High Temperature Low Cycle Fatigue Research Articles

Effect of microstructure rafting on deformation behaviour and crack mechanism during high-temperature low-cycle fatigue of a Ni-based single crystal superalloy

The low cycle fatigue behaviours of a microstructure rafting Ni-based single crystal superalloy have been experimentally investigated at 980 ℃. Deformation of γ/γ’ phases and the corresponding dislocation configurations were investigated, highlighting rafting γ/γ’ morphology that contributes to crack initiation and propagation, as well as macro-scale accumulated plastic strain. Unlike the discrete slip lines of a virgin superalloy, intense slips developed along the parallel {111} slip plane result in crossed slip bands in the rafting superalloy. The decreased resistance of widened γ channels to dislocation movement, along with the prevention of dislocation cutting through γ’ precipitates in pre-existing dense dislocation networks, facilitates crack propagation in the γ channel in the slightly rafting superalloy. As the rafting state increases, the dislocation network loses its protective effect by reducing coherency stress and acting as a superdislocation source, which facilitates crack propagation along the γ/γ’ interface. Finally, a microstructure-based fatigue model is developed considering the reduction of deformation resistance induced by rafting. The fatigue loading control mode effect is introduced by a combination of resolved shear stress and tensile stress effects on crack initiation. The LCF life of rafting Ni-based superalloys significantly decreases under stress-controlled conditions compared to strain-controlled conditions due to the increase in cumulative plastic strain. However, the insignificant impact of the initial surface oxide layer on LCF life is revealed.

High-temperature low-cycle fatigue characteristics of the 12Cr10Co3MoWVNbNB turbine rotor steel

Low-cycle fatigue (LCF) experiments of the 12Cr10Co3MoWVNbNB steel at 598 °C and a strain amplitude ranging from 0.27 % to 0.55 % were executed to reveal the fatigue characteristics, fatigue fracture mechanism, and further to predict the high-temperature fatigue life of the new-style turbine rotor steel. The results indicate that the steel exhibits a high-temperature fatigue softening characteristic. During the high-temperature LCF, the cracks are generated at surface or subsurface defects of the specimens, and the fatigue striations and dimples are apparent on the fracture surfaces of the crack propagation zone and the final fracture zone, respectively. The Smith-Watson-Topper (SWT) and Manson-Coffin models are suitable to predict the high temperature fatigue life of the steel at high/low strain amplitudes, respectively. In terms of finite element analysis of the turbine set rotor, it is determined that the steady state strain of the rotor steel is approximately 0.2 %. And thus, the fatigue life of the steel rotor is estimated as 172,500 cycles at the strain amplitude, based on the Eq. (6) in this work. After 100,000 cycles at the strain amplitude, the yield and tensile strengths of the steel specimens are only reduced by 1 % and 3 %, respectively, indicating a long residual life, which is consistent with the above predicting result.

Dynamic recrystallization mechanisms for improving fatigue life under high temperature low cycle fatigue in FGH95 alloy

High temperature low cycle fatigue (LCF) tests were conducted on the FGH95 nickel-based superalloy at temperatures of 420 °C and 600 °C. The experimental results indicate that the fatigue life of the FGH95 alloy at 600 °C is approximately nine times higher than that at 420 °C under identical stress conditions. This finding differs from the conclusions of previous studies on the effect of temperature on fatigue life. The microstructure of the alloy was characterised, and it was found that there were multiple dynamic recrystallisation (DRX) mechanisms present. Continuous dynamic recrystallisation (CDRX) and discontinuous dynamic recrystallisation (DDRX) were identified as the dominant mechanisms at different temperatures. These different dominant mechanisms contributed to the variation in the fatigue life of the alloys under different temperature conditions at the same stress level.

