Single Crystal Turbine Blades Research Articles

The rapid improvement method of fatigue life reliability for single-crystal aeroengine turbine blades: Casting orientation design technology

The uncertainty in the casting orientation of single-crystal aeroengine turbine blades is an intractable engineering problem, and breaking through the crystal orientation design technology is vitally important to enhance the fatigue lifetime reliability of the blade. In this paper, a three-dimensional decoupling model for casting orientation is established according to the crystallographic characteristics of single-crystal blade, and the distribution information and statistical features of the orientation deviation angle of single-crystal turbine blades are obtained for the first time according to this decoupling model. Subsequently, the orientation-dependent models and generalized regression neural networks (GRNNs) analysis system of reliability are established, and the analysis of fatigue life response for single-crystal blades in four quadrants is carried out based on the neural network analysis system. Finally, three orientation design schemes are developed, and the improvement effect of each scheme on lifetime reliability are investigated. For the secondary orientation design scheme, the lifetime reliability is improved from 62% to more than 95% at the design life of 15,000 cycles; For the integrated orientation design scheme, the life reliability is improved from 50% to more than 95% at the design life of 20,000 cycles. The results show that the orientation design can effectively improve the fatigue life reliability of the product, especially in the long-life design interval of 10,000 to 20,000 cycles. Therefore, the orientation design should be highly emphasized in the design of single-crystal turbine blades for aeroengine.

Surface integrity characterization of third-generation nickel-based single crystal blade tenons after ultrasonic vibration-assisted grinding

Machined surface integrity of workpieces in harsh environments has a remarkable influence on their performance. However, the complexity of the new type of machining hinders a comprehensive understanding of machined surface integrity and its formation mechanism, thereby limiting the study of component performance. With increasing demands for high-quality machined workpieces in aerospace industry applications, researchers from academia and industry are increasingly focusing on post-machining surface characterization. The profile grinding test was conducted on a novel single-crystal superalloy to simulate the formation of blade tenons, and the obtained tenons were characterized for surface integrity elements under various operating conditions. Results revealed that ultrasonic vibration-assisted grinding (UVAG) led to multiple superpositions of abrasive grain trajectories, causing reduced surface roughness (an average reduction of approximately 29.6%) compared with conventional grinding. After examining the subsurface layer of UVAG using transmission electron microscopy, the results revealed that the single-crystal tenon grinding subsurface layer exhibited a gradient evolution from the near-surface to the substrate. This evolution was characterized by an equiaxed nanocrystalline layer measuring 0.34 μm, followed by a sub-microcrystalline grain-forming zone spanning 0.6 μm and finally, a constituent phase-twisted distorted deformation zone over 0.62 μm. Under normal grinding conditions, the tenon exhibited low surface hardening (not exceeding 15%), and residual compressive stresses were observed on its surface. In cases where grinding burns occurred, a white layer appeared on the tenon’s surface, which demonstrated varying thicknesses along the teeth from top to root due to thermal-force-structural coupling effects. Additionally, these burns introduced residual tensile stresses on the tenon’s surface, potentially substantially affecting its fatigue life. This paper enhances our understanding of UVAG processes and establishes a foundation for their application in manufacturing single-crystal turbine blades for next-generation aero-turbine engines.

Surface integrity characterization of third-generation nickel-based single crystal blade tenons after ultrasonic vibration-assisted grinding

