Continuous Softening Research Articles

Revealing the abnormal softening mechanisms of Zn-xCu (x=2, 3) wrought alloys by gradually increasing ECAP numbers

In this study, equal channel angular pressing (ECAP) was employed as a technique to gradually increase the input of strains on Zn-xCu (x = 2, 3; wt%) alloys and investigate their work softening behavior. The microstructure and mechanical properties of Zn–Cu alloys with various processing strains were systematically characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometer (XRD), electron back scattered diffraction (EBSD), transmission electron microscopy (TEM) and tensile tests at room temperature. The results showed that besides η-Zn matrix refined via dynamic recrystallization (DRX), nano-sized precipitated CuZn5 particles were formed in ECAP alloy. Compared with the as-cast alloy, the ultimate tensile strength (UTS) of 1P alloy (after 1 pass of ECAP) was significantly improved to 207 MPa. However, the UTS declined with further increasing ECAP numbers. The 12P alloy (after 12 passes of ECAP) exhibited low UTS of 114 MPa but with an obviously increased elongation of 121% (from 56% of 1P), indicating that the alloy had a softening behavior with increased plastic strains. The main reason for the decline of strength was attributed to the texture weakening, the lower contribution of solid solution strengthening caused by the growth of precipitated CuZn5 phase. The higher ductility of 12P alloy was mainly ascribed from grain refinement, increased continuous DRX softening and the Zn/CuZn5 phase boundary sliding. This work provides a promising insight into the improvement of softening behavior for Zn–Cu based alloys.

Lamellar kinking in primary hot working of titanium alloy: Cross-scale behavior and mechanism

Lamellar kinking, a complex and significant cross-scale heterogeneous deformation behavior during primary hot working of titanium alloys, affects the microstructure evolution, mechanical response as well as optimization of product performance. However, the essence, development processes, formation condition, and contribution of kinking are still controversial. To address these issues, a two-scale crystal plasticity model is developed to analyze the intra-colony strain partitioning and inter-colony interaction. In-situ laser confocal microscope experiments with detailed microstructure characterization enable to track the origin and evolution of lamellar kinking. Kinking is re-recognized as deformation banding rather than the conventionally claimed instability resulting from elastic or plastic buckling or macro plastic flow. Strain partitioning plays a pivotal role in the driving force of kinking by stimulating the continuous orientation softening and leading to a decrease of strain energy. The dependence of kinking’s formation condition on geometrical orientation can be rationalized by the orientation sensitivity of strain partitioning. The formation of kinking contributes to reducing the deformation resistance of colony with initially “hard” orientation and further improve the deformation compatibility among colonies. This work provides new insight into the cross-scale mechanisms of lamellar kinking, which can be utilized to control microstructure homogeneity during hot deformation of titanium alloys aiming at optimizing the performance of forging pieces.

Characterization of the cyclic deformation behavior of simulated HAZs and other constituent microstructural regions of P91 steel weldment

Low cycle fatigue tests were conducted on theweld metal, coarse grain heat-affected zone (CGHAZ), fine grain heat-affected zone (FGHAZ), inter-critical heat-affected zone (ICHAZ), base metaland the actual weldmentof P91 steelat 550 °C using different strain amplitudes.The P91 weldment and its constituent microstructural regions exhibited typical continuous cyclic softening behavior. In the HAZs, the FGHAZ exhibited better fatigue behavior. The CGHAZ and ICHAZ showed higher fatigue lives at the lower and higher strain amplitudes, respectively. An empirical relationship was established for predicting the fatigue life of the weldment using weighted factors of all the constituent regions.

