Continuous Softening Research Articles

Microstructure Evolution of a Precipitation Hardened Cu-Ni-Si Alloy during Low Cycle Fatigue

The low cycle fatigue (LCF) resistance of a precipitation hardened Cu-Ni-Si alloy has been investigated. Fully reversed strain controlled LCF tests were performed in air at room temperature at a total strain variation Δεt included between 0.6 and 1.5% and at a strain rate equal to 4.10-3 s-1. The cyclic accommodation of the alloy is composed of a hardening step (for Δεt > 0.8 %) followed by a continuous softening until the specimen fracture, except at Δεt = 0.6 % where only a slight softening is observed. For all tested strain variations, the microstructure study of the fatigued alloy by transmission electron microscope (TEM) revealed some grains with a high density of isolated short dislocations pinned between two nanoprecipitates. For Δεt ≥ 0.8 %, some grains present also deformation bands observed by TEM. The strain localisation into these bands implies an important shearing of the precipitates. It leads to their dissolution into the copper matrix and forms precipitate-free bands. The resulting macroscale cyclic accommodation depends on the ratio between the two mechanisms of plasticity accommodation at the microscale.

A Study on the Hot Deformation Behavior of 47Zr-45Ti-5Al-3V Alloy with Initial Lamellar α Structure

The hot deformation behavior of the 47Zr-45Ti-5Al-3V (wt pct) alloy with initial lamellar α structure was investigated by compression tests in the temperature range of 823 K to 1073 K (550 °C to 800 °C) and strain rate range of 10−3 to 100 s−1. In the α + β phase field, the flow curves exhibited a continuous flow softening. The extent of flow softening first decreased with increasing strain rate from 10−3 to 10−1 s−1, and then increased with further increasing strain rate to 100 s−1. In the single β phase field, the flow curves exhibited a pronounced stress drop at the very beginning of deformation at low temperatures and high strain rates. The magnitude of the stress drop increased with decreasing deformation temperature and increasing strain rate. At higher temperatures and lower strain rates, the flow curves exhibited typical characteristics of dynamic recrystallization. A hyperbolic-sine Arrhenius-type equation was used to characterize the dependence of the flow stress on deformation temperature and strain rate. The activation energy for deformation at different strains was calculated. In the α + β phase field, the activation energy for deformation increased from 333 to 355 kJ/mol with increasing strain, and then decreased to 312 kJ/mol. In the single β phase, the activation energy for deformation decreased from 169 to 136 kJ/mol with increasing strain.

Strain-controlled fatigue behavior of cold-drawn type 316 austenitic stainless steel at room temperature

The strain-controlled fatigue behavior of cold-drawn type 316 austenitic stainless steel has been investigated at strain amplitudes varying from 0.22% to 0.75% at room temperature. The results showed that continuous cyclic softening occurs over the whole lifetime at strain amplitude higher than 0.35% in the 20% cold-drawn steel, and 0.31% in the 30% cold-drawn steel, which implies that there is a threshold strain required for cyclic softening during fatigue. The 30% cold-drawn steel displays higher elastic strain amplitudes, resulting in higher fatigue life at strain ranges below 0.80%, while the 20% cold-drawn steel exhibits higher plastic strain amplitudes, leading to higher fatigue resistance at strain ranges above 0.80%. The interactions between dislocations and deformation twins activated during prior cold-drawing process were observed under cycle loading, which play a significant role in the evolution of dislocation structure and subsequently influence the fatigue property.

