Single Crystal Nickel-based Superalloy Research Articles

Anodic Dissolution Behavior of DD6 Nickel-Based Single Crystal Superalloy for Electrochemical Machining Applications

Abstract Nickel-based single crystal superalloys have been widely used in turbomachinery components. Electrochemical machining (ECM) is an essential method for shaping such high-performance alloys. Understanding the fundamentals of ECM for these alloys is crucial for the design and optimization of the process. This study investigated the anodic dissolution of DD6 nickel-based single crystal superalloy in concentrated NaCl and NaNO3 electrolytes under ECM conditions. Polarization curves showed a passive-transpassive transition behavior in both electrolytes. Galvanostatic experiments demonstrated unique current efficiency characteristics contradicting the empirical ECM knowledge. Surface analyses revealed that the anomalous current efficiency of >100% in the NaNO3 electrolyte results from the falling of γ’ phases from anodic surfaces due to the preferential dissolution of the γ matrix phase. The transition from selective to uniform dissolution with increased current density and reduced electrolyte flow velocity leads to decreased current efficiency in NaNO3 electrolyte. The removal of both γ and γ’ phases depends entirely on electrochemical dissolution in NaCl electrolyte, ensuring that current efficiency maintains a normal value. A schematical anodic interface model was proposed to describe the dissolution behavior.

Anisotropy cyclic plasticity constitutive modelling for Ni-based single-crystal superalloys based on Kelvin decomposition

Nickel-based single-crystal (SC) superalloys exhibited excellent exceptional mechanical properties at high temperatures due to the elimination of internal grain boundaries, contributed a strong orientation-dependent material response. The anisotropy of SC superalloys was modeled viscoplastically from a macroscopic viewpoint based on the Kelvin decomposition theory [1] which was a decomposition of the stress space according to the elastic matrix eigen-directions to control the viscoplastic flow by Kelvin stress decoupled from each other. Compared to the classical phenomenological macro model, the proposed model effectively captures the slip deformation mechanism of SC superalloys with the inherent ability to simulate anisotropic because of the two criterions framework controlled by Kelvin stress. Compared with others, the proposed model was able to simulate time-dependent inelastic deformation and cyclic deformation behavior under complex loading. The kinematic hardening and isotropic hardening models incorporated microscopic quantities, such as dislocation density and channel phase width, connecting the macroscopic mechanical response with the microscopic state to achieve multiscale constitutive modelling. The parameter identification and finite element implementation were conducted on a SC superalloy [2]. Simulation results demonstrated the accuracy of the proposed model in predicting deformation behavior under various orientations, rate-dependent effects, isothermal and non-isothermal cyclic deformation. Comparison with the classical anisotropic matrix macroscopic phenomenological approaches highlights the superior capability of the proposed model to simulate the orientation-dependent mechanical properties of single-crystal alloys.

Comparative study of temperature-dependent oxidation and interdiffusion behavior on NiCoCrAlYHf-coated nickel-based single-crystal superalloys

Comparative study of temperature-dependent oxidation and interdiffusion behavior on NiCoCrAlYHf-coated nickel-based single-crystal superalloys

Fatigue notch strengthening effect of nickel-based single crystal superalloys under different stress ratios

Nickel-based single crystal superalloys (Ni-SXs) are widely used in turbine blades of aircraft engines. With the increasing demand for higher temperature-bearing capacity, the introduction of film cooling holes, impingement holes, and trailing edge slots for cooling induces stress concentration, which compromises the structural integrity of the blades, generates complex multiaxial stress states, and adversely affects their operational performance. In this study, Ni-SXs, DD6, was utilized to investigate the influence of stress ratios on fatigue performance under complex stress states. Low cycle fatigue (LCF) tests were carried out at different stress ratios with stress concentration coefficient Kt = 1.0, 2.0 and 3.0. The results indicate that the notched specimens exhibit a significant fatigue notch strengthening effect under high stress ratios. The fracture surface and microstructure also indicate that under high stress ratios loading, the notches exhibit significant creep failure characteristics. This means that the main cause of fatigue notch strengthening is similar to creep notch strengthening effect. A macroscopic anisotropic constitutive model coupled with damage was developed and applied to finite element analysis of notched specimens. The results demonstrate that as the stress ratio rises, the stress relaxation effect at the notch becomes more pronounced, yet the level of damage diminishes. Additionally, a stress-equivalent model based on Bridgman stress was proposed, which effectively unifies the lifetime trends of notched and smooth specimens, predicting lifetimes within a threefold error band.

