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The thermal stability and mechanical properties of a gradient-nanograined structure (GNS) CoCrNi medium entropy alloy (MEA) processed by ultrasonic surface rolling were studied by using isothermal/isochronal annealing tests combined with quasi-in-situ electron backscatter diffraction (EBSD) characterization and Vickers micro-hardness (HV) measurements. A layer by layer high-throughput investigation method was used to quantitatively study the grain growth kinetics and grain boundary evolution with different initial grain sizes, which could effectively save specimen and time costs. The grain nucleation and growth, as well as shrink and disappearance process through Σ3 coincidence site lattice boundary migration with slightly lattice rotation during annealing were directly revealed. The layer by layer grain growth kinetics and calculated activation energy indicate that the thermal stability of nano-grained top surface layer is relatively higher than that of nano-twined subsurface layer for the gradient CoCrNi MEA processed by ultrasonic surface rolling. Further analysis show that the grain boundary relaxation and dynamic recrystallization of the topmost nano-grains led to the decrease of grain boundary energy, thus improving their thermal stability. The present work provided theoretical basis for the application of CoCrNi MEA at high temperatures. Moreover, the high-throughput method on the investigation of grain stability by using gradient structure can be easily extended to other materials and it is of great significance for understanding the microstructural evolution of gradient materials.

The microstructural evolution and mechanical properties of ultrafine-grained (UFG) CP-Ti after an innovative large-volume equal channel angular pressing (L-ECAP) and multi-directional forging (MDF) were systematically examined using monotonic tensile tests combined with transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) techniques. Substantially refined and homogeneous microstructures were achieved after L-ECAP (8-pass and 12-pass) and MDF (2-cycle and 3-cycle), respectively, where the grain size distribution conformed to lognormal distribution. The grain refinement of 450 °C L-ECAP is dominated by dynamic recrystallization (DRX) and dynamic recovery (DRV), while that of MDF is dominated by DRX. The iron impurities promote recrystallization by pinning-induced dislocation accumulation so that DRX is prone to occur at iron segregation regions during L-ECAP. The monotonic tensile results show that the strain hardening rate of CP-Ti increases with the decrease of grain size, while the continuous strain hardening ability decreases. The relationship between the average grain size and yield strength is in accordance with Hall-Petch relationship. Meanwhile, the individual strengthening mechanisms were quantitatively examined by the modified model. The results indicate that the strengthening contribution of dislocation accumulation to yield strength is greater than that of grain refinement.

Preferred surface integrity around the hole wall is one of the key parameters to ensure the optimized performance of hole components for nickel-based superalloy. The novel hole cold expansion technique introduced in this work involves the laser texturing process (LTP) followed by the Hertz contact rotary expansion process (HCREP), where the cylindrical sleeve is the critical component connecting the above-mentioned two processes. The purpose of LTP is to obtain the most optimized strengthened cylindrical sleeve surface, preparing for the following HCREP. Hereafter, the HCREP acts on the nickel-based hole components by the rotary extruding movements of the strengthened sleeve and conical mandrel tools. As compared to the as-received GH4169 material, the surface integrity characterization for the strengthened hole shows that a plastic deformation layer with finer grains, higher micro-hardness, deeper compressive residual stress (CRS) distribution and lower surface roughness is formed at the hole wall. In addition, transmission electron microscope (TEM) observations reveal the microstructure evolution mechanism in the strengthened hole. Grain refinement near the hole wall is regarded as the fundamental reason for improving the surface integrity, where the aggregated dislocations and recombined dislocation walls can be clearly observed.

GH4169 at 650 °C in atmosphere was investigated by using single edge notch tensile specimens. The number of main cracks and crack initiation mechanisms at the notch surface strongly depended on the grain size. The crack initiation life accounted for more percentages of the total fatigue life for the alloy with smaller grain size. The fatigue life generally increased with increasing crack initiation life. The small crack transited to long crack when its length reached ˜10 times the grain size.