Machined surface integrity of workpieces in harsh environments has a remarkable influence on their performance. However, the complexity of the new type of machining hinders a comprehensive understanding of machined surface integrity and its formation mechanism, thereby limiting the study of component performance. With increasing demands for high-quality machined workpieces in aerospace industry applications, researchers from academia and industry are increasingly focusing on post-machining surface characterization. The profile grinding test was conducted on a novel single-crystal superalloy to simulate the formation of blade tenons, and the obtained tenons were characterized for surface integrity elements under various operating conditions. Results revealed that ultrasonic vibration-assisted grinding (UVAG) led to multiple superpositions of abrasive grain trajectories, causing reduced surface roughness (an average reduction of approximately 29.6%) compared with conventional grinding. After examining the subsurface layer of UVAG using transmission electron microscopy, the results revealed that the single-crystal tenon grinding subsurface layer exhibited a gradient evolution from the near-surface to the substrate. This evolution was characterized by an equiaxed nanocrystalline layer measuring 0.34 μm, followed by a sub-microcrystalline grain-forming zone spanning 0.6 μm and finally, a constituent phase-twisted distorted deformation zone over 0.62 μm. Under normal grinding conditions, the tenon exhibited low surface hardening (not exceeding 15%), and residual compressive stresses were observed on its surface. In cases where grinding burns occurred, a white layer appeared on the tenon’s surface, which demonstrated varying thicknesses along the teeth from top to root due to thermal-force-structural coupling effects. Additionally, these burns introduced residual tensile stresses on the tenon’s surface, potentially substantially affecting its fatigue life. This paper enhances our understanding of UVAG processes and establishes a foundation for their application in manufacturing single-crystal turbine blades for next-generation aero-turbine engines.

Effect of constraint removal on single-crystal blade dimensions during investment casting

Single-crystal turbine blades, manufactured via investment casting, are key components of high-performance aero engines and gas turbines. The final dimensions of the blades depend on the cumulative effect of heat shrinkage at different stages, including the superalloy, mold shell, and ceramic core, etc. To clarify the effect of these stages on the blade dimensions, we first use the heat transfer coefficient calculated from inverse heat-conduction analysis to simulate the directional solidification. By extracting and mapping the final physical field from the previous stage to the next stage, four successive constraint removal processes are performed at different time points. Subsequently, we establish a time-varying model for the entire process of directional solidification and constraint removal. The errors of the extraction/mapping are consistently less than 0.002 mm in all three directions, thus confirming the feasibility of data transfer. Secondly, we analyze the dimensional deviation patterns of the casting after sequentially removing the mold shell, process rib, grain selector, and ceramic core constraints, with emphasis on the dimensional accuracy of the airfoil while disregarding the dovetail generated from machining. The final average deviations of Section 1, Section 2, and Section 3 of the airfoil are 0.1896 mm, 0.1723 mm, and 0.1909 mm, and the maximum deviations occur consistently at the trailing edge near the tip. Finally, three section deviations are validated via measurements performed using a vacuum furnace and industrial computed tomography. The results show that the proposed method improves the dimensional accuracy of hollow turbine blades.

Microscale stray grains formation in single-crystal turbine blades of Ni-based superalloys

Microscale stray grains (MSGs), which are composed of misoriented and fragmented high-ordered dendrite arms, small-sized equiaxed grains, and columnar grains, are a novel grain defect that occurs within the shrouds of single-crystal (SC) blades made from Ni-based superalloy. State-of-the-art non-destructive testing methods are incapable of detecting this defect, resulting in a high-risk application of SC blades. This study aims to control this defect by systematically investigating directional solidification processing parameters, shroud dimensions, and rhenium (Re) content in the formation of MSG defects. The results reveal that the defect forms exclusively at the overhanging extremities of the upper shrouds on the back side of the blades that face the heater. Increasing the withdrawal rate reduces the occurrence of MSG defects. As the shroud dimensions increase, MSG defects appear in the transition area between the downward suspended extremity of the blade shroud and the side of the blade body, as well as near the corner of the downward suspended extremity of the shroud. The occurrence of MSG defects increases with increasing shroud dimensions. Varying Re content sharply decreases the formation of MSG defects. A concentration-attached Rayleigh-Taylor instability (RTI) superheating (CARTISH) model, considering solidification shrinkage, was proposed to comprehend the formation of MSG defects. Simulation results based on this model are consistent with the experimentally observed distribution and degree of MSG defects under different conditions. Effective control of the CARTISH is critical to managing MSG defects.