High-temperature low-cycle fatigue and fatigue–creep behaviour of Inconel 718 superalloy: Damage and deformation mechanisms

In this article, strain-controlled Low-Cycle Fatigue (LCF) and fatigue–creep tests were performed on Inconel 718 nickel-based superalloy at temperatures of 650 °C and 730 °C. LCF tests at elevated temperatures were performed with a mechanical strain rate of 1×10−3/s, while fatigue–creep tests involved either tensile or compressive strain dwell. Both the LCF and fatigue–creep tests revealed cyclic softening, with the mean stress evolving oppositely to the applied strain dwell in the fatigue–creep tests. Investigations into the damage mechanisms identified intergranular cracking as the predominant failure mode. Fatigue–creep loading with a compressive dwell resulted in multiple crack initiations from transgranular oxide intrusions, along with multiple creep cavities during loading at 730 °C. Deformation features such as persistent slip bands and deformation nanotwins were observed during cycling at 650 °C. In addition, fatigue–creep tests at 730 °C exhibited δ phase precipitation and a coarsening of strengthening precipitates, contributing to additional softening that increased over prolonged test durations. Finally, the observed lifetime during LCF tests decreased with increasing temperatures, and fatigue–creep loading was observed to be more damaging than LCF. On the other hand, fatigue–creep loading with a tensile strain dwell demonstrated a higher lifetime compared to LCF at 730 °C.

Effect of surface modification on the high temperature low cycle fatigue performance of LPBF 316L austenitic steel

In this study, high temperature low cycle fatigue (LCF) tests at 550 °C were conducted on LPBF (Laser powder bed fusion) 316L with three different surface states, including the As-built surface (AB), fill contour (FC) manufacturing surface, and the surface combining FC and electropolishing (FC + EP). The effect of surface treatment on the high temperature LCF performance of LPBF 316L was investigated. Experimental results showed that the FC + EP surface treatment had the most significant effect on enhancing life. The difference in enhancing the fatigue life behavior was mainly attributed to the various crack initiation mechanisms with different surface states. The cracks on the surfaces of AB and FC mostly originate from the powder adhesion areas and the boundaries of the melt pool, while FC + EP surface treatment weaken the damage induced by powder adhesion. Furthermore, a parameter representing the fatigue crack propagation driving force that considers the surface condition has been defined, and a proposed life prediction model can provide guidance for surface modification and post-treatment of additive manufacturing materials as well as other metal materials.

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Investigation of the High-Temperature Low-Cycle fatigue failure characteristics of P91 steel weld joints and their fatigue strength reduction factors under various load control regimes

Low cycle fatigue tests are conducted on the base metal, weld metal, a welded joint under the condition that the strain control region is the joint (WJWJ) and a welded joint under the condition that the strain control region is the base metal (WJBM) of P91 steel at 500℃ using different total strain ranges. The cyclic stress response is highest for the weld metal, followed by the base metal, and is relatively similar for WJWJ and WJBM. At 2.0 % total strain range conditions, the materials generally exhibit non-Masing behavior and dynamic strain hardening aging. At all total strain ranges, WJWJ failed in the heat-affected zone, and WJBM broke in the base metal. For the difference in fatigue life between base metal and welded joints, the fatigue strength reduction factor of P91 steel welded joints at 500 °C is given as a suggested value. The fatigue behavior of P91 steel welded joints is verified based on a hybrid hardening model and a modified damage model. This study presents a theoretical framework for analyzing the fatigue strength reduction factors of weldments and comparatively analyzes the effects of different geometrical features.

Investigation of the high-temperature fatigue mechanism related to altered local short-range order in AL6XN austenitic stainless steel

An increase in crystal local short-range order (SRO), characterized by obvious diffuse intensity maxima at 1/3{422} in the [111]-axis selected area diffraction pattern, is observed in AL6XN austenitic stainless steel subjected to high-temperature low-cycle fatigue deformation. SRO is an ordered fcc structure with nanometer size, which can be well explained by the ordered arrangement of Cr/Mo atomic layers segregated on the {422} crystal plane in austenite. Many planar slip bands are observed in the fatigue sample, the distribution of SROs is highly related with the distribution of dislocations. In situ high-energy X-ray diffraction (HE-XRD) experiments are performed to investigate the deformation behavior of both as-received and fatigued samples under uniaxial loading. The lattice strain evolution reveals an interesting lattice expansion in the initial elastic stage, which provides direct evidence of a decrease in SRO during further uniaxial tensile deformation, indicating that the deformation modes greatly affect the SRO transition. We believe that the high-temperature fatigue damage originates mainly from the accumulation of planar slip bands, accompanied by a local increase in SRO, which promotes work hardening near the final fatigue fracture.

