Single Crystal Nickel-based Superalloy Research Articles

The Microstructure characteristic and Its influence on the stray grains of Nickel-based single crystal superalloys prepared by Laser directed energy deposition

The microstructure adjustment and the stray grains inhibition are important for the preparation of Nickel-based single crystal (SX) superalloys by laser directed energy deposition (L-DED). In this work, the single channel monolayer and single channel five-layers DD6 SX superalloys have been prepared by L-DED. The macroscopic simulations in the deposition processing and multiscale characterizations of deposition structure were used to analyze the formation and evolution mechanisms of unique precipitations and stray grains. Results shows that the larger laser power and higher powder feed rate have a positive impact on the epitaxial growth of columnar dendrite and the elimination of stray grains. The residual stress caused by the complex thermal cycles is not the direct reason that induce the formation of SGs, but the rapid movement of dislocations result in the formation of sub-grain boundary. According to the columnar transition to equiaxed theory (CET), temperature field simulation and analysis of element segregation behaviors, the element segregation results in the unique microstructure, include petal-shaped γ/γ′ eutectic and demarcation band with a thickness of a single γ/γ′ eutectic. Besides, the geometric size of γ/γ′ eutectic decrease with respect to the deposition height. With the change of height at deposition region, dendritic orientation deviations affect the uniformity of element segregation, resulting in variations in γ′ phase and γ/γ′ eutectic dimensions between dendritic core and the inter-dendrite zone at the microscopic level and manifesting as grain boundaries at the mesoscale. At the same time, the formation mechanisms of carbide and adjacent γ/γ′ eutectic leads to the misorientations between the dendrite core and the inter-dendrite zone, which also appears as stray grains at the mesoscopic scale.

Effect of purity on solidification structure and micro-segregation in a nickel-based single crystal superalloy

The purity of nickel-based single crystal alloys plays an important role in the final service performance of turbine blades as it impacts the solidification structure, elemental segregation, and inclusions of castings. The highest purity of superalloys studied before was of altogether around 10 ppm of contents of N and O produced by the triple purification process (VIM+ESR+VAR). However, the impact mechanism still remains controversial, and meanwhile whether a lower content of N and O still keeps the impacts on castings is not yet clear due to the lack of raw materials. A superhigh purity of second generation of nickel-based single crystal superalloy DD98M with the sum of N and O contents as low as 4 ppm (hitherto the highest purity of the superalloy as published), home-produced by electron beam smelting (EBS), was put on focus in this work. The effect of three levels of purity (N-doped, commercial, and EBS, corresponding to 21, 10, and 4 ppm for the sum of N and O contents, respectively) on the microstructure evolution (including primary dendrite arm spacing (PDAS), eutectic, micropores, and γ′ phase) and the solute segregation behavior at dendritic scale and nano scale (γ/γ′ interfaces) were systematically studied and the influencing mechanisms were elucidated. The results show that the PDAS decreased from 406 μm to 322 μm with the reduction of O and N from 21 ppm to 4 ppm, which was irrelevant to precipitation of the nitride or oxide inclusions. Instead, it was due to the decrease of diffusion coefficient D or the increase of equilibrium coefficient k, or the decrease of the critical nucleation supercooling and the incubation time of dendrites. The micro-segregation, the number and volume fraction of eutectic, and the number and voltage percentage of shrinkage pores and gas pores were all greatly decreased with the increase of purity. These changes were closely related to the reduction of PDAS and the reduction of fluidity of the remained liquid in the interdendrites during solidification. The morphology and the size of the γ′ phase were independent from the purity, while the segregation of alloying elements at the γ/γ′ interface changed remarkably with the variation of the purity.