A dislocation-based damage-coupled constitutive model for single crystal superalloy: Unveiling the effect of secondary orientation on creep life of circular hole

Circular holes in single crystal (SX) turbine blades are susceptible to creep failure at high temperatures, due to the stress concentrations near the holes. In this study, a dislocation-based damage-coupled constitutive model for SX superalloys is developed, validated, and applied to unveil the effect of secondary orientation on the creep life of circular holes. The structure-induced geometrically necessary dislocations (GND) density resulting from the macroscopic strain gradient is incorporated. The dissipated energy is employed as the indicator to predict crack nucleation. Initially, the specimens without a hole are employed to calibrate the parameters and validate the model in the absence of structure-induced GND. Creep experiments incorporating the digital image correlation technique are conducted on the specimens with a hole under different secondary orientations, which aims to validate the simulated strain distribution, crack nucleation location, and creep life under the influence of structure-induced GND. The results indicate that the incorporation of structure-induced GND contributes to mitigating the conservatism in the creep life prediction of circular holes. Because, the GND can enhance dislocation hardening near the hole, extending the predicted life and improving the accuracy of life prediction. Meanwhile, both experiments and simulations indicate that the secondary orientation influences the crack nucleation locations at the hole edge. The variation in the creep life at different secondary orientations is attributed to the changes in both the maximum value and uniformity of resolved shear stresses distributed across slip systems. Finally, the model is applied to simulate the creep behavior of a circular hole under various secondary orientations, stresses, and temperatures. An empirical model is developed to rapidly evaluate the creep life of a circular hole in the SX superalloy, which facilitates the engineering application of the proposed model.

Research on low‐cycle fatigue behavior of Ni‐based single crystal superalloy with shaped film cooling holes at high temperature

AbstractFilm cooling holes (FCHs) geometric structure significantly affects low‐cycle fatigue (LCF) of the single crystal turbine blade. This study investigates the LCF behavior of Ni‐based single crystal plate specimens with two different structure FCHs, including elliptical and fan‐shaped FCHs, by high temperature fatigue tests and crystal plasticity simulation. The results show that the LCF life of the fan‐shaped FCHs is shorter than those of the elliptical FCHs. The fracture characteristics and microstructure evolution of different structure FCHs are dissimilar. Oxidation affects the fatigue crack nucleation process at the FCH edge. The resolved shear stress (RSS) around fan‐shaped FCHs is greater than elliptical FCHs under one cyclic stress, and the area with larger RSS is consistent with fatigue crack nucleation position. The early LCF failure is mainly attributed to the high temperature oxidation of the local slip deformation induced by RSS concentration around the FCHs.

An arc-tenon attachment for nickel-based single crystal turbine blade with improved high-temperature fretting fatigue performance

Nickel-based single crystal (NBSC) turbine blade tenon is prone to fretting fatigue failure. In this study, an arc-tenon attachment was proposed and its fretting fatigue life increased by up to 408.18 % at 600 ℃ compared with the plane-tenon structure under the applied peak stress of 65 MPa. The arc-tenon attachment could effectively reduce the maximum von-Mises stresses and resolved shear stresses near the contact region, regulating the stress distribution and reducing the stress gradient, which was beneficial to alleviating the activation of octahedral slip systems and delaying crack initiation, thereby improving the fretting fatigue life at high temperature. Meanwhile, high temperature promoted the formation of oxide layer on the contact surface, providing low coefficient of friction and further reducing the contact stress. The large resolved shear stress near the contact leading edge region facilitated the activation of octahedral slip systems, then the resultant multiple slip lines sheared both the γ matrix phase and γ′ precipitates. The further growth of the slip lines accelerated the fretting fatigue crack initiation and propagation. The arc-tenon attachment could provide long fretting fatigue life and showed great potential in engineering application.