Study on High-Temperature Low-Cycle Fatigue Behavior of the FGH96 Superalloy Based on Internal Stress Division

In order to deeply explore the high-temperature cyclic characteristics of the FGH96 superalloy under different strain amplitudes, the high-temperature low-cycle fatigue behavior of the FGH96 superalloy was analyzed from the perspective of internal stress evolution. Four sets of strain amplitude (0.5%, 0.6%, 0.8%, and 1.2%) controlled high-temperature low-cycle fatigue tests were carried out on the FGH96 superalloy at 550 °C, and the internal stress was divided into back stress and effective stress through the cyclic stress-strain curves. The results show that the cyclic softening/hardening characteristics of the FGH96 superalloy under different strain amplitudes are closely related to the evolution of internal stress. The strain amplitude has a significant effect on the back stress of the FGH96 superalloy but has little effect on effective stress. At low strain amplitudes (0.5% and 0.6%), the back stress evolution rate of the FGH96 superalloy is lower than effective stress, and the material mainly exhibits cyclic softening. At high strain amplitudes (0.8% and 1.2%), the back stress evolution rate of the FGH96 superalloy is significantly higher than effective stress, and the material exhibits cyclic hardening. The combined effect of back stress and effective stress is the main reason for the different low-cycle fatigue behaviors of the FGH96 superalloy under different strain amplitudes.

A universal constitutive model considering strain range dependence effect and transient behaviour for both cyclic softening and hardening steels

High-temperature low-cycle fatigue (HTLCF) behaviour of 2.25CrMoV and 316L were investigated by employing internal stress. Results reveal that 2.25CrMoV and 316L show cyclic softening and hardening–softening behaviour, respectively. Moreover, they exhibit strain range dependence effect (SRDE) and transient Bauschinger effect (TBE). The SRDE of 316L was observed throughout the entire fatigue procedure, whereas that of 2.25CrMoV was only observed in the initial softening stage. It is noteworthy that the SRDE is controlled by both back and effective stresses in the case of 316L, and it is primarily dominated by the back stress for 2.25CrMoV. Based on the internal stress analysis, a universal constitutive model was developed. Compared with existing models, a cycle-dependent kinematic hardening factor φ was coupled into kinematic hardening variables to improve the description of the hysteresis loops. Besides, a unified modelling method for SRDE was developed based on a kinematic-isotropic combined hardening rule extended with a strain memory surface. Furthermore, a novel two-dimensional proportionality factor δ related to the plastic modulus of cyclic and monotonic responses was incorporated into the short- and medium-range kinematic hardening variables to account for TBE. Finally, the validity of the proposed model was discussed from the hysteresis loop, peak stress evolution, and peak internal stress evolution. The good agreement between experimental data and simulated results suggests that the proposed model is robust in predicting cyclic softening and hardening–softening behaviours.

Determination of relationship between high-temperature low cycle fatigue damage and mechanical property degradation: Experimental and theoretical analyses

Mechanical properties of metallic materials gradually degrade under high-temperature low cycle fatigue (HTLCF) loading, which is a manifestation of fatigue damage accumulation. In this study, a mapping modeling is developed to determine the relationship between micro-scale HTLCF damage and macro-scale mechanical property degradation. Strain-controlled HTLCF tests in GH4169 superalloy are carried out at 650 °C and interrupted at various lifetime fractions for subsequent microscopic characterizations and high-temperature tensile tests. Quantitative analysis of electron backscatter diffraction indicates a three-stage increase in kernel average misorientation and geometrically necessary dislocation throughout the cyclic process, along with variations in the fraction of different grain boundaries. Simultaneously, the evolutions of shearing precipitate and de-twinning are characterized using transmission electron microscope. Furthermore, the mechanical properties of GH4169 superalloy, such as yield strength, ultimate tensile strength, elongation and work-hardening rate, demonstrate different degradation characteristics after exposure to HTLCF damage. Based on these results, the mechanism of HTLCF-induced mechanical property degradation is clarified and the mapping modeling is established using tensile plastic strain energy density and kernel average misorientation. Finally, a damage level evaluation method for engineering components is developed after the generality and flexibility of model is verified in different high-temperature materials.

