Single Crystal Superalloy DD6 Research Articles

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.

Temperature-Stress Coupling Fatigue Behavior of Film-Cooling Holes in Complex Temperature Fields.

This research investigates the complex temperature distribution and fatigue behavior of single film-cooling holes manufactured by lasers with different pulse widths in a real flow field. The aerodynamic and heat transfer characteristics of film-cooling holes manufactured using lasers with different pulse widths were analyzed through laser drilling experiments, conjugate heat transfer simulations, and crystal plasticity finite element methods. The study investigated the relationship between changes in the geometric accuracy of the film-cooling holes and the corresponding flow and temperature fields during the film-cooling process. Additionally, the effects of temperature and structural variations on the stress around the holes in a flat plate composed of the second-generation nickel-based single-crystal superalloy DD6 in real flow and temperature fields were studied. The coupling effect of the temperature and stress fields around the holes on the fatigue behavior of the film-cooling holes was examined, and the fatigue damage mechanism of film-cooling holes in complex temperature fields was analyzed. It was found that changes in the blowing ratio do not affect the temperature and stress distributions around the holes but only alter the temperature peak. An increase in the temperature peak results in a decrease in the stress peak. Additionally, the fatigue damage of single film-cooling holes is determined by both the structural defects of the holes and the changes in material behavior due to the temperature around the holes, with the structural influence being more significant.

Orientation dependence of nickel-based single crystal superalloys in VHCF regime

Aimed at research the deformation orientation in VHCF regime under different temperatures, fatigue behavior of [111]-oriented DD6 single crystal superalloy at 1000 °C and 760 °C was studied. Using the stress range criterion, the [111] orientation shows higher fatigue resistance. This difference decreases with lower stress ranges and almost vanishes near 109 cycles. In contrast, the [111] orientation has lower fatigue resistance than the [001] orientation by using the total strain range criterion. Dislocations are restricted from shearing the γ’ phase due to the lower octahedral resolved shear stress in the [111]-oriented, decreasing the deformation in γ’ phase. Combined with distinctive cross-slip behavior, this further increased the deformation along the γ-channel, significantly enhancing deformation inhomogeneity between the γ’ phase and γ-channels. At elevated temperatures, thermally activated dislocation climb and higher climb stress factor lead to a greater reduction in fatigue performance for [111] orientation at lower stress level.

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.

Influence of Short-Term Aging Treatment on Precipitation Behavior of DD6 Single-Crystal Superalloy

Influence of Short-Term Aging Treatment on Precipitation Behavior of DD6 Single-Crystal Superalloy

Fast epitaxial brazing of DD6 single crystal superalloy using novel NiCoCrAlNbTi high entropy filler alloy

In recent years, Ni-based single crystal superalloys are widely applied in the fabrication of blades and vanes in aero-engine industry due to the excellent high temperature properties. In this study, a novel high entropy filler without Si and B was proposed. The melting point of the newly developed Ni-Co-Cr-Al-Nb-Ti filler was 1155ºC and fast epitaxial brazing DD6 single crystal superalloy was realized under the brazing condition of 1200ºC for 30 min. Furthermore, the influence of aging heat treatment on the interface microstructure and joint strength was investigated. The as-brazed joint interface mainly consisted of γ, γ’, γ’’-(Ni,Co,Cr)3Nb and NiAl phase. After aging treatment, element distribution became more uniform and the size of intermetallic phase was refined. The γ+γ’ phase in brazing seam exhibits same [001] orientation with DD6 substrate. The orientation relationship between γ and γ’’ was also analyzed. The joint tensile strength tested at 980ºC was 559 MPa.

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.

