Superalloys In Air Research Articles

Effect of pre-strain on oxidation behaviour of nickel-based superalloys in air at 700 °C

The effect of pre-strain on oxidation behaviour of nickel-based superalloys was analysed using alloys 263 and 740H. Both alloys were subjected to tensile tests followed by oxidation tests at 700 °C in air. At higher pre-strain, both alloys exhibited higher oxidation rates and more oxide spallation. Microstructures were examined using various techniques (XRD, SEM, EBSD, and TEM). It was found that the pre-strain accelerated the formation of internal oxides. The defects that formed during the tensile tests apparently acted as oxygen diffusion channels from the oxide/metal interface to the inside of the metal substrate.

Oxidation behavior of high Hf nickel-based superalloy in air at 900, 1000 and 1100°C

Oxidation behavior of high Hf nickel-based superalloy in air at 900, 1000 and 1100°C

Comparative Study of High-Temperature Oxidation Behavior of Bare and Plasma Sprayed Al2O3-40%TiO2 Coated T-91, A-1 Boiler Steel and Superfer800H Superalloy in Air

Comparative Study of High-Temperature Oxidation Behavior of Bare and Plasma Sprayed Al2O3-40%TiO2 Coated T-91, A-1 Boiler Steel and Superfer800H Superalloy in Air

High temperature oxidation behavior of a novel cobalt-nickel-base superalloy

Oxidation behavior of a novel cobalt-nickel-base superalloy in air from 600 to 900°Cis investigated in details. The results indicate a two-layer oxide film formed on the surface after oxidation: the external layer of oxide film is mainly composed of Cr2O3 and a few of other oxides, and the inner layer dominantly contains Al2O3. During oxidation at higher temperature and long duration, evaporation of Mo-containing oxide leads to void formation and local failure in external oxide layer, characterized by roughening of oxide film. A formation mechanism of oxide film on this alloy during oxidation is proposed for the first time.

On the Pre-oxidation Treatments of Four Commercial Ni-Based Superalloys in Air and in Ar–H2O at 950 °C

The short-term oxidation behaviour of RA 602CA, Inconel 693, Manaurite 40XO and Sumitomo 696, which are four alloys recommended for hydrocarbon processing, was studied both in air and in Ar–H2O to determine the conditions of pre-oxidation treatments. Regardless of the considered material, the oxidation rate at 950 °C was systematically higher in Ar–H2O than in dry air. Surface examination of the Al-containing alloys indicated that they were not uniformly oxidized all over the surface. All Al-containing alloys (from 1.6 to 3.2% wt) formed an external protective alumina scale and behaved as alumina-forming alloys in dry air at 950 °C. In contrast, these alloys developed a rate-controlling chromia scale and severe internal oxidation in a H2O-containing atmosphere. Compared with their oxidation behaviour in air + H2O, the phenomenon was significantly enhanced in the atmosphere that coupled water vapour with a low oxygen pressure. Consequently, the Al-containing alloys should not be pre-oxidized in a water vapour atmosphere prior to long exposure to a corrosive atmosphere. In contrast, the chromia-forming Al-free 696 alloy exhibited identical oxidation behaviour in both atmospheres, demonstrating that the degradation of this alloy was not significantly affected by water vapour.

Isothermal oxidation behavior of a new Re-free nickel-based single-crystal superalloy at 950 °C

The isothermal oxidation behavior of a new Re-free nickel-based single-crystal superalloy in air at 950 °C for 200 h was studied by scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results indicate that oxidation kinetics obeys parabolic law approximately, and the mass gain increases rapidly during initial oxidation stage and then gradually slows down. The oxidation scales are composed of three layers: the outer layer mainly consists of NiO with a small amount of CoO; the intermediate layer is mainly composed of Cr2O3 with a small amount of spinel compounds such as CrTaO4, NiCr2O4, CoCrAl2O4, CoAl2O4, and NiAl2O4; and the inner layer is composed of A12O3. Inner Al2O3 layer suppresses the diffusion of elements between oxygen and alloy elements, slows down the alloy oxidation speed, and also suppresses the growth of the oxide scale and reduces the oxidation rate, which is agreeable with the oxidation kinetics.