Effect of temperature on the tensile deformation mechanisms of a fourth nickel-based single crystal superalloys

This work conducted tensile experiments from room temperature to 1100 °C on a new fourth-generation single crystal superalloys along [001]. The effect of temperature on the tensile properties and deformation mechanism of the alloy were investigated. Results showed that the yield strength of the alloy reached its peak value (1017 MPa) at 850 °C. From room temperature to 760 °C, the occurrence of stacking faults (SFs) was observed in the γ matrix channel. From the room temperature to 850 °C, the occurrence of stacking faults was observed in the γʹ phase. The interface dislocation networks appeared at 980 °C and above. At room temperature, stacking faults sheared γʹ phase, in which three types of stacking faults occurred, which controlled deformation process. The main deformation mechanism from 650 °C to 850 °C was the shearing of γʹ phase by stacking faults and super dislocations, and at 850 °C, stacking faults in different directions of propagation corresponded to the formation of Lomer-Cottrell locks (L-C locks), which greatly increased the yield strength. At 980 °C and 1100 °C, dislocation climbing and bypassing and anti-phase boundary (APB) coupled dislocation pair shear γʹ phase became the main deformation mechanism.

Atomic behavior of nickel-based single crystal superalloy during heat treatment process based on molecular dynamics

The heat treatment process plays a pivotal role in enhancing the characteristics of nickel-based single crystal (NBSC) superalloys. Nevertheless, there exists a paucity of comprehensive investigations concerning the microstructural evolution of NBSC superalloys during heat treatment. This study employs a molecular dynamics simulation method to control the temperature of the NBSC superalloy precisely, aiming to unveil intricate details regarding microstructural evolution, temperature distribution patterns, mechanical properties, and other pertinent aspects during the cooling phase. Additionally, a comparative analysis of internal defect evolution under varying cooling rates is undertaken. The findings highlight the consistently heightened activity of atoms in the γ phase compared to those in the γ′ phase. Notably, the stability disparity between these phases gradually diminishes as the temperature decreases during the cooling process. At elevated temperatures, the prevalence of amorphous phases and dislocations in the γ phase channel diminishes concomitantly with the temperature reduction. Strain distribution in the alloy primarily concentrates in the γ phase channel and the central cross position of the γ′ phase. The temperature reduction correlates with a decline in the alloy model’s strain. In the initial phase of strain reduction, stress fluctuation trends in the X, Y, and Z directions exhibit an initial increase followed by a gradual decrease. Furthermore, the atomic number of HCP defects and dislocation density exhibit distinct patterns of change contingent upon the cooling rates employed.

Fusing image and physical data for fatigue life prediction of nickel-based single crystal superalloys

This paper introduces a method for predicting the fatigue life of single crystals, which fuses image data and physical data by exploiting machine learning techniques. The proposed approach establishes a comprehensive study by progressively contributing a new database to the field, enabling the inference of service conditions, including temperature and σmax, as well as inherent material properties from fracture images, such as crystal orientation. We evaluate the effectiveness of the proposed method via its application to the fatigue life prediction of DD6 alloy, which achieves promising performance across various crystal orientations. The extensive experimental analysis and evaluation suggest that our proposed method has high predictive accuracy and computational efficiency of our proposed method. These results show that our approach holds promise for enhancing our understanding of material fatigue and improving predictive precision in failure analysis.

Improving the fatigue property of diffusive film cooling holes in nickel-based single crystal superalloy via ultrashort pulse laser drilling coupled with abrasive flow machining

A novel hole-making technique, which combines ultrashort pulse (UP) laser drilling with abrasive flow machining (AFM), has been developed to enhance the surface quality and fatigue resistance of diffusive holes in nickel-based single crystal (NBSC) superalloys. This study conducted a comprehensive analysis of the surface morphology and metallurgical characteristics of the hole wall, evaluated by fatigue testing at elevated temperatures and fractography analysis. The findings demonstrate that AFM can effectively eliminate the solidified debris generated during UP laser drilling, significantly reducing surface roughness and inducing a rounded effect at the outlet acute zone of the diffusive hole. Such improvements have been shown to increase the fatigue life of the holes by up to 50.6 % compared to those without polish. Furthermore, the crystal plasticity finite element method (CPFEM) was employed to investigate the localized stress concentration and the accumulation of plastic slip around the diffusive hole, elucidating the mechanisms behind fatigue failure in NBSC superalloys. The study also discusses the influence of the different hole-making technology on the fatigue properties of diffusive holes, integrating CPFEM results with analyses of surface quality and fatigue fractography. The study conducted in this study can provide valuable guidance for the fabrication of diffusive holes in turbine blades. Furthermore, it can also improve our understanding of the fatigue failure mechanism of diffusive holes in NBSC superalloy.

