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• Thermal fatigue behavior of Ni-based single crystal superalloy at 25–900 °C are investigated. • The cracks only initiate at the direction of 45° from [0 0 1] direction in hot corrosion environment, which is different from the multi-point initiation of cracks in air environment. • The invisibility of crack morphology in hot corrosion environment deserves to be highlighted. • The influence mechanism of hot corrosion on the thermal fatigue behavior of Ni-based single crystal superalloy is proposed. As an important failure model for single crystal materials in service, with the increasing complexity of working environments, the thermal corrosion fatigue failure needs to be studied urgently. In this study, the thermal corrosion fatigue crack initiation and propagation behaviors of an Ni-based single crystal superalloy in both air and hot corrosion environments at 25–900 °C were studied by using a designed precise thermal fatigue test device. The microstructure characteristics and crack composition were analyzed by SEM, TEM and EDS, and the internal stress and recrystallization during thermal fatigue were characterized by EBSD. Moreover, the detailed and comprehensive crack morphology was characterized with different surface treatments. The results showed that the crack morphology was invisible in the hot corrosion environment, which is worthy of being highlighted. The initiation and propagation of thermal fatigue cracks were also accelerated in the hot corrosion environment, which contributed to the sufficient thermal stress, the accelerated material degradation of the inner/outer layers in the crack region, and the rapid forward diffusion of O 2 in the inner crack layer. Finally, the influence mechanism of hot corrosion on the thermal fatigue behavior of the Ni-based single crystal superalloy is proposed. [ABSTRACT FROM AUTHOR]

The tensile interruption tests of Nickel-based single crystal (NBSX) superalloys at room temperature were carried out to artificially introduce controllable levels of plastic damage, and linear and nonlinear ultrasonic nondestructive methods were used to evaluate it. The results indicate that ultrasonic nonlinear parameter β shows hypersensitivity to early tensile plastic damage compared with traditional linear parameters. Transmission electron microscope (TEM) and X-ray diffraction (XRD) measurements of dislocation string length and dislocation density were conducted to calculate the theoretical predicted values of β by ultrasonic nonlinearity dislocation monopole model, and they are in well agreement with experimental results. In addition, the actuation of slip systems and evolution of dislocation configuration were observed under optical microscope (OM) and TEM. From the perspective of element distribution and energy, it is revealed that the plastic deformation mechanism of the NBSX superalloys at room temperature is anti-phase boundary (APB) dislocation pairs shear γ′ phase. The measurement results of macro mechanical properties manifested that accumulative tensile plastic deformation lead to increase of strength and decrease of plasticity. • Tensile interruption tests of nickel-based single crystal superalloys were conducted. • The microstructure evolution of materials was quantitatively studied by TEM and XRD. • The changes of strength and plasticity of materials during tensile plastic damage were measured. • The tensile plastic damage degree was evaluated using ultrasonic linear and nonlinear methods. • The variation of nonlinear parameter was explained by ultrasonic nonlinear dislocation string model. [ABSTRACT FROM AUTHOR]

Abstract: Based on the meso- and microstructure change and damage characteristic of single-crystalline superalloys at high temperature, the paper presents a rate-dependent crystallographic creep constitutive theory coupled with a two-state-variable creep damage model, which takes the rafting damage and void damage into consideration simultaneously. Three-dimensional finite element analysis has been employed to investigate multiaxial creep damage and crack initiation behaviour of a single-crystalline compact-tension specimen containing a cylindrical void ahead of the crack-tip at high temperature for three crystallographic orientations: sp="0.25" />01ph name="rbond2" />011glyph name="rbond2" />111octahedral and cubic slip systems into consideration. The emphasis is placed on the role of the effects of crystallographic orientation and the specimen geometry on creep damage and crack initiation as well as stress distribution along the thickness direction. The numerical simulation results show that the time to crack initiation depends strongly on the void location and crystallographic orientation, which also has considerable influence on the place of crack initiation (the notch or void surface). The resolved shear-stress along the thickness direction is nearly constant in the midsections and tends to decrease sharply near the free surfaces. [Copyright &y& Elsevier]