Study of Isothermal Oxidation Behavior of FGH91 Superalloy

Oxidation behavior of FGH91 superalloy in air and the temperature range of 540 to 980 °C for period up to 100h has been studied. The results indicate that oxidation kinetics obeyed parabolic law, and the activation energy of oxidation was calculated to be 98.2 KJ/mol. The scales on the surface were determined by SEM, EDS and XRD, the results show that the oxidation scales were composed of three layers: the outer layer was composed of Cr2O3 layer with a small amount of TiO and NiCr2O4 spinel; the intermediate layer was made up of Al2O3 and matrix alloy; and a small amount of titanium-nitride disperse in the inner layer. Besides, γ’-free layer was formed under the oxidation scales due to the impoverishment of Al in this layer, which was induced by the formations of Al2O3.

Simulation of oxidation-nitridation-induced microstructural degradation in a cracked Ni-based superalloy at high temperature

In turbine engines, high temperature components made of superalloys may crack in a creep process during service. With the inward flux of the gases, e.g. oxygen and nitrogen, along those cracks, the microstructure of the superalloy substrate nearby the cracks may degrade by internal oxidation and nitridation. The aim of this study is to investigate and simulate the oxidation-nitridation-induced microstructural degradation in superalloys by taking a variant of Ni-based superalloy IN-792 as a sample. After the creep testing of the superalloy in air, the microstructures on the cross section of the superalloy were analysed in a scanning electron microscope, equipped with energy/wavelength dispersive systems. Internal oxidation and nitridation, presenting by Al/Ti oxides and nitrides, were observed under a porous and even cracked Cr-oxide scale which was formed on the superalloy surface or along the creep cracks connecting the superalloy surface. Meanwhile, the reinforcing γ′ precipitates were depleted. Such oxidation-nitridation-induced microstructural degradation was simulated by using an oxidation-diffusion model, focusing the diffusion of the alloying elements in metallic phases of the superalloy.

Isothermal Oxidation Behavior Characteristics of a Second Generation Ni-base Single Crystal Superalloy in Air at 1000 and 1100°C

Isothermal Oxidation Behavior Characteristics of a Second Generation Ni-base Single Crystal Superalloy in Air at 1000 and 1100°C

Oxidation Behavior of a New Ni-Base Superalloy in Air at 850°C and 950°C

The oxidation behavior of the Ni-base superalloy was studied at 850 °C and 950 °C for 100 h in air using XRD and SEM/EDAX. The mass gain of the alloy at 850 °C was found to be lower than that of the alloy at 950 °C. The oxidation rate was determined by temperatures. The alloy obeyed a subparabolic rate law during oxidation at 850 °C and 950 °C. The scale consisted of an outer porous layer of TiO2,a middle continuous layer of Cr2O3, and inner Al2O3.

Oxidation behavior of a single-crystal Ni-base superalloy in air at 900 and 1050 °C

The high temperature oxidation behavior of the single-crystal Ni-base superalloy with higher Cr contents was investigated at 900 and 1050 °C in air by using SEM, XRD, and SEM/EDAX. The results show that the mass gain rapidly increases in the initial stage of oxidation at high temperature, and the oxidizing process is mainly controlled by the formation of Cr2O3 and Al2O3 phases. The obvious process of the surface oxidation, internal oxidation and internal nitridation in the alloy occurs during oxidation at high temperature. And the poor-aluminum but rich-tantalum areas appear above the internal oxides layer. The phase containg Ta-richer may restrain the element Al in the matrix diffusing outward, which restrains the growth up of the oxide scales.

Cyclic Oxidation Behavior of IN 718 Superalloy in Air at High Temperatures

Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 °C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol−1. The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 °C. At 850 °C, a continuous and very thin oxide scale was formed. At 950 °C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO2–MnCr2O4 spinels, inner layer of Cr2O3, and the inner most layer composed of Ni3Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO2 at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO2) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese–chromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.