The low cycle fatigue (LCF) and creep-fatigue behaviors of Ni-based GH4169 superalloy are investigated by uniaxial strain-controlled fully-reversed testing at 650 °C. Compared with LCF tests, the effects of tensile and compressive strain hold times on creep-fatigue lifetimes are experimentally explored with varying total strain ranges in the present work. In order to elucidate the damage mechanisms under complex loading waveforms, an additional series of tests with both tensile and compressive hold times are carried out at a given total strain range of 2.0%. Posterior to the cyclic tests, main-crack-failure modes and secondary cracking modes are studied via optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques. Main-crack failure mechanisms are examined by the fracture appearance observations. Cracking modes are explored through quantitative characterization on the distributions of secondary cracks in the longitudinal cross sections under different loading waveforms. Moreover, a generalized life model based on linear damage summation (LDS) framework and energy dissipation criterion (EDC) is elaborated to estimate the damage mechanisms of fatigue, creep and oxidation. The prediction results can well establish the correlations between the reductions of numbers of cycles to failure and the presences of different damage mechanisms under respective loading waveforms.

In this work, a dislocation-based crystal plasticity finite element (CPFE) framework was implemented to investigate the effects of loading conditions on dwell-fatigue crack initiation life. Experimentally, a large number of strain-controlled dwell-fatigue tests were carried out at 650 ℃ in a nickel-based superalloy. The combinations of CPFE simulations and post-test examinations were used to reveal the dwell-fatigue crack initiation mechanisms. Then, a life prediction approach was presented on the basis of accumulated energy dissipation at half-life cycle. Good agreement between the experimental and simulated lives verifies the robustness as well as the accuracy of the present approach. Finally, a new three-dimensional (3-D) damage tolerance diagram was proposed by introducing a CP-based physical parameter to describe the degradation levels and evaluate the residual dwell-fatigue life.

A series of strain-controlled creep-fatigue tests under different loading waveforms are carried out on GH4169 superalloy at 650 °C. In the macroscopic view, a unified viscoplastic constitutive framework is used to describe cyclic deformation process. Particularly, a modified kinematic hardening rule considering loading-dependent effect is developed to simulate the stress relaxation behavior during hold periods. Then, creep-fatigue assessment based on interaction diagram is quantitatively determined by the strain energy density exhaustion (SEDE) approach. In order to elucidate damage mechanisms under various loading conditions, the characterization of damage mechanism is observed from the post-test examination. Main-crack-failure modes from fracture appearance observations and cracking modes from longitudinal sections are studied via scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), respectively. Furthermore, image-based creep-fatigue diagram is proposed based on metallographic interpretation of mechanisms.

In this paper, a dual-scale modelling approach is developed to investigate creep-fatigue behavior and predict crack initiation life for holed structures under multi-axial stress state. The macro-scale simulation supplies local deformation histories to the dual-scale simulation as boundary conditions. In the dual-scale simulation process, the micro-mechanical behavior and damage evolution are described by using crystal plasticity. In order to validate the dual-scale simulation procedures, a series of creep-fatigue tests as well as the post-test characterizations were carried out for nickel-based Inconel 718 at 650 ℃. The detailed results of macro- and micro-scale simulations are presented in terms of stress–strain behavior, damage evolution and life prediction. Regarding the macro-scale simulations as the benchmark, it may provide an assistant support and precognition for the micro-scale damage calculation at higher cycles. The predicted cycle numbers to crack initiation are in agreement with the experimental ones. More advantages are manifested in the potential scientific and engineering significance for the dual-scale modelling approach.

