Nickel-based Superalloy Research Articles

Characterisation of the creep cavitation process on grain boundaries in a polycrystalline nickel-base alloy 247

ABSTRACT Hot gas components such as gas turbine blades are loaded with constant centrifugal stress at high temperatures and thus the formation of creep cavitation on grain boundaries is expedited. Conventionally cast polycrystalline Nickel-base superalloys used as blade materials are prone to creep cavitation as unfavourably orientated grain boundaries exist. This research presents a diffusion-based probabilistic creep model which describes the creep cavitation process on grain boundaries. It includes the three mechanisms: pore nucleation, growth, and coalescence. The calibration of the model has been carried out by analysing Alloy 247 as-received specimens and specimens with pre-strain. For the evaluation of the pore numbers and sizes, a deep learning model for pore detection was trained on light microscopic and scanning electron microscopy images. Electron backscatter diffraction images are analysed for further investigations regarding grain orientations and grain boundary angles to the loading direction. The calibrated model allows predictions of pore size distributions over time.

Porosity suppression of nickel-based superalloy by modulated base temperature in laser welding and mechanism analysis

This paper investigates the distribution pattern of welding porosity defects in annular weld seams of superalloy during laser welding. The results reveal that the initial portion of the annular weld seam exhibits a higher density and larger size of porosity defects compared to the ending portion. At a laser power of P = 360 W, the numerical simulation combination showed that the A1 region's average cooling rate is 8.767 × 103 °C/s and the A2 region is 8.151 × 103 °C/s that the thermal conduction effect at the initial welding position leads to an elevation in the base metal temperature in the subsequent unwelded region, thereby reducing the solidification rate of the molten pool in the latter half of the weld seam and effectively decreasing the porosity rate at the weld seam. Furthermore, the study demonstrates that elevating the base metal temperature can effectively modify the cooling rate of the molten pool, thereby influencing the formation of porosity defects. At a laser power of P = 360 W, increasing the base metal temperature to 600 °C reduces the cooling rate of the molten pool from 8.767 × 103 °C/s at a base metal temperature of T = 20 °C to 7.451 × 103 °C/s, leading to a decrease in the porosity rate from 3.357% to 0.022%. At a laser power of P = 672 W, increasing the base metal temperature to 600 °C reduces the cooling rate of the molten pool from 6.781 × 103 °C/s at a base metal temperature of T = 20 °C to 5.056 × 103 °C/s, leading to a decrease in the porosity rate from 8.214% to 0.002%. The increase in laser power brings more heat input, so the solidification rate of the molten pool decreases, but the porosity increases significantly. However, increasing the base temperature can effectively suppress the porosity defects under different laser powers. The research further reveals the relationship between welding porosity rate and cooling conditions, providing a control strategy for achieving low porosity rate welds.

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.

Tailoring the microstructure and deformation mechanisms of an additively manufactured nickel-based superalloy by post-aging heat treatment

Laser powder bed fusion (LPBF) is a new method for preparing the nickel-based superalloy hot-end components. This study systematically investigated the microstructural characteristics and mechanical performance of a γ′-strengthened Haynes 230 alloy after solution heat treatment (SHT) and aging heat treatment (AHT). The results indicate that γ′ phases continue to grow as the AHT temperature increases. The volume fraction of γ′ phases increases first and then decreases. The maximum γ′ phase fraction can be achieved at 750 °C. The yield strength (YS) of the AHT samples at 25 °C and 800 °C increases first and then decreases, which is mainly caused by γ′ phase fraction evolution. The SHT+750 sample presents an excellent integration of tensile performance at 25 °C and 800 °C. The primary deformation mode at 25 °C is stacking fault (SF) shearing. The δ precipitation and high-density Lomer-Cottrell (L-C) locks are the primary reasons for the higher work hardening rate of the SHT+850 sample. The transition of the dominant deformation mechanism from SF shearing at 25 °C to Orowan bypassing at 800 °C leads to a dramatic reduction in the YS of the SHT+850 sample.