Deformation and damage mechanisms of a bimodal 12Cr-ODS steel under high-temperature cyclic loading

The present study reports on the high-temperature low-cycle fatigue (LCF) behavior of a ferritic bimodal 12Cr oxide dispersion strengthened steel. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The measured cyclic stress response showed four distinct stages which include instant initial cyclic softening followed by gradual cyclic hardening thereafter continuous linear cyclic softening and finally crack initiation and growth stage. The rate and amount of these stages depend on both strain amplitude and testing temperature. The cyclic stress-strain and strain-life relationships were obtained through the test results, and related LCF parameters were calculated. Microstructural investigations, using electron microscopy, were carried out to shed light on the deformation mechanisms. At 550°C, no appreciable microstructural evolution occurred, which is consistent with the manifestation of Masing behavior. A propensity towards cross-slip and hence the formation of three-dimensional dislocation structures increased with increase in loading amplitude and testing temperature, which assisted in accumulating higher inelastic strain and led to slightly higher cyclic hardening at these conditions. Furthermore, W-enriched Laves phase particles precipitated at grain boundaries. The evolution of cell structures with strain amplitude at 650°C resulted in a small deviation from Masing behavior. The damage studies revealed multiple surface initiated transgranular cracks under higher strain amplitudes and a single surface initiated transgranular crack under lower strain amplitudes. The different stages of the crack path, whose durations depend on the testing conditions, revealed distinct fatigue fracture features.

Fatigue performance of dual‐phase steels for automotive wheel application

AbstractFatigue performance of ferrite–martensite (FM) and ferrite–bainite (FB) dual‐phase (DP) steels used in automotive wheels has been compared in terms of (i) high‐cycle fatigue performance and failure mechanisms and (b) low‐cycle fatigue performance (Δεt/2 = 0.002 to 0.01) and associated deformation mechanisms. FBDP steel exhibits moderately better high‐cycle fatigue performance, owing to delay in microcrack initiation. In FBDP steel, microcracks initiate predominantly along ferrite grain boundaries, while that at FB interface is significantly delayed in comparison with FMDP steel, where few microcracks appear at FM interface even below the endurance limit. During low‐cycle fatigue, however, FMDP steel performs considerably better than FBDP steel till Δεt/2 ≤ 0.005 attributed to initial cyclic hardening, followed by cyclically stable behaviour exhibited by FMDP steel. In sharp contrast, at all Δεt/2 > 0.002, FBDP steel undergoes continuous cyclic softening. The latter may cause undesirable deformation of wheels in service.

Mechanism of hot-rolling crack formation in lean duplex stainless steel 2101

The thermoplasticity of duplex stainless steel 2205 (DSS2205) is better than that of lean duplex steel 2101 (LDX2101), which undergoes severe cracking during hot rolling. The microstructure, microhardness, phase ratio, and recrystallization dependence of the deformation compatibility of LDX2101 and DSS2205 were investigated using optical microscopy (OM), electron backscatter diffraction (EBSD), Thermo-Calc software, and transmission electron microscopy (TEM). The results showed that the phase-ratio transformations of LDX2101 and DSS2205 were almost equal under the condition of increasing solution temperature. Thus, the phase transformation was not the main cause for the hot plasticity difference of these two steels. The grain size of LDX2101 was substantially greater than that of DSS2205, and the microhardness difference of LDX2101 was larger than that of DSS2205. This difference hinders the transfer of strain from ferrite to austenite. In the rolling process, the ferrite grains of LDX2101 underwent continuous softening and were substantially refined. However, although little recrystallization occurred at the boundaries of austenite, serious deformation accumulated in the interior of austenite, leading to a substantial increase in hardness. The main cause of crack formation is the microhardness difference between ferrite and austenite.