Characterization of the Isothermal and Thermomechanical Fatigue Behavior of a Duplex Steel Considering the Alloy Microstructure

Isothermal and thermomechanical fatigue behavior of duplex stainless steel (DSS) X2CrNiMoN22-5-3 was investigated. The aim of this work was to understand the fatigue behavior by correlation of the isothermal and thermomechanical fatigue behavior with microstructural observations. Fatigue tests at plastic-strain-amplitude of 0.2% were carried out at 20, 300 and 600 °C, while in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) experiments were performed between 300 and 600 °C. During the 20 °C fatigue test, a continuous softening was observed. Transmission electron microscopy examinations reveal pronounced planar slip behavior in austenite. At 300 °C, deformation concentrates in the ferrite, where strong interactions between CrxN and dislocations were observed that explain the pronounced cyclic hardening. DSS studied exhibits softening throughout the whole isothermal fatigue test at 600 °C. In ferrite, during the 600 °C fatigue test, the G phase, γ′ austenite precipitated, and an unordered dislocation arrangement was observed. The stress responses of the TMF tests can be correlated to those of the isothermal fatigue tests. In IP mode, a positive mean stress resulted in premature failure. No γ′ austenite but the formation of subgrains in the ferrite phase was observed after TMF tests. The plastic deformation of the austenite at high temperatures results in an unordered dislocation arrangement.

The influence of hydrogen on the low cycle fatigue behavior of strain-hardened 316L stainless steel

The present study focuses on the low cycle fatigue (LCF) behavior of strain-hardened 316L stainless steel under plastic strain control, aiming to understand the influence of internal hydrogen on fatigue performance and cyclic deformation behavior for energy-related technologies. The strain-hardened 316L specimens tested at plastic strain amplitudes between 0.1% and 0.7% with and without hydrogen displayed continuous cyclic softening. Internal hydrogen increased the cyclic strength of this steel, with a greater difference in strength at low plastic strain amplitudes. A Bauschinger analysis revealed that effective stresses represented the major contribution to the flow stress for all material conditions and amplitudes. At low plastic strain amplitudes, effective stresses were responsible for differences in cyclic strength between hydrogen precharged and non-charged specimens. In contrast, back stresses became more significant at high amplitudes, and more similar deformation structures were observed. Although internal hydrogen reduced the total LCF lifetime at all amplitudes, this degradation was more significant in the low amplitude regime. This result is attributed to high effective stresses that led to the onset of multiple slip at lower cumulative plastic strains in the hydrogen precharged condition compared to the non-charged condition at low amplitudes. In addition, the evolution of the apparent elastic response of the material suggests that the primary crack(s) nucleated at a smaller percentage of the total lifetime and propagated in fewer number of cycles in the hydrogen precharged than in the non-charged condition. Fracture surface and gage section observations revealed a transgranular crack path and planar slip traces in both material conditions, with internal hydrogen promoting multiple slip at lower values of cumulative plastic strain than in the non-charged condition.

In-phase thermomechanical fatigue studies on P92 steel with different hold time

Abstract The effect of hold time with 0, 20, and 40 s on in-phase thermomechanical fatigue (TMF) behavior and life of P92 steel is investigated in this study. TMF tests are carried out under mechanical strain control with strain amplitudes of 0.4 0.4 , 0.6 0.6 , and 0.8 % 0.8\text{\%} , and temperature range of 550–650°C which is closely relevant to the operating condition in power plant. TMF tests are performed in a mechanical strain ratio of R = − 1 R=-1 and cycle time of 120 s. The fatigue life variation follows the sequence of N f 0 s < N f 20 s < N f 40 s {N}_{\text{f}}^{0\hspace{.1em}\text{s}}\lt {N}_{\text{f}}^{20\hspace{.1em}\text{s}}\lt {N}_{\text{f}}^{40\hspace{.1em}\text{s}} for the same mechanical strain amplitude. In addition, the influence of hold time on fatigue life decreases with the increasing strain amplitude. A continuous softening can be observed from the cyclic stress response under all test conditions. Fractographic and microstructural tests indicate that the fracture surfaces are characterized by a multi-source cracking initiation and an oxidation phenomenon. Furthermore, a modified Ostergren model is used to predict the fatigue life and achieves a good predicted result.

