Nickel-base Superalloy Turbine Blades Research Articles

Microstructural evolution and diffusion mechanism of MCrAlY coated nickel-based superalloy turbine blades after serviced for 47,000 h

The large-sized industrial gas turbine blades exhibit non-uniform microstructural degradation in various parts due to ultra-long term service in high-temperature and high-pressure environments. Determination of degradation behavior at critical locations is important for the safety of gas turbine and blades refurbishment program. The microstructure characterization on the surface and interface of MCrAlY coated Ni-based superalloy blades have been studied. The microstructure evolution near the coating/ substrate interface was studied by scanning electron microscope (SEM) and the difference in degradation of the main precipitates at several locations were quantified. Surface oxidation and interfacial diffusion were analyzed through energy dispersive spectrum (EDS) and electron probe microanalyzer (EPMA) to clarify the influence of elemental diffusion on microstructure. The results indicated that regional microstructure evolution influenced by the spatial structural characteristics of the blades. The migration of Ni, Al, Cr, and Co dominated the diffusion mechanism during service, and reconstructed the microstructure of the coating/substrate interface and surface.

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.

Microstructure and mechanical degradation of K403 Ni-based superalloy from ultra-long-term serviced turbine blade

The effective characterization of the mechanical property degradation of turbine blades in service for an ultra-long-term had a significant impact on the service safety and maintenance decisions of turbine blades. In this study, the microstructure observation and mechanical property study of the K403 polycrystalline Ni-based superalloy turbine blade under four service times were carried out, using microstructure characterization methods to quantify the microstructure evolution of the superalloy at different parts of the turbine blade with increasing service time. The effects of the microstructure degradation of the superalloy under ultra-long-term service on the mechanical properties of the turbine blade were analyzed. The results show that the microstructural degradation characteristics of the superalloy turbine blades under ultra-long-term service were strongly correlated with the spatial structural characteristics of the turbine blades after service, and it was found that the carbon elements and other elements inside and at the boundary of the carbide showed regular migration behavior with the increase in service time. The regular influence of microstructural degradation of superalloy turbine blades under ultra-long-term service on their mechanical properties was demonstrated. These results can provide empirical support for the study of the predicted residual properties of nickel-based superalloy turbine blades under ultra-long-term service.

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.

Correction: Park, K.; Withey, P. General View of Rhenium-Rich Particles along Defect Grain Boundaries Formed in Nickel-Based Single-Crystal Superalloy Turbine Blades: Formation, Dissolution and Comparison with Other Phases. Crystals 2021, 11, 1201

There was an error in the original publication [...]

Compositions of Gamma and Gamma Prime Phases in an As-Cast Nickel-Based Single Crystal Superalloy Turbine Blade

The core and the interdendritic regions of an as-cast nickel based single crystal turbine blade were observed by electron microscopy to understand the microstructural development during an investment casting process. The dendrite core region shows an irregular morphology of gamma prime in gamma due to a relatively short casting time, which prevented the development of gamma prime expected in a solution heat-treated microstructure. By comparison, the interdendritic region comprises three different regions composed of: several elongated gamma prime particles, relatively tiny and irregular gamma prime, and gamma prime with relatively regular morphology. The chemical analysis of these phases showed that, regardless of the analysis point in the core or the interdendritic region, almost the same compositions were acquired in the regular type of gamma and gamma prime phases. This result suggests that if the gamma prime forms in the gamma matrix, the composition of gamma prime is almost uniform regardless of the region and prevailing general chemical composition. In contrast, the composition of the elongated gamma prime in the interdendritic region was slightly different depending on the analysis point even within the same elongated particle.

Additive manufacturing of low-shrinkage alumina cores for single-crystal nickel-based superalloy turbine blade casting

We prepare bimodal particle size photo-curable ceramic pastes with high solid loadings (up to 65 vol %) and fabricate porous alumina ceramic cores with complex shapes via ceramic stereolithography (Cer-SLA) 3D printing technique. The sintering temperature is carefully selected, ranging from 1500 °C to 1650 °C, and a high holding time (>4 h) is applied to guarantee that the materials can withstand the subsequent high temperature (>1500 °C) casting process for single-crystal nickel-based superalloy hollow turbine blades. Herein, the originally spherical fine particles are found to become platelet-like after sintering, and the forming mechanism is discussed in detail. In addition, we explore the influence of platelet-like particles, coarse particles and sintering process on the microstructural evolution of alumina particles, and reveal the relationship between microstructure and properties of ceramic cores. These results illustrate that the proposed materials for SLA 3D printing exhibit a great potential in the fabrication of complex-shaped alumina ceramic cores for high-precision investment casting, e.g., manufacturing single-crystal nickel-based superalloy hollow turbine blades for an advanced aircraft engine.

