Creep Lifetime Research Articles

Unraveling the SCC behavior and enhanced creep strength mechanism of AFA alloy in supercritical CO2

The oxide scale of 20Cr25Ni2.5Al AFA alloy formed under the coupling of stress and supercritical carbon dioxide (sCO2, 650 ℃ / 15MPa) consisted of Fe3O4, Cr2O3, preferential intergranular oxidation zones, but lacked a thin Al2O3 protective layer. The poor oxidation resistance resulted in significant CO2 penetration into the matrix and subsequent formation of Cr2O3, thereby hindering the initiation of Cr - rich σ phase beneath the oxide scale. This competition between oxidation and precipitation effectively reduced crack initiation and delayed the connection between SCC and creep cracks, thus significantly prolonging the creep lifetime in sCO2 compared to that in air.

Role of the γ′′ precipitation at the cell boundaries in enhancing the creep resistance of additively manufactured Inconel 718 alloy using the laser powder bed fusion technique

Laser powder bed fusion (LPBF) manufactured Inconel 718 superalloy (IN718) parts exhibit inferior creep properties compared to their conventionally manufactured (CM) counterparts due to the unique microstructural features associated with them. In this study, two different heat treatment schedules were employed, to critically examine role of distinct microstructural features associated with LPBF on the creep performance of LPBF IN718. Experimental results reveal that the creep performance is controlled by the Laves and γ′′ phases, as well as the cellular structure. Specially, the effective dissolution of Laves phases that are distributed along the grain and cellular boundaries in the as-fabricated state during solution treatment reduces the cavity nucleation sites and hence, extends the creep lifetime. The release of Nb into the matrix contributes to the precipitation of primary strengthening γ′′ phase during subsequent aging, enhancing the creep resistance. More importantly, the formation of a unique cellular structure with densely distributed γ′′ phase along the cellular boundaries, which exhibits high impediment of dislocation movement and inhibition of crack propagation, results in superior creep performance. The findings of the current study provide an effective heat treatment strategy for achieving high creep performance in LPBF nickel-based superalloys.

Rapid assessment of the creep rupture life of metals: A model enabling experimental design

Prediction of the creep rupture life of engineering metals is critical for qualification and design of new materials. The use of long-term creep tests and the need to quantify the performance variability in a priori similar systems hinder the rapid creep assessment of a given material. Therefore, it is essential to develop methods that can extrapolate the long-term performance of alloys and the associated variability from short-term experiments. To this end, this study introduces a new model which enables the estimation of the rupture life of a material for a given stress and temperature. This model relies on two components. First, a new relation for the minimum creep rate (MCR) of materials is introduced. It includes a stress dependent stress exponent allowing the model to capture the variation of MCR across a wide range of temperatures and stresses. Second, employing the Monkman-Grant (MG) law, we establish a relation between stress, temperature and creep rupture life. Together, these two elements yield a new closed-form mathematical expression for the Larson Miller parameter as a function of stress and temperature. This expression captures the creep rupture time for many metals (Gr91, Copper, Gr122 and 347H) and compares favorably with alternate empirical approaches. The model is then used to assess the minimum duration of creep rates necessary to qualify the material up to 100000h. It is found that depending on the material system, creep tests as few as five limited to 5000 h for steels (Gr91, Gr122, 347H) and 100 h for copper are sufficient to model creep lifetimes. Finally, using a Bayesian inference-based approach to calibrate the model, we demonstrate that variability in rupture life can be captured via the quantification of the uncertainty in the model parameters and extrapolated from a limited number of short to moderately short creep tests; thereby paving the way for accelerated creep testing.

Microstructure and creep behavior of electron beam directed energy deposited TC11 titanium alloy

The creep behavior of TC11 titanium alloy prepared by electron beam directed energy deposition (EB-DED) under various temperatures and stresses was investigated. The results indicated that TC11 prepared by EB-DED maintained excellent creep resistance in the temperature range from 550 °C to 600 °C, compared with the forged TC and TA series titanium alloys. The creep process accelerated with increasing temperature and stress, leading to elevated creep rates and shortened creep lifetimes. The improved creep resistance was related to the multiscale microstructures, i.e., the primary β columnar grains, the refined α lamellar microstructure and the solid solution strengthening effect of Si element predominantly existing in a nearly fully dissolved state.