Investigation of damage mechanisms in thermomechanical fatigue of nickel-based single-crystal alloys

Thermomechanical fatigue (TMF) is essential for ensuring the safety and efficiency of various thermal engineering systems, however, the underlying damage mechanisms remain unclear. In the present work, a damage evolution model is used to quantify damage mechanisms in nickel-based single-crystal superalloy DD6. The isothermal creep-fatigue model combining the fatigue and creep damages is developed to elucidate their interactions and validated by the tension-dwelling experiments. Finally, the experimentally measured TMF damage is quantitatively decomposed into fatigue, creep and extra TMF components. Significant TMF damage in the in-phase TMF is identified, especially in the [001] specimens, potentially attributed to the non-isothermal creep or oxidation, while damage in out-of-phase TMF aligns with the spiking oxidation mechanism.

Degradation and dislocation evolution of the nickel-based single crystal superalloy DD6 under the coupling high-speed rotating and high temperature environments

To model the centrifugal loads and complex geometries experienced by turbine blades during service, the nickel-based single crystal superalloy DD6 samples with a groove structure are designed and tested under the coupling conditions of high-speed rotating of 20,000 r/min and high temperature of 980 ℃. Scanning electron microscopy and transmission electron microscopy are used to analyze the microstructural evolution of γ-γ' phases and dislocation morphologies of 400 h and 1000 h samples. The results indicate that microstructural degradation of the γ-γ' phase attributes to the larger centrifugal tensile stress S11 in the applied physical stress compositions until deformation occurs. Our observations suggest that a/2 < 101 > -type dislocations initiate plastic deformation by slipping within the γ phase. However, their movement is constrained at the interface between the γ and γ' phases, resulting in their tendency to aggregate at this boundary. After the formation of stable interfacial dislocation networks, these dislocations then react to generate either a< 010 > -type dislocations or a/2 < 112 > -type dislocations, which cut into the γ' phase. However, under high stress conditions, the a/2 < 101 > -type dislocations are primarily cut directly in the form of dislocation pairs.

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.

Effect of orientation deviation on random vibration fatigue behavior of nickel based single crystal superalloy

Vibration fatigue refers to the failure of a structure that is repeatedly subjected to loads causing resonance. The vibration fatigue performance of DD6 single crystal superalloy, an anisotropic material, is significantly affected by its various crystal orientations. This study investigates the impact of different vibration signal intensities and orientation deviation angles on the vibration fatigue behavior of DD6 single crystal superalloy. The fracture surface of the specimen that failed due to vibration fatigue was examined using scanning electron microscopy. The findings reveal that the primary failure mode for vibration fatigue of DD6 single crystal superalloy is the dislocation motion on the {111} plane with the greatest resolved shear stress in the octahedral slip system. Moreover, various methods, including time domain and frequency domain methods, were employed to predict the random vibration fatigue life of DD6 single crystal superalloy with different orientation deviation angles. Finally, a New Model was established to consider the vibration signal intensity and crystal orientation deviation angle, which improved the sensitivity of the frequency domain method to the crystal orientation deviation angle under different vibration signal intensities. Compared to other methods, the New Model is more suitable for predicting the random vibration fatigue life of DD6 single crystal superalloy with different orientation deviation angles.

Effects of Long-Term Aging on Structure Evolution and Stress Rupture Property of DD6 Single-Crystal Superalloy

For China’s second-generation aero-engine blade DD6 single-crystal high-temperature alloy, the standard solution-treated test rods were subjected to long-term aging experiments (1290 °C, 1 h + 1300 °C, 2 h + 1315 °C, 4 h air cooling + 1120 °C, 4 h air cooling + 870 °C, 32 h air cooling) at 980 °C for 1000 h, 5000 h, and 7500 h, and the effects of different long-term aging times on the organization evolution, phase precipitation morphology, high-temperature mechanical properties, and endurance performance of the alloy were studied. The results show that with the increase of aging time, the γ′ phase coarsens, joins along the &lt;100&gt; and &lt;010&gt; directions, and merges to form irregularly shaped directional growth and rafting. The endurance life shows a decreasing trend; at 980 °C/243 MPa, 980 °C/270 MPa, 980 °C/309 MPa for the alloys after 5000 h aging, the enduring life decreased by 47.97%, 70.98%, and 76.75%, and 81.25%, 73.18%, and 87.00% after 7500 h aging, respectively. The tensile strength of the alloy at 760 °C first decreases and then increases, with a minimum value at 5000 h; there is a gradual increase in elongation; there is a gradual decrease in tensile strength at 980 °C; and there is first an increase and then decrease in elongation, with a maximum value at 1000 h. This is due to the diffusion phenomenon of the elements in the alloy after 5000 h aging, the emergence of W-rich, Re-, Mo-, and Ni-poor phenomena, and the transformation of the μ-phase from needle-like to rod-like and block-like.