General View of Rhenium-Rich Particles along Defect Grain Boundaries Formed in Nickel-Based Single-Crystal Superalloy Turbine Blades: Formation, Dissolution and Comparison with Other Phases

Nickel-based single-crystal superalloy turbine blades have been widely used in engines of aircrafts or power plants, but some defect grains are often found on the surface of the blade after full heat treatment or even after casting. Rhenium-rich particles, as well as an intermediate layer, were almost always detected along any defect grain boundary region, if it existed, from a low-angle grain boundary to a high-angle boundary. The particles were also found on the top surface of the base material. The composition and morphology of the particles were different from the most common topologically close packed phases or a fine particle with similar morphology detected at the boundary region between a recrystallized grain and a matrix grain. An additional heat treatment could completely dissolve the fine particles. Furthermore, any rhenium-rich particles were not re-formed after achieving uniform distribution of the alloying elements.

Study on passive compliance control in robotic belt grinding of nickel-based superalloy blade

To overcome the effect of fluctuation of normal grinding force on belt precision grinding of nickel-based superalloy blade with the robot, a novel passive compliance control method is proposed in this work. A passive compliance grinding device was designed to reduce the fluctuation of robotic normal grinding force, and the mechanical modeling and analysis of this device were performed. In consideration of the effect of elastic deformation of contact wheel, inherent accuracy error of robot, non-uniform allowance distribution and weak rigidity of blade, the control model of the robotic normal grinding force was established to maintain its stability. The experimental results revealed that the presented control method was beneficial to stabilize the normal grinding force in robotic grinding of nickel-based superalloy blade, and control accuracy of normal grinding force with this proposed method increased by 64.81% than that without the control method. This control method significantly improved the surface profile accuracy of nickel-based superalloy turbine blade to 0.10309 mm. Furthermore, the mathematical model of residual height based on the proposed control method was obtained to explain the relationship between residual height and machined surface roughness of concave surface and convex surface.

The whole fatigue crack propagation life prediction of Ni-based single crystal super-alloy specimen with single hole based on EIFS and ‘Fish-eye’ theory

In this work, based on the ‘Fish-eye’ theory and EIFS method, a prediction model of the whole fatigue crack propagation life of the predictable structure under multi-axial stress fatigue is proposed. Then, the fatigue crack propagation life of single-hole nickel-based single crystal superalloy samples under nine working conditions was predicted and verified by finite element simulation. Finally, the effects of three loads and three initial crack lengths on the fatigue crack propagation life of single-hole nickel-based single crystal superalloy were discussed by finite element simulation. The results show that the analysis results of the theoretical model and the finite element model proposed in this work are basically consistent under certain conditions. It is found that the fatigue crack propagation life of the initial crack stage, short crack stage and long crack stage accounts for 2%, 6%–10% and 90% of the whole crack propagation fatigue crack propagation life, respectively. Under the same loading stress condition, the higher the proportion of short crack stage in crack propagation is, the faster the fracture speed of the sample is. At the same time, the progressive crack propagation process of single hole specimen under fatigue load can be seen through the calculation results of finite element model. The above conclusions provide a good theoretical reference for the design and fatigue crack propagation life prediction of nickel-based single crystal superalloy turbine blades with cooling film holes.

Design and optimization of a closed die forging of nickel-based superalloy turbine blade

The nickel-based superalloys belong to widely used materials for most demanding industrial applications. The design and the experimental verification of manufacturing technology of NIMONIC 80A turbine blade is presented in this paper. A finite element (FEM) simulation was exploited for the closed die forging technology optimization. Based on the precision material model and boundary conditions, the deformation behaviour in the range of hot working temperatures was studied. The process conditions including the strain rates were preset according to the industrial scale practise. Based on the FEM simulation results the necessary tools were manufactured and the experimental closed die forging of turbine blades was performed. Subsequently, a heat treatment of forged blades was carried out. The minimum of 1300 MPa tensile strength was achieved. A metallographic survey was carried out to verify the structure homogenity.

Development of Numerical Simulation in Nickel-Based Superalloy Turbine Blade Directional Solidification

Development of Numerical Simulation in Nickel-Based Superalloy Turbine Blade Directional Solidification

Thermomechanical fatigue experiment and failure analysis on a nickel-based superalloy turbine blade

Thermomechanical fatigue (TMF) experiment with dwell times on a nickel-based superalloy turbine blade was conducted. The strain field and temperature field of the test section were simulated well through adjusting gripping fixture and induction coil. In addition to the loading, heating, cooling and control subsystems, high temperature strain measurement subsystem was introduced into the experiment to ensure the accuracy of the simulation results of service condition for the test section. In the whole experiment, the service condition of the test section was reproduced. The experiment result showed that TMF crack initiated at the trailing edge of the test section and propagated along the leading edge direction. Using the scanning electron microscope (SEM), multiple crack sources at the surface of turbine blade were observed and the crack surface was oxidized seriously. In addition, based on the fractographic and metallographic observation, the mixed features of intergranular fracture and transgranular fracture were found. The interaction of oxidation, creep damage and fatigue damage is an important reason for the TMF failure of the turbine blade.