Clean recycling of spent nickel-based single-crystal superalloy by molten magnesium

Nickel-based single-crystal superalloys are designed for extreme conditions due to their superior corrosion and creep resistance properties. However, these pose challenges in the subsequent recycling after reaching their end-of-life. Molten magnesium (Mg) can rapidly corrode the stable spent nickel-based superalloys and selectively dissolve nickel (Ni). This waste-free process represents an effective method for recycling spent superalloys and accomplishing metal regeneration. This study investigates the mechanism of selectively dissolving Ni from DD5, a nickel-based single-crystal superalloy, by optimizing process temperature, time, and Mg content in an inert atmosphere. Vacuum distillation was employed to separate the resulting Mg, residual superalloy (i.e., the material left post-extraction), and Ni-rich alloy (i.e., the metal product selectively extracted). The findings revealed that the residual superalloy after selective Ni dissolution is characterized by a porous skeleton structure with pore sizes predominantly ranging from 2 to 30 nm and a low compressive strength which is 1/10 of the original DD5 superalloy.

Fatigue life estimation of nickel-based single crystal superalloy with different inclined film cooling holes: Initial damage quantification and coupling of damage-fracture mechanics models

Quantitative assessment of the initial damage state of a structure and fatigue life prediction on this basis, especially when it is sensitive to manufacturing quality, is crucial for both engineering application and material science. Traditional solutions based on fracture or damage mechanics either require precise constitutive relationships and cumbersome physical mechanism models or simply ignore the initial damage state. This study developed a novel equivalent initial flaw size (EIFS) quantitative evaluation method for film cooling holes in Nickel-based single crystal superalloy turbine blades to address the strong correlation between the initial damage in and fatigue life of the material. The temperature and stress field differences at the edges of holes with different inclination angles were simulated using a solid-liquid-gas three-phase level-set model and considered to represent the same initial damage. Next, the coupled damage–fracture mechanics model was used to achieve equivalent crack insertion, crack propagation, and fatigue life prediction based on the EIFS. The results indicate that this method can provide a highly robust EIFS distribution interval and that the crack geometry correction factor and EIFS distribution range are weakly correlated with the loading conditions (with an error within 2 %). The EIFS-based fatigue life predictions demonstrated a notable degree of accuracy, with the majority of the predicted and experimental fatigue lives falling within a three-fold dispersion band and all predictions remaining confined within a five-fold dispersion band of the actual lifetime. These results represent a substantial advancement in accuracy compared to traditional damage mechanics predictions. Therefore, this study provides a powerful approach for evaluating the fatigue life of turbine blades considering the initial damage state and can be widely applied to guide drilling process optimization and blade fatigue analysis.

A Study of the Hot Salt Corrosion Behavior of Three Nickel-Based Single-Crystal Superalloys at 900 °C

A study of the hot salt corrosion behavior of three nickel-based single-crystal superalloys at 900 °C was conducted. We discovered that the corrosion layer on each alloy was distinctly enriched with Mo, Ni, S, and O, primarily comprising sulfides and oxides. Notably, variations in oxygen distribution across the alloys revealed that the elemental composition plays a pivotal role in their corrosion resistance. These insights not only advance our understanding of the mechanisms driving thermal corrosion in nickel-based single-crystal superalloys but also lay the groundwork for designing alloys with enhanced durability tailored to high-temperature applications. This research marks a significant step toward the optimal design and utilization of superalloys in sectors demanding exceptional material stability under thermal stress.