Fatigue life prediction of film-cooling Hole specimens with initial damage

This study investigates a Nickel-based single crystal (SX) superalloy with femtosecond laser-drilled film-cooling holes (FCHs) under varying temperatures (room temperature, 850 °C, and 980 °C), employing a novel framework for predicting fatigue life based on initial manufacturing damage quantification. For all tested anisotropic SX superalloy specimens (including smooth and FCH specimens), the initial damage state is characterized as an equivalent initial flaw size (EIFS), and an EIFS calculation model considering stress concentration is established. Subsequently, the fatigue crack paths and microstructural characteristics of the FCH specimens at different temperatures are analyzed, elucidating crack initiation mechanisms and propagation patterns. A novel incremental plasticity J-integral driving force for fatigue crack propagation is introduced. By incorporating the closure effect of small crack propagation and employing Markov Chain Monte Carlo simulations for determining crack growth rate probabilities, a more accurate expression for the crack growth rate in relation to ΔJfat − ΔJth is derived. This expression comprehensively captures crack patterns on crystallographic planes and Type I mixed mode behavior. Finally, the total fatigue life of the FCH structures, featuring a threefold dispersion zone in both room and high-temperature environments, is predicted through experimental observations and description of crack growth rates. The predicted outcomes significantly outperform those of the conventional life prediction models reliant on crystal plasticity theory.

Modified crystal plasticity constitutive model considering tensorial properties of microstructural evolution and creep life prediction model for Ni-based single crystal superalloy with film cooling hole

Accurate assessment of the creep life of film cooling hole structures is critical for long-life design and safe operation of aero engines and gas turbines. Firstly, through the high temperature creep experiment of nickel-based single crystal superalloy with film cooling hole, the microstructure evolution process under multiaxial stress state around film hole is characterized. Then, considering the directional effect of rafting structure and the influence of multiaxial stress, a fourth-order tensor is used to describe the evolution of γ phase width, and the microstructure evolution model accounting for multi-axial stress states is established. The microstructure evolution is coupled into the crystal plasticity constitutive model by Orowan stress. Meanwhile, based on continuous damage mechanics, a new multiaxial damage evolution law is established by introducing a multiaxial ductility factor into the constitutive model. The improved crystal plasticity constitutive model can effectively predict the microstructural evolution under multiaxial stress conditions. Furthermore, the combination of the modified crystal plasticity constitutive model and the critical distance method considering stress gradients is used for life prediction of film cooling hole structures. The prediction results show the effectiveness and necessity of considering the microstructure evolution in the life prediction.

Effect of carbon content on the microstructure and stress rupture properties of a 4th-generation nickel-based single crystal superalloy

The balance between the stress rupture properties and castability of a 4th-generation Ni-based single crystal superalloy can be achieved by addition of appropriate carbon content. In this work, the effects of carbon content on the microstructure and stress rupture properties of a 4th-generation Ni-based single crystal superalloy were investigated in detail. The results showed that minor carbon addition could aggravate the dendrite segregation, while the addition of carbon had no obvious effect on the rafting and distortion of γ′ phase during stress rupture deformation. At 760 °C/810 MPa and 1100 °C/210 MPa, the stress rupture lives of the three experimental alloys initially increased and subsequently decreased with the increasing carbon content. However, under condition of 1140 °C/137 MPa, the increased carbon content resulted in the reduction of the stress rupture lives of alloy. It was discovered that minor carbon elements addition (0.045 wt%) can promote formation of interfacial dislocation networks and improve creep resistance at high temperatures. At intermediate temperature, the introduction of carbon could reduce the stacking fault (SF) energy of γ matrix, thus facilitating the formation of SFs in γ matrix. The M6Cgranular carbides precipitated at high temperature, which was considered to be beneficial to stress rupture properties.

