Crept Samples Research Articles

Cyclic creep behavior and life prediction of DZ125 alloy under different stress ratios

Abstract In order to study the creep behavior and mechanism of DZ125 alloy under cyclic creep under different stress ratios, cyclic creep experiments are performed under a temperature of 850°C and peak stress of 500 MPa, with stress ratios of 0.1, 0.3, 0.5, 0.7, 0.9, and 1, and the mechanical behavior and microstructure of creep samples under different stress ratios are compared and analyzed. The accuracy of several life prediction models is compared. The results show that cyclic hardening or cyclic softening will occur under different stress ratios due to grain boundary slip and dislocation plugging. According to the results of fracture analysis, as the stress ratio rises, the proportion of creep damage increases, and the fracture mode shifts from transgranular fracture to intergranular fracture. Several life prediction models are selected, and the results show that the FS frequency separation model and M-G model have better life prediction accuracy.

AP-GAN-DNN based creep fracture life prediction for 7050 aluminum alloy

7050 aluminum alloy has high strength and excellent fracture toughness. However, damage can be caused to the 7050 aluminum alloy by creep at elevated temperatures. The accuracy of conventional creep life prediction methods may fall short of meeting the desired standards. And the application of machine learning (ML) methods is limited due to the scarcity and dispersion of creep sample data. Generative Adversarial Networks (GAN) are often used for data enhancement, but suffer from “pattern collapse” under small sample conditions. To solve the above problems, this paper proposes a creep fracture life prediction method, which combines the affinity propagation (AP) clustering algorithm, GAN, and deep neural networks (DNN) for accurate creep fracture life prediction. The AP clustering algorithm for adaptive clustering was utilized to better reflect the creep sample distribution. Independent GAN models were trained for each cluster to better capture distribution characteristics of the data and generate synthetic data that was highly similar to the real data. The DNN models were trained using the synthetic data and then predicted using real creep fracture life data. The presented method was compared with traditional physical and machine learning methods, and the method combining K-Means, GAN, and DNN. The experimental results show that the method proposed in this paper has better prediction accuracy in a small sample creep fracture life dataset.

Short-term creep approach to redefining the role of 17-4PH stainless steel for high-temperature applications

The creep response of the 17-4PH martensitic age-hardening steel in H1150 state was investigated at 427 and 482 °C. Hardness measurements of the heads of the creep samples demonstrated that the material underwent additional age hardening during the high-temperature exposure. Microstructural investigations confirmed that the additional precipitation of carbides and the G-phase occurred at the lowest temperature. A set of constitutive equations previously developed to describe the creep response of particle-strengthened alloys was successfully used to obtain a comprehensive description of the experimental data. The value of the particle strengthening term was obtained from the hardness measurements and corresponded to the Orowan stress. The model accurately described the observed minimum creep rate dependence on the applied stress and explained the occurrence of lower values of the minimum strain rate observed during variable-load experiments.

Exploring the evolution of microstructure and mechanical property of a γʹ-strengthened Co-based single crystal superalloy during creep and thermal exposure at 1000 °C

The study provided a systematic investigation into the creep behavior of a γʹ-strengthened Co-based single crystal superalloy Co-7Al-8W-1Ta-4Ti (at.%) under compressive creep conditions at 1000 °C and 137 MPa. Simultaneously, the microstructural changes during thermal exposure at 1000 °C were thoroughly examined. The microstructure analysis of the creep samples revealed the presence of γ/γʹ rafts, twins, stacking faults, and dislocation networks. Statistical analyses were conducted to assess the dimensions and volume fractions of γʹ, alongside Vickers hardness tests and nanoindentation experiments. Results show that γʹ dissolution in the creep samples with substantially higher dissolution rates compared to thermal exposure. Localized Co depletion and Ti enrichment in twin is observed in creep sample, potentially initiating localized phase transformations. Concurrently, lattice rotation occurred during creep, resulting in a progressive deviation of crystal orientation away from the [001] direction. Stacking faults transitioned from network-like structures to parallel configurations, and twins played a significant role in creep deformation, particularly in samples subjected to 382 h creep. The study also explored the evolution of Vickers hardness and the elastic modulus, introducing a systematic linear model to describe Vickers hardness changes during both thermal exposure and creep conditions.

