Ni-based Single Crystal Superalloy Research Articles

Atomistic Evolution of Hf2S/γ′ Interfaces in a Hf-containing Ni-based Single-Crystal Superalloy

Atomistic Evolution of Hf2S/γ′ Interfaces in a Hf-containing Ni-based Single-Crystal Superalloy

An Overview of Thermal Exposure on Microstructural Degradation and Mechanical Properties in Ni-Based Single Crystal Superalloys

Microstructural stability at elevated temperatures is one of the main concerns for the service reliability of aero-engine turbine blades. Thermal exposure, as an important approach to examine the microstructural degradation, has been widely studied in Ni-based single crystal (SX) superalloys for decades. This paper presents a review on the microstructural degradation induced by high-temperature thermal exposure and the associated damage in mechanical properties in some typical Ni-based SX superalloys. The main factors affecting the microstructural evolution during thermal exposure and the influencing factors in the degradation of mechanical properties are also summarized. Insights into the quantitative estimation of the thermal exposure-affected microstructural evolution and the mechanical properties will be beneficial for the understanding and improvement of reliable service in Ni-based SX superalloys.

The asynchrony of orientation rotation at 750 °C/750 MPa creep in a [011]-oriented Ni-based single crystal superalloy

The Ni-based single crystal superalloy in [011] orientation exhibited poor creep properties at intermediate temperatures. Detailed knowledge of the damage mechanism was still not deeply understood. Here, the damage mechanism resulting from the orientation rotation was proposed. The deformation microstructure depended on the distance from the fracture; at the farther position, there were micro-twins, while at the nearer position were stacking faults. Both micro-twins and stacking faults all originated from the activity of a single {111} <112> slip system. The activity of a single {111} <112> slip system would lead to orientation rotation, which was asynchrony due to the sample being laterally constrained. The asynchrony of orientation rotation would cause nonuniform deformation and localized force inhomogeneity, leading to premature failure of the specimen.

Creep-Fatigue Interaction on Small Crack Propagation in a Single Crystal Superalloy under Thermomechanical Fatigue Condition

Recently, the high-efficiency gas turbine combined cycle power generation has become essential in operation as a backup power supply of renewable energy sources. In turbine blades and vane of the gas turbine operated as a backup power supply, there is growing concern about the damage due to thermo-mechanical fatigue (TMF) loading because of expecting increased starting/stopping cycle and frequent load fluctuation. This study investigated the small crack propagation behavior in single crystal Ni-based superalloy under the in-phase TMF loading cycle. The creep-fatigue interaction on the small crack propagation behavior was discussed based on the relationship between the crack growth rate and the creep J-integral, considering the small-scale creep at the crack tip. The experimental results indicated that the crack growth rates under the in-phase TMF loading cycle were significantly higher than those under the out-of-phase TMF condition at a given fatigue J-integral range. In addition, introducing the tension holding at the maximum temperature in the in-phase TMF loading cycle accelerated the crack growth rates. The small-scale creep stress field at the crack tip might dominate the small crack propagation behavior under the in-phase TMF loading with the high-temperature holding as the creep-fatigue interaction.

High-temperature oxidation and hot corrosion of Ni-based single crystal superalloy in the incubation stage

Early stages of high-temperature oxidation and hot corrosion of Ni-based single-crystal superalloy were studied at 900 °C by introducing sulfate coating. Detailed elemental and microstructural analysis revealed layered surface oxide, atomic diffusion, faceted Al-rich precipitation, and γ′-phase depletion, together with grain coarsening and reorientation in the near-surface region. Undulated surface, multiple layered outer oxide scale, inner CrS scale (solely localized in γ-matrix), and an additional γ′-depleted region without crystal reorientation were observed in hot corrosion rather than in high-temperature oxidation. The corrosion behavior is discussed in terms of atomic diffusion and oxide growth. The impact of sulfate-coating is elucidated.

Experimental Study of As-Cast and Heat-Treated Single-Crystal Ni-Based Superalloy Interface Using TEM.