Characterization of the hot deformation behavior and microstructural evolution of Ti–6Al–4V sintered preforms using materials modeling techniques

The hot deformation behavior of Ti–6Al–4V sintered preforms was investigated by hot compression tests over temperatures from 800 to 1100 °C and strain rates from 0.001 to 10.0 s−1. The flow curves for the α + β regimes exhibited continuous flow softening and broad oscillation after the peak stress under all of the processing conditions. Otherwise, for the β field, steady-state flow stress was obtained or stress oscillation appeared after the peak stress. In the α + β field, the dynamic globularization and platelet buckling/kinking were observed at lower and higher strain rates, respectively. Unlike in the α + β regions, dynamic recrystallization was found at higher strain rates in the β ones. The apparent activation energies for hot deformation in the α + β and β regimes from kinetic modeling technique were estimated to 147.35 and 233.95 kJ mol−1, respectively. The efficiency of power dissipation over all of the deformation conditions was also calculated by using dynamic material models and schematized into a processing map. The highest value was predicted to 46% at a temperature of 945 °C and at a strain rate of 0.001 s−1, and the processing condition seemed to be optimal for hot deformation of the Ti–6Al–4V sintered preforms.

Cyclic behaviour of 12% Cr ferritic‐martensitic steel upon long‐term on‐site service in power plants

AbstractThe strain‐controlled and stress‐controlled low‐cycle fatigue behaviour of served 12% Cr ferritic–martensitic steel is conducted at room temperature. Continuous softening is observed at both control modes, and the fitting results show that the fatigue properties of 12% Cr steel are not reduced significantly after 230 000 h service at 550 °C/13.7 MPa. Scanning electron microscopy has been employed to investigate the microstructure evolution after long‐term service. It is proved that the decomposition of martensite laths structure and the coarsening of carbides at grain/lath boundaries are the main reasons why the pipe bursts after 180 000 h service at 550 °C/17.1 MPa. The fracture under both control modes has been observed by using scanning electron microscopy, and it indicates coarse carbides along grain/lath boundaries are favourable sites for micro‐crack nucleation and the secondary cracks along the fatigue striations are formed by the connection of micro‐cracks nucleated during fatigue behaviour.

Assessment of fatigue response of thermally aged reduced activation ferritic-martensitic steel based on finite element analysis

In the present investigation, effect of thermal ageing on low cycle fatigue (LCF) behaviour of Reduced Activation Ferritic Martensitic steel has been assessed by finite element analysis. The steel was thermally aged at 873 K for 3000 hour. Low cycle fatigue tests were carried out on both the as-received and thermally aged material at strain rate of 3×10−3 s−1 at 823 K, over strain amplitudes in the range of ± 0.25 to ± 0.8%. Continuous cyclic softening till final failure, except for initial few cycles especially at relatively lower strain amplitudes, was observed in both the material conditions. Thermal ageing resulted in marginally higher cyclic stress response accompanied by lower fatigue life. The differences in fatigue responses have been attributed to the coarsening of precipitates on thermal ageing. Finite element analysis has been carried out considering combined isotropic and kinematic hardening as material model to estimate the effect of thermal ageing on the response of material under LCF loading. Thermal ageing was found to decrease both the isotropic and kinematic hardening with appreciable effect on isotropic hardening. The predicted cyclic stress response and hysteresis loops were found to be in good agreement with the experimental data. The LCF life of the steel has been estimated based on the hysteresis energy approach.

First-principles modeling of energetic and mechanical properties of Ni–Cr, Ni–Re and Cr–Re random alloys

We apply the exact-muffin-tin-orbitals (EMTO) method to investigate structural properties, formation enthalpies, mechanical stability and polycrystalline moduli in Ni–Re, Ni–Cr and Cr–Re disordered fcc, bcc and hcp phases. Substitutional disorder is treated by using the coherent potential approximation (CPA). We predict the alloy lattice parameters in good agreement with the experiment. We find a continuous softening, as a function of Cr composition, of the tetragonal shear modulus C′ in fcc Ni–Cr phase indicating mechanical instability in Cr-rich Ni–Cr alloys. On the other hand, we show that the mechanical stability of fcc Ni–Re alloys persists through the whole composition range. We observe an intriguing behaviour of the Young’s modulus vs. the intrinsic ductility curve in Ni-rich Ni–Re fcc phase.

