the high-temperature zone, and the stress peak and temperature width of the bell-shaped curve gradually decreased. Finally, the change mechanism of the strain rate effect on the bell-shaped stress curve is explained from a microscopic point of view.

High temperature deformation behavior of Al0.2CoCrFeNiMo0.5 high entropy alloy: Dynamic strain ageing

High temperature deformation behavior of Al0.2CoCrFeNiMo0.5 high entropy alloy was investigated in fully annealed condition. Microstructural studies were performed using SEM and EBSD, prior to testing, clearly shows a fully annealed microstructure consists of secondary sigma phase in the FCC matrix. Uniaxial tensile tests were performed in the temperature range of 25–900 °C at an initial strain rate of 10−3 s−1. Serrated plastic flow behavior was observed in the intermediate temperature range of 400–600 °C. The influence of the strain rate on the critical strain corresponding to the onset of serrations were studied which indicated that the serration behavior was type A and type C. Strain rate sensitivity close to zero, decrease in the elongation and microstructural investigations suggesting that dynamic strain aging (DSA) is the responsible mechanism for the serrated flow and that the Al solute interactions with dislocations might be the possible reason for DSA. Additionally, the fractography analysis clearly indicates that there is transition in the failure mode from the transgranular fracture to the intergranular fracture.

Physics-Based Flow Stress Model for Alloy 718

A dislocation density-based model for alloy 718 in the annealed state is proposed in order to accurately describe the deformation behavior of this alloy for a wide range of thermo-mechanical loadings. The model accounts for numerous microstructural mechanisms, including strain hardening, grain size effect, dynamic strain aging (DSA), solid solution strengthening, as well as phonon and electron drag which affects dislocation movements at high strain rates. Two types of recovery mechanisms are also included: recovery due to dislocation glide and recovery associated with cross-slip of screw dislocations. The model is calibrated using experimentally determined stress–strain curves for both low and high strain rates in the order of 10–3 to 103 s−1, and for temperatures in the range 20 °C to 800 °C. The stress–strain data computed with the model are in good agreement with the experimental data. The inclusion of DSA is found to be effective in the combination of temperatures and strain rates corresponding to experimental observations. The solid solution strengthening contribution increases with decreasing temperature and increasing strain rate. The drag effect in the model proves to be significant only for deformation at high strain rate (~ 103 s−1).

"The Sun Came Up Because You Got Here…": A Qualitative Exploration of Person-Centered Care Strategies Used by Adult Day Care Centers to Manage Behavioral and Psychological Symptoms of Dementia.

In order to reduce care partner strain and support aging in place for people living with Alzheimer's Disease and Alzheimer's Disease Related Dementias (AD/ADRD), adult day centers (ADCs) must manage behavioral and psychological symptoms of dementia (BPSD). The purpose of this paper is to identify person-centered care strategies used by center staff to manage BPSD. Six focus groups with center staff (n = 31) were conducted. Data were analyzed using directed content analysis guided by Kitwood's conceptual approach to cultivating personhood in dementia care. Themes were identified and organized within Kitwood's framework. The results demonstrate that staff incorporate evidence-based person-centered approaches to AD/ADRD care that align with Kitwood's principles of comfort, attachment, inclusion, and identity. Staff individualize their approach to people with AD/ADRD within a group setting. They monitor, engage, socially stimulate, and, when needed, de-stimulate them. Centers are flexible social environments with underrecognized expertise managing BPSD using person-centered approaches.

Dislocation-obstacle interaction evolution in rate dependent plasticity of AlSi10Mg as-built microstructure by laser powder bed fusion

Novel microstructural features obtained from laser powder bed fusion methods challenge our understanding of the rate controlling mechanisms for plasticity which are important for structural applications. The strain rate dependent plasticity of as-built AlSi10Mg laser powder bed fusion samples was systematically studied as a function of stress state (tension and compression) and strain rate at 293 and 78 K. Utilizing constant strain-rate and rate-change testing methodologies, a strong dependence of flow stress on strain rate was revealed for both stress states. The compressive yield stresses have a potent strain rate sensitivity in both quasi-static (10−4-10−1 s−1) and medium-dynamic (1-5 s−1) strain rate regimes. A positive strain rate sensitivity was observed in all strain rates except for a shift to negative values in the 10−2-10−1 s−1 range for both tensile and compression states. This negative strain rate sensitivity was correlated with the deformation-induced microstructural evolution, i.e., manifested as a gradual transition in the dominant dislocation-precipitate interaction mechanism from shearing to looping, along with the disappearance of dislocation-solute interaction in the form of dynamic strain aging (DSA). The precise measurement of activation volume evolution with flow stress further confirmed the transition from shearing to looping in the 10−2-10−1 s−1 range.