EFFECTS OF TEMPERATURE AND STRAIN AMPLITUDE ON LOW-CYCLE FATIGUE PROPERTIES OF NICKEL-BASED SINGLE-CRYSTAL SUPERALLOY DD419

High-temperature and low-cycle fatigue tests were conducted on a nickel-based single-crystal superalloy DD419 under total strain-controlled conditions at 760 °C and 980 °C. The fatigue properties of the alloy are discussed by analysing the fatigue test data. Fracture morphology and dislocation structure were observed using scanning electron microscopy and transmission electron microscopy. At the same strain amplitude, the results indicate that the plastic deformation of the alloy is larger at 980 °C compared to 760 °C. This leads to a lower fatigue strength and shorter fatigue life, along with more severe damage. The value of the strain amplitude affects the cyclic stress response behaviour of the alloy. Under low strain amplitudes, the cyclic stress response behaviour differs between 760 °C and 980 °C. The hysteresis loop exhibits similar shapes at 760 °C and 980 °C, with an increase in the area as the strain amplitude rises. The fatigue fracture analysis indicates that micropores on the surface are the primary fatigue sources at 760 °C, while oxides on the surface are the main fatigue source at 980 °C, leading to cracking due to multiple sources. Moreover, transmission electron microscopy reveals that the deformation mechanism involving dislocations at 760 °C primarily occurs through plane slip and wave slip, whereas at 980 °C, dislocations mainly move through cross slip and climb.

High temperature fatigue behavior of coated and uncoated nickel-based single crystal superalloy DD6: Microstructures evolution, damage mechanisms and lifetime prediction

Thermal barrier coatings (TBCs) are widely used to further extend the lifetime of turbine blades by protecting the blades from high temperature corrosion and oxidation. However, the mechanical behavior of turbine blades is obviously affected by the TBCs. In this study, microstructures evolution, damage mechanisms and life prediction of coated and uncoated nickel-based single crystal (NBSX) superalloy DD6 under isothermal fatigue load were investigated at 980 °C. The effects of TBCs on fatigue failure behavior and lifetime of DD6 were addressed. The results showed that the fatigue lifetime reduced with the increase of load. The effect of TBCs on the fatigue lifetime was related to the stress amplitude, as the effect was beneficial at high stress but almost negligible at low stress. Fracture morphologies showed that the cracks more likely initiated and propagated from basal defects for both coated and uncoated DD6, and the microstructure evolution also showed stress amplitude dependence. The crack density of uncoated DD6 increased first and then decreased with the increase of stress amplitude. However, the TBCs reduced the number of cracks that penetrate into the DD6 substrate, and the stress amplitude exerted a significant effect on crack propagation paths. In addition, the rafting behaviors of the DD6 substrate of coated and uncoated samples was compared, and results showed that TBCs could reduce the rafting degree of DD6. Finally, the fatigue lifetime of coated samples was predicted based on the modified Basquin model, and the prediction results fitted well with the experimental results.

Effect of Zr/Dy on thermal corrosion resistant properties of NiCrAlY coatings

Dy or Zr doped NiCrAlY coatings were fabricated via arc ion plating technology on nickel-based single crystal superalloy. Thermal corrosion tests of NiCrAlY, NiCrAlYZr, and NiCrAlYDy coatings with 75 wt% Na2SO4 + 25 wt% NaCl mixed salt were conducted at 900 °C. The weight gain of the NiCrAlY, NiCrAlYZr, and NiCrAlYDy coating samples after 100 h of thermal corrosion was as follows: −11.32, 1.06 and −6.56 mg∙cm−2. The results indicated that the NiCrAlYZr coating exhibits superior thermal corrosion resistance compared to both NiCrAlY and Dy-doped NiCrAlY coatings due to the beneficial effects of Zr interacting with S and Cl, making it more effectively protect the hot components of aircraft engine from thermal corrosion.