A Crystallographic Basis for Critically Evaluating the Mechanisms for Secondary Grain Formation During Directional Solidification

AbstractA crystallography-based method is presented for the critical appraisal of possible mechanisms that trigger the formation of secondary grains during directional solidification. The method permits an analysis of a large population of defects, while avoiding the pitfalls of the metallographic sectioning approach that is affected by dendrite stereology. Here, the nickel-base superalloy CMSX-4, an alloy commonly used for single crystal turbine blade applications, is studied. All secondary grains originate exclusively at the external surface and when the off-axial primary $$\langle 0\,0\,1\rangle$$ ⟨ 0 0 1 ⟩ crystal orientations are measured, are evident at both the converging and diverging dispositions of the single crystal primary dendrites without a noticeable bias. Almost all of the secondary grains have low misorientations, with an average misorientation between 5 to 15 deg. No systematic deviation between the individual $$\langle 0\,0\,1\rangle$$ ⟨ 0 0 1 ⟩ orientations of the secondary grain and the single crystal is observed. A significant twist contribution about an axis within ~ 30 deg from one of the secondary arms occurs when primary arms converge on the external surface, but both twist and tilt prevail for the diverging case. Both nucleation and buoyancy driven thermo-solutal convection can be eliminated as potential mechanisms. Thermo-mechanical deformation is deduced to be the most likely mechanism; deformation must originate in the vicinity of the primary dendrite tips. It is proposed that dendrite deflection arises primarily from the resistance encountered by the primary tips with the external surface during axial contraction in the presence of a dominant vertical thermal gradient.

A Digital Twin Based Residual Stress Prediction Method for Turbine Blades

The investment casting process is the conventional method for manufacturing turbine blades, and the residual stress generated through this process is widely regarded as a critical factor that markedly impacts the quality and serviceability of turbine blades. However, real-time measurement of residual stress is extremely complex and expensive, especially for single-crystal turbine blades. Current experimental measurement techniques such as X-ray or neutron diffraction can only measure residual stresses at specific points or within a very small area. Studies on residual stress of turbine blades are mainly performed by finite element (FE) simulation, but the accuracy is difficult to guarantee due to the inability to establish accurate mathematical models for complex physical systems. To address this issue, this paper proposes utilizing a digital twin (DT) model to predict residual stresses in turbine blades. By combining real-time sensor and FE simulation data, the DT model is able to update and correct the mathematical models, resulting in more accurate predictions compared to traditional FE simulations.

Optimizing the electrochemical corrosion performance of laser cladded Ni-based single crystal superalloy via manipulation of γ′(Ni3Al) phase

The morphology of γ′(Ni3Al) phase is essential for the performance of laser additive manufacturing repaired Ni-based single crystal superalloy, thereby, it is of crucial significance to manipulate γ′ phase in order to optimize the corrosion performance of laser cladded component. In this research, the morphology of γ′ phase is tuned via different heat treatments. The microstructure and corrosion behaviors of laser repaired superalloy are analyzed by electron microscopy and electrochemical measurements. It is found that the electrochemical corrosion resistance of as-deposited part is superior than that of the substrate, and the dominant morphology of γ′ phase changes from spherical shape to cuboidal shape and to rafted morphology with increasing heat treatment temperature. The corrosion resistance peaks at 1050 °C, attributing to the combined effects of decreased volume fraction of γ′ phase and favored distribution of Cr, W, and Al. Our results offer a practical means for the post-repair heat treatment of single crystal turbine blades.