Study on nickel-based single crystal superalloy DD6 subsurface damage of belt grinding with a large cutting depth of one pass

Single crystal superalloy has been widely utilized as a crucial manufacturing material for aeroengine turbine blades due to its excellent properties. However, machining this alloy is easy to cause subsurface damage, resulting in the engineering failure of blades. To explore a large cutting depth and low damage processing method, this paper established a grinding depth analytical model for the abrasive belt. The model provides insights into the material removal behavior mechanism during belt grinding and predicts the impact of process parameters on the grinding depth. Subsequently, the grinding experiments of this alloy with different abrasive belts were conducted. The experimental results showed that grinding depth increased as normal force amplified and exhibited a nonlinear growth pattern with the reduction of feed velocity. This preliminary verified the prediction model of grinding depth. The material removal properties of the pyramid, diamond, and hollow-sphere belts were analyzed, and the ratio of material removal capacity for the three was found to be 12:8:5. Based on the analysis of the topography and roughness of the grinding surface, the study examined the influence mechanism of grinding force, feed velocity and abrasive grain morphology on grinding depth and surface quality. The subsurface damage behavior mechanism and recrystallization depth change law of belt grinding were revealed. The grinding depth of the pyramid and diamond belts could exceed 160 μm and the recrystallization depth was less than 5 μm. To achieve efficient and low-damage machining, it is recommended to reduce the feed velocity rather than increase the grinding normal force. It is important to note that the feed velocity should not exceed 10 mm/s. This study is to furnish a theoretical groundwork and offer engineering technical recommendations for achieving large grinding depth and low damage machining of single crystal superalloy.

Analysis of laser-induced surface damage of single-crystal Ni-based superalloy towards improving machinability

Single-crystal (SC) nickel-based superalloys provide improved high-temperature strength, and resistance to creep, fatigue and oxidation, compared to equiaxed and columnar-grained components due to their higher solid solution strengthening, precipitation hardening and the absence of grain boundaries. However, the SC nickel-based superalloys are ‘difficult-to-cut’. To enhance the machinability, a concept of laser-induced surface damage (LISD)-assisted machining is being studied. Laser surface modification (LSM) experiments were been performed by employing the design of experiments (DoE) strategy L18, OA (orthogonal array). The LSMed geometries were characterized to identify various laser-induced surface defects including solidification cracks, microcracks, micropores, microstructural changes, phase changes, recrystallization, and deformation. Further, parametric interactions and optimization of the process parameters were performed to manufacture a larger LISD layer. The parameter-structure-property relationship was derived systematically and the mechanisms of LISD on the SC nickel-based superalloy were described in details. The results indicate that the LISD increase with an increase in laser power followed by a decrease in scan speed and beam diameter. LISD geometries possess 1 % to 4 % of crack density and lower hardness of around 27 % than the base metal. Slot milling experiments were then conducted on the LISD specimens, which showed up to a 40 % reduction in cutting forces when compared to untreated specimens. It is evident that this methodology can be adopted to further improve the machinability of various difficult-to-cut materials.

The low-cycle fatigue behavior and an entropy-based life prediction model for Nickel-based single crystal superalloy across an extensive temperature range

Nickel-based single-crystal superalloys (Ni-SX) are prominently utilized in the fabrication of turbine blade materials for advanced aero-engines, owing to their commendable material characteristics. This paper delves into the low cycle fatigue (LCF) characteristics of a second-generation Nickel-based single-crystal superalloy across a temperature spectrum ranging from room temperature to 1000 °C. During this procedure, there is an escalation in the trend towards deformation homogenization, leading to a transition in fracture mode from shear fracture to normal fracture. A novel approach is introduced in the form of an entropy-based fatigue life prediction model, grounded in the correlation between cumulative plastic strain and entropy increase rate. Through a comparative examination with experimental data, the model demonstrates proficiency in precisely forecasting the fatigue life of Ni-SX under diverse strain amplitudes and temperature conditions.

Lattice Rotation and Deformation Mechanisms under Tensile Loading in a Single-Crystal Superalloy with [001] Misorientation.

