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Nickel-based superalloy GH4169 has gained significant prominence in aerospace and power-generation sectors due to its exceptional resistance to fatigue, creep and corrosion at high temperatures. However, wear on the contact surface remains a concern during its operational lifespan, as it can lead to crack initiation and reduce the service life of the alloy. Therefore, this study aims to comprehensively investigate the dry-sliding wear behaviors of the GH4169 superalloy under various conditions, including temperatures, applied loads and sliding speeds via a ball-on-disc tribometer. The results obtained from the study indicate that the wear rate of the GH4169 superalloy is initially high at room temperature. However, as the temperature increases to 525 °C, the wear rate significantly decreases. Subsequently, with further temperature increase to 650 °C, there is a slight rise in the wear rate. However, the findings regarding the friction coefficient present a conflicting trend compared to the wear rate. Under the same testing conditions, the wear rate of the GH4169 superalloy at 525 °C and 650 °C demonstrates a significant reduction of 21% (0.0774 × 10–4 mm3/N.m) and 19% (0.0785 × 10–4 mm3/N.m), respectively; when compared to room temperature (0.0935 × 10–4 mm3/N.m). Besides, at room temperature, the wear track depths measure approximately 34 ± 2 µm and 55 ± 3 µm for applied loads of 20 N and 60 N, respectively. These values indicate a significant wear depth under ambient temperature conditions. On the other hand, at 525 °C, the wear track depths are slightly lower compared to those at RT. The measured depths at 525 °C amount to approximately 25 ± 2 µm and 38 ± 2 µm for applied loads of 20 N and 60 N, respectively. At high temperatures and low applied loads, the wear mechanism of the GH4169 superalloy is mainly abrasion. However, at low temperatures and high applied loads, the wear mechanism becomes more complex, involving abrasion as well as oxidation, delamination, softening and melting. This study offers valuable insights into how different wear testing parameters impact the tribological characteristics of the GH4169 superalloy. [ABSTRACT FROM AUTHOR]

The interference between grits and workpiece material during the nickel-based superalloy robot abrasive belt grinding process can cause plastic deformation on the grinding surface, and the evolution of surface deformation microstructure under high-temperature conditions can affect the surface properties of superalloys. Therefore, a high-temperature exposure experiment is devised in this paper, and electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) are used as characterization methods to study the microstructure evolution characteristics and recrystallization behavior of nickel-based superalloy robot abrasive belt grinding surface under high-temperature exposure. In addition, the effects of grinding factors (abrasive belt grit size and normal contact force) on high-temperature exposure recrystallization of grinding surface are explored. The research results indicate that under high-temperature exposure, the surface deformation microstructure consumes dislocations and releases stored energy, evolving into fine equiaxed recrystallized grains within the original grains through subgrain growth nucleation and grain boundary migration. Meanwhile, the recrystallization process is accompanied by twin formation. The recrystallization process is affected by the exposure temperature and time, and exposure temperature has a decisive influence on recrystallization. Furthermore, both the abrasive belt grit size and normal contact force have a profound influence on the recrystallized grain size and the recrystallization area depth. [ABSTRACT FROM AUTHOR]

Precipitation of the secondary phase is a key factor that influences the strengthening effect of nickel-based superalloy. The precipitation behaviours of the M23C6 carbide and γ′-Ni3(Ti, Al) phase in Inconel 617B alloy aged at 700°C for different times were studied by scanning electron microscopy, transmission electron microscopy and atom probe tomography. Plate-like carbide nucleated in the interior of grain and irregular shape carbide precipitated at grain boundary respectively during the early stage of ageing. γ′ particle co-precipitated around carbide is a common microstructure feature during ageing for 5–160 h. After ageing for 160 h, γ′ phase is the main secondary phase. The carbide only distributes on the grain boundary, high density γ′ particle homogeneously distributes in the matrix after a longer ageing time. Elemental partitioning to the secondary phases and composition profiles across different phases was quantitatively analysed. Enrichments of Ti and Al at the carbide/γ interface are directly observed, which act as the nucleation site of γ′ particles during ageing. Enrichments of B, Si and Co at different interfaces are also detected. Based on the experimental results, the precipitation mechanisms of γ′ particles and carbide were discussed. [ABSTRACT FROM AUTHOR]