Hole cold expansion is an effective anti-fatigue manufacturing technology, which is widely used in hole strengthening of aviation components. In this paper, a novel small-deep hole (with the diameter less than 2 mm and the depth greater than 10 mm) cold expansion process, called the multi-spherical bump rotating cold expansion process (MBR-CEP), was proposed and verified by experiments on the Inconel 718 superalloy. The MBR-CEP tool with multi-spherical bumps produced by the laser texturing process (LTP) was first designed, and the MBR-CEP with different expansion degrees (δ = 2.1%, 2.6%, and 3.1%) was conducted on small-deep hole specimens. The surface integrity of the small-deep holes before and after the MBR-CEP in the inlet, middle, and outlet areas was studied. Results showed that compared with the small-deep hole after reaming, the hole treated by the MBR-CEP with different δ formed a plastic deformation layer with a maximum depth of 32.73–85.54 μm, a minimum surface roughness of 0.115–0.138 μm in Ra, and a compressive residual stress (CRS) layer with a depth of 0.39–1.077 mm. In addition, the CRS increased by up to 84%, and the microhardness increased by up to 13%. The microstructure evolution on the top surface and sub-surface of the small-deep hole wall was observed by transmission electron microscopy (TEM), which showed that nanocrystals with the size of 48–90 nm and a large number of dislocations and deformation twins formed. Finally, the effectiveness of the proposed process was verified by fatigue tests with different δ at 400 ° C . The average fatigue life increased by 3.66 and 8.05 times under δ = 2.1% and 2.6%, respectively. The fatigue fracture surface was examined with scanning electron microscopy (SEM), which revealed that low surface roughness and existence of a plastic deformation layer could effectively reduce the probability of crack initiation.

This paper was concerned with an approach to predict fatigue life based on a multi-scale crack propagation model. The expressions of crack propagation rates in microstructurally small crack (MSC), physically small crack (PSC) and long crack (LC) stages were unified in the multi-scale crack propagation model. Its integral form presented a fatigue life prediction approach. The nickel-based GH4169 alloy was employed to validate the prediction capacity of present approach. The prediction results of lives of specimens with different initial defects sizes were compared with the experimental data.

Creep crack growth behaviors of the Inconel 625 superalloy at 650 °C are investigated through experimental and numerical methods. The simulated data agree well with the experimental results, reflecting that the multi-axial creep performance parameter α obtained by present paper can reasonably predict the creep crack growth behaviors of Inconel 625 superalloy. The crack initiation time takes up the most proportion of the whole life for all the load levels, and intergranular fracture is the dominated failure mechanism. Creep constraint effect is not obvious for the C∗ to characterize the creep crack growth of Inconel 625 superalloy.

The fatigue crack initiation life and damage mechanisms of three heat-treated nickel-based alloy with different volume fractions of δ phase have been investigated based on experiment and crystal plasticity simulation. With the help of fatigue indicator parameters (FIPs), the predicted fatigue lives are agreed well with experimental results, where energy dissipation based FIP shows more accurate life prediction than plastic slip based FIP does. Maximum fatigue damage sites shift from grain boundary to δ phase boundary by increasing δ phase and applied strain level. Furthermore, the existence of δ phase leads to the increase of plastic slip accumulation and decrease of localized stress level.

A series of strain-controlled creep-fatigue tests with various dwell times and strain ratios are carried out on a nickel-based superalloy at 650 °C. Aiming at the mechanical understanding of creep-fatigue behavior, a non-unified constitutive framework is used to describe macroscopic cyclic deformation process to understand creep-fatigue behavior. Afterward, the quantitative evaluation of creep-fatigue damage is quantitatively determined by the cycle-by-cycle strain energy density exhaustion (SEDE) approach. The effects of strain ratio and dwell time on creep-fatigue life endurances can be well revealed by the proposed life model, where all the data points are predicted within ± 1.5 error band. Particularly, life prediction capacities with long dwell times are further improved based on three-regime creep mechanism.

A series of continuous and interrupted cyclic tests in Inconel 718 with the dwell times ranging from 0 s to 10,800 s at 650 °C were carried out in order to study the multi-stage dwell fatigue crack growth (DFCG) behavior. Replica technique was used to measure the intermittent DFCG rates from short to long periods and to retrospect specific damage mechanisms based on microstructural characterizations. From the macroscopic viewpoint, the DFCG curves were classified into short, moderate and long dwell conditions according to the tendencies in DFCG rates. Correspondingly, the dwell fatigue life distribution was divided into three periods, which exhibit different sensitivities in the length of dwell time on dwell fatigue life. From the microscopic viewpoint, the high-scattered points reflected in the DFCG curves were elaborately discussed with the combinations of EBSD mapping and high-resolution TEM analysis. Σ3 twin boundaries (Σ3 TBs) were regarded as the barriers to resist the DFCG due to their special performances. In addition, with the increase of dwell times, the main crack morphologies changed from transgranular to intergranular mode. Simultaneously, local crack growth modes at high-scattered points changed from across to along the Σ3 TBs with extreme low DFCG rates. Also, high-angle crack deflection was another factor that was responsible for the decrease of the DFCG rates.