This study investigates how deviation angles close to the [001] orientation affect the tensile properties and deformation behavior of a nickel-based single-crystal superalloy at room temperature. The research focuses on samples with deviation angles of 3°, 8°, and 13° from the [001] orientation and examines their strength and ductility. We employed scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) to explore the deformation micro-mechanisms at varying angles. Findings reveal that strength decreases and ductility increases as the deviation angle widens within the [001] vicinity. The study emphasizes that <110> octahedral slip-driven crystal slip and rotation are crucial for understanding tensile deformation. The deformation differences in samples at varying angles are attributed to the differential engagement of mechanisms. Specifically, at lower angles, reduced ductility and increased strength are due to short lattice rotation paths and work hardening causing superlattice stacking faults (SSFs) to slip in two directions on the {111} plane within the γ' phase. As the angles increase, the lattice rotation paths extend, and Shockley partial dislocations (a/6<112>) accumulate in γ channels. This process, involving SSFs moving in a single direction within the γ' phase, results in higher ductility and reduced strength.

Temperature dependence of electrochemical characteristics on the growth surface and isomorphous perpendicular surface of DD6

Electrochemical machining (ECM) is a highly effective method for processing nickel-based single crystal (NBSC) superalloys. However, the influence of temperature on the electrochemical corrosion of NBSC remains underexplored. This study investigates the temperature dependence of the corrosion characteristics on the growth surface and its isomorphous perpendicular surface of NBSC DD6 were investigated. Results indicate that temperature significantly impacts the passivation and dissolution processes of the material: at low temperatures, the passivation effect weakens, and the differences in self-corrosion potential between phases decrease, leading to smaller variations in corrosion. Additionally, a mechanism is proposed to elucidate the temperature-dependent corrosion behaviors on different surfaces.

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Stress redistribution and stress triaxiality effect on the fatigue life of notched Ni-based single crystal superalloy at 760℃

The multiaxial stress state caused by geometric discontinuities will result in significant differences in the fatigue performance compared to the uniaxial stress state. It is crucial importance to research the notch fatigue behaviors of nickel-based single crystal (Ni-SX) superalloys at high temperature. This study investigated the impact of stress redistribution and stress triaxiality on the fatigue life of notched Ni-based single crystal superalloy at 760℃. The high temperature fatigue tests were carried out on the specimens under different stress ratios and the stress amplitudes under the same life conditions were obtained. A novel anisotropic damage model, incorporating the influence of stress triaxiality is proposed coupled with crystal plasticity theory. Based on experimental findings, an evaluation of the test parameters was conducted.The results showed that Mises stress, maximum principal stress, normal stress of principal stress along the loading direction, maximum principal strain, and strain along the loading direction were inadequate in reflecting the significant variations of samples observed among different stress ratios. However, the stress triaxiality at the unloading valley exhibited a high sensitivity to changes in fatigue life. To evaluate the fatigue life of notched specimens under different stress ratios, an improved Basquin’s type life prediction model was proposed, utilizing the stress triaxiality at the unloading valley. The proposed model demonstrates a favorable normalization trend for the fatigue life of notched specimens subjected to various stress concentration factor (SCF) conditions.

Size effect of femtosecond laser helical drilling on nickel-based single crystal superalloy

Femtosecond laser has gradually become a reliable tool for processing high-quality film cooling holes (FCHs) of turbine blades in recent years. However, in the case of turbine blades with a variety of aperture drilling requirements, the influence of aperture on drilling quality and its internal causes are ignored. In this paper, the “size effect” of different aperture holes prepared by femtosecond laser helical drilling method was analyzed and compared in terms of geometric features, machining efficiency, hole wall ablation and drilling evolution process, and the formation mechanism of the “size effect” were revealed from the aspects of removal efficiency, laser spot overlap rate, energy shielding effect and residues discharge capacity. The results show that with the increase of laser scanning diameter D, the taper of the hole increases, the drilling efficiency increases first and then decreases, and the surface of hole wall is transformed from microvoids, microcracks and adhesive oxide residues into a regular nanostripes. The increase of D makes the radial spot overlap rate Pr decrease significantly, and Pr seriously affects the removal efficiency and thermal effect on the hole wall, which should be adjusted according to different apertures. Thermal accumulation and laser energy utilization rate attenuation caused by the energy shielding effect are the most important factors that deteriorate the processing quality and reduce the processing efficiency during femtosecond helical drilling. Therefore, for smaller aperture drilling, the strategy of process adjustment is to increase residues discharge capacity.