High and very high cycle fatigue tests were performed to examine the microstructure and fracture mechanism of GH4169 superalloy in combination with techniques including electron‐backscatter diffraction (EBSD). Fractographic analysis revealed that surface failures are induced by surface flaws, whereas internal failures are caused by pores, facets, and inclusions. The three‐dimensional observation shows that fracture surfaces exhibit an irregular texture due to crystallographic mismatch of grains and plastic deformation at the crack tip. Based on EBSD analysis, Euler angles exhibited a complex geometry of grain orientation at the crack tip area, hindering crack propagation as evidenced by lower values of the Schmid factor and misorientation at the crack tip. Furthermore, the threshold values of small and long cracks decrease, whereas the transformation sizes from small to long crack growth increase from surface to internal failure. Finally, a novel microstructure defect‐based life prediction model is established, and the predicted results demonstrate a close resemblance to experimental outcomes. [ABSTRACT FROM AUTHOR]

Corrosion exposure study was conducted on the commercial nickel-based K411 superalloy in a simulated gas turbine operating environment (air + 2 vol% SO2) at 900 °C up to 2000 h. The corrosion behavior of the alloy was quantificationally analyzed from both morphological and chemical points with SEM, XRD, EDS, and EPMA. The results show that the formation of fine TiN inside the oxide layer can be strongly accelerated with the introduction of SO2. Sulfide is assumed as diffusion channels for gas molecules that accelerate internal nitridation. Large-volume variation caused by the TiN formation leads to a stress gradient, which induces Cr and Ni elements from inside to the surface of the alloy. The oxide scales release the compressive stresses generated by internal nitridation through forming protrusions on the surface, which is a potential risk for alloy failure. The corrosion behavior of K411 superalloy is controlled by a combination of oxidation, sulfidation, and internal nitridation whereby the relevant corrosion mechanism has been given. [ABSTRACT FROM AUTHOR]

Surface integrity—comprising abrasive embedding, work hardening, and residual stress—is crucial for the performance of nickel-based superalloy components milled by abrasive waterjet (AWJ). Effective evaluation of this integrity is therefore essential. First, the effects of pure waterjet and small-size AWJ on the milled surface were analyzed, with abrasive embedding as a key indicator. It was found that small-size AWJ significantly reduced both the number and depth of abrasive particles embedded. Next, single-factor experiments determined the influence of AWJ process parameters on abrasive embedding, leading to the identification of optimal modification schemes (modification scheme 1 and modification scheme 2) for minimizing the number and depth of abrasives. Recommended processing parameters are waterjet pressure = 100 MPa, and step-over distance = 0.38 mm. Finally, it was observed that both modification schemes decreased surface hardness while significantly increasing residual compressive stress compared to the unmodified surface. These findings offer a theoretical and practical foundation for achieving high surface integrity in the milling of nickel-based superalloys using AWJ. [ABSTRACT FROM AUTHOR]

By regulating Inconel718‐xRene88DT (x = 0–100%) alloy powder, microstructure evolution and microsegregation of as‐deposited samples fabricated by direct energy deposition (DED) are investigated. The novelty of this approach lies in the systematic regulation of the alloy composition to study its impact on microstructure and microsegregation, which is an important prerequisite to improving the performance of DED superalloys. It is found that the as‐deposited microstructure mainly consists of predominant γ columnar dendrites directionally growing along the deposition direction. With the increase of Rene88DT proportion, thelayered structure tends to be more obvious, and the amount and size of γ′ phase increases. The content of the Laves phase is in decline. As a result of the microsegregation, the needlelike δ (Ni3Nb) and lath‐shaped η (Ni3Ti) precipitate in the interdendritic regions when the proportion of Rene88DT is above 60%. The various microstructures, microsegregation, and their correlations at different proportions of Rene88DT DED samples are discussed. The relationship between composition and microstructure evolution is investigated based on the alloying mechanism, which can guide alloy design and process optimization for enhanced properties in AM‐built Ni‐based superalloys. [ABSTRACT FROM AUTHOR]