Effect of NaCl and Na2SO4 on Low Temperature Corrosion of Vapour- and Pack-Aluminide Coated Single Crystal Turbine Blade Alloys CMSX-4 and RR3010

The current work presents a systematic study of two alloy compositions (RR3010 and CMSX-4) and two types of coatings: inward grown (pack) and outward grown (vapour) deposited aluminides, exposed to 98Na2SO4–2NaCl mixture. Grit blasting was used on some of the samples, prior to coating, to mimic in-service procedures and remove oxides from the surface prior to coating. Two-point bend tests were then performed on the coated samples, with and without applied salt at 550 °C for 100 hours. Samples were pre-strained at 0.6 pct strain to deliberately pre-crack the coating and then strained at 0.3 pct for the heat treatment. Exposure to 98Na2SO4–2NaCl under applied stress of vapour-aluminide coated samples of both alloys, revealed significant coating damage in the form of secondary cracks in the intermetallic-rich inter-diffusion zone, although only CMSX-4 exhibited cracks propagating further into the bulk alloy while RR3010 proved more resistant. The pack-aluminide coating proved more protective for both alloys, with cracks propagating only into the coating and never into the underlying alloy. In addition, grit blasting proved beneficial in reducing spallation and cracking for both types of coating. The findings were used to propose a mechanism based on thermodynamic reactions, to explain the crack width changes through the formation of volatile AlCl3 in the cracks.

Effect of crystal orientation on elastic stresses and vibration characteristics of nickel-based single crystal turbine blade

The impact of crystal orientation on the elastic stress and vibration properties of nickel-based single-crystal turbine blades is complex and can affect their performance. By defining and describing single-crystal blade orientation by Euler angles rotation, this work presents the influence rule of crystal orientation on the elastic stress and dynamic frequency of blade through lotus leaf diagrams. The results shows that the rotations of three Euler angles can affect significantly the distribution of the region where the stress extremum occurs, and the difference between the maximum and minimum of stress is 210.11 MPa. In addition, the degree and mechanism of influence of Euler angles rotations on the several order frequencies are different. From the 1st to 3st order mode, the influence range of the crystal orientation on the dynamic frequency increases gradually, but the frequency ranges of the 3st, 4st and 5st order are roughly the same, and the frequency range of 6st order is the largest variation range. The blade is most likely to generate resonance when the second-order frequency reaches the minimum fmin2. The Euler angle β has a significant influence on the resonance margin, and the larger resonance margin can be obtained easily when the Euler angle β is large enough.

Effect of Withdrawal Rate on Solidification Microstructures of DD9 Single Crystal Turbine Blade.

Single crystal superalloys are widely used in the manufacturing of turbine blades for aero-engines due to their superior performance at high temperatures. The directional solidification process is a key technology for producing single crystal turbine blades with excellent properties. In the directional solidification process, withdrawal rate is one of the critical parameters for microstructure formation and will ultimately determine the blade's properties. In this paper, the as-cast microstructures in the typical sections of a DD9 single crystal (SX) superalloy turbine blade were investigated with 3 mm/min and 5 mm/min withdrawal rates during the directional solidification process. With increased withdrawal rate, the dendrite morphologies tended to become more refined, and the secondary dendritic arms tended to be highly developed. The dendrite in the blade aerofoil section was more refined than that in the tenon section, given the same withdrawal rate. Additionally, with increasing withdrawal rates, the size and dispersity of the γ' precipitates in the inter-dendritic (ID) regions and dendritic core (DC) tended to decrease; furthermore, the size distributions of the γ' precipitates followed a normal distribution law. Compared with the ID regions, an almost 62% reduction in the average γ' sizes was measured in the DC. Meanwhile, given the same withdrawal rate, at the blade's leading edge closest to the heater, the γ' sizes in the aerofoil section (AS) were more refined than those in the tenon section (TS). As compared with the decreasing cross-sectional areas, the increased withdrawal rates clearly brought down the γ' sizes. The sizes of the γ-γ' eutectics decreased with increasing withdrawal rates, with the γ-γ' eutectics showing both lamellar and rosette shapes.

