Single Crystal Nickel-based Superalloy Research Articles

Coupling model for the gradient loading creep behavior of nickel-based single-crystal superalloys

The emergency situation caused by the abnormal service of an aeroengine, such as gradient loading, directly leads to blade rupture. Considering the effect of gradient loading on creep, a coupling creep model of nickel-based single crystals under gradient loading conditions is proposed based on crystal plastic theory and thermodynamic damage theory. Internal state variables—elastic strain, plastic strain and damage—are introduced to describe the thermodynamic irreversible process of a system. An anisotropic creep damage evolution equation represented by the damage strain energy release rate Y is deduced by the energy conversion method. The gradient loading creep behavior predicted by the finite element calculation is in good agreement with the experimental results.

Grinding subsurface damage mechanism of nickel-based single crystal superalloy based on stress-strain

This study investigates the microstructure damage mechanism of the difficult-to-machine material nickel-base single crystal superalloy DD5 by analyzing the working condition load and crystal deformation. The complexity of the macroscopic and microscopic deformation phenomena during the machining process makes it challenging to study the damage mechanism of the workpiece. Hence, crystal kinematics equation is established using true stress-true strain theory, and the internal force balance and constitutive model of the crystal are analyzed. The proposed theory is applied to finite element processing and experimental analysis of the workpiece, and the subsurface deformation behavior is studied from a mechanistic perspective.The stress model is employed to explore the plastic flow phenomenon of the material during the machining process, and the influence of stress and shear force on the subsurface of the workpiece is discussed. The theoretical analysis of crystal deformation, based on the principles of materials science and mechanics, uncovers the relationship between subsurface deformation curvature and stress. The study delves into the effects of the grinding process on subsurface damage and deformation by analyzing the phenomena of plastic flow, plastic slip, and plastic winding in the subsurface microstructure. The results indicate that at grinding speed vs = 25 m/s and grinding depth ap = 60 μm, the damage to the subsurface deformation layer is minimal. The grinding process considerably impacts the stress exerted on the workpiece, with an amplitude ranging from 1.255 × 103 to 1.286 × 103. Conversely, the strain on the workpiece is relatively less affected by the grinding process, exhibiting an amplitude of 2.366–3.568. Moreover, when the normal stress and shear force of the abrasive particles form 45° angle, the subsurface deformation of DD5 manifests a distinct slip distortion at an approximate 45° curvature.

Dislocation density-based modeling of the yield drop phenomenon in nickel-based single crystal superalloy

The phenomenon of yield drop, characterized by a decrease in flow stress after initial yield, has been observed in various nickel-based superalloys. Despite numerous proposed physical mechanisms, there is still a lack of a meso-mechanism-based constitutive model to explain this phenomenon. In this study, the tensile behavior of a nickel-based single crystal superalloy (DDX), was investigated at different strain rates and a temperature of 900 °C. It was observed that the yield drop phenomenon in DDX became more pronounced with increasing strain rate. To predict the yield drop phenomenon during tensile processing, an improved strength law based on continuum dislocation density theory was considered in the crystal plasticity framework. The proposed constitutive model was implemented using nonlinear iteration and incorporated into a finite element analysis software. The simulation results exhibited a good agreement between the experimental data and the stress–strain curve in the vicinity of the yield drop region, affirming the predictive aptitude of the proposed model in elucidating the yield drop phenomenon at various strain rates.

Process Optimization Method for Inhibiting TCP Precipitation in A Nickel-Based Single Crystal Superalloy with High Refractory Element Content

In order to study the optimization method of microstructure stability of high generation single crystal superalloys, the influence of manufacturing process on the microstructure stability of a single crystal superalloy containing high content of refractory elements was studied, and the corresponding process optimization method was obtained. The microstructure characteristics of the alloy at different stages were observed and the corresponding element distribution were tested. The microstructure and element distribution of the alloy at different stages of preparation were analyzed with the assistance of kinetic calculation. Results show that the element dendrite segregation is an important reason to reduce microstructural stability. Reducing the dendrite spacing in directional solidification and increasing element diffusion in solid solution heat treatment can reduce the segregation of refractory elements in the dendrite core. As a result, less TCP precipitates in the dendrite core, and thus improve microstructure stability. Based on analyzed results, the manufacturing process were optimized and proved to be effective.