Electrochemical Characterization Method of Laves Phase in 9Cr Martensitic Heat-Resistant Steel and Creep Life Prediction

The Laves phase formed during the long-term creep of 9% Cr heat-resistant steels plays an important role in their high-temperature mechanical properties. Detecting and quantifying the Laves phase is one of the main problems in predicting the creep residual life. This study detects and quantifies the Laves phase in various 9% Cr heat-resistant steel samples by anodic polarization. Both T92/P92 samples after creep and 9Cr ferritic martensitic samples after thermal ageing precipitate Laves phase particles, and the content of the Laves phase increases with the service time. Comparing the results obtained by the electrochemical method with those obtained by the traditional SEM observation method, it can be found that there is a similar two-stage linear relationship between the two results in all materials, which is related to the diameter change of Laves phase particles during service. According to the correlation between the Laves phase content and creep time, an extrapolation method for the prediction of the residual creep life is proposed. The results show that the creep life of P92 can be predicted by using the dissolved charge density of the Laves phase with high prediction accuracy. This method has the potential to evaluate the service states of materials and predict the long-term creep life in the industrial field.

Creep of IN738LC manufactured with laser powder bed fusion: effect of build orientation and twinning

ABSTRACT The microstructural anisotropy caused by the highly oriented solidification of the metal Laser Powder Bed Fusion (PBF-LB/M) process affects mechanical short- and long-term properties. Component build orientation influences grain morphology and orientation, and thus, mechanical properties. While the creep behaviour of samples manufactured parallel and perpendicular to the build direction are studied intensively, the 45° build orientation remains uncharacterised. In this study, IN738LC creep samples are manufactured via PBF-LB/M in three build orientations (0°, 45° and 90°). While the results of 90° and 0° are as expected, where 90° achieves the longest time to rupture and largest rupture strain, the 45° specimen shows the least fracture time. Differences in microstructure and twinning behaviour are identified as one of the root causes for this unexpected behaviour. This study discusses the correlation between microstructure, twinning and build orientation and their effect on creep behaviour, with special focus on the 45° build orientation.

Effect of Friction Stir Welding on Short-Term Creep Response of Pure Titanium

Friction Stir Welding (FSW) is a recent joining technique that has received considerable attention. FSW causes significant variations in the material microstructure commonly associated with changes in the mechanical properties. The present study deals with the creep response of pure titanium (CP-Ti grade 2) after FSW. Dog-bone creep samples, obtained by machining, which show the longitudinal axis of each sample being perpendicular to the welding direction, were tested in constant load machines at 550 and 600 °C. The creep response of the FSW samples was analyzed and compared with that of the unwelded material. The shape of the creep curves was conventional, although the FSW samples went to rupture for strains lower than the base metal. The minimum creep rates for FSW samples were, in general, lower than for the unwelded metal tested in equivalent conditions. In addition, when the applied stress was high, deformation concentrated in the parent metal. The creep strain became more and more homogeneous along the gauge length as testing stress decreased. A constitutive model, recently developed for describing the creep response of the base metal, was then used to rationalize the observed reduction in the minimum strain rate in FSW samples.

Thermal creep and fatigue failure of the sintered silver solder in a SiC-IGBT module under power cycling

In this work, the creep samples of sintered silver were prepared by the chemical reduction method. According to the creep testing results, the Anand constitutive model was established to demonstrate the creep characteristics of sintered silver within a certain temperature range. According to the finite element electro-thermal–mechanical coupling analysis, the cyclic creep characteristics of sintered silver solder in the SiC-IGBT module under power cycling were simulated. The results shows that the sintering pressure and temperature can greatly promote the integrity of sintered silver by enhancing inert-particle bonding strength, and the established Anand model can effectively describe the creep behavior of sintered silver. The predicted results from electro-thermal–mechanical simulation were in good agreement with the power cycling testing results under the same condition. The finite element-based creep fatigue model is effective in predicting the creep fatigue life of sintered silver solders.

Creep behavior of 3D printed polymer composites

AbstractThe creep behavior of polymer composites containing different weight percentages of poly(lactic) acid (PLA), thermoplastic polyurethane (TPU), and poly(ethylene) glycol (PEG) is experimentally characterized and computationally modeled using finite element analysis (FEA). The composite filaments are manufactured with melt extrusion method by using twin‐screw extruders, and then are employed in the 3D printing of creep samples. The samples are tested under a constant tensile load of 100 N. In this study, the computational model is developed by using the Generalized Voigt‐Kelvin solid model and three terms in Prony series. The equations of Prony series were obtained by the Laplace transformation of Kelvin model. The experimental creep displacements and strains are compared with computational results, and a good agreement between them is observed. The maximum error percentage in computational result is approximately 6% as compared to the experimental result. Hence, it can be said that the relatively simple computational models developed are reliable and can be used to study the creep behavior of similar polymers. The error percentages can still be reduced by considering a higher number of terms of Prony series in the model.Highlights Studied creep behavior for additively manufactured thermoplastic polymer composites. Developed computational model using three terms of relaxation modulus. The experimental results correlate very well with the computational model. Computational models can be used for other polymers.