Re-heat treatment effect on the microstructure and mechanical properties of the Inconel 706 alloy for repair

Although the understanding of microstructure and mechanical property changes in re-heat-treated Inconel alloys is important to evaluate the performance of repaired components, recent research has mainly focused on the heat treatment effect of as-fabricated products. Thus, in this work, the performance change of the used Inconel 706 rotor-disks under varying re-heat treatment conditions was investigated. Because of the heat exposure of the used rotor-disk component, a high fraction of plate-like η phase (6.36%) and compact γ'/γ" co-precipitate structure were initiated in the as-received sample. After re-heat treatment, η phase dissolution breakage and compact to non-compact γ'/γ" co-precipitate structure transition occur, resulting in mechanical properties changes of the Inconel 706 alloy. As a result, η phase fraction affects the tensile strength and ductility of Inconel 706 alloy, while the non-compact γ'/γ" co-precipitate structure in the matrix degrades the creep lifetime. These results indicate that re-heat treatment during the repair of operated components induces microstructural and mechanical properties changes. However, to investigate the detailed history of used Inconel 706 components, additional research on the microstructural degradation during operation of Inconel components is required.

Thermo-mechanical analysis and creep lifetime prediction of coated turbine vanes considering thermal barrier coating spallation characteristics

Quantitative analysis of the influence of thermal barrier coating spallation on the thermo-mechanical behaviors and creep lifetime of turbine vanes is crucial for their maintenance and reliability improvement. The temperature and stress distribution of vanes with preset coating spallation damage are investigated in this study, using the thermal-fluid-solid coupling method. Additionally, a further computational analysis is conducted to predict the hazardous regions and creep lifetime of the vanes. The results indicate that coating spallation leads to significant changes of the temperature and stress distribution at the spalled regions of vanes. The remaining coating on the unspalled regions continues to provide effective protection. Stress concentration primarily occurs at the upstream and downstream of the leading edge film holes, while high-stress regions are observed between adjacent rows of film holes, forming a serrated shape. The creep lifetime of the vanes decreases significantly at the region with coating spallation. When the same spallation area is considered, the coating spallation at the leading edge has a more serious influence on creep lifetime, which is more likely to cause the vanes failure. This study reveals the influence of coating spallation characteristics on the thermo-mechanical behaviors and creep lifetime of vanes, providing valuable insights for durability assessment of coated high-temperature components.

A refined model for the flexural behavior of reinforced UHPC members under sustained loading

The long-term behavior of reinforced Ultra-High Performance Concrete (UHPC) members is significantly influenced by UHPC’s creep properties. To predict the long-term performance of structural members, it is essential to have both a material-level creep model and a constitutive model for structural analysis. This paper presents a refined model for the long-term performance of UHPC, incorporating critical nonlinear deformation mechanisms: instantaneous damage, plastic strain, creep strain, and time-dependent damage. The constitutive model for instantaneous damage and plastic strain mechanisms is based on experimental data from short-term cyclic loading tests on UHPC specimens. Additionally, the creep strain components and time-dependent damage evolution laws are derived from well-established linear and nonlinear creep models, validated for cementitious materials through extensive research. The model parameters were meticulously calibrated using creep damage test results from UHPC. The validation process included linear and nonlinear creep tests on UHPC cylinders under sustained stress levels ranging from 0.2fc to 0.66fc, creep failure tests on UHPC cylinders under sustained stress levels from 0.85fc to 0.95fc, and long-term bending tests of reinforced UHPC beams under sustained load. In the linear and nonlinear creep tests, comparison between the calculated ultimate creep coefficients and the experimental results demonstrates the model’s validity under low and moderate stress levels. In the creep failure test, a comparative analysis of strain evolution and creep lifetime between experimental and model results confirms the model’s validity under high sustained stress levels. Similarly, during the long-term bending test, simulated time-dependent displacement and compressive strain show good agreement with the experimental data, confirming the model’s accuracy in predicting the long-term performance of reinforced UHPC members. Furthermore, to streamline the constitutive model, the instantaneous damage effect was simplified and ultimately neglected. A comparison of simulation results between the original model and the simplified version demonstrates that disregarding the instantaneous damage effect is a safe approach for practical engineering applications.

