Reliable life design and damage level evaluation are essential to ensure safe operation and healthy maintenance of high-temperature components. It can be more challenging when considering the degradation of material mechanical property during low cycle fatigue (LCF) and creep-fatigue (CF). To tackle this problem, experimental data accumulations and theoretical foundations, including damage summation rules representing mechanical property degradation and CF damage models are introduced. Following the modified damage-damage threshold interference principle, a three-dimensional reliability assessment diagram is constructed for a low-pressure turbine disk as an example, where the additional third axis represents mechanical property degradation. Among them, probabilistic damage threshold distributions are obtained by the Latin Hypercube Sampling and linear fitting as well as probabilistic damage distributions at weakness hotspots are acquired with the help of finite element analysis. Joint failure evaluation is more conservative since it incorporates all the failure possibilities within the weakness region. Finally, a procedure of residual lifetime design and damage level evaluation is proposed for components with various applications within the framework of LCF and CF reliability analysis.

Start-up and shut-down transients in turbomachinery impose varying strain rates that affect the in-service life of high-temperature components. However, the quantitative role of strain rate sensitivity in high-temperature fatigue of nickel-based superalloys remains inadequately reflected in damage mechanism and life prediction. In this study, fully-reversed strain-controlled low cycle fatigue (LCF) tests were performed at 650 degrees C over a wide range of strain rates from 5 x 10_ 5 to 1 x 10_2 s_ 1. The experimental results reveal the existence of a threshold strain-rate range for nickel-based superalloy IN718. When the strain rate exceeds this threshold, the fatigue life remains nearly constant with increasing strain rate. The cracking behavior is dominated by the transgranularintergranular mixed mode. In contrast, at strain rates below this threshold, the fatigue life decreases rapidly due to the time available for oxidation- and creep-assisted damage. Based on these observations, a modified energy-based model incorporating the strain rate sensitivity was proposed using tensile-derived plastic strain energy density, achieving a prediction accuracy of 97% within a +/- 2 error band. These findings provide an effective strategy for enhancing the service reliability of high-temperature rotating components.

The stacking fault energy (SFE) of metallic materials is a crucial parameter that significantly determines their deformation mechanisms and mechanical properties. In this study, SFE was manipulated to suppress premature failure and enhance the cryogenic mechanical performances of additively manufactured materials by the minor addition of elements. Additively manufactured CoCrNi medium-entropy alloy (MEA) was selected as the model material due to the presence of intergranular cracking during cryogenic deformation. We manipulated the SFE of CoCrNi MEA by addition of minor Al and Ti, to prolong the ductility from 28% (CoCrNi) to 40% ((CoCr-Ni)94Al3Ti3) at 87 K without sacrifice of strength. The reasons accounting for this improvement mainly lie in the following two aspects: First, the SFE was elevated from 14 mJ/m2 for CoCrNi MEA to 26 mJ/m2 for (CoCr-Ni)94Al3Ti3 MEA at 87 K; consequently, the propensity of deformation twins (DTs) is suppressed, leading to less intersections of DTs and grain boundaries (GBs). Secondly, the element segregation at GBs changed from Cr (1.4 at%) to Ti (2.6 at%), which was believed conducive for the cohesive strength of GBs. In short, the higher GB strength and less stress concentration at GBs collectively suppress the formation of micro-voids therein, therefore, contributing to the strength and ductility synergy at cryogenic temperature. The findings emphasize the need for precise SFE control in modulating the mechanical properties of additively manufactured materials at harsh temperatures.

Small hole structures in aerospace components suffer from severe stress concentration and thus cold expansion processes (CEPs) are required to improve fatigue resistance. In this study, a multi-convex rotating CEP was developed to enhance the fatigue performance of vent holes with small diameter of 2.87 mm in a powder metallurgy nickel-based FGH4109 superalloy at 650 degrees C. High-temperature fatigue tests were carried out to verify the CEP-improved effect on fatigue life by over one order of magnitude under certain stress levels. By integrating experimental characterization and Gaussian process regression-expected improvement (GPR-EI) framework, the optimal expansion degree and rotational speed were identified under limited experimental conditions. A weighted comprehensive surface integrity index, combining surface roughness, microhardness, plastic deformation depth, and residual stress, was established and embedded into a modified Smith-Watson-Topper (SWT) model to predict the fatigue life of CEP-treated holes. The proposed framework successfully bridges processing optimization, surface integrity index and fatigue life. The proposed framework not only provides an efficient strategy to optimize CEP parameters and predict fatigue life of hole structures, but also offers a generalizable approach for other surface strengthening processes.

