Search Results (27)

Nickel-based superalloys are extensively used in high-temperature applications because of their exceptional oxidation and corrosion resistance. However, their performance in aggressive environments containing molten salts, such as Na₂SO₄ and V₂O₅, remains a critical challenge. This study investigated the hot corrosion behavior of Inconel 718, Udimet 710, Nimonic 75, and Inconel 625, focusing on the role of the alloying elements in the corrosion layers and degradation mechanisms. The superalloys were exposed to 50/50 wt.% Na₂SO₄–V₂O₅ at 900 °C for 8, 48, and 96 h, and their corrosion resistance was evaluated through weight gain measurements, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). These results indicate that Mo is a key factor in accelerating degradation, with Inconel 625 exhibiting the highest weight gain owing to the formation of thermally unstable Mo-rich phases. Fe also negatively impacted the stability of the protective scale of Inconel 718, contributing to an increased corrosion rate. In contrast, Nimonic 75 exhibited the best resistance, forming more of the NiCr₂O₄ spinel phase through the reaction of Cr₂O₃ with NiO from the high Ni and Cr contents in the corrosive layers. These findings highlight the importance of alloy composition in optimizing corrosion resistance and suggest that using superalloys with lower Mo and Fe contents and higher Cr and Ni concentrations can significantly enhance the durability of superalloys in molten salt environments. Full article

This study presents the synthesis of sintered composite foams based on the Invar alloy (64Fe-36Ni), using hollow spherical particles from a nickel superalloy (NiCrSiB) in order to generate porosity. The Invar powder was obtained by mechanical alloying (MA), and the NiCrSiB hollow spherical particles were incorporated into the composite at 20 vol %. The sintering was realized using the spark plasma sintering (SPS) process in an argon atmosphere at 600 °C and 5 MPa, with 10 s holding time. The porous structures were structurally characterized by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The coefficient of linear thermal expansion (CTE) of the Invar/NiCrSiB syntactic foams was found to be 2.52 × 10⁻⁶ °C⁻¹ in the 25–150 °C temperature range and 19.68 × 10⁻⁶ °C⁻¹ in the 150–400 °C range. Full article

The combination of cold deformation and solution aging is an important technological route for the bar processing of superalloy fasteners. The microstructure evolution and mechanical properties are intimately related to the process parameters. In this study, we systematically elucidate the mechanical properties and microstructure evolution of Fe-15Cr-25Ni alloys in different treatment processes and conduct in-depth analysis of the synergistic strengthening mechanism of fine-crystal strengthening, second-phase strengthening, and work hardening on Fe-15Cr-25Ni alloys. The results show that the tensile strength and yield strength at room temperature increase with the increase in grain refinement and dislocation density but decrease with the increase in elongation. After solid-solution treatment, most of the precipitates dissolve into the matrix, and the dislocation density is greatly reduced, resulting in a decrease in strength and an increase in plasticity. After aging, a large amount of γ′ phase was precipitated. Due to the two strengthening effects of dislocation strengthening and second-phase strengthening, the strength of the aging state is more improved than that of the cold-drawing state. The purpose of this study is to provide valuable insights for the industrial production of Fe-15Cr-25Ni superalloys. Full article

In order to enhance the resistance of superalloys to high-temperature molten chloride salt corrosion, Fe-Al coatings were prepared on 310S and 347H stainless-steel surfaces via pack aluminizing. Then, the coatings were annealed at different temperatures to explore the influence of temperature on their phase constitution, microstructure, microhardness, and corrosion resistance. The results showed that the annealing temperature had a considerable effect on the corrosion resistance of the Fe-Al coatings, which was related to the change in the phase composition of the coatings that occurred due to the annealing treatment. The growth rate of the coating on 347H steel was higher than that on 310S steel, and their thicknesses from aluminizing at 800 °C for 20 h were 209.6 and 153.5 µm, respectively. When annealing at 900 °C for 30 h, the phase composition of the coatings was completely transformed into (Fe, Cr, Ni) Al. The corrosion loss rate of the annealed coating was clearly reduced, the loss rate of the 310 coating was 6.0 and −0.25 mg/cm² before and after annealing at 900 °C and that of the 347 coating was 4.89 and −0.7 mg/cm² before and after annealing at 750 °C, respectively. The two coatings showed good corrosion resistance to molten chloride salts, as demonstrated by the oxide scale (Al₂O₃) that formed on the surface, which had a thickness of about 30~40 µm. Full article