Microstructure and nanoindentation studies of M23C6 carbides in Fe-18Cr-8Ni steel after long-term ageing at high temperature

ABSTRACT Understanding the M23C6 carbides precipitation sequence aims to determine a correct heat treatment design in order to optimise the mechanical properties of the Fe-18Cr-8Ni steel. In this paper, the ageing influence at 800°C on the mechanical properties and microstructure of this steel has been studied. The aged material was examined with nanoindentation, macrohardness, scanning electron microscopy, light microscopy, and X-ray diffraction techniques. The hardness test results showed a continuous softening of the Fe-18Cr-8Ni steel when increasing the treatment time up to 6000 minutes. The nanoindentation tests revealed a decrease in the nanohardness of the austenite and M23C6 carbides as the ageing time increased. Before the ageing process, the solution treatment produced completely austenitic steel. After 30 and 60 minutes of ageing, the M23C6 carbides were located at the grain boundaries. The precipitation of the M23C6 carbides within the grains was observed from 300 to 6000 minutes of ageing. The coarsening of the M23C6 carbides occurred at high holding times.

A combined elastic–plastic framework unifying the various cyclic softening/hardening behaviors for heat resistant steels: Experiment and modeling

In this study, the stress–strain responses of G115 martensitic steel, Inconel alloy 750H, and 316H austenitic steel were thoroughly investigated at elevated temperatures via low cycle fatigue tests. The evolutions of effective stress and back stress were determined by stress partition method and were related to cumulative plastic strain. The mobile dislocation density, dislocation structure, and sub-grain structure were discussed to reveal the effect of microstructure on stress–strain responses during fatigue process. Furthermore, a unified elastic–plastic framework was established by introducing a peak plastic strain-dependent relaxation factor, a cumulative plastic strain-modified kinematic hardening model, and a modified isotropic hardening model. The validity of the unified model was discussed based on the maximum stress evolution, stress rate factor, and hysteresis loop. Good consistencies were observed between the experimental and predicted results and showed its strong capability to integrate continuous softening, hardening–softening, and hardening–softening-secondary hardening behaviors into a constitutive model.

Investigation on the Effect of Asymmetric Cyclic Loading on Ratcheting Deformation and Bulk Texture Development in HSLA Steel

Ratcheting deformation in association with bulk texture evolution in HSLA steel is studied in this investigation. The cyclic deformation behaviors and damage modes of the specimens are obtained at room temperature, employing asymmetric stress-controlled tests under various combinations of mean stress and stress amplitudes. Bulk texture measurements of the specimens are done adopting an X-ray diffraction analysis. The results exhibit an increase in strain accumulation from 0.23 to 30.61% associated with a sharp reduction in ratcheting life from 3320 to 880 cycles by the increment in mean stress/stress amplitude. A continuous cyclic softening until the final failure is observed for all the ratcheting tests. It is noticed that variation of mean stress is more detrimental than stress amplitude in controlling strain accumulation while the effect of stress amplitude is more pronounced than mean stress for the reduction of ratcheting life. The fracture surface morphology indicates typical striation formation in the crack propagation region along with overload failure. The bulk texture studies indicate that there is a transition between ‘ϒ’ fiber and ‘ζ’ fiber during all loading conditions. The texture intensity is increased from 5.71 to 11.68 with progressive ratcheting strain accumulation. Here also, influence of stress amplitude is more significant than the mean stress.