Constitutive Modeling for Flow Behaviors of Superaustenitic Stainless Steel S32654 during Hot Deformation

Hot deformation behavior of superaustenitic stainless steel S32654 was investigated with hot compression tests at temperatures of 950 — 1250 °C and strain rates of 0.001 — 10 s−1. Above 1150 °C, with strain rate lower than 0.1 s−1, the flow curves exhibit nearly steady-state behavior, while at higher strain rate, continuous flow softening occurs. To provide a precise prediction of flow behavior for the alloy, the constitutive modeling considering effect of strain was derived on the basis of the obtained experimental data and constitutive relationship which incorporated Arrhenius term and hyperbolic-sine type equation. The material constants α, n, Q and lnA are found to be functions of the strain and can be fitted employing eighth-order polynomial. The developed constitutive model can be employed to describe the deformation behavior of superaustenitic stainless steel S32654.

Coupled Influence of Temperature and Strain Rate on Tensile Deformation Characteristics of Hot-Extruded AZ31 Magnesium Alloy

To explore the coupled effect of temperature T and strain rate \(\dot{\varepsilon }\) on the deformation features of AZ31 Mg alloy, mechanical behaviors and microstructural evolutions as well as surface deformation and damage features were systematically examined under uniaxial tension at T spanning from 298 to 523 K and \(\dot{\varepsilon }\) from 10−4 to 10−2 s−1. The increase in T or the decrease in \(\dot{\varepsilon }\) leads to the marked decrease in flow stress, the appearance of a stress quasi-plateau after an initially rapid strain hardening, and even to the occurrence of successive strain softening. Correspondingly, the plastic deformation modes of AZ31 Mg alloy transform from the predominant twinning and a limited amount of dislocation slip into the enhanced non-basal slip and the dynamic recrystallization (DRX) together with the weakened twinning. Meanwhile, the cracking modes also change from along grain boundaries (GBs) and at twin boundaries (TBs) or the end of twins into nearby GBs where the DRX has occurred. The appearance of a stress quasi-plateau, the formation of large-sized cracks nearby GBs, and the occurrence of continuous strain softening, are intimately related to the enhancement of the non-basal slip and the DRX.

Dynamic Strain Aging and Oxidation Effects on the Thermomechanical Fatigue Deformation of Reduced Activation Ferritic-Martensitic Steel

Thermomechanical fatigue (TMF) behavior of a reduced activation ferritic-martensitic steel was investigated under in-phase (IP) and out-of-phase (OP) conditions under different mechanical strain amplitudes and temperature regimes. OP TMF was generally observed to result in the lowest cyclic lives compared to both IP TMF and isothermal low cycle fatigue (IF) at the maximum temperature (T max). The stress–strain hysteresis loops under TMF were marked by extensive serrations associated with dynamic strain aging (DSA) at the strain amplitudes of ±0.4 and ±0.6 pct. The serrations were noticed during the downward ramp of temperature that resulted in IP and OP TMF exhibiting jerky flow in the compressive and tensile portions, respectively. However, no evidence of serrated flow was seen under IF cycling at any of the temperatures within the TMF cycle. The stress response during IP TMF was marked by a near-saturation regime over 65 to 70 pct of life in contrast to continuous cyclic softening in the case of OP TMF. The marked life reduction observed under OP cycling at the strain amplitudes of ±0.4 and ±0.6 pct was attributed to the deleterious influence associated with oxidation, DSA, and tensile mean stress. The findings assume importance in the context of elevated temperature fatigue design, considering the fact that the IF data at T max are deemed adequately conservative in traditional design approaches.