Abstract: The fracture behaviour of compact tension (CT) specimens made of nickel-based single crystal superalloys DD3 has been studied by experiments at 760, 850 and 950°C. Three different crack crystallographic orientations, (001)[100], (011)[100] and (111)[0−11], of the specimens have been considered. The macro crack growth and micro characteristics of the fracture surfaces have been examined by optical microscope (OM) and scan electron microscope (SEM). The emphasis has been put on the crack growth path and fracture toughness as well as the micro characteristics of the fracture surface. The crystallographic plastic finite element method (FEM) has also been used to analyze the resolved shear stress distribution, normal stress and slip systems activated of the specimens, especially ahead of the cracks, in order to have a deep understanding with crack. The experimental and FEM results show that the crystallographic orientations of the CT specimens have a great influence on the crack growth path and fracture toughness of the specimens as well as the stress characteristics ahead of the cracks. The experiments show that at low temperature, e.g. 760 and 850°C, the macroscopic crack growth path appears as zigzag wave, while at high temperature, e.g. 950°C, all the fracture surfaces are flat along the crack direction except the crack initiation. FEM analysis shows that deformation and fracture ahead of the crack take place in specific slip planes in different crack orientations. The resolved shear stress of the slip system is found to be the main factor of specimens fracture. Also, the normal stress of slip planes has some influence on the crack growth. To the three specimens tested at 760°C, the deflected angles of crack initiation direction with respect to crack plane are 45°, 53.7°and 90° in the order of crack orientations of (001)[100], (011)[100] and (111)[0−11]. The growth path of the crack observed in experiment is agreement with the theoretical analyses by FEM. Furthermore, temperature is the other factor affecting the fracture of specimens. It is clear that the fracture mode of the CT specimens transfers from brittle to ductile with the temperature increasing. [Copyright &y& Elsevier]

Abstract: In this paper, a unit cell model with a film cooling hole has been set up to analyze the crystallographic stress characterization and failure behavior under temperature gradient of nickel-base single crystallographic superalloys (SC). The aim of this work is to study the failure behavior of SC blades with film cooling. The distribution of cooling air pressure on the hole side surface and the distribution of the temperature around the hole are obtained from the fluid analysis. The result of the temperature distribution is then transferred to the finite element model (cell model) by the interpolation method. The cell model is analyzed by the crystallographic rate dependent finite element method (FEM). Special attention is put on the influence of temperature gradient. The influence of the loading boundaries, i.e. displacement loading and stress loading, on the stress characterization around hole is also taken into consideration. The results show that temperature gradient hole has much influence on the stress characterization. Different types of loading boundaries result in different types of stress and strain distributions. There is clear stress concentration near the hole under displacement loading, while there is clear strain concentration under stress loading. The failure characterization has been studied by the strain energy density criterion. It is shown that the temperature gradient has influence on the failure behavior. [Copyright &y& Elsevier]

• Elevated temperature thermal exposure tests of nickel-based superalloys were conducted. • The microstructure evolution of materials was quantitatively metallographic studied. • The degradation of Vickers hardness and yield strength during thermal damage were measured. • The thermal damage degree was evaluated using linear and nonlinear ultrasonic parameters. • The variation of nonlinear parameter was explained by dislocation precipitation interaction model. Ultrasonic nondestructive technique was used to assess the thermal degradation in a nickel-based superalloy in this research. Thermal exposure tests were carried out at an elevated temperature of 1050 °C to introduce controlled levels of damage, while the linear ultrasonic parameters (sound velocity and attenuation coefficient) and nonlinear ultrasonic parameters were observed. The evolution of the microstructure and the subsequent degradation in macro-mechanical properties of the material were quantitatively researched. The results suggest that the nonlinear parameters show hypersensitivity and high-credibility compared with the traditional linear parameters. The monotonic increase of the ultrasonic nonlinear parameters was explained by the evolution of the lattice mismatch and the volume fraction of the γ′ and carbide precipitated phase based on the dislocation precipitation interaction model. [ABSTRACT FROM AUTHOR]