Optimized low-cycle fatigue behavior and fracture characteristics of Ti–6Al–4V alloy by Fe microalloying

In the present work, low cycle fatigue (LCF) behavior and fracture characteristic of Ti–6Al–4V-0.55Fe alloy with bimodal microstructure, consisted of equiaxed primary α (αp), lamellar α (αl) and β matrix, are systematically investigated at room temperature. Results indicate that Ti–6Al–4V-0.55Fe alloy mainly exhibits a continuous softening behavior both at high and low strain amplitudes, due to the interaction of back stress(σb) and friction stress(σf) mainly related to the plastic deformation heterogeneity and precipitates shearing, respectively. Compared with Ti–6Al–4V alloy, LCF life of Ti–6Al–4V-0.55Fe is similar at high strain amplitudes (Δεt/2 > 1.0%), while much higher at low strain amplitudes (Δεt/2 < 1.0%), which could be attributed to the extra resistance to dislocation movement and crack propagation produced by Fe microalloying. The fractography shows remarkably different characteristics for these imposed strain amplitudes. At low strain amplitude, there is only one crack initiation site caused by dislocations pile-ups and stress concentration on the specimen surface, accompanied with narrower fatigue striation, while more cracks initiated on the surface with wider fatigue striation due to strain accumulation at high strain amplitudes.

Electrochemical water softening as pretreatment for nitrate electro bioremediation

Electro bioremediation is gaining interest as a sustainable treatment for contaminated groundwater. Nevertheless, the investigation is still at the laboratory level, and before their implementation is necessary to overcome important drawbacks. A prevalent issue is the high groundwater hardness that generates scale deposition on electrodes that irreversibly affects the treatment effectiveness and their lifetime. For this reason, the present study evaluated a novel and sustainable approach combining electrochemical water softening as a preliminary step for electro bioremediation of nitrate-contaminated groundwater. Batch mode tests were performed at mL-scale to determine the optimum reactor configuration (single- or two-chambers) and the suitable applied cathode potential for electrochemical softening. A single-chamber reactor working at a cathode potential of −1.2 V vs. Ag/AgCl was chosen. Continuous groundwater softening under this configuration achieved a hardness removal efficiency of 64 ± 4% at a rate of 305 ± 17 mg CaCO3 m−2cathode h−1. The saturation index at the effluent of the main minerals susceptible to precipitate (aragonite, calcite, and brucite) was reduced up to 90%. Softening activity plummeted after 13 days of operation due to precipitate deposition (mostly calcite) on the cathode surface. Polarity reversal periods were considered to detach the precipitated throughout the continuous operation. Their implementation every 3–4 days increased the softening lifetime by 48%, keeping a stable hardness removal efficiency. The nitrate content of softened groundwater was removed in an electro bioremediation system at a rate of 1269 ± 30 g NO3− m−3NCC d−1 (97% nitrate removal efficiency). The energy consumption of the integrated system (1.4 kWh m−3treated) confirmed the competitiveness of the combined treatment and paves the ground for scaling up the process.

Tailoring the deformation characteristics of commercial purity titanium through hot deformation of a martensitic microstructure

The control of microstructure, texture and deformation inhomogeneity is critical for the hot working of titanium alloys and to optimize their mechanical properties. Herein, a new approach to control the grain refinement, texture and deformation anisotropy of commercial purity titanium (CP–Ti) was established through hot working of a martensitic starting microstructure. The deformation characteristics of the martensitic CP-Ti were investigated through uniaxial hot compression testing at 800 °C and compared with the same alloy with an equiaxed starting microstructure. The associated microstructure, texture development and grain refinement mechanisms were elucidated using electron backscatter diffraction (EBSD) and flow behavior analysis. The equiaxed microstructure displayed a typical flow behaviour of metals undergoing continuous dynamic recrystallization (CDRX). This was accompanied by moderate grain refinement to ∼8.2 μm and macroscopically non-uniform expansion of specimen. In comparison, the martensitic microstructure revealed a stress-strain curve of continuous softening beyond the peak strain. An enhanced grain refinement to ∼5 μm was obtained also via CDRX in the form of kinking of laths and progressive lattice rotation without noticeable macroscopic deformation anisotropy. This distinct deformation behaviour was mainly attributed to the unique microstructure and texture formed through the martensitic transformation, which altered the activation of slip systems, grain refinement kinetics and texture development during hot deformation.

