The influence of varying levels of impurity elements on the hot corrosion resistance of the DD98M alloy in Na2SO4+NaCl salt at 950 degrees C was investigated. The results indicate that the corrosion resistance of the DD98M alloy significantly decreases with an increase in impurity content, and the presence of nitrogen leads to an increase in alloy porosity. These porosities promote the rapid diffusion of molten salt and oxygen into the alloy, resulting in a bilateral diffusion of oxygen and sulfur, which leads to an accumulation of these elements at the oxide-matrix interface. This process contributes to the formation and propagation of interfacial cracks. A growth model was developed for hot corrosion products in alloys with varying impurity elements.

This study presents a synergistic strategy that integrating the functional molecules (-COOH) and charge ions (AlO2- ) of the Metal-Organic Framework (Al-Fum) into the silica sol system by ultrasonic dispersion technique to establish a uniform cross-linking system. The synergistic influences of AlO2- hydrolysis and -COOH groups dissociation on the microstructure, flow properties, dehydration behavior and high-temperature crystal phase transformation behavior of silica sol were carried out to clarify the beneficial effects of Al-Fum modification. Further investigation into the phase transition behavior and chemical bonding changes of silica sol was performed to define the influence mechanism on the surface state of particles within the silica sol. The results indicate that through the compressed bilayer effect of H+ dissociated from -COOH, the particle size of silica sol decreased by 25%. Indirectly, this effect contributed to the increase in the specific surface area, the expansion in its pore size and the reduction in viscosity. AlO2- -hydrolyzed formed the alkali-silica gel phase (silica-aluminate) in the silica-gel lattice structure, such formation reduced the water loss rate during sintering, disrupted the construction of Si-O-Si long chains, as well as induced the transformation from amorphous silica to crystalline silica to alter the composition of the silica sol network structure. The dual-functional strategy developed an advanced silica sol for high application properties.

To resolve the problem of WC particle delamination and decomposition during the preparation of the composite coatings, the WC particles and Ni-based alloy composite powders were employed to fabricate the WC particle reinforced Ni-based composite coatings by laser cladding technology on 34MnB5 steels. The effect of cladding power on the microstructure and properties of the coatings was systematically investigated. The microstructure and phase composition of the powders and coatings were characterized by optical microscope, scanning electron microscope, energy dispersive spectrometer, and X-ray diffractometer. The layer thickness and WC volume fraction were measured by Image J software, while the hardness and tribological properties were evaluated by microhardness testing and ASTM G65 standard wear experiments. The results show that the hardness and surface morphology of the composite coatings are significantly improved with increasing cladding power, however, the WC volume fraction in coatings is decreased from 41.6% to 30.2%. When the cladding power is high, the overheating may lead to the decomposition of WC particles to form free C and W atoms, which can be dissolved into the Ni-based matrix or formed W-rich carbides lead to the hardness elevating of the matrix. The tribological properties testing show that the wear resistance is positively correlated with the volume fraction of WC particles under low-stress abrasion condition. So, the wear resistance of the composite coatings is preferred when the cladding power is 1.4 kW. However, when the cladding power is 1.8 kW, there is almost no cracks and higher surface quality, which is suitable for applications on the balancing condition of surface qualities and impact resistance.

The multi-blade casting technique used in fabricating blades by the superalloy K418B was investigated, which is more efficient and cost-saving in comparison with the traditional polycrystalline superalloy casting technique. In this work, the differences in microstructures and mechanical properties were analyzed by adjusting different thicknesses of the blades. The results of experimental studies and numerical simulations showed that the size of the γ' phase and the stress rupture property were positively correlated with the thickness of the blades and that the cooling rate and the residual stress were negatively correlated with it. Additionally, the residual stress decreased from 200 to 50 MPa as the thickness of the blades increased which followed the same trends as the geometrically necessary dislocation (GND) density. This study provides guidance in the optimization of microstructure in multi-blade casting technology.