Crystal plasticity study of the effect of asymmetric structure of diffusive film cooling hole on heterogeneous creep and fracture behavior of NBSC superalloy

The heterogeneous creep and fracture behavior of nickel-based single crystal (NBSC) superalloys with inclined and diffusive holes are investigated. The analyses are made by creep experiments and crystal plasticity (CP) simulation with a modified creep constitutive model that accounts for both plastic and creep shear slip near the cooling hole. The study indicates that the cooling hole has a creep-strengthening effect on the NBSC superalloy, resulting in an extended creep duration for specimens with cooling holes than those without holes. Meanwhile, the asymmetric structure of diffusive and machined-tapered inclined cooling holes causes localized stress concentration around outlet side of the holes, which leads to asymmetric deformation and decreased rupture strain. Moreover, the interaction between neighboring holes causes shear slip and creep deformation more dominantly at side holes than at the center hole. Thus, preferential initiation of microcrack occurs around the side holes. The specimen without a hole displays typical ductile fracture, whereas the specimen with cooling holes displays a combined cleavage-like and ductile fracture due to the localized stress concentration induced by plastic shear slip around the hole. In addition, as loading stress increased, the creep lifetime and rupture strain decreased, while the characteristics of cleavage-like rupture increased.

A study on the grinding subsurface metamorphic layer influencing factors of nickel-based single crystal superalloy

A study on the grinding subsurface metamorphic layer influencing factors of nickel-based single crystal superalloy

Micro Defects Evolution of Nickel-Based Single Crystal Superalloys during Shear Deformation: A Molecular Dynamics Study

Micro Defects Evolution of Nickel-Based Single Crystal Superalloys during Shear Deformation: A Molecular Dynamics Study

Comparison of Isothermal and Adiabatic Elasticity Characteristics of the Single Crystal Nickel-Based Superalloy CMSX-4 in the Temperature Range Between Room Temperature and 1300°C

Based on the results of performed thermophysical measurements and available experimental data for adiabatic elastic stiffnesses, isothermal elastic characteristics of the single crystal nickel-based superalloy CMSX-4 have been calculated for a wide temperature range, from room temperature to 1300°C. According to the results obtained, the adiabatic and isothermal values of such elastic characteristics as elastic stiffnesses {{c}_{{11}}}, {{c}_{{12}}} and bulk modulus of elasticity B differ significantly at high temperatures. The reasons for this are significant changes in the thermophysical properties of the alloy with temperature and a temperature increase of its Poisson’s ratio approaching the limiting value for cubic crystals, equal to 0.5. It is shown that the use of adiabatic elastic constants instead of isothermal ones in engineering calculations affects the relationship between the volumetric strain and hydrostatic stress, and this effect is similar to introducing a field of thermal dilatation into the analyzed object. At low temperatures, this effect is small, but at high temperatures, typical for the service conditions of the blade material of aircraft gas turbine engines, it increases many times.

Origin of hot cracking formation and suppression method in laser additive manufactured nickel-based superalloys

Although laser additive manufacturing is a promising technique for fabricating nickel-based single crystal superalloy components, reducing hot cracking during the deposition is still a challenge. In this study, the cracking susceptibility and microstructures with different substrates are investigated. Results show that samples exhibit much lower cracking susceptibility when using the directional solidified substrates than the cast polycrystalline substrates. Hot cracking is attributed to a stable liquid film dependent on the misorientation angle. Besides, liquation cracking first occurs in the substrate and could propagate by extension into solidification cracks along the high angle grain boundaries. Based on these findings, a crack-free nickel-based superalloy with a polycrystalline substrate is successfully prepared using a continuous scanning strategy.

Study on creep behaviors of nickel-based single-crystal alloys considering microstructure evolution

Purpose The purpose of this study is to describe the mechanism of single-crystal high-temperature creep deformation, predict the creep life more accurately and study the creep constitutive and lifetime models with microstructure evolution.Design/methodology/approachThe mechanical properties of nickel-based single-crystal superalloy are closely related to the γ' phase. Creep tests under four different temperature and stress conditions were carried out. The relationship between creep temperature, stress and life is fitted by numerical method, and the creep activation energy is obtained. The creep fracture surface, morphology and evolution of strengthening phase (γ') and matrix phase (γ) during different creep periods were observed by scanning electron microscope. With the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by transmission electron microscope (TEM).FindingsWith the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by TEM. Dislocations are mainly concentrated in the γ channel phase, especially at high temperature and low stress.Originality/valueA creep constitutive model based on the evolution of γ' phase size and γ channel width was proposed. Compared with the experimental results, the predicted creep life is within 1.4 times error dispersion band.