Hot dwell-fatigue behaviour of additively manufactured AlSi10Mg alloy: Relaxation, cyclic softening and fracture mechanisms

This paper presents the results of high temperature strain-range controlled low cycle fatigue tests of a laser powder bead fused AlSi10Mg alloy. Following stress relief (2hrs at 300 ℃), two cyclic loading waveforms (standard triangular and dwell-type trapezoidal waveforms) and three temperatures (100 ℃, 250 ℃ and 400 ℃) were applied to investigate both the mechanical response and the related microstructural changes of this additively manufactured (AM) aluminium alloy. The bulk mechanical responses were found to exhibit a continuous cyclic softening, decreasing stress relaxation and decreased energy dissipated per cycle. The stress relaxation is strongly affected by the test temperature rising to almost complete relaxation at 400 ℃. At lower temperatures (100 ℃ and 250 ℃), the higher the temperature the more subgrains are developed during cyclic loading. Up to 250 ℃, the subgrain size increases with temperature and laser powder bead fused defects preferentially act as the fatigue crack initiation sites. While at 400 ℃, coarse Si particles precipitate during cyclic deformation and a high density of microvoids are nucleated from these coarse Si precipitates, which grow and link up to cause failure, resulting in a dimple dominated ductile fracture.

Cyclic deformation and damage mechanisms of 9%Cr steel under hybrid stress-strain controlled creep fatigue interaction loadings

The hybrid stress–strain controlled creep-fatigue interaction (HCFI) loadings at various dwell conditions were conducted on 9%Cr steel at 625 °C to investigate the deformation and damage mechanisms. Results reveal that the HCFI tests with conservative dwell conditions show complicated cyclic responses and low creep strain. The failure specimens demonstrate strong creep-fatigue interaction damage (CFID) with many branched surface cracks. With the increase in dwell stress and time, the HCFI tests show continuous softening and remarkable creep deformation. The creep-dominant damage (CDD) featured by internal creep cavities and blunt surface cracks leads to the final failure.

Studies on creep-fatigue interaction behavior of Grade 92 steel and its weld joints

Creep-fatigue interaction (CFI) behavior of 9Cr-1.8W-0.5Mo-VNb steel (Grade 92 steel) base metal and its weld joint has been investigated at 823 K using a constant strain rate of 3 × 10–3 s−1. Both the base metal and the weld joint exhibited continuous softening with an almost equal cyclic stress response. The fatigue life decreased with increasing hold time. The hold applied in the compressive peak strain proved more deleterious compared to the tensile hold, indicating a compressive dwell sensitivity. Besides, the weld joints showed a greater reduction in fatigue life compared to the base metal. Fracture surfaces of the tested specimens revealed the prevalence of secondary cracks and extensive oxidation in the material. Weld joint specimens failed in the base metal region in all the tests. Evidence of dynamic strain aging in the form of serrations in the stress-strain hysteresis loops was also noted.

Cyclic response and dislocation evolution of a nickel-based superalloy under low cycle fatigue deformation

The low cycle fatigue behavior of Inconel 750H superalloy, a newly designed Nickel-based superalloy, was studied in this work. The superalloy exhibited an initial cyclic hardening and a following continuous cyclic softening behavior at the strain amplitudes from 0.35% to 0.6%, while a stress saturation at the strain amplitude of 0.3%. Cyclic response considering cyclic stress, plastic strain, and hysteresis loop area was studied. Four interrupted cycle tests were conducted at the strain amplitude of 0.35% to investigate the microstructure evolution during fatigue deformation. As fatigue cycles increased, the fraction of γ′ phase decreased while the diameter of γ’ phase increased. The planar slip deformation with dislocation pile-ups at half cycles was observed, and finally evolved into simple dislocation lines when the specimen ruptured. The coarsening γʹ precipitation degraded the strengthening effect, resulting in more uneven slip deformation. Compared with chain-shaped M23C6 carbides, a larger dimension of the block like MC carbides (i.e. TiC and NbC) led to higher stress at the interface between MC and matrix, where cavities preferred to nucleate and propagate to a micro-crack.