The accelerated creep failure of Ni-based single crystal superalloy by hot corrosion is confirmed by creep tests at 900 °C/550 MPa–400MPa in air and salt environments. The analysis of creep curve, microstructure and fracture type shows that there are obvious differences in hot corrosion-creep behavior under high, middle and low creep stress conditions, indicating that there is the stress dependence between the influence of hot corrosion on the creep properties of the alloy. Hot corrosion accelerates creep failure of Ni-based single crystal superalloy by decreasing the deformation resistance, reducing the effective stress area, and affecting the creep fracture mode. [ABSTRACT FROM AUTHOR]

In order to clarify the creep-fatigue interaction properties of nickel-based single crystal superalloys with different orientations, mechanical tests of [001], [011] and [111] orientations were conducted at 980°Cunder various stress states and microstructural characterization was performed. The results demonstrated that the creep-fatigue life was longest for the [111] orientation, followed by the [001] orientation, and the [011] orientation exhibited the lowest life. Furthermore, as stress levels increased, the life of specimens with different orientations decreased. Damage accumulation in specimens with various orientations was primarily influenced by the formation and growth of material micropores. Fracture analysis revealed that the failure of [001] and [111] oriented specimens resulted from a combination of multi-slip systems and micropore aggregation. Conversely, the fracture of [011] oriented specimens occurred due to a single slip system. Molecular dynamics simulation was employed to analyze the evolution of defects under creep-fatigue loading for different orientation models, further supporting the fracture mechanism. In addition, the life of the specimens was predicted using the crystal plasticity theory based on energy dissipation, and the results were within ±1.5 error bands. • The creep-fatigue behavior of single crystal with different orientation at 980℃ has been systematically studied. • The crystal plasticity theory based on energy dissipation is used to predict the creep-fatigue interaction life. • The creep-fatigue behavior of single crystal with different orientation at 980℃ has been systematically studied. (2)The crystal plasticity theory based on energy dissipation is used to predict the creep-fatigue interaction life. (3)Experimental characterization and multi-scale simulation (CP and MD) reveal the defect evolution and fracture behavior. [ABSTRACT FROM AUTHOR]

Hot corrosion behaviour of Ni-base single crystal at 900 ℃ is investigated under three different mixed salts. The result shows that hot corrosion degree in Na 2 SO 4 +NaCl+NaVO 3 is the most serious, followed by Na 2 SO 4 + NaVO 3 , and Na 2 SO 4 +NaCl is the least. The hot corrosion mechanism of superalloy in Na 2 SO 4 +NaCl+NaVO 3 salts is proposed. More importantly, the interaction of Na 2 SO 4 , NaCl and NaVO 3 is a synergistic relationship in which Na 2 SO 4 causes a basic corrosion attack, and NaCl with NaVO 3 strengthen this attack. Therefore, in hot corrosion protection, the type of corrosive salt should be reduced to avoid the synergistic effect caused by multiple salts. [Display omitted] • Hot corrosion behavior of Ni-based single crystal superalloys at 900 ℃ under three different mixed salts was investigated. • Sulfides in γ′-depleted layer are mainly affected by critical oxygen partial pressure and standard free energy of sulfide. • NaCl and NaVO 3 consume the protective oxides and prolong the time of internal sulfidation-oxidation cycle reaction. • There is a synergistic relationship between the corrosion salts of Na 2 SO 4 , NaCl and NaVO 3. [ABSTRACT FROM AUTHOR]