Wear behaviour of abrasive tools and its effect on surface integrity during polishing superalloy GH4169G with abrasive cloth flap wheels

In polishing nickel-based superalloys, the surface quality and dimensional accuracy of the workpiece are closely related to the wear of the abrasive tools. Over the years, several researchers have studied the wear of grinding wheels or abrasive belts, including the effects of various machining methods, abrasive materials, and machining parameters on the wear of abrasive tools. However, there is not enough research on the wear mechanism of the abrasive cloth flap wheel (ACFW). In this paper, during polishing GH4169G with ACFW, the wear evolution mechanism of ACFW and its effect on the surface integrity of GH4169G were investigated. First, the variations in material removal and ACFW wear amounts during the polishing process were analysed, and the different wear stages were divided based on the polishing ratio. Second, the mic-morphology and surface element distribution of ACFW at different wear stages were investigated using SEM and, EDS, and the wear evolution mechanism of ACFW was analysed. Finally, the effects of ACFW wear on the surface morphology, surface oxidation and microstructure of GH4169G were investigated by SEM, EDS, XRD, and EBSD. The results showed that the primary wear forms of ACFW were stepped fracture, cleavage fracture, blunt, pull-out, and adhesion wear. During the initial wear stage, the proportion of each wear form was relatively balanced. In the medium wear stage, fracture wear was the main wear form. Blunt and adhesive wear were the primary forms of wear during the final wear stage. In addition, with ACFW wear, the surface roughness tend to decrease first and then increase, however, the refinement level of the grains, the intensity of plastic deformation and the intensity of the oxidation reaction gradually increased. This study provides effective guidance for developing more wear-resistant abrasive tools and optimising the grinding and polishing processes.

A feasibility study on phase characterisation of nickel-based superalloys using ultrasound

The use of nickel-based superalloys, such as Inconel 718, Udimet 720Li and Waspaloy, is widespread across the engineering industry due to their superior mechanical properties at elevated temperatures and excellent resistance to creep and fatigue under harsh working conditions. The microstructure of these materials, such as the precipitates and grain sizes, strongly influence the materials’ performance. Currently, existing materials characterisation methods are time-consuming, costly and destructive, as well as only capable of taking measurements of surface or near-surface properties.This paper presents a novel ultrasonic technique for bulk materials characterisation of nickel-based superalloys, based on a spherical harmonic convolution theory which has been successfully used to quantify texture in various metals; it is applied here to study a nickel-based superalloy, with a specific focus on the phase changes when subject to thermal loading. The key parameter used to interpret phase information of a material is the zeroth spherical harmonic coefficient of ultrasonic wave velocities, V00, which corresponds to the average velocities across all directions in 3D space and is independent of crystallographic texture. Experimental investigation using samples of a representative material, Inconel 718, with contrasting microstructures, has shown a clear and quantifiable sensitivity of this coefficient to microstructural changes occurring within the material, and has thus laid the foundations for the development of a rapid phase characterisation technique for nickel-based superalloys using ultrasound.

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.

New insights on dislocation forming mechanism of nickel-based superalloy fabricated by laser powder bed fusion

In this work, the forming mechanism of dislocation in laser powder bed fusion (LPBF) built IN738LC alloy was elucidated by studying the intrinsic relationships between the density and distribution of dislocation and the orientation and size of dendrite. The substructures with different dislocation densities and cell spacings were customized by controlling solidification conditions, and analyzed by multi-scale characterizing methods. The results show that an increase in geometrically necessary dislocation (GND) density from 1.48 × 1014 to 1.82 × 1014m−2 is accompanied by a decrease in cell spacing from 1046 nm to 640 nm as the solidification rate increases. However, the relationship between GND density and cell spacing doesn't follow Ashby's model. This indicates that GNDs are not entirely generated by thermal strain itself. On the other hand, the characterization results of cell structures show that a number of misfit dislocations are formed by the thermal strain induced deflection growth of the dendrites. These prove that the dislocations are comprehensively generated by thermal strain itself and dendritic deflection growth.

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.