A large number of strain-controlled creep-fatigue tests under wide loading waveforms are conducted at 650 ℃ in nickel-based forged GH4169 superalloy. Comprehensive characterizations, including scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM), are observed from the post-test examinations. Particular focus is brought to the physical understanding of damage mechanisms under wide creep-fatigue loading conditions using EBSD analysis. The representative misorientation parameters are calculated for constructing diffraction-based misorientation mapping. Semi-quantitative analysis of longitudinal EBSD observations is conducted to prove that strain ratio has little influence on creep-fatigue damage degrees, while dwell time causes noticeable changes to damage progressions. In particular for geometrically necessary dislocation (GND) map explored in this work, more fundamental information based on failure physics is obtained to analyze the creep-fatigue crack initiation mechanism.

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.

Fatigue behaviors of 2205 duplex stainless steel (initially composed of austenite and ferrite phase) with a gradient nanostructured (GNS) surface layer induced by the ultrasonic rolling process (USRP), are investigated in both strain-controlled high-cycle fatigue (HCF) and low-cycle fatigue (LCF) tests. Results showed that the fatigue life is improved in HCF (~5 times longer under 0.3% strain) but decreased in LCF (~53% under 0.8% strain). This is different from the simultaneous enhancement of fatigue behaviors in both HCF and LCF in other materials with GNS. The effect of martensite phase transformation (MPT), residual compressive stress as well as the GNS layers have been investigated to clarify the mechanisms of the fatigue behavior of the USRP samples. Besides the suppression of the surface crack initiation and propagation by residual compressive stress and GNS layers, the MPT during the USRP and cyclic loading processes greatly affects the fatigue behaviors. For HCF tests, not only the hard-brittle martensite phase induced by the USRP process improved the material strength but also the MPT process during cyclic loading absorbed the strain energy released from the crack growth, which increased the HCF life. However, for LCF tests, the hard-brittle martensite phase decreased the ductility of the GNS layer and accelerated the crack growth rate under higher strain amplitude, resulting in the decrement of LCF life. This work presents the first investigation on fatigue behaviors of duplex stainless steel with GNS structure.

The aim of the present work is to propose a generalized strain energy density exhaustion model to predict the creep-fatigue lives. The oxidation damage, which is described by a correction factor representing the non-linear oxidation damage mechanism, is considered in the proposed model. All the 77 experimental data sets of GH4169 superalloy at 650 °C, Inconel 738 superalloy at 850 °C and P91 steel at 550 °C in both tension-hold-only and compression-hold-only tests are used to validate the prediction capabilities of the model. The fatigue, creep and oxidation damages per cycle are separately calculated. Then a three-dimensional damage interaction diagram combined with a suitable enveloping surface is proposed.

The purposes of the present work are to extend the previous energy-based model on the basis of strain energy density exhaustion and to estimate creep-fatigue endurance and accumulated damage using cycle-by-cycle concept in tension-hold-only tests. The nickel-based GH4169 superalloy at 650 °C is employed to fit material constants and to verify the prediction capacity of the present model under various loading conditions. The present model exhibits a higher accuracy than some existing models, especially for certain loading waveforms, where the half-life cycle cannot be considered as a representative one. In addition, the trajectory for time-dependent accumulated damage during the operating period can be monitored in the creep-fatigue interaction diagram.