This study investigates the high-temperature creep-fatigue interaction behavior of a newly developed martensitic steel, 13Cr10Mo1W1VNbN (X13), for heavy-duty gas turbine disk applications. Hybrid stress-strain controlled creep-fatigue interaction (HCFI) tests were conducted to identify its cyclic response, precipitate evolution, and martensitic lath degradation under service-relevant conditions. The as-received steel is dominated by chain-like M23C6 carbides along lath boundaries and coarse, sparsely dispersed MX particles, while the Laves phase forms only gradually after HCFI exposure but remains at a relatively low quantity within the loading range examined in this work. M23C6 carbides primarily stabilize the lath structure, whereas coarse MX particles mainly act as local barriers to recovery rather than classical strengthening precipitates. With increasing precipitate coarsening ratio, X13 steel exhibits a two-stage microstructural response. At low-to-intermediate coarsening ratios, reduced boundary pinning promotes lath widening, but heterogeneous precipitate constraint still induces local strain incompatibility, increasing GND density and LAB fraction. At high coarsening ratios, the pinning stress falls below the critical range for lath stability, while Fe-rich MX formation further weakens stabilization, triggering dislocation annihilation, boundary migration/coalescence, rapid lath widening, and partial microstructural equiaxation.

Several probabilistic fatigue evaluation models have been proposed, but the effectiveness and accuracy of current models have not yet been examined. In this work, probabilistic fatigue life is linked to an energy-based damage parameter, where the statistic of logarithmic fatigue life is represented by this parameter. Taking into account the variability in loading amplitudes and material properties, a novel fatigue reliability assessment method for structural system is proposed, based on the probabilistic fatigue model and loading cycle-failure life interference principle. A series of strain-controlled fatigue tests of Inconel 718 alloy are conducted for model development and fatigue data with different strain ratios are collected from open literature to verify the model applicability. The results show that the proposed model aligns most closely with experimental data when compared to traditional probability-strain-life models, Xie’s modified model, Castillo’s model, and Correia’s model. Furthermore, a turbine fir-tree attachment is taken as an instance to illustrate the implementation procedure for fatigue reliability analysis. It is evident that the proposed method mitigates the overly conservative estimates typically provided by independent treatments in structural system reliability assessments. This research proposes a method for accurate evaluations of material-level fatigue life and system-level reliability supports reliability-centered design and life management of crucial structures.

Additive manufacturing (AM) provides a novel avenue for the fabrication of metal matrix nanocomposites with a uniform distribution of reinforcements and excellent static mechanical properties, while few studies have been conducted on their fatigue behavior. In this study, TiCnp/ (CoCrNi)94Al3Ti3 nanocomposites were fabricated using powder bed fusion (PBF), with another representative AM processes-directed energy deposition (DED) as a reference for comparison. In the case of similar low-density defects, though PBF-sample had finer microstructure and higher tensile strength than DED-sample, its fatigue endurance limit (350 MPa) was markedly lower than that of the DED sample (550 MPa). Further investigation revealed that the fatigue initiation sources for DED-samples were pores, while fatigue failure of PBF-samples were mainly initiated from manufactured microcracks. Though the average volume of pores in DED (1.9 x 105 mu m3) was significantly larger than microcracks in PBF (1.6 x 104 mu m3), the latter posed a more serious threat to fatigue performance. Microcracks were associated with Ti segregation at grain boundaries (GBs) and strong solidification shrinkage, both induced by higher solidification rate of PBF. Finally, two methods were applied to reduce the risk of GB cracking in nanocomposites by adjusting the alloy composition. As a result, segregation at GBs in PBF-fabricated nanocomposites was mitigated, reducing the microcrack density and significantly improving the fatigue resistance. The work reveals the origin of microcrack susceptivity in PBF and offers a microstructural strategy for designing high-strength and fatigue-resistant nanocomposites.