Hot deformation behaviors and microstructure evolution of a supersaturated nickel-based superalloy

The hot deformation behaviors and microstructure evolution of a supersaturated Ni-based superalloy GH4742 were investigated by isothermal compression tests. The deformation temperature range encompassed γ' sub-solvus (1000–1075 °C) and super-solvus (1100–1150 °C), with strain rates ranging from 0.001 s−1 to 1 s−1. Separate Arrhenius-type constitutive equations were established considering the re-precipitation of the γ' phase. The correlation coefficient between the calculated stresses and the experimental data was as high as 0.997, which indicated that the rheological behaviors of the material during hot deformation processes could be accurately analyzed by the constitutive relationship. Additionally, the activation energies for the γ + γ' two-phase region and the γ single region were calculated to be 949.08 kJ.mol−1 and 437.66 kJ.mol−1, respectively. Microstructural analysis revealed that the partial dynamic recrystallisation (DRX) occurred in the γ' sub-solvus regime whereas the full DRX occurred in the γ' super-solvus regime. The low DRX fractions in the γ' sub-solvus regime were attributed to the presence of near-spherical re-precipitated γ' phase, which effectively suppresses the nucleation and growth of new grains. The deformed grains were oriented in the <101> direction parallel to the compression direction. Meanwhile, the non-monotonic relationship between the degrees of DRX and strain rate was observed due to the adiabatic temperature rise. The retarding effects of solute elements on the grain growth in the γ' super-solvus regime were determined by comparing the recrystallized grain sizes from theoretical analysis and experiment values. This work can provide guidance for understanding the deformation behaviors of the alloy and achieving desired microstructures.

Controlling of microstructures and mechanical properties based on the non-equilibrium microstructures of a nickel-based superalloy fabricated by laser powder bed fusion

In this paper, we propose a novel idea of controlling microstructure and mechanical properties based on the non-equilibrium microstructure of nickel-based superalloy fabricated by laser powder bed fusion (LPBF). The network microstructures were firstly built in LPBF nickel-based superalloys by direct aging (DA) heat treatment. Thermodynamic calculations were conducted to design the DA and homogenizing + DA (HDA) heat treatments for elucidating the control mechanism. The results showed that the non-equilibrium microstructures in as-built nickel-based superalloy contributed to the network microstructure formation and recrystallization during DA heat treatment. The grains were refined and the <001> texture was eliminated after DA heat treatment. The non-equilibrium microstructures were removed and the recrystallized grains tended to coarsen after HDA heat treatment due to high temperature homogenizing treatment. The in-situ nano γ′ phase uniformly distributed in the grains of HAD-treated microstructures, which was also found inside network microstructures of DA-treated microstructures. Due to the additional strengthening effect of refined grains and network microstructures, the strength of DA-treated sample was higher than those of as-built and HAD-treated samples, while the ductility was lower than that of as-built sample, and but was close to that of HAD-treated sample. This study will provide guidance for developing a suitable heat treatment process of LPBF nickel-based superalloys.

Investigation on Mechanical Properties of MAR-M247 Superalloy for Turbine Blades by Experiment and Simulation

This study aimed to investigate the metallographic structure and the impact of the heat treatment process on the MAR-M247 superalloy, a high-temperature nickel-based superalloy commonly used in turbine blades. The heat treatment process can potentially influence the mechanical properties of the MAR-M247 superalloy at different temperatures. A strength simulation analysis of gas turbine blades should include the variations in the mechanical properties of the material. The effect of heat treatment on grain size was investigated by metallographic experiments, and numerical calculations of material mechanical properties were conducted. The mechanical property parameters necessary for finite element analysis of turbine blades were determined. Finally, a finite element simulation model of the blade was established based on these mechanical property parameters, and strength analysis was performed. The simulation results provided the stress distribution and the strength of the turbine blade.