Ni-based single-crystal superalloys weaken or even eliminate the influence of weak grain boundaries at high temperatures and contain >= 60% (volume fraction) of L12-type coherent ordering gamma'- Ni3(Al, Ti) precipitation strengthening phase. These superalloys exhibit excellent properties at high temperatures such as, high resistance to oxidation, creep, and fatigue resistance, making them the preferred materials for manufacturing advanced aviation engine turbine blades. The inner cavity structure of engine turbine blades has become complex with the rapid development of the engine manufacturing industry, making investment casting technology as a key technology in blade production. Si-based ceramics are selected as core materials owing to their low thermal expansion coefficient, good dimensional stability, and easy solubility. However, during pouring, active elements such as Hf, Al, and Cr, in the superalloy liquid, undergo thermo-physicochemical and thermomechanical infiltration with the cores when they come in contact with Si-based ceramic cores for extended period at high temperatures. This results in interface reactions and sand formation on the casting surface, thereby reducing the quality of the blade's inner surface and increasing subsequent processes such as eliminating the reaction layer through certain chemical methods. To suppress the interface reaction between the superalloy liquid and Si-based ceramic cores during blade casting and improve the surface quality of the blade inner cavity, the effect of Al2O3 coating on the surface of Si-based ceramic cores were investigated using the multi-arc ion plating method. Furthermore, the effect of Al2O3 coating on the interface reaction and wettability between Si-based ceramic cores and the superalloy were explored using the in situ droplet method. The surface quality, morphology, element distribution, and reaction products of the interface reaction were analyzed via optical profilometry, SEM, and XRD, respectively. It has been found Al2O3 and silicides are generated in few areas at the bottom of the superalloy after high-temperature contact between the Al2O3-coated Si-based ceramic cores and superalloy melt. However, a continuous and dense Al2O3 reaction layer is formed at the bottom of the superalloy after contact between the unmodified Si-based ceramic cores and superalloy melt. The wetting angles of the superalloy melt on the Al2O3-coated and unmodified Si-based ceramic cores are 89.1 degrees and 100.4 degrees, respectively, indicating that the wettability is substantially improved by the Al2O3 coating. Results indicate that applying Al2O3 coating on Si-based ceramic cores can effectively suppress the interface reaction between Ni-based single-crystal superalloy and Si-based ceramic cores and improve the filling ability of the superalloy liquid during casting.

In gas turbines, critical components such as turbine blades operate under prolonged high‐temperature conditions, which often result in the degradation of the microstructure in a nonuniform manner. In the article, the relationship between the microstructural evolution and mechanical properties of a novel second‐generation nickel‐based single‐crystal superalloy subjected to extended thermal exposure at 900 °C is examined. The results demonstrate that, with increasing the duration of thermal exposure, the γ′ phase size increases from 0.35 μm (0 h, edge length) to 0.46 μm (1000 h, thickness), transitioning from a regular cubic shape to a rafting structure. The tensile properties of the alloy first increase and then decrease. The coarsening behavior of the γ′ phase in the tested alloy aligns with the Lifshitz–Slyozov–Wagner theory (LSW), with a coarsening rate constant of 5.51 × 10 4 nm 3 h −1 . Additionally, the Kirkendall effect, driven by the uneven diffusion of alloying elements, results in a rise in both the quantity and size of micropores. Microcracks are more likely to start and spread from the pointed corners of micropores. The coarsening of the γ′ phase, combined with the growing presence of large, irregularly shaped micropores, ultimately contributes to the degradation of the alloy's tensile properties.

This research applied Cr2O3 coating on the surface of silicon-based ceramic core through multi-arc ion plating method. The effect of Cr2O3 coating on wetting behavior and interface reaction behavior of silicon-based ceramic core and nickel-based single crystal superalloy after contact at 1550 degrees C were studied using high-temperature in-situ droplet method. The interface morphology, element distribution, and reaction products after the interface reaction were analyzed using SEM, EDS, and XRD. The results show that the wetting angle between nickel-based single crystal superalloy and silicon-based ceramic core coated with Cr2O3 is 98.29 degrees. Hf and Al in superalloy melt react with Cr2O3 coating, generating HfO2, Al2O3, and free Cr at the bottom of superalloy. The generated Al2O3 forms a protective layer to prevent the diffusion of active elements in superalloy to the interface. However, a small amount of superalloy melt still reacts with the silicon-based ceramic substrate without coating protection, generating Al2O3 and free Si at the interface. The generated Cr and Si are enriched at the interface and form CrSi2 on the superalloy surface. Part of Si diffuses from the surface of the superalloy to the interior, forming (Mo,W,Re)(5)Si-3 with refractory elements such as W near the surface of superalloy. The results indicate that the wetting angle of nickel-based single crystal superalloy on silicon-based ceramic core coated with Cr2O3 is smaller than that on unmodified silicon-based ceramic cores, and its wettability is better. Based on the above analyses, the Cr(2)O(3)coating on the surface of silicon-based cores is beneficial to improve the filling of alloy at local positions of castings, but its control effect on interface reactions is limited.