This paper focuses on the difficulties in high-quality and high-efficiency micro-milling nickel-based superalloy micro thin-walled parts. The second-generation Non-dominated Sorting Genetic Algorithm (NSGA-II) is improved. A central composite experiment is designed, and a surface roughness prediction model is developed for micro-milling thin-walled parts. A prediction model for surface residual stress on thin-walled parts is developed using an L9(34) orthogonal simulation experiment. Using the NSGA-II algorithm, the four cutting parameters (spindle speed, feed per tooth, axial cutting depth, and radial cutting depth) are optimized to achieve low surface roughness and high material removal rate, while stable cutting and surface compressive residual stress are considered constraints. Finally, the high-quality and high-efficiency micro-milling of the Inconel 718 cross-shaped thin-walled parts is realized. [ABSTRACT FROM AUTHOR]

The electrochemical machining post-processing of additively manufactured nickel-based superalloys was studied to solve the contradiction between material processing efficiency and accuracy. The research results show that the surface of the fabricated specimens presented an uneven undulating morphology due to the overlapping of the cladding tracks and the stacking of the cladding layer in the process of laser directed energy deposition. After electrochemical machining post-processing, the levelling degree of the specimen surface gradually increased with the processing. However, at the same time, the levelling efficiency of the electrochemical machining post-processing gradually decreased due to the gradual decrease of the current density difference between the peaks and valleys on the specimen surface. The current efficiency remained basically unchanged at 67.6% throughout the electrochemical machining post-processing. This study provides experimental basis and theoretical foundation for the development of new combined machining technology of nickel-based superalloys based on laser directed energy deposition and electrochemical machining. [ABSTRACT FROM AUTHOR]

Microstructure, tensile property and creep performance of a novel powder metallurgy nickel-based superalloy FGH4113A were systematically studied under two different prepared conditions. The experimental results show that the yield strengths and ultimate tensile strengths of the designed superalloy of A3-HIP and A3-HEX excess 1200 and 1650 MPa at room temperature, respectively, and still reach 900 and 1050 MPa at 800 °C. The 0.2% creep lifetime of A3-HEX is longer than that of A3-HIP. Prior particle boundaries are responsible for inferior creep property of A3-HIP. FGH4113A shows excellent tensile property, which is better than that of René104 and RR1000. The strengthening mechanism shows that superior strength of FGH4113A is attributed to precipitation strengthening. [ABSTRACT FROM AUTHOR]

The flow behavior and deformation mechanisms of the [001]-oriented single-crystal nickel-based superalloy PWA1483 are investigated under quasi-static compressive and tensile loading conditions at 750°C. We found that the tensile yield strength is larger as compared to the compressive strength, and the serrated flow only appears on the compressive stress–strain curves. Microstructural observations uncover that stacking fault shearing and microtwinning are prone to initiate in PWA1483 during tensile deformation, and the appearance of the Protevin–Le Châtelier effect in compression derives from the abrupt frequent bursts of the two mechanisms on a mesoscale, rather than the simple occurrence of these procedures. [ABSTRACT FROM AUTHOR]

The tensile behavior of single-crystal superalloys was investigated at room temperature (RT) and 850 °C, focusing on various secondary orientations. Transmission electron microscopy (TEM) and quasi in situ electron backscatter diffraction (EBSD) were employed to study the deformation mechanisms across length scales. Deformation at 850 °C enhanced the tensile ductility of the samples, evidenced by the more uniform coverage of dislocations across the γ and γ′ phases, and the fracture mode switched from pure cleavage at room temperature to mixed mode due to accelerated void growth. The influence of secondary orientations on mechanical properties is insignificant at room temperature. However, the ductility of the different secondary orientation samples shows significant variations at 850 °C, among which the one with [001] rotated 37° demonstrated superior ductility compared to others. [ABSTRACT FROM AUTHOR]