Creep-fatigue behavior of nickel-based single crystal superalloy with different orientations: Experimental characterization and multi-scale simulation

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.

Plastic deformation mechanism of grinding subsurface of nickel-based single-crystal superalloy

Plastic deformation mechanism of grinding subsurface of nickel-based single-crystal superalloy

Optimizing process for pulsed laser additive manufacturing of nickel-based single crystal superalloy

The relationship between pulsed laser processing parameters and epitaxial growth of alloy is essential to additive manufacturing technology in repairing and manufacturing nickel-based single crystal (SX) superalloys. In this paper, orthogonal experiments of Laser Direct Energy Deposition (DED-L) process have been designed to optimize the process for the epitaxial growth of the SX superalloy. The relationship between process parameters and epitaxial growth of SX superalloy is established in a radar map, which shows that low laser power, pulse width and powder feeding rate help epitaxial growth in the DED-L process. It is implied that increasing the powder feeding rate value in the process range decreases the epitaxial growth rate of the molten pool and increases manufacturing efficiency. The size of the cladding layer width is greatly influenced by laser power (reached 44%) and pulse width (reached 38%). The deposited heigh of the cladding layer is mainly influenced by pulse width (reached 45%) and powder feeding rate (reached 42%). The process parameters have a similar level (approximately 33%) of influence on the powder using efficiency.

Effect of molybdenum on cyclic oxidation behavior of 4th generation nickel-based single crystal superalloys

The cyclic oxidation behavior of fourth-generation nickel-based single crystal superalloys with 2–4 wt% Mo content were studied at 900 ℃ and 1100 ℃. At 900 ℃, with the increase of Mo content, the oxide layer accumulates more growth stress, resulting in the interface between the outer oxide layer and the inner oxide layer more prone to crack initiation. At 1100 ℃, the oxidation layer accumulates more growth stress and thermal stress, so the oxidation layer occurs large scale spalling, which is more severe in 4Mo alloy. The results show that the increase of Mo content of alloy promotes the oxidation of containing W-Re alloy, which promotes the volatilization of oxides and the spalling of oxide, decreases the protection of oxide layer, and deteriorates the oxidation resistance of the alloy.

The Stray Grains from Fragments in the Rejoined Platforms of Ni-Based Single-Crystal Superalloy

Nickel-based single crystal superalloy is the most important material for blade preparation. However, some solidification defects inevitably occur during the process of preparing single-crystal blades through directional solidification. In this study, in order to study the origin of misorientation defects during solidification, a model with rejoined platforms was designed according to the geometry of single-crystal guide vanes. Electron Back-Scattering Diffraction (EBSD) was used to quantify the orientation deviation of the dendrites and identify the solidification defects in the rejoined platforms. The results showed that stray grain defects appeared in the platforms and their misorientation changed gradually, not abruptly. Combined with the simulation results, it was proposed that the stray grains formed as the result of the dendrites fragment, which was induced by solute enrichment in the mushy zone during solidification. Meanwhile, it was accompanied by a obvious dendritic deformation, which was caused by solidification shrinkage stress. This suggested that the fragmentation was induced by multiple factors, among which, the concave interface shape provided favorable conditions for solute enrichment, and the dynamic variability in the local thermal gradient and fluctuations of the solidification rate might play catalytic roles.

Creep properties and life model of anisotropic Ni-based single crystal superalloys over a wide temperature range

The current study is centered on the creep anisotropy of a nickel-based single crystal superalloy across a wide temperature range and with orientations of [001], [011], and [111]. Based on continuous damage mechanics, a new damage evolution law of anisotropic nickel-based single crystal materials is proposed. Based on the crystal slip theory, creep resistance is innovatively introduced and the modified creep constitutive model of nickel-based single crystal superalloy is presented. Meanwhile, in order to facilitate engineering application, an anisotropic creep life prediction model for single crystal superalloy was established based on continuous damage mechanics and crystal plasticity theory. By establishing the relationship between deformation gradient and shear strain rate on the sliding system, the combination of macroscopic creep deformation and microscopic crystal slip is realized. According to the working state of single crystal blade, a wide range of temperature creep tests at different load points were carried out. The creep parameters A (temperature dependent parameter), n (apparent stress index), Q (apparent activation energy) and τres(α) (creep resistance) are obtained by fitting on the basis of the slip theory of finite deformation crystals and test results. The creep behavior of anisotropic Ni-based single crystal superalloy is simulated by using a modified life prediction model considering creep resistance in finite element framework, and the numerical results are compared with the experimental results. The results show that the constitutive formula proposed in this paper can be used to simulate the mechanical properties of Ni-based single crystal superalloys at different orientations in the wide temperature range, and the constitutive formula also can accurately predict the creep life and creep deformation with different temperatures and directions. The life prediction error is in the range of 1.2 times.