Occurrence of dynamic strain aging in intercritically annealed low carbon high aluminum medium manganese steels

Medium manganese steel grades heat treated with the quench and partition (Q&P) treatment have shown promising results of strength and formability, but the number of studies on high aluminum medium manganese Q&P steels still remains rather limited. Consequently, this study investigates the mechanical properties, behavior and microstructures of a low carbon-high aluminum medium manganese (3%) steel after intercritical annealing and subsequent Q&P heat treatments with varying heat treatment parameters. Samples that were intercritically annealed above 760 °C showed an ultimate tensile strength (UTS) of around 1500 MPa with uniform elongation values of approximately 10%. On the other hand, the samples annealed at 740 °C showed somewhat lower UTS values (in the range of 1000 … 1200 MPa) but much higher uniform and total elongations, i.e., considerably improved formability. However, the stress-strain curves of the samples annealed at 740 °C showed rather severe serrations that can be connected to the dynamic strain aging (DSA) phenomena, which limits the usability of the steel especially in applications where good surface quality of the deformed (sheet) is required. Therefore, the main focus of this article is on the samples annealed at temperatures close to A1, where DSA serrations tend to appear. The evidence gathered from these investigations leads to the conjecture that the DSA serrations are due to the free carbon and nitrogen originating from the dissolution of M(C,N) and M2(C,N) type carbides, which were formed during the intercritical annealing at temperatures producing less than 50% of the austenite phase. Other reasons for the occurrence of DSA in the current test materials can be the scarcity of martensite and the morphology of the retained austenite.

Ambient, elevated temperature tensile properties and origin of strengthening in friction stir welded 6 mm thick reduced activation ferritic-martensitic steel plates in as-welded and post-weld normalised conditions

Reduced Activation Ferritic-Martensitic (RAFM) steels are currently being considered for test blanket modules of International Thermonuclear Experimental Reactor. This study is aimed at understanding the microstructure and mechanical properties of Indian RAFM steel during friction stir welding (FSW) of 6 mm thick plates. The full penetration bead-on-plate welds were fabricated employing PcBN tool at a rotational speed of 200 rpm. The FSW joint consisted of stir zone, thermomechanically affected zone and BM. Microstructure, hardness and tensile properties were evaluated in as-received (base metal, BM), as-welded and post weld normalised + tempered states. The as-received microstructure was composed of Cr-rich M23C6 on prior austenite grain and tempered lath martensite boundaries with intra-lath V-and Ta -rich monocarbides. Substantial changes occurred during welding leading to destruction and dissolution of M23C6 and precipitation of Fe3C in SZ. The microstructure was restored to the BM level by giving a post weld normalising and tempering treatment. Comparative study on tensile properties has been carried out at room and elevated temperatures (up to 550 °C) in all the three states and the observed variations have been explained on the basis of initial microstructure and evolving substructures. The as-welded samples showed higher strength and low percentage elongation values. The ductility was minimum in all the states at an intermediate temperature and ascribed to the dynamic strain ageing. All the states revealed falling strength with increasing temperature up to 550 °C. Failure occurred in AW and PWNT states at the interface between TMAZ and BM. Irrespective of material state and tensile test temperature, the transgranular ductile fracture prevailed. The contribution of the different strengthening mechanisms to the yield strength of the SZ region has been estimated theoretically and it matches with the experimental results. The influence of low angle grain boundaries (LAGB) and high angle grain boundaries (HAGB) on the tensile behaviour of the SZ was well explained using the Electron Back Scattered Diffraction (EBSD) maps of these regions.