The filling and solidification processes in investment casting of typical K4169 nickel-based superalloy complex thin-walled casting are simulated by ProCAST finite element software.The temperature field during the casting process is analysed to predict the formation of defects. The complex thin-walled casting is produced by investment casting with the same process parameters， and its shrinkage level， microstructure， and tensile mechanical properties are studied.The results show that when the pouring temperature is 1530 ℃ and the preheating temperature of the mold shell is 1000 ℃， the filling of the molten metal is stable. The solidification conforms to the principle of sequential solidification. The overall defects of the casting are less； the micro-shrinkage average volume fraction of different parts is counted， and the highest is only 1.01%； the minimum secondary dendrite arm spacing（SDAS） in the thin-walled region is only 18.4 μm， and the maximum secondary dendrite spacing in the thick part is 38.8 μm； the as-cast microstructure of K4169 alloy is dendritic structure. After standard heat treatment， the dendrites grow and coarsen， and the dendritic morphology in the grains is not obvious. The average room temperature tensile strength of the standard heat-treated K4169 alloy is 785.0 MPa， the average yield strength is 659.7 MPa， and the average elongation is 13.9%. The difference in shrinkage porosity level significantly affects the tensile strength and yield strength， but the elongation change is not obvious.

Nickel-based superalloys exhibit excellent high strength and thermal fatigue resistance at 650 ℃, making them widely used for manufacturing elevated-temperature components such as turbine blades for aero-engines. Laser-powder bed fusion (L-PBF) is a rapidly developing metal additive manufacturing technology that is increasingly important for producing nickel-based superalloy products. The design and service life of aero-engine turbine blades typically require more than 107 load cycles. Therefore, it is crucial to investigate the very-high-cycle fatigue characteristics of L-PBF nickel-based superalloys at increased temperatures. Internal failure is a common elevated-temperature fatigue mode of L-PBF nickel-based superalloys that is currently not well understood. To address this issue, first, axial fatigue tests with stress ratios of −1 and 0.1 are conducted at 650 ℃. Specifically, partial typical internal failure fractures at a stress ratio of 0.1 are selected as the focus of this study. Second, scanning electron microscopy and ultra-depth field microscopy are employed to observe the 2D and 3D morphology of the fatigue fracture surfaces and analyze the crack nucleation areas and growth paths. The results show that irrespective of the presence of defects, the emergence and aggregation of numerous facets occur in the “facetted cracking area (FCA),” a typical internal failure characteristic of L-PBF nickel-based superalloys. Measurements indicate that the size of these facets leading to cracking is comparable to that of large grains and correlates with variations in grain orientation. Therefore, internal failures are categorized into two modes of cracking: “defect-assisted faceted cracking” and “non-defect-assisted faceted cracking.” Third, the FCA exhibiting typical internal failure fractures is sectioned and subjected to electron backscatter diffraction analysis to observe surface and subsurface crystallographic features related to crack nucleation and growth behavior. The analysis reveals that microcracks mainly originate from large grains with softer orientations, propagate through slips, expand along the direction of maximum shear stress, and ultimately form a perforated fracture pattern. Fourth, subsurface microcrack features beneath the FCA are examined via focused ion beam milling and imaging. Transmission electron microscopy is then employed to observe slip bands and dislocation structures near the microcracks. The results confirm that the fatigue deformation mechanism of facets at 650 ℃ is mainly controlled by a combination of anti-phase boundary shearing, precipitate bypassing, and stacking fault shearing. This mechanism is especially evident under stress concentration effects induced by cracks or defects. Finally, according to the definition of the crack tip stress intensity factor, a crack nucleation life prediction method that accounts for the characteristics of faceted cracks is proposed. The predicted results align well with the experimental findings.

The flow behavior and deformation mechanisms of the [001]-oriented single-crystal nickel-based superalloy PWA1483 are investigated under quasi-static compressive and tensile loading conditions at 750°C. We found that the tensile yield strength is larger as compared to the compressive strength, and the serrated flow only appears on the compressive stress–strain curves. Microstructural observations uncover that stacking fault shearing and microtwinning are prone to initiate in PWA1483 during tensile deformation, and the appearance of the Protevin–Le Châtelier effect in compression derives from the abrupt frequent bursts of the two mechanisms on a mesoscale, rather than the simple occurrence of these procedures.