The purity of nickel-based single crystal alloys plays an important role in the final service performance of turbine blades as it impacts the solidification structure, elemental segregation, and inclusions of castings. The highest purity of superalloys studied before was of altogether around 10 ppm of contents of N and O produced by the triple purification process (VIM+ESR+VAR). However, the impact mechanism still remains controversial, and meanwhile whether a lower content of N and O still keeps the impacts on castings is not yet clear due to the lack of raw materials. A superhigh purity of second generation of nickel-based single crystal superalloy DD98M with the sum of N and O contents as low as 4 ppm (hitherto the highest purity of the superalloy as published), home-produced by electron beam smelting (EBS), was put on focus in this work. The effect of three levels of purity (N-doped, commercial, and EBS, corresponding to 21, 10, and 4 ppm for the sum of N and O contents, respectively) on the microstructure evolution (including primary dendrite arm spacing (PDAS), eutectic, micropores, and γ′ phase) and the solute segregation behavior at dendritic scale and nano scale (γ/γ′ interfaces) were systematically studied and the influencing mechanisms were elucidated. The results show that the PDAS decreased from 406 μm to 322 μm with the reduction of O and N from 21 ppm to 4 ppm, which was irrelevant to precipitation of the nitride or oxide inclusions. Instead, it was due to the decrease of diffusion coefficient D or the increase of equilibrium coefficient k, or the decrease of the critical nucleation supercooling and the incubation time of dendrites. The micro-segregation, the number and volume fraction of eutectic, and the number and voltage percentage of shrinkage pores and gas pores were all greatly decreased with the increase of purity. These changes were closely related to the reduction of PDAS and the reduction of fluidity of the remained liquid in the interdendrites during solidification. The morphology and the size of the γ′ phase were independent from the purity, while the segregation of alloying elements at the γ/γ′ interface changed remarkably with the variation of the purity.

Nitrogen (N) is an unavoidable element in single-crystal superalloys that combines with other elements to form nitrides. Large-size nitrides often become the sites of crack initiation and propagation, which adversely affect the microstructures and mechanical properties of superalloys. However, in this study, it was found that the addition of trace amounts of N could improve the creep properties of superalloys. The results indicated that the size and number of nitrides gradually increased as the N contents increased from 3 ppm to 25 ppm. In addition, the porosity first decreased and then increased. The cause of the increase and subsequent decrease in creep lives as N contents increased was the lattice expansion caused by the formation of nitrides, which reduced the porosity by compensating for volume contraction. Meanwhile, the small-size nitrides could impede dislocation movement, which benefited the creep life of the specimens, resulting in the best creep life of the specimens containing 12 ppm of N.

Nickel-based superalloys have been widely used in gas turbines, aerospace, and other fields owing to their excellent high-temperature strength and creep resistance. Advanced directional-solidification techniques allow crystals to grow along specific directions, which can eliminate most or all of the transverse grain boundaries to obtain columnar- or single-crystal superalloys, which further improve the high-temperature mechanical properties. A strong magnetic field can modify the mass-transfer behavior during solidification via magnetic-damping or thermoelectromagnetic effect without contacting the material, thus improving the microstructure and microscopic segregation. In order to further refine the microstructure of nickel-based single crystal superalloys and improve the degree of homogenization of element distribution, the influence of longitudinal static magnetic field with a magnetic field intensity (B) that ranges from 0 to 4 T on the microstructure and microsegregation of liquid-metal-cooling directionally solidified nickel-based single-crystal superalloy DD98M was investigated. OM and SEM were applied to characterize the microstructure. Microsegregation was evaluated using a microsegregation coefficient and isoconcentration contour maps based on different data collection modes embedded in EDS. The results showed that with an increase in B, the primary dendrite spacing, average size of gamma/gamma' eutectic organization, and size of the gamma' phases decreased. Meanwhile, the gamma' phase in the interdendrite became more regularized. The microstructure refinement under static magnetic fields was attributed to the decrease in Delta T' / G (ratio of the temperature difference between the nonequilibrium solid-phase line and dendrite tip to the temperature gradient based on the Kurz-Fisher model) or the increase in sub-cooling of the melt surrounding the dendrites due to thermoelectric-magnetic convection. The relationship between Delta T' / G and B was revealed. The reduction in the gamma' phase size was caused by the increase in the nucleation rate of the gamma' phase due to the introduction of magnetic free energy difference (Delta G(M)) under a magnetic field. The magnetic field depressed the microsegregation of solutes, i.e., as B increased, the segregations of Al, Ta, Co, and W decreased. The effective partition coefficient (k(e)) of the dendritic scale and the average effective partition coefficients of the dendritic and interdendritic areas were obtained. It was found that the decrease in macrosegregation was essentially due to the effective distribution coefficient that approached 1 that due to the magnetic field.