The influence of nickel (Ni) on the mechanical properties and deformation performance of 316L austenitic stainless steel is multifaceted, significantly impacting its overall suitability for engineering applications. Three kinds of 316L-based alloys with different Ni contents were fabricated via in-situ alloying during laser powder bed fusion. The results indicate that Ni addition had a negligible effect on the original microstructure. Compared with the 316L sample (yield strength 688 MPa, elongation 40%), the alloy with 1 wt.% Ni addition exhibited a simultaneous increase in strength and ductility at 87 K, reaching a yield stress of 801 MPa, an ultimate tensile stress of 1467 MPa, and a total elongation of 42.2%. Increasing the Ni addition to 2.5 wt.% further enhanced the yield strength to 958 MPa; however, the ultimate tensile strength (1334 MPa) and elongation (32%) decreased due to the suppressed martensitic transformation. Higher Ni content increased the stacking fault energy (SFE) thus stabilised the austenite and suppressed the martensitic transformation. Consequently, a balanced martensitic transformation accounted for the excellent strength-ductility synergy in 316L+1Ni sample. This finding provides an insight in manufacturing metastable materials with superior strength-ductility synergy in cryogenic environment by manipulating the SFE through in-situ alloying during additive manufacturing.

The stability of microstructures during fatigue loading has emerged as a pervasive issue in both engineering practice and fundamental materials science. Addressing this challenge and elucidating the underlying mechanical mechanisms are essential prerequisites for ensuring the long-term reliable service of high-performance engineering materials and for advancing existing safety assessment frameworks. Here, a gradient nanolamellar structure was fabricated in commercially pure titanium via ultrasonic surface rolling processing, exhibiting superior fatigue performance compared with a gradient equiaxed ultrafine-grained counterpart. Post-fatigue microstructural characterization reveals that the nanolamellar layer undergoes limited coarsening during cyclic loading, indicative of excellent cyclic mechanical stability, whereas the equiaxed ultrafine-grained layer undergoes pronounced abnormal grain growth. The results of large-scale atomistic simulations attribute the enhanced cyclic stability of the nanolamellar architecture to a coupled grain-boundary configuration, in which vertical low-angle grain boundaries (LAGBs) accommodate cyclic deformation through highly reversible glide and absorption of grain-boundary dislocations, while horizontal high-angle grain boundaries (HAGBs) remain essentially immobile. Furthermore, extensive parametric investigations demonstrate that both the inclination of LAGBs and the grain-boundary topology strongly influence the cyclic stability of the lamellar structure. The results suggest that, although plastic deformation-induced microstructural evolution during fatigue is unavoidable, such evolution should not disrupt the key microstructural parameters governing strength in order to achieve a favorable balance between mechanical performance and fatigue stability. These findings further advance the mechanistic understanding of cyclic mechanical stability and may provide a new strategy for tailoring nanostructured metastable microstructures in metals for fatigue-critical applications.

Fatigue crack growth (FCG) in Ni-based superalloys at elevated temperature is governed by the coupled effects of cyclic loading, dwell and environmental oxygen availability. In this work, the effects of atmosphere, stress-intensity-factor range (ΔK), and peak-load dwell on FCG of IN718 at 650 °C were systematically investigated. Compact-tension specimens were tested at ΔKin = 15–25 MPa·m0.5 under trapezoidal loading with peak-load dwells of 0–600 s in high vacuum (∼10⁻³ Pa), laboratory air and high-purity oxygen. Crack growth rates were measured by direct-current potential drop and correlated with SEM, EBSD, EPMA and site-specific FIB-TEM observations. Under no-dwell conditions, oxygen-containing atmospheres increased da/dN by approximately 3–5 times relative to vacuum across the tested ΔK range. For the air tests initiated at ΔKin = 15 MPa·m0.5, the specimens subjected to 60 and 600 s dwells exhibited limited crack extension, and their specimen-average crack-growth rates were below the minimum rate recorded for the no-dwell vacuum baseline at the same ΔKin. This low crack-growth response was associated with a compact Cr/Al/Nb-bearing oxide assembly at the examined crack-tip region. For test series initiated at ΔKin = 20–25 MPa·m0.5, a different atmosphere-dwell response was observed: dwells in oxygen-containing atmospheres, where tested, were associated with accelerated mixed transgranular/intergranular crack growth, whereas a 600 s dwell in vacuum was associated with lower crack-growth rates and creep-cavity-associated boundary damage. These results identify condition-specific differences among the investigated atmosphere–dwell–initial-loading series. A condition-specific qualitative schematic is proposed to organize the three experimentally supported response categories within the investigated experimental window.