A statistical microstructures-based method for the prediction of mechanical properties in nickel-based single crystal alloys

ABSTRACT The γ/γ’ microstructure is a key factor for the mechanical performance of nickel-based single crystal superalloys. Experimental results have shown that the size and shape of the γ’ precipitates are not uniform and follow certain probability distributions. In order to study the effect of the microstructure on the mechanical behavior of the superalloys, a method is proposed to reproduce its initial microstructure with the distribution function of its γ′ particle size. The effect of different microstructures on the creep behavior at different locations of a real blade is predicted to demonstrate the application of the method. It can be found from the simulation results that the mechanical behavior of the blade is related to not only the thickness of the blade wall but also the position in the blade.

Research on the Melt Pool Shape Formation Mechanism of the Laser Surface Remelting of Nickel-Based Single-Crystal Superalloy

By numerical simulation and experimental analysis, the melt pool shapes for the laser surface remelting of nickel-based single-crystal superalloy under different processing parameters are investigated. The results show that heat conduction and heat convection work together to determine the formation of the melt pool during the laser surface remelting, and the melt pool shape can be controlled by adjusting the laser power and laser scanning speed. For processing with large laser power and low scanning speed, the alloy vaporizes in the melt pool, which makes the melt pool shape unstable. For laser surface remelting with smaller laser power or higher scanning speed, one can have a stable “ω” shape melt pool, which is because the Peclet number is large, and the heat convection plays the dominant role. For the condition with further smaller laser power or higher scanning speed, the Peclet number in the melt pool is much lower, and the heat convection is the weakest, which produces the semi-elliptical melt pool shape that has no essential difference from that of the pure heat conduction model. The present study offers theoretical support to our previous research and the future parameters selection of processing parameters for the laser repairing of nickel-based single-crystal superalloys.

Microstructure Evolution of CET-Free Epitaxial Growth NiCoCrAlYTa Coating by Electron Beam Cladding

Some unavoidable factors in the operating environment could damage single-crystal components of nickel-based single-crystal superalloys. This work prepared an epitaxial growth NiCoCrAlYTa repaired coating without the columnar-to-equiaxed transition (CET) phenomenon on a nickel-based single-crystal superalloy by electron beam cladding. The microstructure of two cross-sections and two surfaces at different depths were characterized. Moreover, the formation mechanism of coating dendrite was revealed by studying the relationship between coating dendrite size, growth direction, and solidification rate. The microstructure evolution and crystal growth orientation of the coating were investigated. The microstructural investigation of the sample revealed that the dendrites’ orientations on the coating’s horizontal section were different, and its characteristics were highly visible on the surface of the coating. The crystal growth orientations of the coating on the vertical cross-section parallel/perpendicular to the scanning direction of the electron beam were also different. Moreover, the average primary dendrite arm spacing (PDAS) of the columnar dendrite in the different areas of the coating was different and increased from 2 μm to 4 μm. The oxidation resistance of the coating at 1000 °C was about three times higher than that of the substrate.

Failure mechanism of single-crystal superalloy NiAlReRu components with film cooling holes under the coupling high-speed rotating and high temperature condition

In order to make up for the deficiency that the laboratory testing of standard samples cannot approximately simulate the turbine blade geometry and service environment, the creep experiment of the nickel-based single crystal superalloy NiAlReRu components with film cooling holes (FCHs) is carried out under the coupling high-speed rotating and high temperature condition. The component with the FCHs exposed to the coupling high-speed rotating of 20000 rpm and high temperature of 1000 ℃ continuously lasts 255.92 h until it breaks. The microstructural evolution and fracture process are analyzed. The results indicate that the existence of the FCHs evidently changes the stress distribution of the region with the FCHs, which makes the microstructural evolution of γ′ phase significantly different. The component is fractured along the second row of the FCHs and the normal direction of the fracture surface is basically parallel to the centrifugal force direction. The fracture process can be divided into three typical stages. The coupling effect of temperature and stress on the failure mechanism is discussed.