Fatigue Analysis of the Nickel-Based Superalloy Inconel 617 by Fatigue Experiments and EBSD Data-Based Finite Element Simulations in Correlation with E·m Theories

Nickel-based superalloys exhibit pronounced elastic anisotropy and, hence, the local grain orientation strongly affects the stress and strain distribution in the material under mechanical loadings. Therefore, the crack initiation and failure behaviour of components made from nickel-based superalloys are complex and hardly predictable. A better fundamental understanding of the phenomena that occur in nickel-based superalloys under a quasistatic and cyclic load is therefore desired. Previously, a continuum mechanics-based model has been successfully developed, considering the grain structure, the elastic anisotropy, and the Schmid factor, based on data from electron backscatter diffraction (EBSD). The E·m model was confirmed by the finite element method (FEM) simulations and experimental observations regarding the resulting average stresses and strains in the individual grains as well as the formation of slip bands under a quasistatic load with few restrictions. The behaviour under cyclic loadings has been investigated in this work to correlate the mechanical behaviour, simulated by the previously developed FE models, with the local stiffness and Schmid factors considering fatigue failure. For this purpose, the fatigue behaviour of Inconel 617 samples was characterised up to the high-cycle fatigue (HCF) regime, accompanied by EBSD measurements for stress amplitudes that resulted in strains close to the elastic–plastic regime. The EBSD data were used to create digital twins of the samples to simulate the mechanical reaction to a displacement similar to the associated strain of the fatigue tests. An analysis of the fractured samples by scanning electron microscopy was performed to retrace the location of the crack initiation supported by the EBSD measurements before and after fatigue testing. Two samples were investigated in detail that showed different fracture types. Sample 1 showed transcrystalline failure in a grain that showed a high Young’s modulus, Schmid factor, and resolved shear stress that indicates a failure due to the properties of the grain itself. In contrast, an intercrystalline failure was observed for sample 2 that showed large differences in the orientation and, hence, largely different mechanical properties in the area of failure as well. The observed failure types, the resulting stresses and strains calculated by the FE model, and the consideration of the E·m model showed an agreement of all the methods. Therefore, the findings of this work complement previous investigations of the mechanical behaviour of coarse-grained anisotropic nickel-based superalloys with a focus on the orientations of the grains towards the loading direction.

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.

Effect of interrupted cooling on microstructural evolution and microhardness of FGH97 nickel-based PM superalloy

Interrupted cooling experiments of FGH97 nickel-based powder metallurgy (PM) superalloy with different cooling rates were carried out using a DIL805A quenching dilatometer. The microstructural evolution of the γ′ precipitates during cooling after solution heat-treatment was investigated by means of field emission scanning electron microscopy (FE-SEM), energy dispersive spectrometer (EDS), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Vickers microhardness tests were performed on the samples that were interrupted at different temperatures to establish a relationship between microstructure and mechanical properties. The results revealed that the secondary γ′ precipitates preferentially nucleated at a higher temperature, and gradually grew during the cooling stage. Meanwhile, the morphology of the secondary γ′ precipitates transitioned from spherical to cubic, to rounded cuboidal, to a butterfly-like shape, and ultimately splitting into octo-cubes, whose driving force was attributed to the mutual competition between surface energy and elastic strain energy. During the initial stages of the cooling process, the microhardness decreased due to the growth and coarsening of the secondary γ′ precipitates. As the temperature decreased, the supersaturation within the matrix became sufficient to overcome the energy barrier for a second burst of nucleation, the tertiary γ′ phases precipitated, resulting in an increase in microhardness.

Process parameter optimization for laser powder directed energy deposition of Inconel 738LC

Processing nickel-based superalloys is one of the major challenges in the additive manufacturing (AM) field. Nickel-based superalloys consist of several alloying elements, which make them prone to the formation of undesired phases and cracking due to the complex thermodynamics during the metal AM processes. In order to produce defect-free parts from nickel-based superalloys via metal AM processes, optimization of process parameters is required. In this study, the effect of laser powder directed energy deposition (LP-DED) process parameters on the geometrical properties and porosity ratio of Inconel 738 LC (low carbon) is investigated. Geometrical specifications, including the track height, width, and wetting angle are the major factors that indicate the quality of the final product. The analysis of variance (ANOVA) and response surface methodology (RSM) were developed in order to predict the geometrical specifications of single tracks from the key process parameters, namely laser power, scan speed, and powder feed rate. An optimum set of process parameters was obtained through multi-objective optimization by minimizing the porosity ratio, crack formation and maximizing the deposition rate. The results were verified and a good agreement with the experimental measurements was achieved. The produced bulk sample showed less than one percent porosity content, which indicates the effectiveness of single-track optimization prior to bulk sample production. The overall microstructure of the bulk sample showed a high content of MC carbides resulting from the abundance of alloying elements.