Effect of secondary crystal orientations on the deformation anisotropy for nickel-based single-crystal plate with notch feature

The effects of the secondary crystal orientations on the nickel-based single-crystal superalloy turbine blades were investigated. The stress concentration features were used for investigation of the optimal secondary crystal orientation leading to the higher strength of the single-crystal turbine blades. The crystal plastic finite element method coupled with micromechanics constitutive model is applied to study the effect of secondary crystal orientation on plastic deformation and mechanical behavior around the cooling holes and notches with the primary (load) orientation fixed at [001] direction. For nickel-based superalloy plates with holes or notches, the secondary crystal orientation effect on the strength needs to be clarified at various load levels. The maximum von Mises stress in the single-crystal alloy varies significantly with variation in the secondary crystal orientations. It was found that only two slip systems dominate the deformation process of the material owing to their favorable orientation with loading. The secondary orientation of 45° was identified with lowest resolved shear stress in the dominating slip systems and potential of producing higher strength for single-crystal turbine blades.

Formation of Secondary Phases in the Boundary between Surface Defect Grains and Matrix in Third Generation Nickel-Based Single Crystal Superalloy Turbine Blades

Ni-based single crystal superalloy turbine blades have excellent mechanical strength and resistance to corrosion and oxidation due to a uniformly distributed gamma prime phase in a gamma matrix. However, defect grains have been often found on the surface of turbine blades after manufacturing, which can be potential sites of crack initiation. In this study, several different types of surface defect grains formed in third generation Ni-based single crystal turbine blades, such as stray grains, freckle chain grains, equiax grains, and a new grain formed in surface scale, had been investigated. The grain boundary regions were observed by high resolution electron microscopy. Although the formation mechanism of each grain defect is different, secondary phases, such as rhenium-rich particles, have been always found in each grain boundary. In addition, depending on the existence of the secondary phases as well as the size of defect grains, different microstructures were observed even in the same defect grain boundary. Finally, the observed results suggest that if there is any boundary region in a turbine blade, secondary phases, such as Re-rich particles, can be found.

Investigation of Microstructure Properties and Quantitative Metallography by Different Etchants in the Service-Exposed Nickel-Based Superalloy Turbine Blade

In this study, the effect of etchant type and etching conditions on the root and airfoil microstructure of a service-exposed IN738 turbine blade has been investigated. The microstructure of superalloy components used at high temperatures, in addition to the usual microstructural changes, experiences deterioration in micrometer dimensions. In order to investigate these changes, electrochemical etching was performed on the samples with the chemical solution including 80% phosphoric acid, solution containing Cr2O3 and 55% glycerol. Chemical etching was performed with marble and etchant solution containing 60% glycerol. The results in terms of specifying the deterioration effects on microstructure of the blade applied at high temperature, the amount of γ′ phase and the best etchant were investigated. Among the solutions used for chemical etching, the solution containing 10 ml HNO3, 50 ml HCl and 60 ml glycerol was appropriate for detection of segregations and dendrites, and among the electrochemical etching solutions, the Cr2O3 solution was found suitable for specifying γ′ precipitates’ morphology by scanning electron microscopy. In this research, the results of the quantitative analysis of the images provided by these etchants were also investigated.

Response to Discussion of \u201cFormation of an Intermediate Layer Between Grains in Nickel-Based Superalloy Turbine Blades\u201d

Response to Discussion of \u201cFormation of an Intermediate Layer Between Grains in Nickel-Based Superalloy Turbine Blades\u201d

Formation of an Intermediate Layer Between Grains in Nickel-Based Superalloy Turbine Blades

The boundary region formed on the surface of nickel-based single-crystal turbine blades was investigated by high-resolution microscopy observation. There was a distinguishable intermediate layer with the size of about 2 to 5 μm between the matrix and surface defect grains such as stray grains, multiple grains, freckle grains, and even low-angle grain boundaries which were formed during the solidification of turbine blades. The intermediate layer was composed of many elongated γ′ as well as γ phases. In addition, only one side of the intermediate layer was coherent to the matrix grain or defect grain due to good orientation match. At the coherent interface, the γ′ (as well as γ) phase started to extend from the parent grain and coincidently, rhenium-rich particles were detected. Furthermore, the particles existed within both elongated gamma prime and gamma phases, and even at their boundary. Based on experimental observations, the formation mechanism of this intermediate layer was discussed.

Investigation on microstructure and mechanical properties on varying surface region of a service-exposed IN738 turbine blade

The study investigates the microstructural transformation and mechanical properties of nickel-based IN738 super-alloy turbine blade after service exposure of 52,000 h at temperature of ±720°C for the degradations and stress rupture profile. The grain boundary carbides, decomposition and the transformation of MC carbides (gamma prime (γ′)) and creep cavities are analysed. The growth of γ′ precipitates in the microstructure was investigated using transmission electron microscopy. It was observed that gamma prime (γ′) possesses high coarsening at inter-dendritic area than in the dendrite core. Thus implies a contributing factor in the interface between the carbides, grain boundaries and the gamma prime (γ′) films preferential location for nucleation and propagation of cracks and creep cavities. The coarsening behaviour of γ′ precipitates adhere with the Ostwald ripening. The average hardness values of the inside and outside portion of the turbine blade was 325 and 298 HV10, respectively.

Detection of Rhenium-Rich Particles at Grain Boundaries in Nickel-Base Superalloy Turbine Blades

Detection of Rhenium-Rich Particles at Grain Boundaries in Nickel-Base Superalloy Turbine Blades