Search Results (13)

Alloy 625 is a widely utilized nickel-based superalloy known for its excellent mechanical strength and corrosion resistance in aggressive environments. However, its high niobium (Nb) content can lead to the formation of detrimental phases, such as Laves and MC carbides, during welding processes, compromising the mechanical integrity and long-term performance of the weld overlay. This review systematically examines recent research findings on the implications of reducing Nb content in Alloy 625 weld overlays, particularly with respect to microstructure evolution, mechanical behavior, and corrosion performance. Key advancements, including the understanding of segregation behavior, solidification paths, and secondary phase formation, are presented based on recent studies. This paper aims to provide a discussion on the trade-offs and future directions for optimizing Alloy 625 weld overlay claddings through Nb content modification. Full article

Preheating is important to improve the mechanical properties of nickel-based superalloys processed by additive manufacturing. The microstructure of IN718 was found to be influenced by the preheating temperature. Different preheating temperatures affect mechanical properties by changing microstructures. This work aims to clarify the thermal behavior for two preheated base plate temperatures (200 °C and 600 °C) on the IN718 superalloy built by the selective laser melting (SLM) process using the finite element method and experiments. The simulation findings indicate that the preheated 600 °C model has a deeper melt pool, a slower transformation of liquid to solid, and a slower cooling rate compared to the 200 °C model. As a result, the interdendritic Niobium (Nb) segregation of IN718 is reduced, thus improving the mechanical properties of additive-manufactured IN718 using the laser. The solidification map derived from the simulation indicates a columnar microstructure for the IN718 superalloy. Preheating increased the size of the dendrite structure and reduced elemental segregation, but it did not affect the morphology or size of crystal grains. We focused on comparing the temperature gradient and cooling rate for the two preheated base plate temperatures using the solidification map of IN718. The simulation confirmed that preheating does not affect the grain structure. Full article

This study applied plasma transferred arc (PTA) welding to fabricate hard cladding layers by using nickel-based superalloy (NCR7) and niobium carbide (NbC) powders as filler material. The resultant composite claddings were coated onto ductile iron and then analyzed to understand the effect of different quantities of NbC on the solidification structures of the cladding layers and on the characteristics of the interface between the cladding layers and the ductile iron. Cutting tests were also conducted to assess the morphology and mechanism of flank wear on PTA NbC/NCR7 composite cladding tools. The results revealed that the cladding specimens’ microstructures comprised a mixture of dendrites and interdendritic eutectics along with a considerable quantity of carbides (MC, M7C3, and M23C6) scattered within the γ-Ni matrix. Incorporating considerable NbC carbide enhanced the claddings’ surface hardness, but it had a limited effect on improving the flank wear on the turning tools. The flank wear on the composite cladding tools intensified as the NbC content was increased. The wear behavior, defined by brittle fractures and stripped NbC particles, led to a decline in turning tool performance. Accordingly, the Ni-based alloy composite cladding with larger NbC particles appears more suitable for sliding or erosion applications under normal stress conditions. Full article

The prediction of the equilibrium and metastable morphologies during the solidification of Ni-based superalloys on the mesoscopic scale can be performed using phase-field modeling. In the present paper, we apply the phase-field model to simulate the evolution of solidification microstructures depending on undercooling in a quasi-binary approximation. The results of modeling are compared with experimental data obtained on samples of the alloy Inconel 718 (IN718) processed using the electromagnetic leviatation (EML) technique. The final microstructure, concentration profiles of niobium, and the interface-velocity–undercooling relationship predicted by the phase field modeling are in good agreement with the experimental findings. The simulated microstructures and concentration fields can be used as inputs for the simulation of the precipitation of secondary phases. Full article

Inconel 718 is a widely used superalloy, due to its unique corrosion resistance and mechanical strength properties at very high temperatures. Hot metal extrusion is the most widely used forming technique, if the manufacturing of slender components is required. As the current scientific literature does not comprehensively cover the fundamental aspects related to the process–structure relationships, in the present work, a combined numerical and experimental approach is employed. A finite element (FE) model was established to answer three key questions: (1) predicting the required extrusion force at different extrusion speeds; (2) evaluating the influence of the main processing parameters on the formation of surface cracks using the normalized Cockcroft Latham’s (nCL) damage criterion; and (3) quantitatively assessing the amount of recrystallized microstructure through Avrami’s equation. For the sake of modeling validation, several experimental investigations were carried out under different processing conditions. Particularly, it was found that the higher the initial temperature of the billet, the lower the extrusion force, although a trade-off must be sought to avoid the formation of surface cracks occurring at excessive temperatures, while limiting the required extrusion payload. The extrusion speed also plays a relevant role. Similarly to the role of the temperature, an optimal extrusion speed value must be identified to minimize the possibility of surface crack formation (high speeds) and to minimize the melting of intergranular niobium carbides (low speeds). Full article