Effects of hot isostatic pressing on the micron-scale residual stress of nickel-based single-crystal superalloys

Quantifying the residual stress at micron-scale is crucial for comprehending the trans- and inter-granular deformation mechanisms and the influence of heat treatment, but remains technically challenging. This study utilized focused ion beam and digital image correlation (FIB-DIC) techniques to assess residual stress within the dendrite stem and arm of nickel-based single-crystal superalloys. The influence of hot isostatic pressing (HIP) on the microstructure and residual stress was also elucidated. Our results revealed that the residual stresses in the dendrite stem and arm regions manifest as tensile stress along the x-axis and compressive stress along the y-axis, with a range of -720 MPa to 680 MPa. HIP treatment effectively improved microstructure and regulated residual stress in nickel-based single-crystal superalloys, leading to a rapid reduction in residual stress levels. The present study lays a solid theoretical groundwork for optimizing processing strategies to regulate residual stress and enhance mechanical properties in next-generation single-crystal superalloys.

Enhancement of creep properties with trace nitrogen in a low-cost nickel-based single crystal superalloy

Nitrogen (N) is an unavoidable element in single-crystal superalloys that combines with other elements to form nitrides. Large-size nitrides often become the sites of crack initiation and propagation, which adversely affect the microstructures and mechanical properties of superalloys. However, in this study, it was found that the addition of trace amounts of N could improve the creep properties of superalloys. The results indicated that the size and number of nitrides gradually increased as the N contents increased from 3 ppm to 25 ppm. In addition, the porosity first decreased and then increased. The cause of the increase and subsequent decrease in creep lives as N contents increased was the lattice expansion caused by the formation of nitrides, which reduced the porosity by compensating for volume contraction. Meanwhile, the small-size nitrides could impede dislocation movement, which benefited the creep life of the specimens, resulting in the best creep life of the specimens containing 12 ppm of N.

Study on the tensile behaviour and nano deformation mechanism of Nickel-based single crystal superalloys with γ/γ' structure at atomic scale

To study the deformation mechanism of nickel-based single crystal superalloys (NBSC) during tensile process, tensile experiments on NBSC are carried out by molecular dynamics to systematically analyze stress-strain, shear strain, atomic migration and defect evolution. The effects of the structure of the two phases and the strengthening elements on the tensile behavior are analyzed by comparing the pure Ni phase, the pure Ni3Al phase and the Ni phase containing the strengthening elements Cr and Co from a microscopic view. The results show that the interaction between the two-phase structural dislocations during tensile process balances the stress concentration and reduces the degree of plastic deformation that occurs within the matrix. The γ/γ' interface separates the strain morphology and inhibits the slip of the shear band between the two phases, resulting in enhanced deformation resistance of the matrix. By comparing the three single-phase structures, the γ/γ' structure has the highest yield point. From the shear strain distribution pattern, the portion of the two-phase structure with a larger degree of shear strain mainly occurs at the interface as well as at the edge of the matrix.

Exploring the evolution and damage behaviors of multidimensional defects surrounding film holes in single-crystal nickel-based superalloys

Exploring the evolution and damage behaviors of multidimensional defects surrounding film holes in single-crystal nickel-based superalloys

A mechanism on multiscale heterogenous deformation of nickel-based single crystal superalloys unraveled by time-of-flight neutron diffraction technology

Time-of-flight (TOF) neutron diffraction technology is a powerful tool for acquiring rich diffraction information on the multiscale microstructures and stresses of nickel-based single crystal superalloys. In this study, the tensile deformation behaviors of DD426 superalloy at 760 °C were investigated by TOF neutron diffraction. Combined with electron backscatter diffraction technology, the multiscale deformation heterogeneity between γ/γ' phases and dendritic/interdendritic regions was simultaneously characterized for the first time. Our experiments revealed a significant misorientation difference and the existence of a long-range stress field in the adjacent area between the dendritic and interdendritic regions after tensile deformation at 760 °C. The misorientation angles of crystal planes along the loading direction between the dendritic and interdendritic regions in the deformed alloy was approximately 1.6 ± 0.6° This misorientation can be attributed to the geometrically necessary dislocations (GNDs) concentrated in the adjacent area between the two regions with densities of approximately 1014 m-2. In addition, the lattice strains of both γ' and γ showed a compressive–tensile transition from the dendritic to interdendritic regions, balanced with a long-range stress field. The pile-up GNDs in the adjacent area between the two regions contributed to the existence of long-range stress fields. This study highlights that TOF neutron diffraction technology may reveal well the heterogeneous deformation mechanisms via the formation of both GNDs and pile-up in the adjacent interdendritic/dendritic area of single crystal superalloys.