Isothermal Oxidation Behavior of a New Directionally Solidified Ni-Base Superalloy in Air at Different Temperature

The isothermal oxidation behaviour under static atmosphere of a new directionally solidified Ni-base superalloy was investigated. The results showed that the oxidation kinetics curves of the alloy follow parabolic law in the temperature range of 750-950°C. The diffusion activation energy Q is to be about 244.86 kJ•mol-1 and the oxidation within this temperature range is mainly controlled by Cr3+ diffusion among Cr2O3 oxidation film. The oxidation resistance grade of the alloy is perfect anti-oxidation within 750-850°C, and anti-oxidation within 900-950°C. The oxidation film can be divided into TiO2, Cr2O3+TiTaO4+NiCr2O4, Al2O3 and TiN layers, from the surface to inside. Priority oxidation and the inside oxidation of Al are considered with the oxidation of Ti and the depletion of Cr, and it is the key point for the improving of oxidation resistance.

A study on the oxidation behavior of HVOF sprayed NiCrAlY–0.4 wt.% CeO 2 coatings on superalloys at elevated temperature

The high temperature oxidation behavior of HVOF sprayed ceria added NiCrAlY coatings on superalloys has been studied. Oxidation kinetics of the bare and coated superalloys in air at 900 °C under cyclic conditions was investigated. X-ray diffraction, field emission scanning electron microscope with energy dispersive spectroscopy and X-ray mapping were used to analyse the scales formed on the surface of the oxidised samples. The NiCrAlY–0.4 wt.% CeO 2 coated specimen showed negligible microspalling of the scales. The incorporation of ceria in NiCrAlY powder has contributed to the development of adherent oxide scale, in the coating, at elevated temperature and provided the better oxidation resistance.

High temperature oxidation studies of detonation-gun-sprayed Cr 3C 2–NiCr coating on Fe- and Ni-based superalloys in air under cyclic condition at 900 °C

The cyclic oxidation behavior of detonation-gun-sprayed Cr 3C 2–NiCr coating on three different superalloys namely Superni 75, Superni 718 and Superfer 800H at 900 °C for 100 cycles in air under cyclic heating and cooling conditions has been investigated in the present work. The kinetics of oxidation of coated and bare superalloys was analysed, using thermogravimetric technique. It was observed that all the coated and bare superalloys obey a parabolic rate law of oxidation. X-ray diffraction, FE-SEM/EDAX and X-ray mapping techniques were used to analyse the oxidation products of coated and bare superalloys. The results on the Cr 3C 2–NiCr-coated superalloys showed better oxidation resistance due to the formation of a compact and adhesive thin Cr 2O 3 scale on the surface of the coating during oxidation. The scale remained intact and adherent to the partially oxidised coating during cyclic oxidation due to its good compatibility and similar thermal expansion coefficient between Cr 3C 2–NiCr coating and the superalloy substrates. In all the coated superalloys, the chromium, iron, silicon and titanium were oxidised in the inter-splat region, whereas splats which consisted mainly of Ni remained unoxidised. The parabolic rate constants of Cr 3C 2–NiCr-coated alloys were lower than that of the bare superalloys as observed in the present work.

Oxidation behaviour of selected Ni- and Fe- based superalloys in air at 900°C under cyclic conditions

Oxidation behavior of Ni- and Fe- based superalloys has been studied in air at 900°C under cyclic conditions. The mass change measurement with time was used to establish the oxidation kinetics. The oxide scales formed on the surface of the superalloys were characterised by various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDAX) analyses. The oxide scales formed on the superalloys contained NiO, Cr2O3 and Fe2O3. The scales were adherent and only microspalling was observed. Superfer 800 has provided best resistance to oxidation among the alloys studied.