Nickel-based superalloys have excellent properties at high temperatures, which makes them appropriate for applications such as turbocharges and aeronautic gas turbines. MAR-M246 is a superalloy developed for these kinds of applications. The objective of this work was to study the total replacement of Ta by Nb in atomic percentage for this superalloy, regarding the oxidation resistance. Although both elements have the same role (form the γ’ precipitates) in nickel-based superalloys, Ta is more expensive than Nb. Thus, this replacement leads to the possibility of a price reduction. This work studied both conventional MAR-M246(Ta) and experimental MAR-M246(Nb) on cyclic oxidation tests at 900 and 1000 °C for up to 180 cycles. The oxide products were characterized by SEM/EDS and XRD analysis. The products of oxidation were mainly TiO₂, Al₂O₃, Cr₂O₃, NiO, and Ni(Co)Cr₂O₄. Mass gain variation per unit area was stable for both materials and temperatures. However, spalled areas were detected for tests performed at 1000 °C. The results obtained here suggest that use of Nb instead of Ta can be considered regarding high temperature oxidation for MAR-M246 superalloy. Full article

Nb-Si-based superalloys are considered as the most promising high-temperature structural material to replace the Ni-based superalloys. Unfortunately, the poor oxidation resistance is still a major obstacle to the application of Nb-Si-based alloys. Alloying is a promising method to overcome this problem. In this work, the effects of Hf, Cr, Zr, B, and V on the oxidation resistance of Nb-Si-based superalloys were discussed. Furthermore, the microstructure, phase composition, and oxidation characteristics of Nb-Si series alloys were analyzed. The oxidation reaction and failure mechanism of Nb-Si-based alloys were summarized. The significance of this work is to provide some references for further research on high-temperature niobium alloys. 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

Nickel-based superalloys contain various elements which are added in order to make the alloys more resistant to thermal and mechanical stress and to the adverse operating environments in jet engines. In particular, higher combustion temperatures in the gas turbine are important, since they result in higher fuel efficiency and thus in lower CO₂ emissions. In this paper, a semi-quantitative assessment scheme is used to evaluate the relative supply risks associated with elements contained in various Ni-based superalloys: aluminium, titanium, chromium, iron, cobalt, niobium, molybdenum, ruthenium, tantalum, tungsten, and rhenium. Twelve indicators on the elemental level and four aggregation methods are applied in order to obtain the supply risk at the alloy level. The supply risks for the elements rhenium, molybdenum and cobalt are found to be the highest. For three of the aggregation schemes, the spread in supply risk values for the different alloy types (as characterized by chemical composition and the endurance temperature) is generally narrow. The fourth, namely the cost-share’ aggregation scheme, gives rise to a broader distribution of supply risk values. This is mainly due to the introduction of rhenium as a component starting with second-generation single crystal alloys. The resulting higher supply risk appears, however, to be acceptable for jet engine applications due to the higher temperatures these alloys can endure. Full article

In a previous work, the chemical elements necessary for the achievement of Niobium base superalloys were defined in order to get a structure equivalent to that of Nickel base superalloys, which contain ordered precipitates within a disordered solid-solution. It was especially emphasized that precipitation hardening in the Niobium matrix would be possible with the addition of Ni. The remaining question about the design of such Niobium superalloys concerned the achievement of ordered precipitates in crystalline coherence with the Nb matrix i.e., with a crystalline structure equivalent to the Nb crystal prototype and with a lattice parameter in coherency with that of the Nb matrix. In order to reduce the trial/error experimental work, a reasoning based on various data for the achievement of coherency is presented. Then, starting from the Nb-Hf-Ni ternary alloy thus defined, this paper demonstrates that the precipitation of an ordered Nb phase within a disordered Nb matrix can be achieved with lattice parameter coherency between the ordered precipitates and the disordered matrix. The chemistry and the crystallographic structure of the precipitates were characterized using Transmission Electron Microscopy and Atom Probe Tomography. These results can help to conceive a new family of Nb base superalloys. Full article