The aim of the present paper is to identify the effects of sampling locations and loading waveforms on high-temperature low-cycle fatigue (HTLCF) and creep-fatigue life of a forged and precipitation hardened nickel-based GH4169 superalloy. Both the deformation and failure mechanisms are considered here. It has been revealed that HTLCF and creep-fatigue life of specimens were influenced by inhomogeneous microstructures at different locations. Compared with the HTLCF tests, the presence of dwell times in creep-fatigue tests tended to reduce number of cycles to failure. Intergranular damage was observed at both crack initiation and propagation stages. For the dwell times under tension, the intergranular damage was mainly associated with precipitate-assist voids. However, oxidation accounted for the presence of intergranular damage for the dwell times under compression.

Raising the operating temperature to increase electricity production in the nuclear power industry is a practical solution in the aspect of efficiency and emissions reduction. However, regarding structural integrity, such high temperature has a significant impact on the life of nuclear power plant components. The superheater tube plate is one of the most critical parts under exposures of very high temperature in the boiler system of the power plant. This study investigates creep-cyclic plasticity behaviour of a superheater outlet tube plate under extremely sophisticated thermo-mechanical loading and evaluates creep-fatigue damage endurance, using the modified LMM eDSCA. Temperature-dependent material properties are employed to calculate more practicable structural behaviour and lifetime prediction. The results clearly present the criticality of the effects of non-isothermal loading and multiple dwells on the structural integrity of the tube plate. Also, this study clarifies critical factors that cause dominant damage to the tube plate by providing total damages using various damage assessment models, of which results show the trend of the conservativeness of each damage assessment model, compared to experiment result.

The cyclic plasticity and the low cycle fatigue failure behavior of the weld metal (WM) and base metal (BM) of the CrNiMoV steel welded joint under the strain and stress-control modes were investigated respectively. Significant cyclic softening was observed for both the WM and BM under the low cycle fatigue tests with the two control modes. Besides, obvious ratcheting happened in the WM and BM under the stress-controlled cyclic loading conditions. It is shown that both the WM and BM exhibited lower fatigue strength at the stress control mode than that at the strain control mode due to the influence of tension-compression asymmetry. Meanwhile, the WM showed larger cyclic softening rate, lower ratchetting deformation and fatigue strength than the BM under the same loading levels. The failure location of the WM specimens shifted from BM region (nearby the heat affected zone) to the center of WM with the increasing of strain amplitude under the strain-controlled tests, which can be explained with the similar maximum equivalent plastic strain amplitude location shifting behavior observed from the corresponding finite element simulations.

A numerical process based on crystal plasticity finite element (CPFE) was implemented to predict creep-fatigue crack initiation life. CPFE-based model can describe the macroscopic cyclic deformation and reveal grain-level damage mechanism. A new life prediction approach was then constructed by introducing fatigue and creep indicator parameters. Furthermore, a series of strain-controlled creep-fatigue tests in GH4169 superalloy at 650℃ were used to validate predicted accuracy of this model, where most of the data points lied within ±1.5 error band. Finally, a potential methodology for conservative creep-fatigue life evaluation was provided by flexible creep-fatigue damage summation rule in engineering applications.

Molecular dynamics (MD) simulations were performed to explore the structures, energies and tensile properties of grain boundaries (GBs) as well as crack nucleation in nickel nanolaminated (Ni NL) structures. Four typical textures observed in experiments (Liu et al., 2013) [6] were considered in this paper. Results showed that low-angle GBs consist of a series of periodically arranged dislocations while high-angle GBs are composed of disordered phase for all textures. GB energies increase linearly with increasing misorientation in the range of low-angle GBs (0–10°). Then, the growth rate of energies slows down and finally stabilizes when the misorientation angle reaches 30°. Among four textures, {1 1 1} 〈1 1 0〉 texture has the highest GB energy while {1 1 0} 〈1 1 1〉 texture the lowest one. The tensile properties of NL structures with different textures and misorientations were further investigated. Results showed that the tensile properties depend primarily on textures, while weakly on GB energies and dislocation densities. {1 1 1} 〈1 1 2〉 texture possesses the best combination of high-strength and ductility among the four textures. Furthermore, the evolution of dislocation density and structure as well as the nucleation of crack were analysed. Crack nucleates at the junction between Shockley partial dislocations and twin boundaries generated during deformation for low-angle GBs, while at the location where Shockley partial dislocations and the original disordered GBs intersect for high-angle GBs. Our results provide a fundamental understanding of GB structures and deformation mechanisms of Ni NL structures.