Effect of thermal exposure on subsurface microstructure evolution of nickel-based single crystal superalloy DD5 after milling

Thermal exposure experiments were performed on the second-generation nickel-based single crystal superalloy DD5 under different conditions, and the evolution of microstructure and mechanical properties in the milling subsurface were systematically investigated. The results show that when the thermal exposure temperature is lower than 1000 °C, the thickness of the heat-affected layer is 5–20 μm, which does not affect the performance of DD5. When the thermal exposure temperature is 1100 °C, the milling subsurface of DD5 is composed of a surface recrystallization (SRX) layer and a cellular recrystallization (CRX) layer both with lower nano-hardness than the DD5 matrix. The thicknesses of both SRX and CRX layers increase monotonically with the extension of holding time, and the thickness of the heat-affected layer (composed of SRX and CRX layers) is 20–40 μm. As the thermal exposure temperature increases to 1200 °C, the milling subsurface of DD5 is composed of an SRX structure with large size topologically close-packed (TCP) precipitates and a transition layer with refined γ/γ′ structure and TCP precipitates. The transition layer with higher nano-hardness than other heat-affected layers is formed between the recrystallization region and DD5 matrix, and its thickness increases from 44.5 μm to 66.8 μm with the extension of holding time increasing from 6 h to 24 h. The results will provide an experimental basis and a theoretical reference for the determination of removal allowance in subsequent processing and the applicable service conditions of DD5.

Experimental investigations into effect of milling parameters on milling force and burr micromorphology for Zr-based bulk metallic glass by using groove milling

Zr-based bulk metallic glass is an amorphous alloy. In order to study the burr problem in the groove milling process of Zr-based bulk metallic glass, the milling force in the process is studied experimentally. Then the effect of milling parameters on the burr of Zr-based bulk metallic glass was studied, and the relationship between milling force and burr was analyzed. In order to further study the milling burr of amorphous alloy, under the same test parameters, the single crystal alloy material (nickel base single crystal superalloy DD5) and the traditional polycrystalline metal material (304 stainless steel) were selected for comparative test and analysis. The results show that Zr-based bulk metallic glass has good groove milling machinability. The test results provide some experimental basis for groove milling of Zr-based bulk metallic glass.

Experimental investigation on surface integrity and fatigue of nickel-based single-crystal superalloy DD6 during grinding-shot peening composite manufacturing

Experimental investigation on surface integrity and fatigue of nickel-based single-crystal superalloy DD6 during grinding-shot peening composite manufacturing

Study on milling material removal mechanism and surface integrity of nickel-based single crystal superalloy DD5

Study on milling material removal mechanism and surface integrity of nickel-based single crystal superalloy DD5

Low cycle fatigue lifetime and deformation behaviour prediction of nickel-based single crystal superalloy considering thickness debit effect

Through stress-controlled low cycle fatigue (LCF) experiments, LCF performances for thickness debit effect of nickel-based single crystal superalloy DD6 with [001] orientation are investigated. Based on scanning electron microscope (SEM) observation, the formation mechanism of thickness debit effect and the damage mechanism under LCF loads are revealed. Then, considering the thickness debit effect of nickel-based single crystal superalloy DD6, an improved slip-based damage model is developed. Predicted LCF lifetimes are mainly within a scatter band of factor 2, and the predicted laws between LCF lifetimes and wall-thicknesses are in good agreement with the experimental data, which verifies the rationality and accuracy of the thickness-sensitive LCF damage model established in this paper. Furthermore, a damage-coupled crystallographic constitutive model reflecting thickness debit effect is developed. For DD6 specimens with different wall-thicknesses, the predicted results of the stress-controlled LCF deformation behaviours including the whole-lifetime ratcheting behaviours show good agreement with the experimental data.