Influence of build orientation on the creep behavior of IN738LC manufactured with laser powder bed fusion

While metal Laser Powder Bed Fusion (PBF-LB/M) offers many advantages, such as increased design freedom, it is yet to be accepted as a reliable manufacturing route. Due to the highly oriented solidification of the PBF-LB/M process, the microstructural anisotropy significantly affects mechanical short- and long-term properties. Component build orientation directly influences grain morphology and orientation and thus mechanical properties.In this study, IN738LC creep samples are manufactured in three build orientations (0°, 45° and 90°) using PBF-LB/M. Creep tests are carried out at 750 °C and 850 °C with applied stresses up to 350 MPa. While the results of 90° and 0° agree with the creep results of other Ni-superalloys manufactured with PBF-LB/M, where the 90° specimen achieves the longest time to rupture and largest creep strain, the 45° specimen shows differing creep results compared to other Ni-superalloys with the least time to fracture. This study discusses the correlation between microstructure and build direction and their effect on creep behavior, especially on the 45° specimen.

Microstructure evolution and precipitation behavior in a Y2O3-bearing TiAl alloy during creep

The paper studies the microstructure stability of a Y2O3-bearing Ti–48Al–2Cr–2Nb alloy during creep. Tensile creep tests were performed at 800–850 °C under 100 or 275 MPa. Compared to thermal exposure, the low creep stress promotes the microstructure degeneration of lamellar colonies, mainly including α2-lath dissolution, γ-lath coarsening and βo phase precipitation. The high creep stress further accelerates the α2 → γ transformation, but goes against the precipitation of βo grains inside α2 laths. Similarly, more Cr-rich precipitates with the composition of Ti(Al, Cr)2 are observed in creep samples, which are identified as the C14-Laves phase. It has a {0001}C14//{111}γ and <101¯0>C14//<11¯0>γ orientation relationship with the γ phase. The high imposed stress also facilitates the precipitation behavior. It should be noted that there are a few C14-Laves phases precipitating along the Y2O3 interface by heterogeneous nucleation. The high strain energy around Y2O3 particles, especially under the high creep stress, contributes to the precipitation of C14-Laves phase not only at the Y2O3 interface, but also in the γ matrix.

Microstructural insights into creep of Ni-based alloy 617 at 700 °C provided by electron microscopy and modelling

In this work, microstructural changes during creep of Ni-based alloy 617 at 700 °C and 165 MPa have been investigated by electron microscopy, and complementarily modelled. Precipitate types, sizes and chemistry were determined by transmission- (TEM) and scanning electron microscopy (SEM). Apart from γ’ particles, MX and carbides, coarse μ-phase was found. Grain size, frequency of twins, deformation patterns and geometrically necessary dislocations were characterized by electron backscatter diffraction (EBSD). Based on measurements and literature data, creep behavior and a time-to-rupture (TTR) diagram of A617 have been numerically simulated at 700 °C in a range of 165 to 215 MPa with a new physical model. Our new creep model achieved excellent agreement with measured data and literature in terms of predicted creep life, times to 1% strain, minimum creep rate and microstructural evolution. We also succeeded in considering the varying ductility of the material in a novel damage law by implementing the reduction of area from fractured creep samples. Diffusion creep (coble creep) is considered in addition to dislocation creep in the model. The impact of diffusion creep is mostly visible at low stresses, leading to significant improvements within the TTR diagram.