Turbine bladed disks normally operate under complex loadings coupling with uncertainties originate from multiple sources, including material variability, load variation and geometrical uncertainty. The influence of these uncertainties on mechanical response of engineering components are critical for their fatigue assessment and reliability evaluation. In this work, a general framework for fatigue reliability analysis is developed by coupling the Latin hypercube sampling with FE analysis to describe the combined effects of multi-source uncertainties. Fatigue reliability analysis of a full-scale bladed disk under multi-source uncertainties was performed as well as sensitivity analysis for fatigue design. In order to describe the manufacturing errors or tolerances, random dimensions are inputted. Comparing the predicted fatigue lifetime distributions with/without geometrical uncertainty, it shows that geometrical uncertainty matters in structural fatigue reliability. Particularly, sensitivity analysis indicates that the geometrical uncertainty exerts more critical influences on the fatigue lifetime and reliability of the turbine bladed disk than others. The sensitivity factors of three typical dimensions emerges the influence of designed sizes and dimensional tolerances on the failure probability, which provides a reference for engineering design.

The accumulated creep–fatigue damage is expected to be an important failure mechanism for lots of high-temperature components. The aim of this paper is to propose a modified strain energy density exhaustion model to predict the tension-hold-only creep–fatigue life. This model exhibits high accuracy due to the reasonable evaluation of creep damage. The proposed model elaborates the determinations of mean stress, stress relaxation rate and creep damage. A few existing experimental data sets of Grade 91 steel, Alloy 617 and 304 stainless steel are used to verify the prediction capacity of the present model under different temperatures and loading conditions. Results show that most of the experimental data falls into a range within a scatter band of ±1.5 on life.

Small fatigue crack initiation behaviour in a series of specimens containing different sizes of micro-notch has been observed through RepliSet system during interrupted low cycle fatigue tests. From the replicating observations, very initial single crack or multiple cracks near the micro-notch area can be captured. Since then, the crack initiation life for each specimen can be determined. Computationally, a numerical procedure with a combination of crystal plasticity theory and finite element implementation has been constructed for predicting small fatigue crack initiation life as well as identifying the crack initiation sites near the micro-notches. With the help of fatigue indicator parameter represented by accumulated plastic slip, the predicted numbers of cycles for the submodels with various micro-notch sizes agreed with the experimental ones. In addition, the simulation results can successfully reveal the mechanism in crack initiation sites. The stress concentration effect was found to be responsible for corner crack initiation, while the combined effects of stress concentration and grain orientation around micro-notch area showed great influence on edge crack initiation.

The aim of this paper is to investigate different factors, including dwell time, strain range, and strain ratio on creep-fatigue endurances in nickel-based Inconel 718 and GH4169 superalloys. We also summarize classic approaches for life assessments based on the generalizations of Coffin–Manson equation, linear damage summation (LDS), and strain-range partitioning (SRP) method. Each approach does have some degree of success in dealing with a specific set of creep–fatigue data. In order to evaluate the prediction capabilities of the validated approaches, a Bayesian information criterion (BIC) allowing for maximum likelihood and principle of parsimony is used to select the best performing model.

Polymer and montmorillonite (MMT) clay composites are the hot topic of research because of its excellent performances, a kind of hyper-dispersant(MA) was synthesized as compatible component, and organo-montmorillonite(OMMT)/PP/MA composites were prepared by means of melt blending. The results showed that the PP /OMMT /MA were of better mechanical properties compared to (OMMT) /PP when the same content of MMT. When the weight content of PP/OMMT/MA is 100:2:0.5, the composites exhibited excellent machine performances, and the scanning electron microscopy showed the composites of (OMMT)/PP/MA were consistent., XRD test shows that PP has entered into layers of montmorillonite to form intercalated composites.