Effect of PWHT on microstructure and mechanical properties of welded joint of a new Fe-Ni-based superalloy

In this study, the effect post weld heat treatment (PWHT) on microstructural evolution and mechanical properties of weld metal of HT700P alloy with Inconel 617 filler metal was investigated. After welding, columnar grain, dendritic microstructure and segregation of Mo, Co and Ti elements related to the equilibrium distribution coefficient were observed in the weld metal. The PWHT was conducted in both direct-aging treatment with different time and solution treatment with different cooling methods. The results showed that direct-aging treatment had a limit impact on homogenization of weld metal, while microstructural homogenization and elemental segregation were significantly improved after solution treatment and massive M6C and M23C6 carbides precipitated at the grain boundaries and interdendritic region of weld metal. It was found that the elongation at 700 °C of welded joints after solution treatment was relatively increased, which was related to the decrease of residual stress and the precipitation of Mo-rich carbides at the grain boundaries. The fracture occurred in the weld metal presented interdendritic fracture characteristics, and the fracture mechanism for stress relaxation cracking was the strong grain interior induced by the precipitation of γ′ phase, leading to the stress concentration and nucleation of cavities at grain boundaries. After solution treatment, the creep lifetime of furnace cooling was much greater than that of water quenching. The superior creep properties of furnace-cooled joint could be attributed to the lower residual stress and the migration of carbon element in the fusion line, which inhibited the nucleation and aggregation of cavities in the partially melted zone.

Achieving superior elevated temperature properties in additive manufactured nickel-based superalloys through unique alloy design

Additive manufacturing (AM) offers a new solution for the integrated fabrication of complex components, supplementing the shortcomings of conventional subtractive manufacturing. However, the forming defects owing to alloy unsuitability and the lack of long-term service stability of the components seriously limit its widespread engineering applications. Here we developed a nickel-based superalloy, Haynes 230AM, suitable for AM by using Zirconium (Zr) alloying strategy in Haynes 230 alloy, which is crack-free and has outstanding elevated temperature properties. Compared with commercial Haynes 230 alloy fabricated by rolling and AM, the printed Haynes 230AM in our work achieves about 700 % improvement in creep lifetime, breaking the status quo that the creep properties of AM fabricated alloys are generally inferior to the corresponding conventional manufactured alloys. Our results reveal that the introduction of Zr element optimized the microstructure of the alloy. The superior creep resistance of Haynes 230AM stems from the uniform distribution of fine ZrC carbide, the precipitation of Ni7Zr2 phase along close-packed planes and the formation of unique matrix distortion stripes, which impede dislocation motion and delay the formation of creep defects. Moreover, the Zr element can also improve internal oxide scales, reduce oxidation weight gain by 20 % and does not deteriorate the mechanical properties after long-term thermal exposure. These results can provide guidance for the design of high-performance AM-specific superalloys.

Prediction of long-term creep response: accelerated characterization of industrial nylon6,6 and HMLS PET filaments

The performance of polymeric materials in dynamic loading conditions and their long-term reliability are the two main issues practitioners in the composite field face today. The purpose of this study was to construct master creep curves for industrial nylon6,6 and high modulus-low shrinkage polyethylene terephthalate (HMLS PET) filaments in order to predict their long-term performance. Here, the procedure to develop the master creep curves with short-term creep strain of industrial filaments was investigated under different isothermal conditions in the glass transition region. Nylon consistently exhibits a higher short-term creep strain than PET by around 5–10% based on its lower tensile modulus. Then, master creep curves were developed to foresee the creep of yarns for almost two decades (∼20 years) by the Time-Temperature Superposition principle shift factors. Based on the shift factors derived from master curves, PET and nylon have constant activation energies over the temperature range tested, implying that their creep mechanisms remain constant regardless of the temperature. Overall, rapid prediction of filament creep behavior and lifetime using this method is proposed.