The interface plays an important role in determining strength and toughness in multiphase systems and the accurate measurement of the interface structure in single crystal (SX) Ni-based superalloy is also essential. In this work, the γ and γ' lattice constant, γ/γ' interface width at dendritic and interdendritic region of casting and solution treatment SX Ni-based superalloy is measured. Various advanced equipment is used to characterize γ/γ' interface nanostructure. A typical correlation between interface width and γ/γ' misfit is also summarized. The interface width in the dendritic region of the as-cast sample is larger than that in the interdendritic region. The misfit in the dendritic region is larger than that in the interdendritic region, which has a trend of negative development. There is a common law of the as-cast interdendritic and dendrite interface sample, where the absolute value of the misfit between the two phases is increasing with the phase interface broadening. The comparison of the as-cast and heat-treated interdendritic sample shows that after heat treatment, the phase interface width increases, the misfit decreases, the lattice constant of γ phase increases, and the lattice constant of the γ' phase decreases. By comparing the as-cast and heat treated dendrites, the absolute value of the misfit of the as-cast dendrite sample is significantly smaller than that of the heat-treated sample, and the misfit increases with the interface broadening. The comparison between interdendritic and dendritic heat-treated samples shows that the absolute value of the misfit between the two phases is smaller than that of the dendritic as-cast samples, and the absolute value of the misfit also increases with the phase interface broadening. In conclusion, property heat treatment can significantly increase the lattice constants of the γ and γ' phases, reduce the lattice mismatch at the interface of the two phases, and improve the high temperature stability of the alloy. A better understanding of the microstructure of Ni-based single crystal superalloys will provide guidance for the subsequent design of more advanced nickel-based single-crystal superalloys.

Generation mechanism and motion behavior of sliver defect in single crystal Ni-based superalloy

Sliver is a common but easily neglected defect in single crystal Ni-based superalloy castings. To date, there is still no unified viewpoint on its formation mechanism and generation causes. In this work, the orientation discontinuity and motion behavior of sliver defects were studied through experiments and numerical simulations. The ultrathin wedge-shaped specimen containing the grain boundary of the sliver and the matrix was prepared at the initial position of the sliver defect for the observation of equal thickness fringes. The discontinuity of equal thickness fringes on both sides of the grain boundary was observed through a transmission electron microscope, which directly confirms the abrupt change in the orientation between the sliver and matrix from the nanoscale. The crystal lattices at the smooth area and the bulging area of the grain boundary were found to have unusually different arrangements. The irregular lattice arrangement at the bulging area shows that the grain boundary has experienced high-stress deformation. Statistical results of sliver orientation deviation with a further composition analysis show the micro protuberance of the mold shell has a noticeable inductive effect on the sliver generation. Furthermore, a self-developed three-dimensional phase-field simulation model coupled with the spatial topology algorithm is established to simulate the orientation deflection behavior and orientation deviation threshold of fractured dendrites. The simulation results indicated that there is an upper limit of the cross-section solid fraction at the fracture position for the motion of the fractured dendrites. When the cross-section solid fraction at the fracture position is higher than this upper limit, it will be difficult to produce large deviation slivers due to the structural limitation of surrounding dendrites. This upper limit does not change with the solidification temperature gradient.

Effects of rhenium and high-angle grain boundaries upon the elemental distribution and microstructure of Ni-based single-crystal superalloys

Effects of rhenium and high-angle grain boundaries upon the elemental distribution and microstructure of Ni-based single-crystal superalloys

Temperature dependent transition from Mode I to crystallographic cracking in a Ni-base single crystal superalloy

To study the temperature dependent transition from Mode I to crystallographic cracking, fatigue crack propagation tests at room temperature, 300 °C and 450 °C, and crystal plasticity finite element analysis are conducted for a Ni-base single crystal superalloy by adopting compact specimens with four different crystallographic orientations. Through experimental examination, the stress intensity factor range (ΔKI) at the critical point of transition scatters for different crystallographic orientations and cannot explain the transition behavior. The slip plane for transition at different temperatures keeps the same. By crystal plasticity finite element assessment, the slip systems on the slip plane for transition are quite activated comparing with other slip systems. The distributions of the plastic shear strains along 12 slip systems at the critical point of transition exhibit the same pattern at different temperatures, which explains the slip plane for transition at different temperatures keeping the same. A damage parameter considering slip plane activity and normal stress on the slip plane can explain the transition regardless of crystallographic orientations. The damage parameter at the critical point of transition increases with the temperature, which indicates that the resistance for transition to crystallographic cracking increases with the temperature.