Influence of uniaxial and biaxial pre-straining on the low cycle fatigue performance of DP590 steel

The present work aims to examine the low cycle fatigue (LCF) behavior of dual-phase (DP590) steel subjected to different pre-straining (uniaxial and equi-biaxial) paths. The LCF tests reveal all specimens showed continuous cyclic softening throughout the fatigue life. However, the rate of cyclic softening is higher for the equi-biaxial pre-strain specimen. The rotation of one maximum shear stress plane during fatigue cycling after equi-biaxial pre-straining introduces non-proportional loading condition, thereby significantly reducing the fatigue life. The larger lattice rotation and in-grain misorientation due to the non-proportional loading is notable for the equi-biaxial pre-strain specimen.

Low Cycle Fatigue Behavior and Failure Mechanism of Wire Arc Additive Manufacturing 16MND5 Bainitic Steel

In this study, the low cycle fatigue behavior and failure mechanism of WAAM 16MND5 bainitic steel, which is commonly used in the nuclear reactor, were investigated thoroughly via a series of strain-controlled and stress-controlled low cycle fatigue tests. Results show that the microstructure of WAAM 16MND5 steel is granular bainite with a certain number of banded pro-eutectoid ferrite. WAAM 16MND5 steel exhibits similar cyclic deformation behavior under different strain amplitudes, i.e., a long stage of continuous cyclic softening and rapid cyclic softening till failure. The fatigue life decreases with the increasing strain amplitude. Moreover, the persistent slip markings in ferrite grains, the interface of pro-eutectoid ferrite and large Al2O3 inclusions are the preferred locations for microcrack initiation. The crack initiated from the edge of the specimen is observed to develop into a main crack. The main crack propagates mainly in transgranular mode and can also propagate along the pro-eutectoid ferrite interface.

A full-stage creep model for rocks based on the variable-order fractional calculus

An accurate characterization of rocks for the modelling of creep is an essential step toward ensuring the safety with respect to reliability of underground rock engineering. In this work, a novel variable fractional order rheological model is established to describe the full-stage creep behavior of rocks. With simpler form and fewer parameters, the proposed model is verified to have the ability for the description of trimodal creep behavior of rocks. The evolution of mechanical properties during creep is quantitatively described by the variable fractional order including the creep hardening, softening and the transition part between them. Moreover, the physical significance of fractional order is further explored by the commonly accepted competition mechanism of creep hardening and softening for brittle creep, which indicates that the mechanical property of rocks during creep is harder with lower water contents but softens faster with larger applied stresses. Furthermore, the linear form of the variable order function is determined by applying the new variable order fractional operator. It is shown that the slope of order function confirms the creep strain rate and the intercept of order function primarily affects the critical time for entering the nonlinear creep phase. Finally, the rising tendency of fractional order reveals that the accelerating creep is a continuous softening process of mechanical properties since the larger values of fractional order exhibits the property of rocks is more viscous.

The creep-fatigue behavior of a nickel-based superalloy: Experiments study and cyclic plastic analysis

Cyclic behavior of a nickel-based GH4169 superalloy under fully-reversed and asymmetric creep-fatigue tests with strain-controlled is investigated at 650℃. Significant tension–compression asymmetry and continuous cyclic softening are observed for all loading waveforms. The material exhibits mean stress relaxation under asymmetric loading and fully-reversed loading with valley strain dwells. Moreover, stress relaxation behavior during the dwell period is almost independent of the number of cycles. In order to identify the deformation mechanism, the evolutions of mean value and amplitude of the internal stress (back stress and effective stress) based on the partition of hysteresis loops are analyzed. Results show that both the cyclic softening and mean stress relaxation are primarily dominated by the evolution of back stress and secondarily affected by that of effective stress. Such findings lay a solid foundation for further investigations of reasonable cyclic deformation description and accurate life prediction in GH4169 alloy under creep-fatigue loadings.