Low Cycle Fatigue Behaviour of Ex-Service P92 Steel at Elevated Temperature

The influence of strain amplitude and strain rate on low cycle fatigue (LCF) behaviour of the ex-service P92 steel at temperature of 600°C has been examined. Fully reversed strain-controlled LCF tests were conducted at the strain amplitude between ±0.4% and ±0.8% employing strain rate of 2.4x10-3s-1 to 2.4x10-5s-1. The material has been found to exhibit continuous cyclic softening behaviour throughout the LCF tests, and there was no saturation stage observed. The number of cycles to failure decreased with lowering the strain rate and increasing strain amplitude. Mathematically, the relationship between time to failure and strain rate can be expressed by a power law relation. Elastic-plastic finite element (FE) analysis was carried out to obtain the hysteresis loop and cyclic stress response of the material. The computation results are shown to be in good agreement with the experimental data. Fractographic examinations of the fatigued specimens were performed using scanning electron microscopy (SEM). Under the LCF condition at higher strain rates, the crack propagated intergranularly which due to fatigue while at lower strain rates the crack may be propagated in both inter- and transgranular manner.

Defect Recovery in Severely Deformed Ferrite Lamellae During Annealing and Its Impact on the Softening of Cold-Drawn Pearlitic Steel Wires

Cold-drawn pearlitic steel wires with a drawing true strain of 3 were annealed at temperatures (Tann) ranging from 423 K to 723 K (150 °C to 450 °C) with an interval of 50 K. Recovery of the lattice defects in the severely deformed ferrite lamellae were characterized by means of high-energy X-ray diffraction and positron annihilation techniques (including positron annihilation spectroscopy and coincidence Doppler broadening spectroscopy). Accordingly, the impact of defect recovery on the softening of the annealed wires was investigated. It is found that at low temperatures [Tann ≤ 523 K (250 °C)], the recovery of the lattice defects in ferrite lamellae is dominated by the agglomeration and annihilation of vacancy clusters, while at Tann > 523 K (250 °C), the recovery process is controlled by the annihilation of dislocations. Further analyses on the softening of the annealed wires indicate that the evolutions of dislocation density and concentration of vacancy clusters, and the strain age hardening in ferrite lamellae play important roles in changing the strength of the wires. The strain aging hardening leads to a maximum strength at 473 K (150 °C). Above 523 K (250 °C), the annihilations of vacancy clusters and dislocations in ferrite lamellae cause a continuous softening of the wires, where the decrease in dislocation density plays a major role.

Material characterization of high-frequency mechanical impact (HFMI)-treated high-strength steel

Fatigue strength improvement due to high-frequency mechanical impact (HFMI) is based on compressive residual stresses, improved weld toe geometry and strain hardening of the treated surface region. In order to understand the combined effects of these factors on the weld toe elastic–plastic response and residual stress stability, the material properties in the HFMI-treated weld toe region need to be determined. In this study, five different material conditions were studied: high-strength steel base material, Gleeble simulated heat-affected zone, and three HFMI-treated material conditions. The local material characteristics were determined by microhardness measurements, electron backscatter diffraction and mechanical testing. The specimens were fatigue tested under fully reversed strain-controlled loading. Surface residual stresses in the fatigue test specimens were measured with X-ray diffraction. The results showed that the heat-affected zone had lower fatigue and yield strength than the base material due to larger grain size. HFMI-treatment increased the steel yield strength up to 1–1.5mm. This observation was supported by change in grain shape in the treated surface region. This lead to an increase in fatigue strength at low strain amplitudes and a decrease in fatigue strength at high strain amplitudes. Continuous cyclic softening was observed for all material conditions.

‘Excess’ of primary cosmic ray electrons

With the accurate cosmic ray (CR) electron and positron spectra (denoted as Φe− and Φe+, respectively) measured by AMS-02 Collaboration, the difference between the electron and positron fluxes (i.e., ΔΦ=Φe−−Φe+), dominated by the propagated primary electrons, can be reliably inferred. In the standard model, the spectrum of propagated primary CR electrons at energies ≥30GeV softens with the increase of energy. The absence of any evidence for such a continuous spectral softening in ΔΦ strongly suggests a significant ‘excess’ of primary CR electrons and at energies of 100–400GeV the identified excess component has a flux comparable to that of the observed positron excess. Middle-age but ‘nearby’ supernova remnants (e.g., Monogem and Geminga) are favored sources for such an excess.