Additively manufactured alloys typically exhibit higher strength than their cast counterparts, owing to the formation of cellular substructures composed of high-density dislocations during the printing process. The high strength level increases the risk of hydrogen embrittlement (HE) of which the mechanisms and the mitigating strategies are unclear in additively manufactured materials. Here we show that an equiatomic CoCrFeMnNi alloy produced by additive manufacturing, in contrast to the cast and thermomechanically treated conditions, is rather susceptible to HE. This susceptibility arises from orientation-dependent, H-enhanced decohesion of specific high-angle grain boundary segments aligned relative to the loading direction. We further show that this decohesion process is mitigated by an in situ reprecipitation strategy enabled by rapid additive manufacturing of a carefully designed composite powder feedstock containing TiC nanoparticles. Beyond the intrinsic H trapping associated with TiC precipitates, this strategy also induces a set of correlated microstructural changes, including reduced Mn segregation at grain boundaries and modified deformation behavior and local stress states. These coupled effects collectively contribute to the suppression of H-induced decohesion. As a result, this in situ reprecipitation strategy yields a substantial increase in HE resistance compared to a microstructure devoid of such nanoprecipitates, together with a marked increase in strength (∼290 MPa). The successful reconciliation of these two competing properties provides valuable insights into the design of alloys with similar microstructure and deformation behavior, particularly for lightweight and safety-critical components used in H-rich environments.

Bipolar plate is a critical component of solid oxide fuel cell (SOFC), which have a great impact on electrochemical performance and thermal stress distribution of SOFC stack. Focusing on the synergistic improvement of electrochemical performance and mechanical properties, a flexible bipolar plate structure is designed, and a multiphysics field coupling of electrochemical reaction-gas transfer-material diffusion-heat transfer-mechanical analysis is developed by software of COMSOL to explore the electrochemical performance and thermal stress distribution of SOFC. Compared with the conventional bipolar plate, the results indicate that the SOFC with proposed bipolar plate increases current density by approximately 5.2 % at high operating voltages, and reduces the tensile stress at the PEN by10.3 %. Based on a comprehensive consideration of both the electrochemical performance and stress distribution in SOFC, the optimized geometric parameters of the flexible bipolar plate are determined as follows: the ratio of flow channel width to rib width is set to 1.5, the thickness of the bipolar plate is 0.4 mm on the anode side and 0.2 mm on cathode side, the inclination angle of the bipolar plate is optimized to 7 degrees, and the chamfer radius is specified as 0.5 mm.

Counter-rotating turbine technology is widely applied in advanced aeroengines due to its superior aerodynamic performance. Ensuring safe operation typically requires a substantial supply of sealing air to prevent the intrusion of high-temperature mainstream gas into the turbine disk cavity. However, the introduction of sealing flow significantly intensifies secondary flow structures in the hub region, thus limiting further aerodynamic improvements. Therefore, this study investigates the impact of different J-type bowing designs on the aerodynamic performance of low-pressure guide vanes in a counter-rotating turbine. Initially, the bending structure of the blade hub region in the J-type bowing designs was modified by varying the bowing angle and bowing height. Subsequently, numerical simulations and topological analysis methods were utilized to investigate the effects of bowing design on the hub region flow field. Numerical results based on different J-type bowing designs demonstrate that the reverse J-type bowing design exhibits superior aerodynamic performance, effectively delaying horseshoe vortex formation and reducing its scale. A 4.23% reduction in the streamwise size of the leading-edge horseshoe vortex validates the effectiveness of the proposed design approach. The results of this study effectively address the challenge of high aerodynamic losses within the vane passage of the counterrotating low-pressure turbine guide vanes and provide an effective approach for controlling hub region losses under the influence of sealing flow.

Austenitic stainless steel 316H is a candidate structural material for lead-bismuth-cooled fast reactors (LFRs). Understanding its creep-fatigue-oxidation (CFO) behavior in liquid lead-bismuth eutectic (LBE) is therefore essential for assessing structural reliability. This study systematically investigates CFO behavior of 316H stainless steel in oxygen-saturated LBE at 550 degrees C-600 degrees C, with an emphasis on the coupled influence of environmental exposure and holding time during cyclic loading. The results demonstrate a distinct, time-controlled transition in damage mechanisms. Short holding times promote oxidation-assisted transgranular cracking, driven by repeated rupture of a discontinuous oxide film and rapid ingress of LBE, leading to a 5-10-fold reduction in fatigue life compared with air. At intermediate holding durations, enhanced grain-boundary oxidation and stress-assisted diffusion cause mixed transgranular-intergranular cracking. Under long holding time, a dense duplex oxide develops and remains stable within the crack cavity, producing crack-tip blunting, suppressing LBE penetration, and shifting the dominant mechanism toward creep-assisted intergranular fracture. Microstructural, compositional, and kinetic analyses collectively demonstrate that oxide-film evolution, grain-boundary degradation, and time-dependent relaxation act in concert to regulate CFO behavior. These mechanistic findings delineate the time-dependent pathways of environmental degradation in oxygen-saturated LBE and form a scientific basis for evaluating material performance and guiding structural design for LFR applications.