The solution-treated second-generation single crystal (SX) superalloys underwent hot isostatic pressing (HIP) at 1316 °C and 105 MPa for varying durations. This study investigates the effect of HIP on micropores and stress-rupture behavior under conditions of 980 °C and 250 MPa. The results illustrate that the application of HIP treatment significantly reduces the porosity and volume of micropores, thereby increasing the rupture life of the alloys. With the disappearance of micropores, the crack initiation site shifts from the micropores to the γ/γʹ interface, resulting in reduced crack dimensions and decelerated propagation. Consequently, the smaller size of cracks in HIPed specimens slow down fracture. Nonetheless, the crack propagation pattern remains consistent: cracks expand perpendicular to the stress axis, connecting with cracks at other heights via secondary cracks along the <112> direction.

Single-crystal superalloys have been developed to the 5th generation to improve their temperature capacity. Thus, a rare metal Ru is doped to the 4th and 5th generations based on the 6%Re (with the same mass fraction) contained in third-generation superalloys. Compared with Re addition in low-generation superalloys, improvement of temperature capacity decreases with Ru addition in high-generation superalloys; however, the cost of superalloys containing Ru has increased significantly. Therefore, considerable attention must be paid to the development of third-generation single-crystal superalloys because of their superior cost performance. Thus, considering the slight precipitation of the topologically close-packed (TCP) phase and low properties at the intermediate temperature of third-generation single-crystal superalloys, Al is considered as a significant element affecting microstructure stability, which is determined by calculating the number of electron vacancy (N-v). By reducing 0.4%Al, no TCP phase is precipitated in the superalloy after long-term thermal exposure at 1100 degrees C for 1000 h; therefore, good microstructure stability is obtained. The concentration of Re and Co is decreased slightly to increase the stacking fault energy of the superalloy and to enhance the properties at intermediate temperature. The stress rupture life at 760 degrees C and 800 MPa extends from 40 h to 150 h by reducing 0.4%Al followed with reduction of 0.25%Re and 1%Co. Moreover, the stress rupture properties at high temperature remain unchanged. Based on the abovementioned research, a third-generation single-crystal superalloy is developed, and the causes of stabilization of the microstructure and improvement to properties at intermediate temperature are also discussed.

The effect of wall thickness on the microporosity and stress-rupture properties of a second-generation singlecrystal nickel-based superalloy was investigated using optical microscopy, scanning electron microscopy, energydispersive spectroscopy, and X-ray computed tomography. Specimens with various thicknesses (0.8, 1, 1.5, and 3 mm) were subjected to stress-rupture experiments at 980 degrees C and 250 MPa. The stress-rupture lives of the 0.8- and 1-mm-thick samples were shorter than those of the 1.5- and 3-mm-thick samples. The results indicate that increasing wall thickness results in an increased eutectic fraction, primary dendrite arm spacing, and degree of dendritic segregation of the as-cast alloys, which promotes porosity growth during solution heat treatment. The reduction in the real load-bearing cross-sections and a discontinuous Al2O3 layer caused by the oxidation behavior significantly influence the fracture mechanism of the thin-walled specimens, whereas preexisting micropores significantly affect the stress-rupture properties of the thicker specimens.

The anisotropy of Poisson's ratio of a single crystal superalloy is essential to understand its mechanical behavior, e.g. calculating the contact stress between blade tenon and turbine disc. However, it is difficult to determine the Poisson's ratio of single crystal superalloy of every orientation. In this research, one simple experimental method was employed to measure the stiffness constants and then the Poisson's ratio of different orientations was calculated. The slabs of a third generation single crystal superalloy in two orientations <001>< 100> and <011><110> were prepared by seeding technique in Bridgman method. The Young's modulus and shear modulus of the first specimen and the shear modulus of the second specimen were measured by resonance method from room temperature to 1100 degrees C. The three stiffness constants C-11, C-12 and C-44 of this superalloy were calculated from the measured moduli. The Poisson's ratio in any orientation can be calculated based on the stiffness constants. Further, the 3D distribution map of maximum and minimum of Poisson's ratio of every primary orientation can be drawn, so the distribution feature of Poisson's ratio in 3D space can be acquired conveniently. When the primary orientations are along <001> and <111>, the Poisson's ratio in plane is isotropy with secondary orientation. When the primary orientation is along <011>, the Poisson's ratio demonstrates significant anisotropy with secondary orientation, the Poisson's ratio reach minimum with secondary orientation <110> with negative value, while maximum is obtained in secondary orientation <100>.

