high-velocity solution plasma spray (HVSPS) process. Graded coatings were formed by reactions between the Si(OH) 4 sprayed liquid precursor and the C/C substrate; these reactions were promoted by the high temperature of the plasma torch. The morphologies, microstructures, and chemical compositions of the coatings were investigated by X-ray diffraction, Raman spectroscopy, Fourier-transform infrared spectroscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy. By altering the deposition time, the coating thickness was controlled, therefore demonstrating SiC formation and realizing graded SiO 2 /SiC coatings.
The evaluation of stability of secondary phases in superalloys is of great importance because of its application under high homologue temperatures. The precipitation hardening Inconel 718 superalloy has high stress rupture and creep resistance, good LCF behavior and high corrosion resistance. Those properties can be obtained due to the high stable matrix based on Ni, Fe and Cr, as well as secondary phases promoters elements such as Ti, Al and Nb. This study aims to analyze the phase stability among a range of temperature from 500°C to approximately 1400°C. The thermodynamic modeling has a high importance to predict the behavior of alloys among equilibrium transition. The microstructure of the system was evaluated using Thermo-Calc software and the selected databases: SSOL4 and TTNI6. This analysis made possible the prediction of the phases presented, compositions and amounts. Results from SEM analysis were used to compare the results obtained, showing good coherence between them.
The application of heat treatment, by solid solution and precipitation hardening, is very important to optimize the mechanical properties of superalloys.The main phases present in Inconel 718 are: gamma prime γ′ face ordered Ni 3 (Al, Ti); gamma double prime γ″ bct ordered Ni 3 Nb; eta η hexagonal ordered Ni 3 Ti; delta δ orthorhombic Ni 3 Nb intermetallic compounds and other topologically closed-packed structures such as μ and Laves phases. δ, μ and Laves phases have low ductility, which causes losses in mechanical and corrosion properties (Fu et al. in Mater Sci Eng A 499:215-220, 2009 [1]). The heat treatment applied to Inconel 718, precipitation hardening, has two steps: solid solution and aging treatment. In first step the secondary (hardening) phases are dissolved along the matrix, as well as carbides. It is important to note that after 650°C (Durand-Charre in The microstructure of superalloys. CRC Press, Boca Raton, 1997 [2]) and with long exposure times, γ″ transforms in the stable phase δ, which results in a loss of mechanical resistance. In this study we aim to characterize the microstructure and phases in superalloy Inconel 718 during the steps of heat treatment with double aging. The double aging treatment performed followed the steps of solid solution to 1095°C/1 h and double aging at 955°C/1 h to 720°C/8 h + 620°C/8 h. The characterizations were performed through the techniques of XRD and SEM/EDS. It was possible to obtain the microstructural and phases characterizations before and after heat treatment in all steps.
This work presents a plasma torch able to operate at supersonic regime with axial injection of feedstock. In contrast to commonly used linear scheme, the principal axis of the plasma torch is perpendicular to feedstock injection direction, which is aligned with coming out plasma jet. The plasma torch has slightly ascending current voltage characteristics and fixed arc length. Electrical, thermal and kinetic characteristics outlined from comparison with conventional linear plasma spray torches are intermediate between APS, HVOF and VPS. The plasma torch developed in this work has an elevated arc voltage (370 V) and low arc current (100 A), which contribute to increase the electrode life and decrease the arc voltage relative fluctuation (10%). According to in-flight particle monitoring the CoNiCrAlY particles were sprayed at 500 m/s and temperature of 2400°C, whereas the 7%YSZ at 491-683 m/s and 2535-2636°C.Thermal spray embodies a family of processes where a layer of material (metallic, ceramic and/or polymer) is applied on a substrate in order to get protection against oxidation, corrosion, abrasion, thermal loads, high temperature fatigue and creep. Hermanek (2001) defined the thermal spraying as "a group of coating processes in which finely divided metallic or non-metallic materials are deposited in a molten or semi-molten condition to form a coating". Thus the coatings are produced when the particles are heated and deformed at impact with the substrate, which happens if they are not only softened but have sufficient kinetic energy. The particles may be in the form of a powder, solution or suspension, and from here on it shall be called generically as feedstock.The thermal spraying encloses several processes that uses the thermal energy generated chemically (by combustion) or electrically (mainly by electric arc discharge) to soften and/or melt and accelerate the feedstock at high velocities from few tens to thousand meters per second. Fauchais et al (2007) highlighted that among the thermal spraying processes, the atmospheric plasma spray (APS) is the technique most commonly used due to its versatility and cost-efficiency. The high temperature of the plasma jet is suitable, but is not limited, to materials with high melting point like ceramics and refractory materials. The process usually is accomplished in the open-air environment, but to improve the coating quality a controlled atmosphere chamber (LPPS -low pressure plasma spray or VPS -vacuum plasma spray) may be used.As described by Pawlowski (2008) thermal energy converted from electrical determines the temperature of the flame (or jet), however, besides the efficiency of fusion of the particles, the adhesion of droplets to the substrate and porosity depends also of the plasma velocity. The conventional plasma spray (APS) has higher thermal loads and relatively low particles velocities when compared to High Velocity Oxygen Fuel (HVOF), Detonation Gun (D-Gun) and Vacuum Plasma Spray process (VPS).Most conventional torches for plasma spraying were develope...
Inconel 718 is one of the most important superalloys, and it is mainly used in the aerospace field on account of its high mechanical strength, good resistance to fatigue and creep, good corrosion resistance and ability to operate continuously at elevated temperatures. In this work the resistance to pitting corrosion of a superalloy, Inconel 718, is analyzed before and after double aging heat treatment. The used heat treatment increases the creep resistance of the alloy, which usually is used up to 0.6 T m . Samples were subjected to pitting corrosion tests in chloride-containing aqueous solution, according to ASTM-F746-04 and the procedure described by Yashiro et al. The results of these trials show that after heat treatment the superalloy presents higher corrosion resistance, i.e., the pitting corrosion currents of the as received surfaces are about 6 (six) times bigger (~0.15 mA) than those of double aged surfaces (~0.025 mA). Keywords: Double aging heat treatment; Pitting corrosion; Inconel 718. ANÁLISE DE CORROSÃO POR PITE NA LIGA INCONEL 718 SUBMETIDA A TRATAMENTO TÉRMICO DE ENVELHECIMENTOResumo O Inconel 718 é uma das superligas mais importantes, sendo utilizada principalmente no setor aeroespacial devido a alta resistência mecânica, boa resistência a fadiga e fluência, boa resistência a corrosão e capaz de operar continuamente em temperaturas elevadas. Neste trabalho a resistência à corrosão por pite da superliga Inconel 718 é analisada antes e após o tratamento térmico de duplo envelhecimento. O tratamento térmico utilizado aumenta a resistência à fluência desta superliga, que normalmente é utilizada até 0,6 T m . Amostras foram submetidas ao teste de corrosão por pite em solução de cloreto de sódio, de acordo com a norma ASTM-F746-04 e o procedimento descrito por Yashiro et al. Os resultados mostram que a superliga apresenta maior resistência a corrosão após o tratamento térmico, isto é, a corrente de corrosão por pite da condição como recebida é 6 vezes maior (~0,15mA) do que a envelhecida (~0,025 mA). Palavras-chave: Duplo envelhecimento; Corrosão por pite; Inconel 718. *Dedicated to the memory of Prof. Carlos de Moura Neto.
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