Short-time oxidation exposures of the Ni-based TROPEA single crystal superalloy was implemented to determine the nature and quantities of transient oxides in the 680–1000 °C temperature range. Experiments were carried out in situ in the SEM with reduced air pressure (150 Pa, P O 2 ~ 31.5 Pa) compared to atmospheric conditions (105 Pa, P O 2 ~ 2.1 104 Pa). TEM characterization after oxidation showed the complexity of the oxidation products developed. Aluminum underwent internal oxidation between 680 and 1000 °C. During the limited duration of oxidation, the TROPEA alloy only formed a continuous alumina layer at 1000 °C. At 680 and 850 °C, the low diffusion rate and small amount of Al in the Ni-based single crystal led to the formation of a significant amount of transient oxides such as (Ni,Co)O, compared to the desired chromia or alumina protective oxides. The lower the temperature, the smaller the size of the internal Al2O3 precipitates and the larger the transient oxide amount, which would lower the resistance of TROPEA to Type II hot corrosion. In contrast after a transient period shorter than 22 h, during which multiple transient oxide developed, the oxidation resistance would be ensured at 1000 °C by the formation of a continuous Al2O3 scale. [ABSTRACT FROM AUTHOR]

The interface heat transfer coefficient (IHTC) at the different casting/shell mold interfaces during the vacuum investment casting process was investigated by measuring the temperatures of K4169 superalloy melt and mold shell during the preheating, transferring, pouring, and solidification processes. The inverse model of ProCAST was used to calculate the interface heat transfer coefficient (IHTC) based on the temperature measurement results. The inverse calculation results show that the heat transfer between the shell mold and the outside air changes from natural convection to forced convection during the transfer process, but the heat transfer between the shell mold and the inside air is consistently natural convection heat transfer. Then, the convective heat transfer coefficient gradually decreases to 0.1 W/m2·K with the vacuum increases and is finally converted to radiation heat transfer at a vacuum of 10-3 Pa. In the early stages of solidification, the IHTC between the shell mold and the casting can reach 16,000 W/m2·K due to the close contact and the large temperature difference between them, and with the thermal expansion of the shell mold and cooling contraction of the casting, the formation of the gap leads to the interfacial heat transfer coefficient gradually stabilizing at 100–200 W/m2·K. The experimental validation results show that the IHTC has a great influence on the accuracy of the prediction of the location and shape of shrinkage porosities in the vacuum investment castings. [ABSTRACT FROM AUTHOR]

Copyright of Chinese Journal of Computational Mechanics / Jisuan Lixue Xuebao is the property of Chinese Journal of Computational Mechanics Editorial Office, Dalian University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

New developments in nickel-based superalloys and production methods, such as the use of additive manufacturing (AM), can result in innovative designs for turbines. It is crucial to understand how the material behaves during the AM process to advance the industrial use of these techniques. An analytical model based on reaction–diffusion formalism is developed to better explain the solidification behavior of the material during laser metal deposition (LMD). The well-known Scheil–Gulliver theory has some drawbacks, such as the assumption of equilibrium at the solid–liquid interface, which is addressed by this method. The solidified fractions under the Scheil model and the pure equilibrium model are calculated using CALPHAD simulations. A differential scanning calorimeter is used to measure the heat flow during the solid–liquid phase transformation, the result of which is further converted to solidified fractions. The analytical model is compared with all the other models for validation. [ABSTRACT FROM AUTHOR]

Nickel-based superalloy grinding, as well as selecting the appropriate operational parameters, is one of the major challenges in machining engineering. The conducted research consists of two sections. The first section presents a systematic experimentation, which has been carried out to qualitatively and quantitatively investigate nickel-based superalloy grinding. Vitrified CBN grinding wheels, along with a rotary diamond cup dresser, were utilised to conduct the experiments with the workpiece of Inconel 738. Specific grinding energy, G-ratio and surface roughness were indicated as three major machining outputs, while the inputs were the dressing conditioning and grinding parameters. In the following, the effects of different operational parameters were relatively ranked according to their weighted impact on the grinding performance using hybrid multi-attribute decision-making (MADM) methods. For this purpose, a combination of three techniques: Analytic Hierarchy Process (AHP), Entropy and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) was utilised to select the most appropriate grinding conditions, while considering the conflicting effect of the different dressing and grinding parameters. The results indicate that specific energy can be an important criterion for evaluating the process efficiency. The changes in G-ratio and surface roughness have the same trends, as opposed to specific grinding energy mutation. The grinding parameters are summarised in equivalent chip thickness. Our findings show that dresser cross feed rate, along with equivalent chip thickness, has the greatest impact on the grinding process efficiency. [ABSTRACT FROM AUTHOR]