Investigation of Hafnium Oxide Containing Zirconium in the Scaled Region on the Surface of As-Cast Nickel-Based Single Crystal Superalloy Turbine Blades

Surface scale is usually formed in the aerofoil part of as-cast nickel-based single crystal turbine blades by the strong interaction between the mould wall and the melt, and the subsequent oxidation of the fresh metallic surface of the casting. For better understanding of the scaling, the scaled region was investigated, and an interesting region containing hafnium oxides and a rhenium-rich particle was found. Generally, a continuous aluminium oxide layer was detected on the outer surface of the base material and covered the surface of an unscaled region. In contrast, there was no oxide on the surface of a scaled region, but it was replaced by several tiny particles remaining locally on the outer surface of the base material. SEM-EDX and TEM-EDX point analysis of these particles indicated not only the existence of high amounts of hafnium, but also several particles such as hafnium oxide, aluminium oxide, and even tiny metallic particles. Most of all, STEM-EDX point analysis clearly detected zirconium in the hafnium oxide. Furthermore, a rhenium-rich particle was also detected towards the outer surface of the base material, which suggested that the surface of the scaled region might be exposed to high enough temperatures to allow the diffusion of heavy alloying elements. Based on the observation, the formation mechanism of hafnium oxide containing zirconium and its meaning was discussed.

Potential eutectic accumulation in single crystal turbine blade due to geometry effect: a numerical study

Inhomogeneous distribution of the eutectic phase in single crystal (SC) turbine blades is detrimental to its mechanical properties. In a recent publication, it was reported that an accumulation of eutectics was observed on the upper surface of solidification units (e.g. shroud of a turbine blade), whereas fewer eutectics were detected at the bottom. This kind of casting defect brings a huge challenge to the production of turbine blades since the accumulated eutectics cannot be dissolved completely by heat treatment. A sample with three pairs of platforms/shrouds was designed to study the geometry effect on eutectic accumulation. In the current study, the simulation was performed by coupling ProCAST with our previously developed multiphase volume-average solidification model. In the first step, the thermal field including radiation in the furnace and heat conductivity in the casing and mould was calculated via ProCAST. Then, the obtained temperature profiles were set as thermal boundary conditions of the casting to predict the eutectic accumulation and formation of freckles. Based on the simulation results, convective plumes arising from the thermosolutal buoyancy transport solute-enriched liquid upward, which causes the solute pile-up and the final eutectic accumulation at the top surface in each platform. Freckles accompanied with eutectic accumulation was also observed at the corner of the sample. The calculated inhomogeneous distribution of the eutectics agrees well with experimental observations. Knowledge about the geometrical effect on eutectic accumulation is extended.

Digital twin for directional solidification of a single-crystal turbine blade

Single-crystal superalloy turbine blades (TBs) fabricated using directional solidification are key components of aeroengines and gas turbines. Owing to thermal–solutal convection during solidification, such components are susceptible to flow-induced defects such as freckles and/or eutectic accumulation. The formation mechanisms of the above defects are well understood, but reliable theories or empirical laws are unavailable to guide the engineering production process as the thermal–solutal convection is sensitive to the alloy, Bridgeman furnace design, shape and internal structure of the TB, withdrawal parameters, etc. This study proposes a novel method to ‘digitally twin’ the directional solidification of the TB, i.e. to utilise a physically based numerical model to quantitatively simulate the solidification process, including freckle formation and eutectic accumulation. It includes two simulations: one for the global thermal field in the Bridgeman furnace, including the casting system, and the other for the flow and solidification within the casting component. The former is modelled using ProCAST, the latter is modelled using a volume-average-based multiphase solidification model, and both are coupled. To verify the digital twin concept, an actual industrial TB with slight geometrical modification (removal of the fins while maintaining the inner surface profile) was cast in a Bridgman furnace, and the as-solidified TB was inspected for freckles. An excellent agreement between the simulation and experimental results was obtained. Typically, an actual TB features a complex inner structure (fins) that connects the front and back blades with an average wall thickness of 1.5 mm. A fresh simulation was performed for the TB with inner fins. It was observed that the inner fins of the TB along with other process conditions, such as the shadowing effect of the furnace, play an important role in freckle formation. This study demonstrates the necessity of the digital twin in future TB production.