Effect of Stress Ratio on Fatigue Crack Propagation Behavior of Single Crystal Superalloy

In order to study the impact of different stress ratio on the fatigue crack growth behavior of DD6 single crystal superalloy, the fatigue crack growth rate test of different stress ratio (R = 0.05, 0.2, 0.5, 0.6) at 530 °C was conducted, the influence of stress ratio on crack growth rate was studied, and the growth of crack extension rate was corrected by normalization method. The results show that the section includes the fatigue fracture section perpendicular to the load and an angle to the load. The flat section is the main section representing the crack extension. The oblique section is formed with the change of the crack extension load state, and its occurrence is related to the dispersion of (da/dN)−Δk data, reflecting the uniqueness of single crystal. The Paris formula better characterized the crack expansion behavior of DD6 alloy, and the stress is more sensitive than the crack growth rate in the early stable growth phase. Crack growth rate normalized characterization equation constructed under different stress ratios are predicted to be within the 1.3-fold dispersion band.

Research on fretting regime transition of DD6 single- crystal superalloy via femtosecond laser-induced asperity and hardened layer

In this study, the fretting wear test was performed on DD6 single-crystal superalloy after femtosecond laser modification (FLM) to evaluate the influence of femtosecond laser-induced asperity and hardened layer on its fretting regime. Results show that the ablation layer with a depth of ∼ 3.2 μm and the deformed layer are formed beneath the surface, with an increase in surface roughness and hardness. The ablation layer is composed of Al2O3, Cr2O3, and spinel structures. However, the diffraction peak and near-surface orientation after FLM remain similar to those in DD6 single-crystal superalloy substrate, indicating that no remarkable phase change and recrystallization occur despite thermal effect during FLM. The hardened layer has lower plasticity, and the asperities are easy to remove, which results in material removal predominating during the friction process. Therefore, the asperity and hardened layer induced by femtosecond laser could change the fretting regime of DD6 single-crystal superalloy from mixed slip regime (MSR) to gross slip regime (GSR). FLM provides a possibility to replace crack initiation with acceptable material removal in special conditions.

Kinetics and Microstructures of Cellular Recrystallization of Single Crystal Superalloy DD6

The samples of single crystal superalloy DD6 were grit blasted and heat treated at 1100°C for 0.5h, 1h, 2h, 4h, 8h, 16h at vacuum atmosphere, respectively, then the recrystallization microstructure and kinetics of DD6 alloy was investigated. The results showed that the cellular recrystallization grains nucleated in grit blasted single crystal samples heat treated at 1100°Cfor 0.5h, 1h, 2h, 4h, 8h, 16h. With the increase of the duration of heat heating process, the configuration of cellular recrystallization cleared up, and the depth of cellular recrystallization increased. While the coarse γ phase formed in the cellular recrystallization, and the shape of γ phases in the cellular recrystallization was almost equiaxed near the surface of the grit blasted samples and lamellar at the interface between cellular recrystallization and the original zone, respectively. The lamellar γ phase in the cellular grains was radial, and perpendicular to the cellular grain boundary. The recrystallization kinetics of single crystal superalloy DD6 was disclosed, with the increase of the duration of heat heating process, the depth of cellular recrystallization increase quickly and then almost keeps stable at last. The velocity of growth of cellular recrystallization increase very quickly at first, and then decrease at some stage, at last, the velocity tends to zero.