Dissimilar welding joints of Inconel 718 (IN718) nickel superalloy with low-carbon AISI 304L austenitic steel (SS304L) were conducted using the Tungsten Inert Gas (TIG) welding process. The present investigation focuses on the effect of different welding currents on the produced dissimilar joints’ microstructure and mechanical properties. The microstructure study was carried out by light optical (LOM) and scanning electron microscopy (SEM), coupled with energy-dispersive spectroscopy (EDS) analysis. The nanoscale investigation was performed via a high-resolution transmission electron microscope (TEM). The mechanical behavior of the TIG joints was investigated via Vickers hardness testing. In all cases, the morphology and the microstructure of the fusion zone (FZ) and the corresponding heat-affected zones (HAZ) of the TIG-welded IN718 and SS304L verified the absence of porosity or other metallurgical defects. Except for carbides and carbonitrides, hard and brittle Laves phases ((FeNiCr)₂(NbMoTiSi)) were also identified, which were dispersed in the interdendritic spaces in the form of elongated islands. Prolonged exposure to high temperatures and a slower cooling rate due to higher initial heat input led to the precipitates’ coarsening both in FZ and HAZ and, thus, to the consequent gradual embrittle of the dissimilar joints. Full article

The development of high-entropy alloys (HEAs) for high-temperature applications has been driven by the limitation of nickel-based superalloys in achieving optimal efficiency at higher temperatures for higher efficiency in power generation engines. The alloys must have high oxidation resistance and microstructural stability at high temperatures. Relatively equimolar multi elements involved in HEAs produce microstructure containing a single solid solution or multiphase that improves the mechanical properties and oxidation resistance resulting from sluggish diffusion and core effects. In this study, the oxidation behavior and microstructural changes of Al_0.75CoCrFeNi HEA at 900, 1000, and 1100 °C in air atmosphere were investigated. Based on the XRD and SEM-EDS analysis, the mechanism of oxide scale formation and microstructural changes of the substrate are proposed. The results show that the oxidation behavior of the alloy follows a logarithmic rate law. Different oxide compounds of CoO, NiO, Cr₂O₃, and CrO₃, θ-Al₂O₃, α-Al₂O₃, and Ni(Cr,Al)₂O₄ with semicontinuous oxides of Al₂O₃ with Cr₂O₃ subscale and an oxide mixture consisting of spinel of Ni(Cr,Al)₂O₄ and Co(Cr,Al)₂O₄ were found. During oxidation, Widmanstätten of FCC-A1 and BCC-B2/A2 phases in the substrate have changed. Spheroidization of B2 and a reduction in volume fraction decrease the hardness of the substrates. Full article

The Si atom diffusion behavior in Ni-based superalloys was evaluated based on first-principles calculations. Also, the site occupation of Si atoms as the melting point depressant elements in Cr, Mo, and W atom doped γ-Ni and γ′-Ni₃Fe supercells was discussed and Si atom diffusion behaviors between both adjacent octahedral interstices were analyzed. Calculation results indicated that formation enthalpy (∆H_f) was decreased, stability was improved by doping alloying elements Cr, Mo, and W in γ-Ni and γ′-Ni₃Fe supercells, Si atoms were more inclined to occupy octahedral interstices and the diffusion energy barrier was increased by increasing the radius of the doped alloy element. Especially, two diffusion paths were available for Si atoms in the γ′-Ni₃Fe and Si diffusion energy barrier around the shared Fe atoms between adjacent octahedral interstices and was significantly lower than that around the shared Ni atoms. The increase of interaction strength between the doped M atom/octahedron constituent atom and Si atom increased Si atom diffusion and decreased the diffusion energy barrier. The Si atom diffusion behavior provides a theoretical basis for the phase structure evolution in wide-gap brazed joints. Full article

The present study describes the laser welding of Co-based superalloy L605 (52Co-20Cr-10Ni-15W) equivalent to Haynes-25 or Stellite-25. The influence of laser welding process input parameters such as laser beam power and welding speed on mechanical and metallurgical properties of weld joints were investigated. Epitaxial grain growth and dendritic structures were visible in the weld zone. The phase analysis results indicate the formation of hard phases like CrFeNi, CoC, FeNi, and CFe in the weld zone. These hard phases are responsible for the increase in microhardness up to 321 HV_0.1 in the weld zone, which is very close to the microhardness of the parent material. From the tensile strength tests, the ductile failure of welded specimens was confirmed due to the presence of dimples, inter-granular cleavage, and micro voids in the fracture zone. The maximum tensile residual stress along the weld line is 450 MPa, whereas the maximum compressive residual stress across the weld line is 500 MPa. On successful application of Response Surface methodology (RSM), laser power of 1448.5 W and welding speed of 600 mm/min i.e., line energy or heat input equal to 144 J/mm, were found to be optimum values for getting sound weld joint properties. The EBSD analysis reveals the elongated grain growth in the weld pool and very narrow grain growth in the heat-affected zone. Full article