In order to study the tension/compression asymmetry of (001) single-crystal nickel-based superalloy DD6 during low cycle fatigue, fully reversed cycle fatigue tests were conducted at 760°C and 980°C, with strain rates of 10−3 and 5×10−3 s−1, respectively. The total strain ranges were varied from ±0.5% to ±1.2%. The DD6 single crystals show that tensile stress (T) is higher than compressive stress (C) at the initial cycles in the above conditions. Under the strain rate 10−3 s−1at strain ranges ±0.7%/760°C and ±0.55%/980°C or under the strain rate 5×10−3 s−1at ±0.7%/980°C, the tension–compression asymmetry of C>T was observed. According to the above experiments, an advanced rate-dependent constitutive model based on the crystal plasticity theory was proposed. The effects of elastic deformation (for T>C at the initial cycles of LCF), dislocation accumulation in the matrix channel (for kinematic hardening) and the shearing of precipitates by Superlattice Intrinsic Stacking Faults (SISF) (for kinematic softening) were taken into account. Numerical simulations were conducted using the proposed constitutive equations, such as monotonic tension, compression and cyclic deformation. The present model is shown to be successful in simulating the inelastic behavior of DD6 single crystals under low cycle fatigue. [ABSTRACT FROM AUTHOR]

Abstract: In present study, low cycle fatigue model based on rate dependent constitutive formulation with kinematic hardening and fatigue damage rule is introduced to investigate the fatigue behavior of single crystal superalloys with temperature gradient. Low cycle fatigue tests with uniform temperature and with temperature gradient are carried out to investigate the influence of temperature gradient. The microstructure reveals that the slip deformation is a principal mechanism of low cycle fatigue with temperature gradient. Complex stress experiments of low cycle fatigue are carried out to verify the life prediction rule. Bauschinger effect, ratcheting effect and fatigue damage of single crystal superalloy at different temperatures are studied. Special attention is placed on the simulation of low cycle fatigue behavior with temperature gradient. The simulation results show that the ratcheting effect at high temperature is more remarkable than that at low temperature due to the influence of temperature gradient, which results in the difference of damage evolution between high temperature zone and low temperature zone. Comparison between experiments and simulations with temperature gradient proves that the predicted model based on the damage of the second cycle is reasonable. [Copyright &y& Elsevier]

Abstract: In this research, the double notched plate mini-specimens have been designed to study the notch effect during creep. The experimental results show that the notch has strengthening effect. The scanning electron microscopy (SEM) photos of fractured surfaces reveal that void growth is the primary mechanism of creep rupture. Finite element analysis (FEA) with a modified form of Kachanov–Rabotnov damage law was carried out to simulate the damage evolution in the specimens. The creep damage calculations show that the creep is a process of stress redistribution. The 3-D voided unit cell model with the constant maximum principal stress was used to explore the mechanism of notch strength. The results show that the initial stress triaxility has remarkable influence on void growth. The greater the initial stress triaxility is, the slower the growth rate is. The stress state plays a main role in void deformation. The void grows remarkably in transverse direction when the stress triaxility is high. The void growth and coalescence is the main mechanism of creep rupture. The multiaxial stress state can inhibit the void growth, and this constraint effect is beneficial to the creep life. [Copyright &y& Elsevier]

Abstract: Plate specimens both with holes and without holes were used to model air-cooled turbine blades. Experimental study on the creep behavior of a second generation nickel-based single crystal has been performed at 980°C with these modeling specimens in two crystallographic orientations [001] and [111]. The results of creep tests show that crystallographic orientations and cooling holes have remarkable influence on the stress rupture behavior. The scanning electron microscopy (SEM) observation on the fractured surface shows that the cavitation-controlled damage is the main mechanism inducing fracture. So the finite element analysis (FEA) with a modified form of Kachanov–Rabotnov damage law was carried out to simulate the damage evolution in modeling specimen with cooling holes in crystallographic orientation [001]. The FEM results reveal that the existence of cooling holes causes the stress and strain concentrations near the holes. The creep damage is localized in the cooling holes region, where the fracture will happen easily. The numerical result is coincident with the fracture appearance of the specimen. [Copyright &y& Elsevier]