Effect of laser shock peening on the tensile behavior and notch sensitivity of Ni-based single-crystal superalloy

Effect of laser shock peening on the tensile behavior and notch sensitivity of Ni-based single-crystal superalloy

Thickness Effects on Oxidation Behavior and Consequent γ’ Degradation of a High-Al Ni-Based Single Crystal Superalloy

High temperature oxidation is considered to play an essential role in the thickness debit effect on the creep rupture life of Ni-based single crystal (SC) superalloys. In order to clarify thickness effects, thin-walled specimens of different thickness (t = 0.1, 0.3, 1.0 mm) were prepared. Cyclic and isothermal oxidation tests of a high-Al Ni-based SC superalloy IC21 were carried out at 900 °C and 1100 °C in order to study the thickness effects on the oxidation behavior and consequent microstructural degradation. Thin-walled specimens of S01-N exhibited a good oxidation resistance when the protective scale-forming elements were enough in the matrix. Specimen thickness tuned the oxidation kinetics by changing the spallation behavior but had few influences on the thermodynamics. The easier stress relief via creep deformation in thin specimens is the main reason behind this phenomenon. Moreover, the obvious temperature effects on the multilayer oxide scales caused the different thickness-related spallation behaviors. Weaker microstructural degradation appeared and was further mitigated by reduced specimen thickness. Sufficient Al content is considered to be indispensable for the formation of protective α-Al2O3 scale with less degradation of matrix during the high temperature oxidation of thin-walled Ni-based SC superalloy castings.

The effect of amorphous coating on high temperature oxidation resistance of Ni-based single crystal superalloy

Amorphous Al2O3 coating was prepared on Ni-based single crystal superalloy via atomic layer deposition (ALD) for high temperature oxidation protection. The oxygen inward transport and outward-cation diffusion were hindered by amorphous Al2O3 coating, and the coated alloy showed high oxidation resistance. The low oxygen partial pressure at the alloy/coating interface altered the alloy oxidation behavior, allowing the priority oxidation of protective Al and Cr elements. The oxidation process was controlled by the growth of Cr2O3 and Al2O3. The coating exhibited high spallation resistance. Degradation of the coating, the formation of NiAl2O4 phase in amorphous Al2O3 layer, was observed.

Molecular dynamics study of shock-induced deformation phenomena and spallation failure in Ni-based single crystal superalloys

Shock-induced dynamic mechanical behavior and spallation failure of Ni-based single crystal superalloys are studied via non-equilibrium molecular dynamic simulations. The results show that the spallation failure of Ni-based single crystal superalloys exhibits two modes of classical spallation and micro-spallation. At low shock velocity, the deformation mechanism is dominated by slip and drag of dislocations. As the shock velocity increases, the atomic structure undergoes complex phase transitions from FCC to BCC or disordered structures, and the spallation strength gradually decreases. The decreasing tendency of the spallation strength corresponds to the characteristics of the deformation process. Moreover, the spallation strength and spallation threshold velocity drop significantly with increasing initial temperature, which is due to the softening of Ni-based single crystal superalloys at high temperatures. In addition, the plastic mechanism dominated by shear stress leads to void collapse along the vertical direction at lower shock velocities, while the internal jetting mechanism, controlled by normal stress, induces void collapse in the horizontal direction under higher shock velocities. The existence of voids significantly lowers the spallation strength, and the amplitude reduction of the strength is directly proportional to the radius of the voids.

Microstructure-Sensitive Crystal Plasticity Modeling for Austenitic Steel and Nickel-Based Superalloy Under Isothermal Fatigue Loading

Intermittent mechanical loads combined with high temperatures appear during the operation of turbines in jet engines or in power plants, which can lead to high-temperature fatigue or to thermomechanical fatigue. Since the assessment of fatigue properties is a complex and time-consuming process, it is essential to develop validated material models that are capable of predicting fatigue behavior, thus allowing the extrapolation of experimental results into a broader range of thermomechanical conditions. To accomplish this, two representative volume elements (RVEs), mimicking the typical microstructure of single crystal Ni-based superalloys and polycrystalline austenitic steels, respectively, are introduced. With the help of these RVEs, the temperature and deformation-dependent internal stresses in the microstructure can be taken into account. In the next step, phenomenological crystal plasticity models are implemented and parameterized for cyclic deformation of these two materials. The RVE, constitutive model, and the material parameters for the Ni-based superalloy are taken from a former study. For the austenitic steel, however, an inverse procedure has been used to identify its material parameters based on several isothermal fatigue tests in a wide temperature range. With the identified material parameters, a valid description of the isothermal fatigue behavior at different temperatures is possible. The most important conclusion from the comparison of the isothermal fatigue behavior of the two different materials is that the kinematic hardening, which is responsible for the shape of the hysteresis loops, is entirely described by the internal stresses within the typical microstructure of the Ni-based superalloy, which is modeled in a scale-bridging approach. Hence, no additional terms for kinematic hardening need to be introduced to describe the cyclic plasticity in the superalloy. For the austenitic steel, in contrast, the Ohno–Wang model for kinematic hardening needs to be considered additionally to the internal stresses in the polycrystalline microstructure to obtain a correct description of its cyclic plasticity.