Creep strengthening mechanisms of a superlattice γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy at steady-state conditions under compressive stresses

The strengthening mechanism of steady-state creep is a critical factor in developing advanced superalloys. In this study, the steady-state creep mechanisms of a superlattice γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy are investigated with the creep conditions of 750 °C/800 MPa, 900 °C/420 MPa and 1000 °C/137 MPa. Results suggest that the microstructures of steady-state creep samples are significantly different under the above three experimental conditions. A detailed TEM study shows that, Lomer-Cottrell locks and stacking faults interactions are responsible for the low creep rate at low temperature and high stress creep regime, dislocations cross-slip and the dislocation tangles contribute the main creep resistance at intermediate temperatures and moderate stress creep regime, interactions of stacking faults and dislocation networks are the strengthening mechanism for the high temperature and low stress creep regime. The steady-state creep rate of investigated alloy follows the classical power law equation, and the creep activation energy calculated by the power law equation is 378.46 kJ/mol. The results of this study provide insights into the effects of temperature and stress on steady-state creep mechanisms, and high activation energy makes the γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy an attractive candidate for further study as a high temperature structural material.

Study on the creep properties and crack propagation behavior of single-fissure sandstone based on the damage bond model

As geotechnical engineering expands to deeper formations, the creep behavior of surrounding rock becomes more significant. To better understand the creep deformation and creep propagation of fissured rocks, this study conducts triaxial compression tests and triaxial creep tests of sandstone samples containing a single fissure. The experimental results show that fissured sandstone has a time-dependent failure mode, and creep loading not only causes the crack that dominates the failure of the fissured sandstone to change from a wing crack to an anti-wing crack but also produces more secondary cracks, which are obvious in samples with a low-angle fissure. The steady-creep rate of fissured samples is higher than that of intact samples and first increases and then decreases with increasing fissure angle. The sample with a 45° fissure produces the maximum axial deformation before entering the accelerated creep stage. The theory of the damage bond model (DBM) applied in Particle Flow Code software is proposed, combining the Kachanov damage theory and linear parallel bond model. The DBM effectively simulates the creep deformation and crack propagation process of sandstone in all creep stages, and the failure mode is consistent with the test results. During the initial and steady-state creep stages, the number of microcracks slowly increases, generating a dispersed distribution that rarely exhibits coalescence. Then, the microcracks propagate unstably and coalesce macroscopic cracks, and the accelerated creep stage starts, which is both stress- and time-dependent. Under the same deviatoric stress, the creep simulation samples generate more microcracks than the triaxial compression samples and exhibit a greater reduction in the width of the preexisting fissure after failure. Creep damage accumulation with time significantly affects the evolution of contact forces and displacement vectors. A stress concentration is more likely to occur along the propagation path of an anti-wing crack in creep samples, and the anti-wing cracks present a trend of shear failure. This study plays an important role in predicting the deformation of fissured surrounding rock in underground engineering and explaining the mechanisms of catastrophic events due to the evolution of damage.

Resilient deformation characteristics of subgrade silty filler under intermittent train loading

The intermittent loading method has been proved to be more realistic to simulate the long-term dynamic loading of trains on the subgrade, because the time interval of adjacent trains could be considered in this loading method. The plastic deformation characteristics under intermittent loading have been elaborated in previous studies, but the resilient deformation characteristics have not been reported. By carrying out a series of cyclic triaxial tests with intermittent loading, the resilient deformation characteristics of silty filler under intermittent loading were investigated. The test results indicate that the loading intermittence had a reduction effect on the resilient deformation, and this reduction effect was most significant for incremental collapse samples, followed by the plastic creep samples, and the plastic shakedown samples were the least. There was also a critical resilient strain for this silty filler under intermittent cyclic loading, beyond which the plastic strain developed rapidly to sample failure. A classification criterion for deformation behaviors based on the difference of resilient strain between the first and second loading stages Δεe,1-2, was proposed and then verified by other intermittent loading modes in which the intermittent duration and loading cycles per stage changed. Meanwhile, the limitations of this classification criterion were clarified, and a simpler method to identify the deformation behaviors of subgrade filler was presented based on the stable value of resilient strain under intermittent cyclic loading, which could satisfy the needs of railway engineers and workers to quickly evaluate the railway subgrade in service.

Microstructural stability and creep properties of middle Nb γ-TiAl alloy with a modulated microstructure

In this work, a modulated structure composed of nano-scale Ti2AlNb (O phase) and α2 phase was obtained in middle Nb γ-TiAl alloy. The O phase nucleates at the α2/γ interface and is orthogonal to the α2/γ interface. The creep testing results show that the modulated structure exhibited the better creep performance compared with the fully lamellar microstructure, manifesting in a smaller minimum creep rate and creep strain after creep for 200 h. The O phase in α2 lamellae suppressed the α2 →γ phase transformation and constructed the coherent interface with adjacent γ lamellae, which replaced part of the semi coherent α2/γ interface, to reduce the generation of interfacial dislocation loops, thereby improving the creep resistance. Besides, discontinuous precipitations (DPs) caused by (α2/γ) → β0(ω0) + γ phase transformation were observed in the crept samples, contributing to the microstructure degradation. The present work shows significant engineering significance and scientific value for improving the creep performance and microstructure stability for middle Nb–TiAl alloys.