The purpose of this work is to study the effect of heat treatments on the microstructure of the nickel-based superalloy Inconel 713C. Three different conditions were studied and the results compared: (1) as cast; (2) solid solution treatment (1179 °C/2 h) and (3) stabilization heat treatment (1179 °C/2 h plus 926 °C/16 h). Inconel 713C is normally used in the as-cast condition, an improvement in the 980 °C stress-rupture life is often obtained by a solution heat treatment. However, the material in this condition tested under high stress at 730 °C shows a marked decreased in rupture life and ductility. The mechanical resistance in creep increases in Inconel 713C by precipitation hardening phase, with γ’ (Ni₃Al) formed during the heat treatments. The characterization techniques used were: chemical analysis, hardness testing, X-ray diffraction, optical microscopy and scanning electron microscopy (SEM), EDS analyzes and thermocalculation. The SEM and EDS analysis illustrated the γ, γ’ and carbides. The matrix phase (γ), has in its constitution the precipitation of the γ’ phase, in a cubic form, and in some regions, carbides were modified through the heat treatments. (M₂₃C₆-type) and boride (M₃B₂ type) identified with the use of the thermocalculation. The heat treatments increase the relative intensity of niobium in the carbides. The hardness test was not achieved because the material was overaged. Full article

Surface corrosion resistance of nickel-based superalloys after grinding is an important consideration to ensure the service performance. In this work, robotic belt grinding is adopted because it offers controllable material processing by dynamically controlling process parameters and tool-workpiece contact state. Surface corrosion behavior of Inconel 718 after robotic belt grinding was investigated by electrochemical testing in 3.5 wt % NaCl solution at room temperature. Specimens were characterized by morphology, surface roughness and residual stress systematically. The potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) analysis indicate the corrosion resistance of the specimen surface improves remarkably with the decrease of abrasive particle size. It can be attributed to the change of surface roughness and residual stress. The energy dispersive X-ray spectroscopy (EDS) indicates that niobium (Nb) is preferentially attacked in the corrosion process. A plausible electrochemical dissolution behavior for Inconel 718 processed by robotic belt grinding is proposed. This study is of significance for achieving desired corrosion property of work surface by optimizing grinding process parameters. Full article

Superalloys are used primarily for the aerospace, automotive, and petrochemical industries. These applications require materials with high creep resistance. In this work, evaluation of creep resistance and microstructural characterization were carried out at two new nickel intermediate content alloys for application in aerospace industry and in high performance valves for automotive applications (alloys VAT 32 and VAT 36). The alloys are based on a high nickel chromium austenitic matrix with dispersion of intermetallic L1₂ and phases containing different (Nb,Ti)C carbides. Creep tests were performed at constant load, in the temperature range of 675–750 °C and stress range of 500–600 MPa. Microstructural characterization and failure analysis of fractured surfaces of crept samples were carried out with optical and scanning electron microscopy with EDS. Phases were identified by Rietveld refinement. The results showed that the superalloy VAT 32 has higher creep resistance than the VAT 36. The superior creep resistance of the alloy VAT 32 is related to its higher fraction of carbides (Nb,Ti)C and intermetallic L1₂ provided by the amount of carbon, titanium, and niobium in its chemical composition and subsequent heat treatment. During creep deformation these precipitates produce anchoring effect of grain boundaries, hindering relative slide between grains and therefore inhibiting crack formation. These volume defects act also as obstacles to dislocation slip and climb, decreasing the creep rate. Failure analysis of surface fractures of crept samples showed intergranular failure mechanism at crack origin for both alloys VAT 36 and VAT 32. Intergranular fracture involves nucleation, growth, and subsequent binding of voids. The final fractured portion showed transgranular ductile failure, with dimples of different shapes, generated by the formation and coalescence of microcavities with dissimilar shape and sizes. The occurrence of a given creep mechanism depends on the test conditions. At creep tests of VAT 32 and VAT 36, for lower stresses and higher temperature, possible dislocation climb over carbides and precipitates would prevail. For higher stresses and intermediate temperatures shear mechanisms involving stacking faults presumably occur over a wide range of experimental conditions. Full article