• The thermal fatigue failure mechanism of Ni-based single crystal superalloys with hole structure has been revealed. • The strain–stress constitutive equation under transient thermal shock based on the crystallographic theory was established. • The thermal stress location and evolution around the hole at different temperatures was obtained by FEM. • A thermal fatigue crack initiation life model considering transient thermal stress and cyclic oxidation has been proposed. • There is good consistency between the experimental results and the simulation results. The transient thermal fatigue behavior of Ni-based single crystal superalloys with film cooling holes under different peak temperatures (25 °C → 980 °C and 25 °C → 1050 °C) were investigated by a combination of test and finite element simulation. The initiation position and propagation direction of thermal fatigue cracks in both single hole and multi-hole specimens exhibit the angles of ± 45° with respect to the horizontal direction at the four corners of the holes. Octahedral slip failure mainly occurred at the crack initiation location and crack tip. The cyclic oxidation around the film cooling hole promotes the initiation and propagation of cracks. Based on the crystallographic theory, the strain–stress constitutive equation of the Ni-based single crystal superalloy under transient thermal shock was established. The location and evolution of thermal stress around the hole at different temperatures were obtained by finite element calculation. The kinetic behavior of cyclic oxidation has been quantitatively characterized. A thermal fatigue crack initiation life model considering heating temperature, transient thermal stress, and cyclic oxidation has been established. The simulation results are in good agreement with the tests. [ABSTRACT FROM AUTHOR]

• The combined high and low cycle fatigue (CCF) response of DD6 alloy was studied under different low cycle stress amplitudes at a temperature of 760 °C. • Accurately established the correlation between DD6 alloy microstructure characteristics and its fatigue performance, revealed the evolution law of microstructure characteristics and the fracture mechanism of DD6 alloy under CCF conditions. • The CCF life distribution of DD6 alloy at 760 ℃ was statistically verified using the dual-parameters Weibull distribution theory. • Based on crystal plasticity theory, a prediction model for high and low cycle composite fatigue of DD6 alloy at 760 ℃ was established. The impact of high and low cycle combined loads on the fatigue performance of DD6 alloy was investigated in this work. Specifically, the combined high- and low-cycle fatigue response of DD6 alloy was examined at a temperature of 760 °C under different low cycle stress amplitudes (960, 1000, and 1040 MPa). The findings revealed that higher low cycle stress amplitudes result in shorter combined high and low cycle fatigue life and less dispersion in life. Correlations between the microstructural features (crystal plane distribution, fracture elevation difference, and the area proportion of crack propagation zone) and fatigue performance were accurately established, demonstrating a linear relationship between these microstructural features and the logarithm of fatigue life. The study also uncovered the evolution rule of microstructural features in high- and low-cycle combined fatigue. Furthermore, the fracture mechanism was analyzed and the distribution of high and low cycle combined fatigue life of DD6 alloy at 760 °C was statistically validated using the dual-parameter Weibull distribution theory. Building upon the crystal plasticity theory, a fatigue life prediction model was developed, and the combined cycle fatigue lifespan under various low cycle stress amplitudes was compared. The comparison between the predicted life and actual fatigue life indicated that the predicted lifespan falls in three times the scatter band. [ABSTRACT FROM AUTHOR]

The circumferential V-type notched round bar has been designed to investigate the effect of multi-axial stress state on creep rupture behavior of DD6 Ni-based single crystal superalloys at 1100 °C. Creep test on both smooth and notched specimens were carried out under 160 MPa and 200 MPa on the minimum net-section. Time to creep fracture was found to be higher in notched specimens than in smooth specimens. Finite element (FE) analysis coupled with continuum damage mechanics (CDM) was carried out to understand the stress distribution and the creep damage evolution under uniaxial and multi-axial stress states. The creep rupture life of the smooth specimen was successfully predicted by the classical Kachanov–Rabotnov damage law. The creep rupture life of the notched specimen predicted by finite element calculation incorporating a multi-axial creep CDM model was in good agreement with the experimental results. Both the fracture morphology and FE damage analysis indicate that rupture initiates first at the notch root in the notched specimen and finally towards to the center location. [ABSTRACT FROM AUTHOR]