Copyright of Foundry Technology (1000-8365) is the property of Foundry Technology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Foundry Technology (1000-8365) is the property of Foundry Technology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Microstructure degradation and phase transformations are critical concerns in nickel-based superalloys during thermal exposure. Understanding the phase transformation mechanism requires the detailed mapping of the distribution of each phase at different degradation stages and in various precipitation sizes. However, differentiating between phases in large areas, typically on the scale of millimeters and often relying on scanning electron microscopy (SEM) techniques, has traditionally been a challenging task. In this study, we present a novel and efficient phase mapping method that leverages multiple imaging detectors and modes in SEM. This approach allows for the relatively rapid and explicit differentiation and mapping of the distribution of various phases, including MC, M23C6, γ′, and η phases, as demonstrated in a typical superalloy subjected to aging experiments at 800 °C. [ABSTRACT FROM AUTHOR]

Micro drilling using Electrical Discharge Machining (MD-EDM) has emerged as a prominent technique in precision manufacturing, offering the capability to create small and intricate holes. This study focuses on investigating the influence of input process parameters on nickel-based superalloy (Ni3 (Ti, Al)) material, while the electrode materials studied are copper, aluminum and tungsten, with varying electrode diameters of 0.4 mm, 0.5 mm and 0.6 mm. The research encompasses an L9-orthogonal array (L9-OA) in which the Machining Voltage, Peak Current (IP), Pulse off time (Poff) and Servo Standard Voltage (SVR) are varied. Three levels of Machining Voltage (4 volts, 6 volts and 8 volts) and IP are selected. Moreover, scanning electron microscope (SEM) imaging is employed to observe and analyze the morphological characteristics of the drilled holes. The experimental results reveal that the choice of electrode material and diameter significantly influences the SR of the drilled holes. Copper electrodes demonstrate the lowest SR, followed by tungsten and aluminum electrodes. Smaller electrode diameters result in improved surface finish due to reduced discharge energy concentration. Additionally, increasing the Machining Voltage leads to larger discharge energy and heat, resulting in increased SR. SEM imaging demonstrates distinct differences in hole morphology based on the electrode material and diameter, with copper electrodes producing cleaner and more precise holes with minimal recast layer formation compared to aluminum and tungsten electrodes. The influence of Machining Voltage is observed through the presence of craters and debris in higher voltage conditions.

The combination of thermodynamic calculation and machine learning was used to reverse design nickel-based superalloys for thermodynamic performance requirements. The results show that the thermodynamic calculation dataset of nickel-based superalloys is successfully constructed by high-throughput thermodynamic calculations， which provides the data basis for the reverse design of nickel-based superalloys with thermodynamic performance requirements by using machine learning methods. Several C2P models are established for the thermodynamic target performance， and the accuracy of the models is higher than 99%. MLDS method is used to reverse design the alloy composition， and the eight alloys are recommended to meet the performance requirements （Vγ'，1100 ℃≥60%，Vγ，1100 ℃+Vγ'，1100 ℃≥99% and Tγ′≥1300 ℃）. The experimental verification of the three alloys with the smallest prediction error of thermomechanical properties shows that the Vγ'，1100 ℃ are greater than 80%， Vγ，1100 ℃+Vγ'，1100 ℃≥99% in the microstructure after aging， and Tγ′≥1300 ℃， which meet the design requirements.