The precipitation behavior of η phase with lamellar morphology in a newly designed K439B nickel-based superalloy

The microstructure of nickel-based superalloys significantly influences their mechanical properties and performance. The precipitation behavior of η phase during heat treatment of a newly designed K439B superalloy was studied. The results demonstrate that the precipitation temperature of η phase in this alloy varies from 1040 to 1070 °C. A detailed analysis of the microstructure indicates that the η phase exhibits a ∼10 nm lamellar structure, unlike the needle-like form reported in previous research. The γ' phase is distributed on both sides of the η phase, and the Ti content in the η phase is higher than that in the γ' phase, although the content of Al and Ti in the η and γ' phases are higher than that in the matrix γ phase. Moreover, the γ′ phase demonstrates an orientation relationship of {111}γ||{0001}η and [110]γ/γ′||[11—20]η with the η phase. This study can be used as reference for heat treatment process design and optimization.

Investigation of microstructural evolution and mechanical properties for in-service nickel-based superalloy

In this study, the microstructural evolutions are systematically investigated in the IN718 alloy that has been in-service for eight years. Correspondingly, the effect of in-service process on mechanical properties are explored in terms of tensile properties, fatigue performance and crack growth behavior. Results show that the grain sizes at different sampling locations vary from 9.8 to 15.5 μm, exhibiting the inhomogeneous microstructures for the in-service IN718 alloy. The volume fraction of δ phase and kernel average misorientation (KAM) at outermost layer are the maximum ones, followed by the middle and innermost layers. In addition, the high magnitude of δ phase is caused by the transformation from strengthening phase γ′′. From the perspective of mechanical properties, the in-service IN718 alloy can maintain excellent yield strength and elongation at the high temperature of 650 °C, while the tensile performance is degenerated at room temperature (RT) compared with those of new IN718 alloy. The fatigue life at 650 °C is similar to that at RT under the stress levels lower than 600 MPa. As the stress level increases to 800 MPa, an obvious decrease in fatigue life at 650 °C compared with that at RT. This special fatigue performance is revealed based on the fractography examinations under different loading conditions. Moreover, the investigation of fatigue crack growth for the in-service IN718 alloy is carried out under different stress ratio and temperatures.

Temperature dependence tensile behaviors of additively manufactured GH4099 Ni-based superalloy

Additive manufacturing was currently widely used to fabricate complex-shaped superalloy components for extreme environment applications. To replace the components prepared by the traditional approaches, it is crucial to ensure the reliability of AMed superalloys, especially in a thermal-stress environment. In this work, the high-temperature tensile properties of AMed GH4099 nickel-based superalloy are studied to establish the relationships among deformation temperatures, mechanisms, and failure responses. The results show that the deformation mechanisms at 600 °C are mainly cross-slip of the slip bands and arrays, which induced a ductile fracture. Increasing the tensile temperature will cause a shift of the dislocation movement from cross-slip to Orowan looping and stacking fault shearing to dislocation entangles-recovery-recrystallization, and the fracture mode transformed from pure ductile to mixed ductile-brittle. Additionally, the neighboring grain boundaries and the carbide-grain boundary bondings will also change with the temperature, which is further linked to the mediate-temperature brittleness and high-temperature abnormal plasticity of this superalloy.

Hot Deformation Characteristics of GH4975 Nickel-Based Superalloy in the Coexistence Region of γ and γ′ Phases

Hot Deformation Characteristics of GH4975 Nickel-Based Superalloy in the Coexistence Region of γ and γ′ Phases