Scandium-tungsten (Sc-W) cathodes have garnered a lot of research attention due to their high emission current density. This study investigates the electronic properties of Sc-W cathodes using the VASP code to explain the operating mechanism of hot electron emission. The results reveal that the work function is decreased by 0.58 eV, whereas the charge density in the crystal structure, the number of high valence electrons and the energy of electrons increase when Sc content is increased from 1% to 2%. Moreover, the bandgap and energy required for electrons to jump from the valence band to the conduction band decreased with the increase of Sc content, enhancing the current emission density and reducing the work function of the cathode.

The surface morphology and chemical states of W-2%ThO2 thermionic cathode during vacuum high-temperature treatment were investigated in this research. The W-2%ThO2 thermionic cathode was prepared by a solid-liquid doping method combined with high-temperature sintering. The morphology and distribution of thorium oxide were observed using a transmission electron microscope and scanning electron microscope. The chemical states of elements at different temperatures were analyzed by X-ray photoelectron spectroscopy. Results indicate that the surface morphology and chemical form of the alloy evolve with the increase of temperature. The matrix had a lamellar structure at low temperatures, and the surface was relatively flat. The samples were heated to 500 °C, 1100 °C, and 1300 °C for 1 h. During the heating process, thorium oxide changed from granular to spherical, and the matrix was recrystallized. As the heating temperature rises, diffusion channels appear inside the cathode. As the temperature increases, the high-priced tungsten gradually decreases, and the zero-valent tungsten content increases. The adsorbed oxygen left the cathode surface, and the lattice oxygen increased. The surface oxygen content decreased, and the thorium and tungsten content increased.

Various cooling scenarios (water, oil, air and furnace) were employed to study the impacts of the solution cooling rate (SCR) on the microstructure and creep behavior of a novel single-crystal (SX) superalloy. The results showed that the cubic degree and size of the gamma' phases were inversely proportional to the SCR. The creep life first increased and then dropped dramatically with a reduction in the SCR. The creep life of the sample cooled with air cooling (AC) was the highest, up to 144.90 h at 800 degrees C/750 MPa and 160.15 h at 1100 degrees C/137 MPa. During creep at 800 degrees C/750 MPa, the improved creep life of the AC sample was mainly attributed to the fine cubic gamma' phases, which decreased the rate of gamma'-phase coarsening and favoured plastic deformation by promoting the active movement of dislocations. The AC helped the gamma' phases become rich in Al, Ti and Ta while depleted in Co and Cr, which enhanced its stacking fault energy, thus promoting the formation of dislocation locks. Meanwhile, the largest negative lattice misfit caused by AC induced denser gamma/gamma' interface dislocation networks at 1100 degrees C/137 MPa, which efficiently reduced the minimum creep rate. The calculated average dislocation spacing results indicated that the smallest density of excess dislocations corresponded to the AC sample, proving its greatest creep resistance. Interestingly, the size of the secondary gamma' phases first decreased and then increased sharply with decreasing SCR during creep at 1100 degrees C/137 MPa, when fine secondary gamma' phases had a positive role in the blockage of dislocation movement in the matrix. Eventually, the comprehensive SCR effect was explored to provide more guidance in the design of Re-free SX superalloys. (C) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Effects of boron (B) addition on microstructure and high-temperature stress rupture properties of a heat-treated high chromium polycrystalline Nickel-based superalloy were investigated by adjusting B content (0, 0.016 wt%, 0.048 wt%, 0.09 wt%). The results indicated that the grain boundary characteristics were altered by B addition, such as the morphologies of the MC carbides, M23C6 carbides and precipitation of gamma' particles along grain boundaries. Meanwhile, it was confirmed that B existed in the form of Cr-rich M5B3 borides in the boron-containing alloys. Moreover, boron addition aggravated the inhomogeneity of microstructures, and reduced the volume fraction of secondary gamma' particles. Additionally, the rupture life was remarkably improved as the B content was increased to 0.016 wt%, whilst the elongation tended to improve as the B content was increased to 0.048 wt%. The improvement of stress rupture properties was mainly contributed to the modification of the grain boundary characteristics which retarded crack nucleation and propagation at grain boundaries. However, with excessive addition of B, the stress rupture properties were reduced significantly, as the solid solution strengthening and precipitation strengthening tended to be weakened by B addition, in addition the incipient melting regions and lager size borides promoted the intragranular crack nucleation and propagation. (C) 2020 Elsevier B.V. All rights reserved.