Validation of applicability of induction bending process to P91 piping of prototype Gen-IV sodium-cooled fast reactor (PGSFR)

The application of the induction bending process to pipe systems in various industrial fields is increasing. Recently, efforts have also been made to apply this bending process to nuclear power plants because it can innovatively reduce welded parts of the curved pipes, such as elbows. However, there have been no cases of the application of induction bending to the piping of nuclear power plants.In this study, the applicability of the P91 induction bending piping for the sodium-cooled fast reactor PGSFR was validated through high temperature low cycle fatigue tests and creep tests using P91 induction bending pipe specimens. The tests confirmed that the materials sufficiently satisfied the fatigue life and the creep rupture life requirements for P91 steel at 550 °C in the ASME B&PV Code, Sec. III, Div. 5. The results show that the effects of heating and bending by the induction bending process on the material properties were not significant and the induction bending process could be applicable to piping system of PGSFR well.

Cyclic microstructure analysis, crack propagation and life prediction of Inconel 750H considering the slip fracture energy

The cyclic stress responses and microstructure analyses were carried out of short-term aged Inconel 750H under high temperature low cycle fatigue tests, the influence of strain amplitude on fatigue fracture mechanism of aged IN 750H was systematically analyzed. According to the slip traces, Schmid factor, and geometrically necessary dislocation density distribution on the fracture frontier, the paper revealed that the local slip band provided a preferred path for the fatigue crack growth of Inconel 750H. Due to the limitation of the inability to measure energy of slip system through experiments, a probability based statistical method on average fracture energy of the slip system was constructed. Coupling with the macro-scale constitutive models, a microstructural-sensitive strategy based on fracture energy exhaustion theory was established. Based on the proposed model, the dynamic process of fatigue crack initiation and growth stage were simulated. The predictive fatigue lives showed a good consistence with the experimental results of both as-received state and short-term aged Inconel 750H under various amplitudes. In addition, the validity of the proposed model was further confirmed by fatigue crack length under the initiation and propagation stages, accompanying by the 3D X-ray computed tomography.

A machine learning method for HTLCF life prediction of titanium aluminum alloys with consideration of manufacturing processes

Most conventional methods only consider the effects of materials and loading conditions when predicting the high-temperature low-cycle fatigue life of titanium aluminum alloys. However, the effects of manufacturing processes are not considered. To address this limitation, this paper proposes a machine learning method. First, the loading conditions are preliminarily filtered by physics knowledge. Second, the manufacturing processes and loading conditions are analyzed by using the average value of multiple mean impact values. Then, the variables that have little contribution to the output are filtered out, and the remaining variables are used as inputs. Finally, an optimized genetic algorithm-back propagation artificial neural network is established to predict the low-cycle fatigue life of the materials. The optimization methods are Levy flight and an adaptive adjustment mechanism of probability. Min-Max normalization is used to normalize and denormalize the data in the proposed method. A set of experimental data for Ti-685 is used to validate the proposed method. Moreover, the results show that the proposed method has better prediction accuracy than other methods.

High Temperature Fatigue Properties of Boron-doped Superalloy B1914

The B1914 superalloy is engaged in the power and aerospace industry as a material for critical hot section of gas turbines. Typical findings in these critical components with a high number of operations are damages by low cycle fatigue where high stresses comparable to the material yield strength are developed due to mechanical loading or thermal gradients causing large plastic deformation. The present contribution refers to the high-temperature low cycle fatigue behaviour of polycrystalline nickel-based superalloy B1914. Cylindrical specimens were fatigued under strain control with constant total strain amplitude in symmetrical cycling at 800°C and 900°C in air. The microstructure is based on a γ matrix (face-centered cubic lattice) with coherent L12 γ´ precipitates and γ/γ´ eutectic. The higher content of grain boundary stabilizing boron allows for lower carbon contents and thus reduces the carbide content that can facilitate fatigue crack initiation. Cyclic hardening/softening curves and fatigue life diagrams were determined. An increase in testing temperature accelerated fatigue crack initiation at grain boundaries associated with carbides. The SEM and TEM analysis of fatigued material further the discussion of fatigue behaviour of the B1914 superalloy.