Micro-mechanics investigation of heterogeneous deformation fields and crack initiation driven by the local stored energy density in austenitic stainless steel welded joints

This work investigates the heterogeneous deformation and failure of HR3C austenitic stainless steel welded joints at room and typical service temperatures. Such types of welded joints are widely used in the new generation of fossil fuel power stations and are known to suffer from premature high temperature failure. Observation of in-service failures revealed that cracks may nucleate either in the heat affected zone or in the weld metal. The main objective of this work is to identify the local microstructural conditions and stress–strain fields responsible for both ductile failure and micro-crack nucleation within the weldment at low and high temperatures, respectively. To that purpose, a novel multi-scale micromechanics-based modelling framework is proposed. It relies on representative weld microstructure models digitally reconstructed from electron backscatter diffraction measurements, and dislocation density-based crystal plasticity models to describe the behaviour of individual grains in each weld region. A novel thermo-dynamically consistent relation for the configuration stored energy per unit volume is derived from the crystal plasticity framework.The single crystal models are calibrated and validated from a combination of uniaxial tensile tests on cross-weld specimens using high resolution digital image correlation techniques, representative volume elements of the polycrystals at the macro-scale, as well as micropillar compression tests at the scale of the individual grains. Predictions of heterogeneous inelastic deformation fields within the weldment at both 22 °C and 665 °C, and of micropillar compression behaviour of the individual weld single crystal phases are consistent with experimental data. The experimentally observed ductile failure of the weld metal at room temperature is successfully predicted using a Gurson-type void nucleation, growth and coalescence constitutive model. The suitability of the stored strain energy density and the accumulated plastic strain as crack initiation/creep failure indicators is investigated. It was found that predicted creep lifetimes with either indicator were very similar, and that they were relatively accurate for low stress levels.

Balancing creep resistance and resilience in composites: Leveraging multidirectional and hierarchical structure of collagen fibres

Conventional methods of enhancing the creep resistance of polyvinyl chloride (PVC) often compromise its resilience. This study proposes a novel strategy to achieve a balance between creep resistance and resilience in PVC-matrix composites by incorporating epoxidised soybean oil-modified collagen fibres (MCFs). A comparative analysis of creep and recovery behaviour was conducted among MCF/PVC, pure PVC, and other conventional modified systems. Results revealed that MCF/PVC exhibited a lower total creep strain (12.82%) than pure PVC and a higher recoverable deformation (10.80%) than the other conventional modified systems. Moreover, MCF/PVC had the longest predicted creep lifetime among all the modification systems, which was 103 times longer than that of pure PVC. These improvements were attributed to the natural multidirectional and hierarchical structure of MCF, which hindered the movement of PVC chains and provided sufficient gaps for recoverable deformation. This work provides a new perspective on developing resilient creep-resistant modifications of polymers by leveraging the structural advantages of natural products.

Experimental Study on the Accelerating Creep of Rock Under Dry and Wet Conditions Based on a New Test Method

Creep tests under constant stress are among the most important tests for investigating the time-dependent behavior of rock. In previous studies, creep tests were conducted under various loading and environmental conditions; however, it was very difficult to demonstrate accelerating creep at failure at stresses less than 50% of strength in conventional creep tests. In this study, a new test method is proposed that combines an alternating loading-rate test and a creep test in the post-failure region. The accelerating creep as well as the loading-rate dependence of strength were successfully obtained simultaneously from a single specimen. This test method was applied to three rock types under dry and wet conditions. The results of Sanjome andesite under both dry and wet conditions, Kimachi sandstone under dry conditions, and Tage tuff under dry conditions showed a similar relationship between creep strain, creep strain rate, and residual time (i.e., creep lifetime minus elapsed time), from high to low creep stress levels, and a close relationship between accelerating creep and the loading-rate dependence of strength. Further, the mechanism responsible for the time dependence was found not to change significantly from high to low stress levels. In contrast, different results were obtained when high and low creep stress levels were compared in Kimachi sandstone and Tage tuff under wet conditions. The larger increase in creep strain with increasing creep strain rate at lower creep stress levels was related to the shape of the stress–strain curve in the post-failure region.