The Effect of Optimized Substrate Orientation on Layer Step in Laser Metal Deposition of Single-Crystal Nickel-Base Superalloys.

Laser metal deposition is a promising way to repair the surface defects of single-crystal components in turbo engines. Understanding the mechanisms and improving the efficiency of the repair have been long-standing problems. In this study, the influence of the substrate orientation on the laser metal deposition (LMD) was investigated and its effect on repair layer-step was examined. LMD experiments were conducted on single crystal superalloys with a normal substrate orientation (001)/[100] and with an optimized substrate orientation (101)/[101¯]. It reveals that the laser cladding with the optimized orientation leads to a larger height of the [001] dendrite region than that with the normal orientation. The calculated results of the growth velocity, thermal gradient, and susceptibility to CET in the dendrite-preferred growth direction indicate that, for the (101)/[101¯] orientation, the [001]/[100] boundary is located at relative high position in each layer, which not only decreases the formation ability of stray grain significantly, but also eliminates the appearance of the maximum susceptibility. This makes the necessary dilution position much higher, and thus, a large cladding step can be selected. Our findings could find potential applications in laser repair of single-crystal components.

Experimental study on the effect of minimum quantity lubrication on the grinding surface/subsurface of nickel-based single-crystal superalloys

Experimental study on the effect of minimum quantity lubrication on the grinding surface/subsurface of nickel-based single-crystal superalloys

Hot Corrosion Behaviour by Na2SO4 Deposits of the 1st Generation AM1 Single-Crystal Nickel-Based Superalloy at 750 °C

Hot Corrosion Behaviour by Na2SO4 Deposits of the 1st Generation AM1 Single-Crystal Nickel-Based Superalloy at 750 °C

Effect of Overheating Treatment on Microstructure and Stress Rupture Properties of a Nickel-Based Single-Crystal Superalloy

Effect of Overheating Treatment on Microstructure and Stress Rupture Properties of a Nickel-Based Single-Crystal Superalloy

Creep properties of a low-cost 3rd generation nickel-based single crystal superalloy with orientation deviated from [001] direction

In this paper, the creep properties and deformation mechanisms of a low-cost 3rd generation nickel-based single crystal superalloy deviating 1.5°, 7.8°, and 14.7° from [001] direction were studied at 800 °C/760 MPa and 1100 °C/137 MPa. The microstructure after full heat treatment and creep rupture of specimens were observed by scanning electron microscopy (SEM). The dislocation configurations of specimens after creep rupture were observed by transmission electron microscopy (TEM). The results indicate that specimens with different orientation deviations exhibit strong anisotropy at 800 °C/760 MPa, and the creep lives of specimens decrease with the increase of θ angle. In specimens A (1.5°) and B (7.8°), the creep deformation is controlled by multiple <112>{111} slip systems, which would produce working hardening. Multi-directional stacking faults are apt to form stacking fault (SF) locks, which inhibit the slipping and cross-slip of dislocations and then increase creep life of the specimen. In specimen C (14.7°), the creep deformation is controlled by single <112>{111} slip system, unidirectional stacking faults cut into γ′ particles, causing rapid failure of the alloy. The creep anisotropy of specimens with different orientation deviations is insignificant at 1100 °C/137 MPa. The same N-type rafted structures and the dislocation networks with similar spacing form in three specimens, which have the same ability to hinder dislocations cutting into γ′ particles.