Microcrystalline and nanocrystalline AlCrFeCoNi high-entropy alloy (HEA) coatings were applied on Inconel 718 superalloy using the atmospheric plasma spraying (APS) process. The high-temperature oxidation behavior of the microcrystalline and nanocrystalline AlCrFeCoNi HEA-coated superalloy was examined at 1100 °C under the air atmosphere for 50 cycles under cyclic heating and cooling (1 h for each cycle). The oxidation kinetics of both nanocrystalline- and microcrystalline-coated superalloys were accordingly analyzed by weight change measurements. We noted that the uncoated and coated samples followed the parabolic rate law of the oxidation. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDS), elemental mapping and X-ray photoelectron spectroscopy (XPS) were used to analyze the oxidized coated and uncoated samples. In the HEA-coated superalloy, Fe, Ni, Co and Al were oxidized in the inter-splat region, whereas the splats, which consisted mainly of Ni and Cr, remained unoxidized. Due to the formation of compact and adhesive thin NiO, CoO oxides and spinels together with the Al₂O₃ oxide scale on the surface of the coating during oxidation, the developed nanocrystalline HEA coating showed better oxidation resistance compared with the microcrystalline HEA coating. Full article

Iron-nickel-chromium (Fe-Ni-Cr) alloy Haynes HR120 is an iron-nickel-based superalloy, which is extensively used in gas turbines. Hence, the materials for the fabrication of steam turbine blades should present great mechanical characteristics and creep properties. In this study, Fe-40Ni-24Cr was heat-treated at temperatures from 950 to 1250 °C. High temperature creep behavior and microstructure evolution of the selected heat-treated (1050 °C, 1200 °C, 1225 °C and 1250 °C) Fe-40Ni-24Cr alloy were assessed at temperatures of 800 °C and 900 °C under 100 MPa stress. The alloy consisted of titanium and niobium rich precipitates, namely NbC, (Nb,Ti)C, TiN and Ti(C,N) distributed in the matrix grain boundaries, which enhance the creep properties of the alloy. The hardness of heat-treated Fe-40Ni-24Cr alloy decreased with increasing temperature and grain size. The creep strain of the Fe-40Ni-24Cr alloy increased with escalation in the creep time and the temperature being under constant applied stress. Fe-40Ni-24Cr alloy shows a decrease in steady-state creep rate with an increase in grain size from 62 μm to 183 μm due to the grain boundary sliding mechanism and 183 μm to 312 μm due to the occurrence of dislocation climb. This result exhibited that grain size has a significant influence on the alloys’ high temperature creep properties. Full article

Various alloy compositions were cast as single crystals in a Bridgman vacuum induction furnace and creep tested at 980 °C: pure Ni, the equiatomic alloys CoCrNi and CrMnFeCoNi (Cantor alloy), single-phase fcc (Ni) solid solution alloys (with the composition of the matrix-phase of CMSX-3 and CMSX-4), and two-phase Ni-based superalloys CMSX-3 and CMSX-4. Due to the single-crystal state, grain size effects, grain boundary sliding, and grain boundary diffusion can be excluded. The results identify two major strengthening mechanisms: solid solution strengthening and other mechanisms summarized as precipitation hardening. Configurational entropy does not increase creep strength: The Cantor alloy, with the highest configurational entropy of all alloys tested, shows a weak and similar creep strength at 980 °C in comparison to pure Ni with zero configurational entropy. The element Re is a very effective strengthener, both in single-phase fcc (Ni) solid solution alloys as well as in two-phase superalloys. Quantitative estimations of different strengthening mechanisms: internal back stress, misfit stresses, Orowan bowing, and γ′-phase cutting (in the case of two-phase superalloys) are presented. Finite element simulations allow estimating the influence of solid solution strengthening of the matrix on the creep behavior of the two-phase superalloys. Full article