Effects of strain rate on the tensile and creep-fatigue properties of 316H stainless steel

Since the unstable start-ups and shutdown processes of nuclear power plants, the effect of strain rate on tensile and creep-fatigue (CF) properties has attracted attention. This paper aims to study the strain rate dependent tensile and CF behavior of 316H stainless steel. The tensile and CF tests were independently performed with different strain rates of 0.0001, 0.0002, 0.0005, 0.001 and 0.005 s−1 at 600 °C. Due to the dynamic strain aging (DSA), the tensile stress-strain curve appears serrated flow phenomenon and shows negative strain rate sensitivity (SRS), which means the tensile strength of the material does not increase significantly with an increase of the strain rate, and even decreases. Meanwhile, it was found that peak tensile stress and inelastic strain increase in CF tests under low strain rates. The CF life of 316H steel was predicted using the hysteresis energy method. At the same time, based on microstructure and fracture observation, different dominant damage mechanisms were discussed.

Effect of thermal aging on the microstructure and mechanical property of 410S ferritic stainless steel

The low chromium ferritic stainless steel, 410S, as a candidate control rod drive mechanism (CRDM) material used in nuclear power plants, was subjected to accelerated thermal aging at 400 °C in air up to an exposure period of 10,000 h (∼417 days). A series of analytical techniques including optical microscope (OM), electron back scatter diffraction (EBSD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and 3D atom probe tomography (3D-APT) were used to characterize the evolution of the microstructure with thermal aging time, whilst a couple of tests (i.e., hardness, Charpy impact and tensile tests) were employed to link the changes of mechanical properties at different thermal aging times with the microstructure. After ≤3000 h thermal aging treatment, Cr-rich α′ phase formed near the ferritic grain boundaries, leading to minor change in the hardness and strength of 410S but sharp decrease in impact energy. When the thermal aging time increased to 10,000 h, the formation of Cr-rich α′ phase spread from near the ferritic grain boundary to the inside of ferritic grain and the growth of carbides precipitated led to the slight increase in the hardness and strength but the decrease in the impact energy and plasticity. Under these circumstances, the decrease of free substitutional atoms (i.e., Cr) caused by the formation of Cr-rich α′ phase and the growth of carbides changed the dynamic strain aging type from type B to type A.

A dislocation density-based model considering dynamic strain aging for annealed and water-quenched aluminum alloys under low-temperature conditions

Low-temperature forming is a novel manufacturing method for aluminum alloy components. To better understand the flow characteristics and deformation behavior of aluminum alloys at low temperatures, uniaxial tensile tests were conducted with an initial strain rate range of 2.5 × 10−4–1 × 10−2 s−1 and a temperature range of 77–298 K for annealed (O) and water-quenched (WQ) 2060 Al–Cu–Li alloys. The results showed that the two tempered alloys exhibited excellent work hardening at lower temperatures, resulting in improved ductility, especially in the WQ-tempered alloy. Dynamic strain aging (DSA) caused by interactions between solute atoms and moving dislocations occurred at higher temperatures and lower strain rates, which resulted in serrated flow. Because moving dislocations were additionally pinned by solute clusters, the flow stress increased. The stress increase and negative strain rate caused by DSA could not be captured using the typical dislocation density-based K-M model. Thus, a modified K-M model that considered DSA was proposed to describe the tensile behavior of the studied alloys, and the stress-strain values predicted by the proposed model agreed well with the experimental ones.

Effects of hydrostatic preload on strain hardening and strain aging of boiler tubes

Tests were conducted to identify the effects of existing defects on strain hardening and strain aging of boiler tubes. Electron backscatter diffraction (EBSD) and finite element analysis (FEA) were used to quantify the local plastic strain near the pre-existing defects. Strain hardening and strain aging effects depend on the degree of plastic deformation. Any existing defects in the boiler tubes can act as stress raisers during the room temperature hydrostatic test. For the morphology and orientations of the defects, a preload of 95 MPa which is equivalent to the hydrostatic test pressure was enough to induce local plasticity. When heated to boiler service temperature and tested to fracture, the mechanical properties of boiler tubes were affected. The test results justify a recommendation that the hydrostatic pressure to be kept near 110% of maximum allowable operating pressure (MAWP) to avoid significant alteration of mechanical properties at stress concentration points in defect-containing boiler membranes.