Oxidation behavior of a single-crystal Ni-base superalloy in air at 900, 1000 and 1100 °C

The isothermal oxidation behavior of a single-crystal (SC) Ni-base superalloy was studied over the temperature range from 900 to 1100 °C and analyzed by OM, TGA, XRD, EDX, SEM and EPMA. The alloy obeyed subparabolic rate law during oxidation at 900 and 1000 °C, whereas the alloy showed parabolic rate law at 1100 °C exposure. The variations in the chemical composition due to segregations, which resulted from the solidification process, led to the formation of different kinds of oxide scale on the dendritic and interdendritic area during oxidation at 900 and 1000 °C, while the alloy exhibited relatively uniform oxidation at 1100 °C. The growth mechanism of the scale was discussed.

Characterization of oxide scales to evaluate high temperature oxidation behavior of Ni–20Cr coated superalloys

Modern thermal spray processes such as plasma spraying are usually considered to deposit high-chromium, nickel–chromium coatings onto the superalloys to enhance their high temperature oxidation resistance. The purpose of these coatings is to form long-lasting oxidation protective scales. In the current investigation, Ni–20Cr alloy powder was deposited on three Ni-base superalloys; Superni 75, Superni 600 and Superni 601 by shrouded plasma spray process. Oxidation kinetics was established for the uncoated as well as the coated superalloys in air at 900 °C under cyclic conditions for 50 cycles by thermogravimetric technique. Each cycle consisted of 1 h heating followed by 20 min of cooling in air. All the coated superalloys nearly followed the parabolic rate law of oxidation. X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray (SEM/EDAX) techniques were used to characterise the oxide scales. The coating was found to be successful in maintaining its integrity with the superalloy substrates in all the cases. The oxide scales formed on the oxidized coated superalloys were found to be intact and spallation-free in general. The XRD analysis revealed the presence of phase like NiO, Cr 2O 3 and NiCr 2O 4 in the oxide scales. The XRD results were further supported by the SEM/EDAX analyses.

On the high-temperature oxidation protection behavior of plasma-sprayed Stellite-6 coatings

High-temperature oxidation resistance of the superalloys can be greatly increased by plasma-sprayed coatings, and this is a growing industry of considerable economic importance. The purpose of these coatings is to form long-lasting oxidation protective scales. In the current investigation, Stellite-6 coatings were deposited on two Ni-base superalloys, Superni 601 and Superni 718, and one Fe-base superalloy, Superfer 800H, by a shrouded plasma spray process. Oxidation studies were conducted on the coated superalloys in air at 900 °C under cyclic conditions for 50 cycles. Each cycle consisted of 1 hour heating followed by 20 minutes of cooling in air. The thermogravimetric technique was used to approximate the kinetics of oxidation. All of the coated superalloys nearly followed the parabolic rate law of oxidation. X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray (SEM/EDAX), and electron probe microanalysis (EPMA) techniques were used to analyze the oxidation products. The Stellite-6 coating was found to be successful in maintaining its continuous surface contact with the superalloy substrates in all cases. The oxide scales formed on the oxidized coated superalloys were found to be intact and spallation free. The main phases analyzed for the coated superalloys were oxides of cobalt and chromium and spinel of cobalt and chromium, which are suggested to be useful for developing oxidation resistance at high temperatures.

Oxidation Behavior of a Single-Crystal Ni-base Superalloy in Air at 900, 1000 and 1100°C

The oxidation behavior of a single-crystal (SC) Ni-base superalloy was studied over the temperature range from 1000–1150°C and analysed by TGA, XRD, EDAX, and SEM. The results indicated that the SC Ni-base superalloy exhibited parabolic oxidation kinetics, which were controlled by the growth of the inner α-Al2O3 layer. A mixed scale formed on the SC Ni-base superalloy after prolonged oxidation. The scale consisted of an outer layer of spinel, a sublayer of mainly α-Al2O3 with small amount of spinel adjoined by a very thin and even discontinuous layer of CrTaO4-rich oxide, and an inner α-Al2O3 layer. The inner α-Al2O3 layer provided good protection. No internal oxides or nitrides were observed below the inner α-Al2O3 layer after 1000 hr at 1000°C, and after 200 hr at 1100 and 1150°C.