High cycle fatigue life prediction model based on crystal plasticity and continuum damage mechanics for Ni-based single crystal superalloys under a multiaxial stress state

A high cycle fatigue life prediction model based on continuum damage mechanics, crystal plasticity and critical distance theory is developed. A novel damage evolution law for anisotropic Ni-based single crystal materials built on continuum damage mechanics is proposed. The crystal plasticity framework was introduced to take into account the particular characteristics of high cycle fatigue. The critical distance theory was innovatively applied to the structure of film cooling holes to predict the lifetime for materials under a multiaxial stress state. High cycle fatigue tests considering drilling technology, stress ratio and creep time were performed using Ni-based single crystal superalloy plates with film cooling holes. The constitutive equations were implemented into a finite element framework to simulate the high cycle fatigue properties of Ni-based single crystal superalloys, and the results of numerical calculations were compared with experimental observations. It was demonstrated that the constitutive formulations proposed in the present study can be used to model the mechanical behavior of Ni-based single crystal superalloys under a multiaxial stress state. The life prediction errors are found to be within the factor-of-three scatter band. The fatigue life of the specimen with ultrafast laser drilled film cooling holes is larger than that with electrostream machined film cooling holes under the same test conditions. The fatigue strength of the specimen with film cooling holes at a stress ratio R = 0.95 is higher than that at a stress ratio R = -1. The life of the specimen with film cooling holes is found to decrease with increasing creep time before high cycle fatigue. The γ/γ'-microstructure evolution in the specimens with film cooling holes is related to the kinetics of the stress redistribution during the fatigue process. A minimum stress level is essential for rafting to occur. The damage distribution for the plate model is in line with the crack initiation location and propagation region of the specimen.

Oxidation-induced recrystallization and damage mechanism of a Ni-based single-crystal superalloy during creep

The creep tests for Ni-based single-crystal superalloy are carried out under 980 °C, and the microstructure at the crack tip is characterized at micro- and nano- scales. Numerous γ’ phase dissolution, nucleation and growth of fine grains are observed near the crack. In addition, γ’ phase did not effectively hinder the nucleation and growth of recrystallization grains at the oxidation-affected zone. Instead, recrystallization promoted the degradation of the γ’ phase to dissolve along the recrystallized grain boundaries. Combining the sub-grain boundary growth model proposed by Winning et al. and the discontinuous recrystallization model suggested by D.G. Cram, a recrystallization and oxidative damage model for Ni-based single-crystal superalloy is proposed. • Local polycrystallization or recrystallization will greatly damage the high-temperature properties of the alloy. From the point of view of Ni-based single-crystal superalloy design, polycrystallization during medium-high temperature creep is very rare and should be strictly avoided. • Many researches have paid attention to the recrystallization of polycrystalline Ni-based superalloys. However, there is no report about the appearance of polycrystallization region in Ni-based single crystal superalloys during creep. Additionally, systematic research and explanation on the mechanism of discontinuous recrystallization of Ni-based SC superalloy are also very lacking. • In this paper, it is found for the first time that in the process of medium-high temperature (980 °C) creep under low stress level, polycrystallization region and a large amount of γ’ phase dissolve appeared at the crack tip, which leads to a sharp decline in the properties of the alloy. Researchers believed that the γ’ phase is the main factor that hinder the recrystallization of Ni-based single-crystal alloy. Nevertheless, this paper propose that the local strain and depletion of solute atoms makes the sub-grain boundaries tend to become the nucleation of recrystallization.

Temperature dependence of compressive behavior and deformation microstructure of a Ni-based single crystal superalloy with low stacking fault energy

The effect of temperature on the compressive behavior and deformation mechanism of a Ni-based single crystal superalloy with low stacking fault energy was investigated in the temperature range from room temperature to 1000 °C. The results indicated that both the compressive behavior and deformation microstructure were temperature- dependent. There was a higher yield strength at room temperature and then the yield strength decreased at 600 °C. After that, the yield strength would increase continuously to the maximum at 800 °C and then decrease rapidly. Furthermore, the deformation mechanisms were revealed by transmission electron microscope observation. The dislocation tangle and dislocation pairs pile-up were the main reasons for the higher yield strength at room temperature. At 600 °C, the transition in the deformation mechanisms from anti-phase boundary shearing to stacking fault shearing accounted for the slight decrease of the yield strength. At 800 °C, the deformation mechanism was mainly controlled by stacking fault shearing and the reaction of stacking faults along different directions as well as Lomer−Cottrell locks was responsible for the maximum yield strength. Above 900 °C, the primary deformation mechanism was the by-passing of dislocations, although there were still some stacking faults. Finally, the temperature dependence of deformation mechanism and compressive behavior was discussed.