Mathematical Modeling of Heating and Strain Aging of Steel during High-Speed Wire Drawing

In this article, a mathematical model of the wire’s average temperature change in the process of multiple drawing on high-speed straight-line drawing machines has been developed. The calculation results showed that the average temperature of the wire during a drawing at a speed of up to 45 m/s on straight-line drawing machines could reach 400 °C. Deformation heating of the wire during drawing does not exceed 60 °C, and heating due to sliding friction can reach 300 °C, depending on the friction coefficient, which ranges from 0.05 to 0.15. The average strain rates under the conditions of the modern high-speed drawing process reach 7000 s−1. Over the course of the research, it was found that there are no conditions for the occurrence of dynamic deformation aging due to impurity atoms of carbon, nitrogen and oxygen. At the same time, at the temperature and speed parameters of the high-speed wire drawing, conditions are created for the onset of the dynamic strain 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 strain aging of steel during drawing, it is necessary to prevent heating the wire above 180–200 °C.

Deformation behaviour and strain rate sensitivity of 304L SS at elevated temperatures

ABSTRACT Tensile deformation behaviour of nuclear grade 304 L SS (PHWR calandria vessel material) was presented at temperatures 300 K to 1123 K and at three different strain rates. The deformation behaviour is influenced by the strain-induced martensitic transformation at 300 K, the occurrence of dynamic strain ageing (DSA) at intermediate temperatures (573–873 K) and the dominance of strain softening at high temperatures (973–1123 K). The occurrence of DSA caused anomalies in flow behaviour and ductility for the steel. At high temperatures, strain softening manifested in a rapid decrease in flow strength with increasing temperature and decreasing strain rate. Further, instantaneous (mi) and quasi-steady state strain rate sensitivity (ms) with strain were evaluated using strain rate jump tests. At low and DSA temperatures, the initial and quasi-steady transient response differs. Contrary to this, the value of mi is equal to ms at high temperatures.

Mechanical properties of plasma arc welded AISI304LN austenitic stainless steel at various temperatures

ABSTRACT AISI 304 grade austenitic stainless steels are widely used in a wide variety of fields such as chemical, automobile, food industries, etc. due to their good mechanical properties and corrosion resistivity. In this study, AISI 304LN austenitic stainless-steel couples were joined with plasma arc welding. The effect of temperature on the mechanical properties, formability and fracture morphologies of joints was evaluated. For this purpose, tensile testing of the joints was carried out in the temperature range of –50°C, and 25°C–850°C. Thus, the ultimate tensile strength, (UTS), yield strength, (YS) and elongation (EL%) amounts of welded joints were determined. The fracture morphologies of the tensile test samples were examined. It was observed that the rupture (except 850°C) occurred from the weld metal. The highest formability was obtained in the samples tested at 850°C. In addition, the existence range of dynamic strain aging (DSA) of the plasma arc welded samples was also investigated. The unsteady plastic flow between the yield and the ultimate tensile of the sample subjected to the tensile test at 250–550°C are evaluated as signs of DSA.

Improving peening efficacy through high-amplitude short duration pulsed current

The efficacy of peening treatment depends on the plasticity of the target metal. In this study, the effectiveness of high-amplitude short duration pulsed current in improving the peening efficacy was examined in a process called electropulsing-assisted ultrasonic nanocrystal surface modification (EP-UNSM). During the EP-UNSM process, the target metal, i.e., Ti64, is subjected to simultaneous ultrasonic peening and electropulsing. The high energy pulsed current can generate a critical magnetic field that can induce the transition of the radical pairs formed by dislocations and the pinning obstacles from the singlet state to the triplet state. This leads to higher dislocation mobility and thus higher plasticity for more effective peening treatment. The results show that the sample treated with EP-UNSM had a deeper plastically deformed layer than that for samples subjected to UNSM and continuous current–assisted UNSM (CC-UNSM), and the maximum depth of plastic deformation was obtained when using the highest peak current density. Due to microstructure refinement, work hardening, and dynamic strain aging, the EP-UNSM sample had a 50% higher surface hardness compared with the control sample. Moreover, the compressive residual stresses generated by EP-UNSM were higher in magnitude and greater in depth compared to those generated by traditional UNSM. These results demonstrate that pulsed current can effectively improve the peening efficacy and EP-UNSM is an effective method for strengthening Ti64.

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.