Reliability study on the fatigue life of film cooling blades in advanced aero-engine turbines: Neglected crystal orientation uncertainty in casting

The orientation uncertainty of single-crystal turbine blades is a neglected practical engineering problem, and previous research has paid little attention to it. The work innovatively presents the random distribution regularity of the spatial orientation formed in the casting of single crystal blades and its influence on the structural reliability, which establish a foundation for the orientation design of blades. In this work, the uncertain characteristics of the spatial orientation for single crystal cast blades are mathematically quantified. Meanwhile, based on the classical material constitutive and life equations, the elastic-plastic theoretical and fatigue life evaluation models related to orientation are deduced and firstly applied to turbine blade engineering analysis. The elastic-plastic stress distributions of single-crystal turbine blades considering thermal effects are obtained by the fluid-structure interaction simulation. The spatial orientation distribution characteristics is presented from analyzing hundreds of samples. Finally, the influence of the uncertain orientation on the low cycle fatigue life reliability and structural failure probability of single-crystal turbine blades are clarified for the first time by using the BP neural network and stepwise response method. Our results demonstrate that crystal orientation uncertainty, especially in long-life design, can cause a serious failure probability, proving the necessity of orientation control for single crystal turbine blades clearly.

Deformation and fracture initiation of Ni3Al intermetallic single crystal turbine blade

The present study deals with the determination of fracture properties of a cracked Ni3Al-based intermetallic single crystal turbine blade subjected to tensile load. The analysis has been carried out by accounting the effect of microstructure, strain hardening and damage development under monotonic loading. A continuum damage criterion combined with crystal plasticity model incorporating non-Schmid effects was employed to evaluate material damage owing to highly localised deformation. The damage evolution was defined as the function of a microstructure variable. The present coupled model was used to replicate the stress-strain behaviour of Ni3Al single crystals until material softening due to damage after necking. The model parameters were ascertained by a close match of the computed data with the available experimental data. Using the model, damage evolution near crack tip of a cracked Ni3Al turbine blade was assessed for randomly varying secondary orientations, while maintaining primary orientations same as that of along the solidification direction. The analysis helped to calculate the fracture initiation toughness (Ji) of turbine blade for various crystallographic orientations. Combined by damage formulation, analysis was continued to estimate the Ji values for different crack lengths in all the cases of orientation. Finally, triaxiality quotient (q) was computed to characterise the estimated Ji values as a function of crack tip constraints. The approach provides a useful way to analyse orientation dependent fracture behaviour of single crystal components with crack under mechanical loading.

Tensile behavior of single-crystal superalloy with different structured cooling holes

Film cooling hole structures significantly influence the mechanical behavior of a single crystal turbine blade. This study investigates the tensile behavior of nickel-based single crystal plate specimens with three different structure holes, including straight, inclined, and diffusive holes, by in-situ tensile tests and crystal plasticity simulation. The results show the cooling holes play a strengthening effect on the single crystal superalloy, and the tensile strengths of the single crystal specimen with holes were increased. At the same time, fracture strains were decreased compared to those without hole. In addition, the cooling hole structure causes the difference in stress concentration and subsequent local plastic deformation around a hole, which further affects the tensile strength, fracture strain, crack initiation and propagation of the single crystal specimens. The asymmetric diffusive cooling hole induces larger resolved shear stresses and localized plastic deformation near the hole, which results in lower tensile strength and decreased fracture strain. Surface microcracks are observed along the acute angle zone for the inclined and diffusive holes, while at an angle of 65° to the loading direction for the straight hole. These orientations are consistent with the maximum accumulated plastic slip in the crystal plasticity simulations. And all the surface microcracks propagated along the direction of the dominant slip system. In addition, the non-uniform distribution of accumulated plastic slips along the hole depth causes the macrocrack initiation on the specimen surface, which propagates along <110> direction, and leads to final failure on {111} plane.