Enhancement of Creep Lifetime of Aluminum through Severe Plastic Deformation

This work investigates the creep behavior of severely deformed commercial aluminum. The commercial aluminum was processed by helical rolling (HR) and equal-channel angular pressing (ECAP) at room temperature. During these processes, the equivalent strain up to about 4 was imposed into the as-received material. The creep testing at 200 °C revealed that HR and ECAP significantly increased the time to fracture compared to the as-received material. The stress dependences showed that the value of stress exponent n decreased with the value of the imposed strain. The stress-change tests showed that as-received and severely deformed states exhibited different recovery rates after unloading. The microstructure analysis showed that creep behavior was influenced by the microstructure formed during severe plastic deformation. The relationships between creep behavior and microstructure in the investigated states are discussed.

Creep failure of hierarchical materials

Creep failure of hierarchical materials is investigated by simulation of beam network models. Such models are idealizations of hierarchical fibrous materials where bundles of load-carrying fibers are held together by multi-level (hierarchical) cross-links. Failure of individual beams is assumed to be governed by stress-assisted thermal activation over local barriers, and beam stresses are computed by solving the global balance equations of linear and angular momentum across the network. Disorder is mimicked by a statistical distribution of barrier heights. Both initially intact samples and samples containing side notches of various length are considered. Samples with hierarchical cross-link patterns are simulated alongside reference samples where cross-links are placed randomly without hierarchical organization. The results demonstrate that hierarchical patterning may strongly increase creep strain and creep lifetime while reducing the lifetime variation. This is due to the fact that hierarchical patterning induces a failure mode that differs significantly from the standard scenario of failure by nucleation and growth of a critical crack. Characterization of this failure mode demonstrates good agreement between the present simulations and experimental findings on hierarchically patterned paper sheets.

A breakthrough in creep lifetime prediction: Leveraging machine learning and service data

Improvement of data-driven techniques, specifically machine learning (ML), in material science turned it into a powerful tool for predicting materials behavior. Accordingly, this study provides a ML prediction of empirical creep lifetimes of 9Cr-1Mo ex-service heater tubes that have been used in industry for up to 47 years. Data from over 90,000 h of stress rupture tests shows that the service parameters influence creep lifetime similar to mechanical properties. Employing six different ML algorithms, viz., K Nearest Neighbors (KNN), Support Vector Regressor (SVR), Random Forest (RF), Gradient Boosting (GB), Gaussian Process (GP), and Multi-Layer Perceptron (MLP) demonstrated that the GP and MLP methods performed significantly better in predicting the creep lifetimes rather than other algorithms. Finally, a validation set involving 12 samples was conducted, and the GP algorithm showed better agreement with experimental values than other ML and Larson-Miller Parameter approaches, illustrating the capability of this model to predict creep lifetimes.

The application of self-healing concept in Ni superalloys: Theoretical design and experimental validation

The self-healing mechanism has been newly applied in Ni superalloys to extend the creep lifetime during service. Quantitative selection criteria are proposed to screen the potential self-healing elements in Ni alloying system. Ti atoms and η-Ni3Ti precipitates are thus selected as the candidate healing atoms and the healing agent, respectively. The ideal composition interval for Ti to achieve the effective healing effects is computationally calculated as 0.076–0.1 mol fraction. On this basis, SHNi (self-healing Ni alloy) has been developed and compared with nSHNi (no self-healing Ni alloy). The creep lifetimes of SHNi alloys are observed 8–10 times higher than that of nSHNi alloys. Microstructure characterizations have shown that η-Ni3Ti healing agents in SHNi alloys can form site-specifically at the cavity sites during service. Meanwhile, the creep cavities in SHNi alloys present much smaller average sizes during creep, and the cavity growth rates are generally one magnitude lower compared to that of nSHNi alloys. The excellent creep properties of SHNi alloys can be well ascribed to their creep cavitation resistance brought by the self-healing behavior. The effectiveness of self-healing mechanism in improving the creep properties has proposed a new strategy in developing novel Ni superalloys.