Bearing significant concentrations of high value and critical metals, superalloy scraps require comprehensive recycling for metal reclamation. In this study, nickel-based superalloy was treated with molten Mg-Zn for the selective extraction of nickel. The influence of heating temperature, the molar ratio of Mg to Zn in the molten metal, Mg-Zn/superalloy mass ratio, and heating time on metal extraction were investigated. Using the heating temperature of 800 °C, the Mg/Zn molar ratio of 9/1, the Mg-Zn/superalloy mass ratio of 5/1, and heating time of 240 min, the extraction rate of 97.1% was achieved for Ni, and the extraction rates of Fe, Cr and refractory metals (Nb, Mo and Ti) were all less than 1%. In the subsequent vacuum distillation process, nickel with a purity of 98.3 wt% was obtained. Therefore, the proposed method is a short, clean, and efficient process for selectively extracting nickel from the superalloy scraps. Full article

In order to explore novel light-weight Co-Nb-based superalloys with excellent performance, we studied the effects of alloying elements including Sc, Ti, V, Cr, Mn, Fe, Ni, Y, Zr, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir and Pt on the structural stability, elastic and thermodynamic properties of γ′-Co₃Nb through first-principles calculations. The results of transfer energy indicate that Y, Zr, Hf and Ta have a strong preference for Nb sites, while Ni, Rh, Pd, Ir and Pt have a strong tendency to occupy the Co sites. In the ground state, the addition of alloying elements plays a positive role in improving the stability of γ′-Co₃Nb compound. The order of stabilizing effect is as follows: Ti > Ta > Hf > Pt > Ir > Zr > Rh > V > Ni > W > Sc > Mo > Pd > Re > Ru. Combining the calculation results of elastic properties and electronic structure, we found that the addition of alloying elements can strengthen the mechanical properties of γ′-Co₃Nb, and the higher spatial symmetry of electrons accounts for improving the shear modulus of γ′-Co₃Nb compound. At finite temperatures, Ti, Ta, Hf, Pt, Ir, Zr and V significantly expand the stabilization temperature range of the γ′ phase and are potential alloying elements to improve the high-temperature stability of the γ′-Co₃Nb compounds. Full article

Nickel superalloy Hastelloy-N, alloy X-750, stainless steel 316 (SS316), and stainless steel 304 (SS304) are among the alloys used in the construction of molten salt reactor (MSR). These alloys were analyzed for their corrosion resistance behavior in molten fluoride salt, a coolant used in MSR reactors with 46.5% LiF+ 11.5% NaF+ 42% KF. The corrosion tests were run at 700 °C for 100 h under the Ar cover gas. After corrosion, significant weight loss was observed in the alloy X750. Weight loss registered in SS316 and SS304 was also high. However, Hastelloy-N gained weight after exposure to molten salt corrosion. This could be attributed to electrochemical plating of corrosion products from other alloys on Hastelloy-N surface. SEM–energy-dispersive X-ray spectroscopy (EDXS) scans of cross-section of alloys revealed maximum corrosion damage to the depth of 250 µm in X750, in contrast to only 20 µm on Hastelloy-N. XPS wide survey scans revealed the presence of Fe, Cr, and Ni elements on the surface of all corroded alloys. In addition, Cr clusters were formed at the triple junctions of grains, as confirmed by SEM–EBSD (Electron Back Scattered Diffraction) analysis. The order of corrosion resistance in FLiNaK environment was X750 < SS316 < SS304 < Hastelloy-N. Full article

Monel K-500, a nickel–copper based alloy, is a very hard and tough material. Machining of such hard and tough materials always becomes a challenge for industry and this has been resolved by wire electric discharge machining (WEDM), a popular non-conventional machining method used for machining tough and hard materials having complex shapes. For the first time reported in this present research work is an experimental investigation executed on Ni-27Cu-3.15Al-2Fe-1.5Mn based superalloy using WEDM to model cutting rate (CR) and surface roughness (SR) using response surface methodology (RSM). The process parameters have been selected as pulse-on time, pulse-off time, spark-gap voltage and wire-feed rate. Experiments have been planned according to the central composite design (CCD). The results show that pulse-on time has a direct effect on CR while the pulse-off time has a reverse effect. The CR increases as pulse-on time increases, and decreases as pulse-off time increases. SR increases as pulse-on time increases, and decreases as pulse-off time increases. Furthermore, increase in spark-gap voltage decreases CR and SR both. The wire feed-rate has a negligible effect for both the response parameters. The optimized values of CR and SR achieved through multi-response optimization are 2.48 mm/min and 2.12 µm, respectively. Full article