Grinding surface and subsurface stress load of nickel-based single crystal superalloy DD5

Nickel-based single crystal superalloys have broad application prospects in the aerospace field. Their excellent physical properties enable the workpiece to serve in complex environments for a long time, and are often used in the most complex stress and the worst working conditions. However, the damage caused by mechanical material removal deteriorates the wafer surface flatness and die strength, while its surface and subsurface formation principle has yet to be revealed. This study aims to analyze the influence of abrasive particles on the load of the workpiece during machining was scrutinized through the lens of stress tensor, yield performance, and third invariant. The results indicate that, a substantial stress load is exerted on the front end of the abrasive grain, leading to the yield deformation of the workpiece. Conversely, the stress load at the bottom of the abrasive grain is comparatively lower, resulting in tensile deformation and the formation of a discernible plastic damage layer. To delve deeper into the workpiece's damage phenomenon, this study conducts a comparative analysis from the standpoints of chips, surface morphology, and subsurface characteristics of DD5. Experimental results indicate that the chips on the contact surface exhibits a relatively smooth texture, while the grinding surface manifests grooves of varying depths. Concurrently, subsurface analysis reveals the presence of slip deformation and elemental flow. This investigation elucidates the intricate interplay between the grinding process and subsurface loading, offering valuable insights to enhance the quality and precision of nickel-based single-crystal superalloy workpiece manufacturing.

Effect of thickness on microstructure of thin-walled nickel-based single-crystal superalloy castings

With increasing turbine inlet temperatures in aero-engines, the demand for temperature-resistant turbine blades has led to the widespread use of nickel-based single crystal superalloys. These alloys, known for their exceptional performance in high-temperature and high-stress environments, incorporate advanced cooling structures like lamilloy and double wall cooling to enhance temperature tolerance. As the development of aviation technology, thin-walled structures are widely used in turbine blades of aero-engines. However, the adoption of ultra-thin wall structures, particularly in lamilloy turbine blades with thicknesses of 0.5 mm or less, presents a significant manufacturing challenge and will cause the degradation of mechanical properties. This study delves into the growth and evolution of dendrites in the nickel-based single-crystal superalloy DD403, exploring thin-walled specimens with varying thicknesses during directional solidification. Findings reveal that decreasing wall thickness correlates with a reduction in average primary dendrite arm spacing, smaller γ′ precipitates sizes within the dendrite core and interdendritic regions, and reduced microsegregation levels of Al, Ti, Co and W. These insights contribute to optimizing thin-walled turbine blade performance in aero-engines applications.

Understanding the formation and evolution of low-angle grain boundaries in Nickel-Based single crystal superalloys

Grain boundaries have long been of utmost importance in the research and application of single-crystal superalloys. However, there are still many unsolved mysteries regarding the understanding of grain boundaries throughout the solidification process. The work focuses on investigating the formation, the evolution, and the disappearance of low-angle grain boundaries, including subgrain boundaries. The findings show that the low-angle grain boundaries and subgrain boundaries in various dendritic arrays exhibit morphological variances and different development processes. The subgrain boundaries in the aligned array appear to be straight, where irregularly fine-grained γ' precipitates have formed. In contrast, low-angle grain boundaries show a curved and sawtooth-like structure in misaligned arrays. The γ' precipitates locating in the region are regularly long strips. The generation of grain boundaries is a result of the adjustment in dendritic spacing, where the influence of solute leads to dendrite deflection. The misorientation of grain boundary is constantly changing in response to dynamic changes in dendrite spacing. The low-angle grain boundaries eventually dissolve as the dendritic array evolves. This study enriches understanding of the interplay between dendrite arrays and grain boundaries, offering theoretical insights into the evolutionary patterns of grain boundaries within single-crystal superalloys.