Tensile creep mechanisms of Al-Mn-Sc alloy fabricated by additive manufacturing

The incorporation of Sc/Zr elements into aluminum (Al) alloys provides great opportunities for high temperature applications, due to the high coherency with the Al matrix, the high thermal stability and the high strengthening potentials of the generated Al3(Sc,Zr) precipitates. In this study, the microstructure, thermal stability, high temperature tensile and creep behaviors of additively manufactured Al-Mn-Sc alloy were systematically evaluated. The Al-Mn-Sc alloy demonstrated sound high temperature yield strength as compared with other reported Al alloys made by additive manufacturing. However, the formed large area fraction (approximately 60%) of fine grain boundaries significantly promoted the dislocation annihilation and reduced the creep threshold stress, though the inherent thermally stable solutes and/or precipitates contributed to its high creep activation energy. On the other hand, the primary Alx(Mn,Fe) and Aly(Sc,Zr) particles and the fine grain boundaries severed as the potential cavity nucleation sites, enhancing the cavity kinetics during creep loading. The cavity nucleation and coalescence process, which basically dominated the tertiary stage of the creep lifetime and led to the final fracture, were further revealed through a cavity evolution model. Future creep-resistant Al alloy design outlook, especially for the inoculates modified Al alloys with fine grain structures, were proposed based on the present findings.

Crystal plasticity study of the effect of asymmetric structure of diffusive film cooling hole on heterogeneous creep and fracture behavior of NBSC superalloy

The heterogeneous creep and fracture behavior of nickel-based single crystal (NBSC) superalloys with inclined and diffusive holes are investigated. The analyses are made by creep experiments and crystal plasticity (CP) simulation with a modified creep constitutive model that accounts for both plastic and creep shear slip near the cooling hole. The study indicates that the cooling hole has a creep-strengthening effect on the NBSC superalloy, resulting in an extended creep duration for specimens with cooling holes than those without holes. Meanwhile, the asymmetric structure of diffusive and machined-tapered inclined cooling holes causes localized stress concentration around outlet side of the holes, which leads to asymmetric deformation and decreased rupture strain. Moreover, the interaction between neighboring holes causes shear slip and creep deformation more dominantly at side holes than at the center hole. Thus, preferential initiation of microcrack occurs around the side holes. The specimen without a hole displays typical ductile fracture, whereas the specimen with cooling holes displays a combined cleavage-like and ductile fracture due to the localized stress concentration induced by plastic shear slip around the hole. In addition, as loading stress increased, the creep lifetime and rupture strain decreased, while the characteristics of cleavage-like rupture increased.

Creep mechanism in ductile dual-phase aluminum alloys with a fully continuous fiber state

To understand the creep behavior of a practical alloy that has a multiphase consisting of a matrix and several types of precipitate phases, the role of individual phases needs to be studied. However, it is difficult to elucidate the contribution of each phase to creep owing to their complex microstructural morphologies and very complex creep behavior. To understand the role of individual phases and the creep behavior, fundamental creep theories for ductile dual-phase (DDP) alloys must be established initially for specimens with simple microstructural morphologies. To apply these theories practically, they must also be experimentally verified. In this study, DDP alloys composed of 99.99 % Al (matrix phase) and an Al–Mg solid-solution alloy (reinforced phase) were fabricated as a fully continuous fiber state by accumulative roll bonding. Tensile and creep tests at elevated temperatures were performed on alloys composed of systematically varying volume fractions of the reinforced phase. Tensile data at 546 K indicated a linear relationship between the maximum stress and volume fraction of the reinforced phase; the lines were also connected to the data from single-phase materials of the matrix and reinforced phase. Thus, the amount of strain in the matrix and reinforced phases was the same as that during the tensile test, and the relationship between the maximum stress and volume fraction of a DDP alloy followed the classical linear law of mixtures (CLLM). The creep data for Al + Al–2Mg (DDP alloy) at 546 K were nearly equal to those calculated using the CLLM. Thus, the matrix and reinforced phases deformed at the same creep rate. Creep tests were also conducted for Al + Al-1M(5ARB), which had a fine-grained reinforced phase. The creep data for Al + Al-1M(5ARB) at 523 K were also a good approximation to the lines representing the data calculated using the CLLM. Hence, although the grain sizes of the two phases were very different, corresponding to different rate-controlling mechanisms, the creep data for a DDP alloy could be analyzed using the CLLM. Consequently, the creep behavior of the DDP alloys was a superposition of the creep behavior in both phases, and the interpretation of the creep data for DDP alloys using the creep theory of single-phase materials would lead to incorrect conclusions. These findings will contribute to a better understanding of the creep behaviors of practical alloys and will aid in the correct estimation of the creep lifetime.