Microstructural stability of ODS steels in cyclic loading

AbstractThe remarkable microstructural stability of high chromium steels prepared by powder metallurgy and strengthened by dispersion of nanometric yttrium oxides in cyclic loading at high temperatures is reported. Contrary to the continuous cyclic softening and profound changes in the microstructure during fatigue of common high chromium steels, the addition of 0.3 wt% Y2O3 stabilizes the microstructure and significantly reduces cyclic softening of investigated steels. The evolution of microstructure as a result of fatigue loading at room temperature, 650 and 750 °C, was examined by means of transmission electron microscopy. Only minor changes in the microstructure were detected. The stability of oxide particles after high‐temperature exposure was confirmed by energy dispersion spectroscopy chemical analysis. The microstructural features are discussed in relation to the cyclic behaviour of the oxide dispersion strengthened steels. The analysis of the hysteresis loop indicates that oxide nanoclusters are intersected and dissolved in slip bands of ODS Eurofer steel. This process contributes to cyclic softening.

Selective enzymatic removal of elastin and collagen from human abdominal aortas: Uniaxial mechanical response and constitutive modeling

The ability to selectively remove the structurally most relevant components of arterial wall tissues such as collagen and elastin enables ex vivo biomechanical testing of the remaining tissues, with the aim of assessing their individual mechanical contributions. Resulting passive material parameters can be utilized in mathematical models of the cardiovascular system. Using eighteen wall specimens fromnon-atherosclerotic human abdominal aortas (55±11years; 9female, 9male), we tested enzymatic approaches for the selective digestion of collagen and elastin, focusing on their application to human abdominal aortic wall tissues from different patients with varying sample morphologies. The study resulted in an improved protocol for elastin removal, showing how the enzymatic process is affected by inadequate addition of trypsin inhibitor. We applied the resulting protocol to circumferential and axial specimens from the media and the adventitia, and performed cyclic uniaxial extension tests in the physiological and supra-physiological loading domain. The collagenase-treated samples showed a (linear) response without distinct softening behavior, while the elastase-treated samples exhibited a nonlinear, anisotropic response with pronounced remanent deformations (continuous softening), presumably caused by some sliding of collagen fibers within the damaged regions of the collagen network. In addition, our data showed that the stiffness in the initial linear stress–stretch regime at low loads is lower in elastin-free tissue compared to control samples (i.e. collagen uncrimping requires less force than the stretching of elastin), experimentally confirming that elastin is responsible for the initial stiffness in elastic arteries. Utilizing a continuum mechanical description to mathematically capture the experimental results we concluded that the inclusion of a damage model for the non-collagenous matrix material is, in general, not necessary. To model the softening behavior, continuous damage was included in the fibers by adding a damage variable which led to remanent strains through the consideration of damage.

INFLUENCE OF TEMPERATURE ON LOW-CYCLE FATIGUE BEHAVIOR OF INCONEL 625 NICKEL-BASED SUPERALLOY WELDING JOINT

The lowcycle fatigue tests of Inconel 625 nickelbased superalloy welding joints at 25 and 760 °C were performed . The strain-fatigue life data and cyclic stress-strain data were analyzed to determine the strain fatigue parameters of Inconel 625 superalloy welding joints. The result showed that the relationship between elastic strain amplitude, plastic strain amplitude and reversals to failure could be described by Basquin and CoffinManson equations, respectively. Under the strain control, the continuous cyclic softening was observed at 25 °C , however, the cyclic hardening appeared at 760 °C. At 25 °C, the fatigue crack of Inconel 625 superalloy welding joints initiated transgranularly at the specimen surface and propagated in the transgranular mode. Differently, at 760 °C, the fatigue crack initiated transgranularly at the specimen surface, but propagated in mixed transgranular and intergranular modes.