Failure mechanism and interfacial diffusion behavior of Ru-doped NiAlHf coatings at 1200 °C

Ru-doped NiAlHf coatings were deposited on Ni-based single crystal substrate by arc ion plating technology. The failure mechanism and interfacial diffusion behavior were comparatively investigated with NiAlHf coating using scanning electron microscopy, electron probe micro-analyzer, and transmission electron microscopy. The results indicated that microstructure evolution of oxide scale induced by element diffusion process significantly affected oxidation resistance of NiAlHf/Ru coatings, resulting in formation of cracks and voids, thereby accelerating failure process. The precipitates in interdiffusion zone and secondary reaction zone of the substrate initiated by interfacial element diffusion were P phase and σ phase, respectively. And the discrepancy in content was elucidated from the perspective of thermodynamics and kinetics. Besides, microstructural evolution between NiAlHf/Ru coatings and substrate was also deliberated. The research could not only provide profound understanding of NiAlHf/Ru coatings failure mechanism, but also had significant guidance for suppressing precipitation of topological close-packed phases and facilitating development of single crystal Ni-based superalloys.

Enhancing hot oxidation resistance of the HVOF-sprayed NiCoCrAlTaY coating by alumina nanoparticles via a modified suspension route

The effect of alumina (Al2O3) nanoparticles on the hot oxidation behavior of NiCoCrAlTaY coating is investigated. High velocity oxygen fuel (HVOF) technique was employed to deposit coatings on a single crystal CMSX-4 Ni-based superalloy. Different amounts of α-Al2O3 nanoparticles (3, 6 and 9 wt%) were added to NiCoCrAlTaY powder via a modified suspension route. By this modification, the Al2O3 nanoparticles were adhered on the surface of the atomized metallic NiCoCrAlTaY powders with a relatively uniform distribution. The composite powder was then sprayed on the superalloy substrate by an industrial HVOF route. The cyclic hot oxidation resistance of the coatings was evaluated at 1100 °C. Results showed that the coating with 6 wt% nano-Al2O3 experienced significantly better oxidation performance. The presence of nano-Al2O3 in the coating accelerated the formation of α-Al2O3 dense oxide layer and, thereby, delayed diffusion of other elements to the surface. Adding more Al₂O₃ nanoparticles up to 9 wt% led to an increase in porosity and surface roughness of the coating which decreased oxidation resistance.

Duration-affected creep behaviors of Ni-based single crystal superalloys with/without rhenium addition designed for IGT application

In this study, the creep behaviors of two Ni-based single crystal superalloys with/without Re addition designed for industrial gas turbine application were investigated at 900 °C under a range of applied stresses (200 MPa–330 MPa), and the examinations of the microstructures and dislocation configurations were performed at the minimum creep rate and creep rupture under each condition of both alloys. The results indicated that although the creep lives spanned over thousands of hours under different stresses, dislocations underwent the similar mechanisms including the proliferation and movements by gliding-climbing process in decelerating stage, followed by the progressive increase in dislocations shearing into the γ′ phases until creep rupture in each alloy. This led to a special rafting regime which was characterized by the sluggish formation of the rafting structure in the present superalloy, considering the relatively lower γ′ volume fraction, lower lattice misfit, and lower elemental diffusion of the experimental superalloys than those of traditional Ni-based superalloys. Thus, the “duration effect” was proposed to describe the strong correlation between the creep duration and microstructural evolution, which showed insensitivity to the applied stress and accumulation strain, resulting in the consistently exponential relationships between the time to the minimum creep rate and the applied stress or creep life of each alloy within a wide range of stresses. The addition of Re gave rise to an inversion of the rafting tendency during high-temperature creep without changing the deformation mechanism under different stresses. Nevertheless, the addition of Re could still enhance the creep property under each stress, which contributes to provide insights for optimizing the alloying design and enhancing long-term reliable service of industrial gas turbine blades.