High Temperature Low Cycle Fatigue of Co–Cr–Ni–W–Ta Heat Resistant Alloy Produced by Additive Technological Processes

High Temperature Low Cycle Fatigue of Co–Cr–Ni–W–Ta Heat Resistant Alloy Produced by Additive Technological Processes

In-Situ Characterization of Microstructural Changes in Alloy 718 during High-Temperature Low-Cycle Fatigue

Components made of nickel-based alloys are typically used for high-temperature applications because of their high corrosion resistance and very good creep and fatigue strength, even at temperatures around 1000 °C. Corrosive damage can significantly reduce the mechanical properties and the expected remaining service life of components. In the present study, a new method was introduced to continuously determine the change in microstructure occurring as a result of exposure to high temperature and cyclic mechanical loading. For this purpose, the conventional low-cycle fatigue test procedure was modified and a non-destructive, electromagnetic testing technique was integrated into a servohydraulic test rig to monitor the microstructural changes. The measured values correlate with the magnetic material properties of the specimen, allowing the microstructural changes in the specimen’s subsurface zone to be analyzed upon high-temperature fatigue. Specifically, it was possible to show how different loading parameters affect the maximum chromium depletion as well as the depth of chromium depletion, which influences the magnetic properties of the nickel-based material. It was also observed that specimen failure is preceded by a certain degree of microstructural change in the subsurface zone. Thus, the integration of the testing technology into a test rig opens up new possibilities for improved prediction of fatigue failure via the continuous recording of the microstructural changes.

Turning/Shot peening of Nickel-based powder metallurgy superalloy: Effect on surface integrity and high-temperature low-cycle fatigue properties

In this study, a nickel-based powder metallurgy alloy was processed via turning, ceramic bead peening, and shot peening by high-intensity cast-iron shots and low-intensity ceramic beads (denoted as compound shot peening). We investigated the surface morphology (surface stress concentration factor, Kst), the surface residual stress profile, microstructure, and the hardness gradient of three surface states and compared the high-temperature low-cycle fatigue lives. The results showed that the surface stress concentration coefficients after turning, ceramic bead peening, and compound shot peening were 2.39, 1.98, and 2.12, respectively. The average surface roughness, Sa, after turning was low, but the bottom of the tool mark was sharp, so the Kst value was higher than that after shot peening, which formed a round bottom by bead blasting. Compared with ceramic bead peening, a surface structure with a deeper compressive residual stress profile and a higher hardness gradient was formed after compound shot peening, and nanostructured features appeared on the outermost surface. The estimated low-cycle fatigue lives after the ceramic bead peening and compound shot peening were 0.99 and 3.13 times longer than after the turning process respectively with strain amplitude 0.5% at 538℃. Moreover the gain of compound shot peening on low cycle fatigue performance gradually decreases with the increase of the strain amplitude.

Enhancement of high temperature low cycle fatigue lifetime of PWA1484 superalloy artificial bicrystals via boron-induced grain boundary strengthening

High temperature fatigue performance plays a critical role in the service of turbine blades, and casting defects like low angle grain boundary (LAGB) in single crystal superalloys would severely affect the service performance of blades. This study demonstrates B-induced pronounced improvement in low cycle fatigue (LCF) lifetime at 980 °C for the PWA1484 artificial bicrystal with 6° LAGB. Such an effective increment in LCF lifetime was achieved by stabilising LAGB through a delicate modulation of LAGB composition without boride. Moreover, B-induced matrix softening featured by homogenous distribution of fatigue deform zones with high density of dislocations was evidenced.

The low cycle fatigue behaviour of MAR-M247 superalloy under different strain rates and cycle shapes at 750 °C

High-temperature low-cycle fatigue experiments were performed at different strain rates on nickel-based superalloy MAR-M247. Furthermore, slow-fast, fast-slow and dwell-fatigue tests were conducted. The deformation, damage and lifetime behaviour as a function of strain rate and cycle shape were observed. Results show that the lifetime decreases with decreasing strain rate, indicating the negative influence of time-dependent creep and oxidation mechanisms. The lifetimes of slow-fast and dwell-fatigue tests are lower than those for fast-slow tests, although the mean stress is compressive in slow-fast and dwell-fatigue tests and tensile in fast-slow tests. Slow-fast and dwell-fatigue tests lead to grain boundary cavitation.