The freckle formation and distribution in large-size directional solidified nickel-based superalloy blades at different withdrawal rates were investigated. The microstructure of freckles was analyzed by optical microscope (OM), and scanning electron microscope (SEM). The crystallographic orientation in the freckle regions was characterized by electron backscattered diffraction (EBSD). The solute segregation patterns on the surface of freckle regions were obtained by X-ray fluorescence (XRF). Numerical simulations for the temperature field were performed to analyze the mushy zone evolution during directional solidification. The experimental results indicated that the root region is prone to form freckles. The freckle chains were only distributed on the outward surface of the root region at low and middle withdrawal rates, and the inward side was free of freckles. When a high withdrawal rate was applied, a few freckles occurred on the inward side of the root region. The number and length of freckle chains increased as the withdrawal rate increased. The XRF results indicated that channel segregation formed on the surface of the root region. The segregation channels predicted by the macrosegregation simulation agreed well with the experimental observation. The concave or inclined solidification interface increased the propensity for freckle formation.

Mar-M247 is widely used in several fields as a material with excellent overall performance at high temperatures. However, its complex fabrication process and extremely poor weldability make it very difficult to repair and fabricate. In this study, a novel flux-cored wire with microstructures and composition similar to that of Mar-M247 nickel-based superalloy had been successfully developed, and the superalloy had been effectively repaired by cold metal transfer welding technique. Under the strict control of welding parameters, good metallurgical bonding between the weld and the base metal can be realized, and the hot cracking problem of the joint was successfully suppressed. In the weld, there were closely arranged γ′ phase precipitates with nanoscale and γ/γ′ eutectic structure, granular carbides dispersed at grain boundaries. The hardness of the weld was about 410–430 HV. It had been found that solidification cracks in welds were caused by intergranular liquid films, and grain orientation was found to be an important factor affecting solidification cracking. Optimized welding parameters of cold metal transfer could control the grain orientation of weld and reduce the size of intergranular liquid film, which could effectively suppress the size of weld solidification cracks. Additionally, liquefaction cracks were caused by eutectic reaction liquefaction in the heat-affected zone. The cold metal transfer welding process with optimized parameters can effectively reduce welding heat input, weaken eutectic reactions, and successfully obtain joints without crack in heat-affected zone.

In this paper, the effects of hot isostatic pressing (HIP) and heat treatment (HT) on the stress rupture properties of nickel-based superalloy K417G were investigated. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were employed to analyze the microstructure, fracture morphology, determination of crack source and dislocation evolution after stress rupture fracture. The results indicate that, HIP specimens demonstrate a 19.5% reduction in stress rupture life compared to as-cast specimens, whereas HIP + HT specimens exhibit a 41% increase in stress rupture life, reaching a value of 63.04 h. The fracture mechanism of the samples under different heat treatment conditions is different. There are a large number of pores in the as-cast specimen, which leads to the initiation and propagation of cracks and eventually result in fracture. The pores in the HIP specimen are almost eliminated. However, the coarsening and uneven distribution of γ′ phase led to the weakening of the hindrance to dislocation movement. Additionally, the increase in elongated carbides at grain boundaries accelerates crack initiation and propagation. Under HIP and HT, the cubic γ′ phase is uniformly precipitated, which effectively hinders the movement of dislocation lines. Granular M23C6 carbides precipitate at grain boundaries, acting as pinning points to retard grain boundary sliding and crack propagation, thus improving the stress rupture life.

Nickel-based superalloys have been widely used in the aerospace industry, and regulating the reinforcing phases is the key to improving the high-temperature strength of the alloy. In this study, a series of aging treatments (650 °C, 750 °C, 850 °C and 950 °C for 8 h) were designed to study different thermal deformation behaviors and microstructure evolutions for a novel nickel-based superalloy. Among the aged samples, the 950 °C aged sample achieved the peak stress of ~323 MPa during the thermal deformation and the highest microhardness of ~315 HV after thermal compression, which were the greatest differences compared to before deformation. In addition, the grains of the 950 °C sample exhibit deformed fibrous shapes, and the grain orientation is isotropic, while the other samples exhibited isotropy. In the 850 °C and 950 °C high-temperature aging samples, the γ′ precipitate (about 20 nm in size) is gradually precipitated, which inhibits the movement of dislocation in the grain during compression, thus inhibiting the occurrence of dynamic recrystallization and improving the high-temperature mechanical properties of the alloy. [ABSTRACT FROM AUTHOR]