Oxidation plays a critical role in the fatigue failure of high-temperature alloys, yet its synergy with creep during fatigue crack growth (FCG) remains insufficiently resolved. Here, FCG behavior of P92 steel at 600 degrees C was quantified under high vacuum (similar to 10(-3) Pa) and atmosphere using trapezoidal loading with varying hold times (0-3600 s), to isolate coupled oxidation-creep effects. Relative to vacuum, oxidation in atmosphere accelerated FCG: to reach a 1 mm crack extension, atmosphere required 1585 cycles versus 3051 in vacuum (51.9 % of the cycles). The acceleration intensified with extended holding: at Delta K = 38.4 MPa center dot m(0.5) with a hold time of 3600 s, the atmospheric da/dN was approximately 4.28 times the vacuum value. A Delta K-dependent transition was identified: oxidation elevated da/dN below approximately 35-40 MPa center dot m(0.5), whereas rates in atmosphere and vacuum converged at higher Delta K (dominated by mechanical driving force). Mechanistically, atmosphere promoted fatigue-dominated transgranular cracking via cyclic crack-tip oxide fracture/reformation, whereas longer hold time in vacuum shift the path from transgranular to mixed trans/intergranular with creep cavity nucleation along boundaries. Oxide analysis showed a stratified scale with an Fe-rich outer layer and a Cr-enriched inner layer adjacent to the substrate, consistent with outward Cr diffusion. The proposed failure mechanism maps offered insights into the interaction between oxidation, creep, and fatigue, providing a framework for understanding FCG of P92 under service conditions.

The urgent need to decarbonize the energy and transport sectors motivates the use of hydrogen-containing fuels in gas turbines for power generation and aviation applications, exposing safety-critical components to hydrogen environments at elevated temperatures. Ambient-temperature hydrogen embrittlement has long been interpreted through the physical interactions between hydrogen and microstructural defects like interfaces and dislocations. Here we show that this understanding does not fully capture the behaviour at elevated temperatures, where vacancy-driven chemical reactions between hydrogen and specific microstructural constituents can markedly intensify embrittlement compared with ambient conditions. In a prototypical face-centred cubic Ni-based superalloy, our near-atomic-scale characterization and ab initio calculations reveal strong trapping of hydrogen atoms in carbon vacancies in carbides, driving their partial decomposition while simultaneously triggering localized methane formation at the carbide-matrix interface. As a result, the heterointerfaces are weakened, rendering them vulnerable to deformation-induced damage. Our work provides a physical foundation for mechanistic modelling of elevated-temperature hydrogen embrittlement in Ni-based alloys, an emerging area critical to hydrogen-fuelled turbines and related high-temperature technologies.

Microstructural understanding of heterogeneous high temperature deformation and microstructural evolution in nuclear grade 316H stainless steel welded joints is essential to ensure structural integrity in nuclear power systems. In this study, an integrated experimental-numerical approach was adopted, coupling high temperature in-situ digital image correlation (DIC) and in-situ electron backscatter diffraction (EBSD) tensile experiments with microstructure-based crystal plasticity finite element (CPFE) simulations. This integrated approach elucidates the heterogeneous high temperature deformation and strain localization behavior across the weld metal (WM) and heat affected zone (HAZ). The WM exhibits earlier slip activation and more rapid plastic strain accumulation than the HAZ, leading to pronounced strain localization. Consistent with this behavior, grain boundary statistics show a rapid early increase and subsequent saturation of the low-angle grain boundary (LAGB) fraction in the WM. At the grain scale, geometrically necessary dislocation (GND) density maps combined with geometrical compatibility (m ') analysis were used to assess dislocation transfer across grain boundaries. Although higher m ' values generally facilitate dislocation transmission, the fine-grained microstructure in WM effectively impedes slip transfer, promoting dislocation pile-up and pinning. These mechanistic insights were incorporated into a dislocation-based CPFE model by introducing a minimum dislocation mean free path. The simulations accurately reproduce the observed asynchronous deformation and heterogeneous dislocation density distributions. Furthermore, the model predicts that strain localization bands coalesce primarily within these fine-grained clusters, identifying them as preferential sites for void nucleation and failure. These combined experimental and modelling insights provide a mechanistic basis at the microstructural level for optimizing weld microstructures and improving the fracture resistance of nuclear grade components.