Microstructural Evolution of the TLP Joints of RAFM Steel during Aging and Creep

In this study, transient liquid-phase (TLP) bonding was adopted to obtain a reliable reduced-activation ferritic/martensitic (RAFM) steel joint with Fe-Si-B amorphous foil. The aging tests and creep tests of the TLP joints were carried out at 550 °C to study the microstructural evolution in the service process. The effect of stress loading on the microstructural evolution of the TLP joint was investigated. The results show that creep fractures in the TLP joints occur in the base material. The main factors affecting the creep performance of TLP joints are the recovery of substructures and the coarsening and deformation of martensitic laths. In addition, the M23C6 carbides in the base material were coarser than in the weld zone. Compared with aging samples and creep samples undergoing the same test temperature, the dislocation density in the isothermal solidification zone (ISZ) increased significantly with increases in the stress level. Furthermore, it is worth noting that the microstructure of the weld zone changed from large-sized ferrite to a mixed, fine microstructure of ferrite and martensite, which increases the heat resistance of the TLP joints, and thus results in creep fractures in the base metal.

Design of a novel creep testing machine to investigate the creep life history of austenitic steel foil at elevated temperature

AbstractA novel creep testing machine was designed to test austenitic steel foil AISI 347 under the elevated temperature of 700°C. The creep test was repeated in five stress levels from low‐stress 55 MPa to severe applied stress 220 MPa for 0.25 mm thickness samples. The machine was equipped with a shield inert gas injection system, and tests were repeated in a nonoxidative area. The creep rupture behavior of the foil is well represented using the modified three‐parameter Theta Projection Concept model. Each model parameter is best represented as a function of the applied stress. Comparison between the results of creep life in different mediums shows that the oxidation effect was the dominant phenomenon in the creep process. Moreover, avoiding oxidation affects the steady‐state stage growth, and afterward, the creep life was increased by more than 40%. Post analyses of ruptured creep samples compare the creep cavitation at triple grain boundaries in a different environment.

Effect of Immersion Media for Polyester Composite Reinforced with Chicken Feathers on Creep Behavior

The use of composite materials has expanded in industrial applications around the world, reinforcing composites with waste materials like (chicken feather) is one of the effective ways to solve environmental concerns compared to traditional methods, as it is considered one of the renewable and inexpensive methods in addition to its presence in abundance. The main added features of composite materials over classic materials are the light-weight. This paper investigates the effect of immersion media for a polyester composite reinforced with chicken feather fillers to enhance the creep characteristic. The composite was manufactured using a hand-layup process. The samples were produced by varying the weight ratios of fiber loading (0 %, 0.2 %, 0.4 %, 0.6 % and 0.8 %) and immersed in three different media (KCl (salt media), NaOH (alkali media) and HNO3 (acid media)). The creep samples were tested according to ASTM standards. The results show that chicken feather–reinforced composites immersed in various media have probable applications because of their superior mechanical properties over non-reinforced composites.

Microstructural characterisation of creep tested advanced austenitic stainless steel super 304H

ABSTRACT Microstructural characterisation was performed on the gauge section of two creep rupture samples manufactured from Super 304H; an advanced austenitic stainless steel that is used as superheater and reheater tubes in power plants. The samples had been exposed to uniaxial creep tests (Sample A: 700°C and 75Mpa, Sample B: 650°C and 120Mpa) where both exhibited low creep ductility. Six phases were identified: MX (Nb-rich), sigma, Cu-rich, M23C6, modified Z phase and Fe (BCC). Quantification of Nb-rich and Cu-rich particles revealed ~10% and ~16% more Nb-rich and Cu-rich particles respectively in Sample A versus Sample B. The amount of sigma and the average particle size for the Nb and Cu-rich particles was similar for both samples. There appears to be an association between the sigma phase and creep cavities as shown in 2D microstructural characterisation which could have contributed to the low creep ductility exhibited by these two creep samples.