Chemical composition of superalloy C-263.
The present investigation was conducted to study the oxidation kinetics of nickel-based superalloy 263, used in the manufacture of rings for aircraft engines. For carrying out this study, we first conducted microstructural characterization of the pieces using the techniques of optical microscopy, scanning electronic microscopy, and X-ray diffraction. Subsequently, using the thermogravimetric analysis, the kinetic oxidation of the metal was performed in a temperature range between 700 and 1000°C, using atmospheres of O2. The results of the micrographs show the formation of a protective oxide film on the surface of the material in different oxidizing agents. Finally, it was found that the kinetics of high-temperature oxidation of the superalloy C-263 obeys the parabolic rate law.
- aircraft engines
In order to meet the current demand for the aerospace industry, the production of advanced materials is required that ensure the design and manufacture of a robust product and that meet customer expectations, increased engine performance for fuel efficiency, mechanical strength of the components, and at the same time be lighter and less expensive. Therefore, there is a great interest in the development and study of superalloys that will generate mechanically resistant aircraft, capable of withstanding corrosive environments at high temperatures. Inside a combustion chamber at duty, on a gas turbine engine of aircraft, very aggressive corrosive agents exist. One of these is sulfur, which can deteriorate the materials and reduce their useful life .
Nickel-based superalloys are generally used at temperatures above 540°C in very aggressive environments. Seamless rolled rings are made of superalloy Haynes C-230 or C-263 and used in the combustion chamber of gas turbine engines, which are exposed to corrosive gas mixtures. One of those gases is SO2. However, because of its machining process, its corrosion-resistant properties may be compromised [2, 3].
Sulfur is generally present as an impurity in fuels and air. When combustion takes place with excess of air to ensure complete combustion of fuel, sulfur reacts with oxygen to form mainly SO2, which is a corrosive gas. However, sulfidation in oxidizing environments is frequently accelerated by other impurities such as sodium, potassium, and chlorine, making possible the so-called hot corrosion mechanism .
Superalloys exposed in SO2 environments generally form oxides and/or sulfides as corrosion products, which depend strongly on temperature. The highest corrosion rate is normally related to the formation of sulfides . Sulfides provide paths for rapid outward diffusion of metals resulting in rapid corrosion attack. For nickel, the corrosion rate in this environment is around 600°C, and above 800°C, the rate decreases with increasing temperature.
Haynes 263 is a nickel-chromium-tungsten-molybdenum alloy that combines high-temperature strength, resistance to oxidizing environments up to 1149°C for prolonged exposures, premier resistance to nitriding environments, and excellent long-term thermal stability .
The manufacturing process for forging rings  of superalloys may be used in turboreactors or components of the high-temperature section of a gas turbine such as the combustion chambers of the intermediate-temperature section (Figure 1) .
There are a large number of nickel-based superalloys in the world such as the C-263 which contains multiple phases, depending on the alloying elements. The most prevalent phase in this superalloy is the gamma (γ) phase, with a cubic crystalline structure centered on the faces. Ni-based superalloys are hardened, due to the formation of solid solutions, using a variety of substitution alloying elements in the γ-structure such as molybdenum. In the case of superalloy C-263, aluminum and titanium only make up about 3% of the total composition. However, these components are important for the properties of the materials associated with superalloys, due to the formation of the γʹ-phase .
In metallic corrosion, the oxidation of alloys at high temperatures can be treated as a special case, which considers the material destruction due to chemical causes, in which solid phases interact either with a liquid agent or a gaseous agent. The latter case involves reducing and oxidizing gases, steam, and even free oxygen. The oxidation is determined by the internal mobility of the solid phases and mainly diffusion in solid state. It is well known that alloying with chromium improves the oxidation resistance of iron at high temperatures .
The principal objective in this research was to study the oxidation kinetics of nickel-based superalloy C-263 in a temperature range of 700–1000°C using atmospheres of O2 for 48 h.
2. Experimental procedure
The material used in this research was Haynes C-263 superalloy. The specimens were sections of 4 × 4 × 2.5 mm. The surface of samples was ground using 2000 grade grit paper, rinsed with distilled water, and degreased with acetone .
The oxidation test was performed in a thermogravimetric analyzer TA Instruments Hi Res TGA 2950 (Figure 2), for 48 h. Experiments were conducted in gaseous atmosphere of O2–N2 with a flow of 75 ml/min at an interval of temperatures of 700–1100°C (+2°C). For each specimen a weight gain measurement in an electronic microbalance (sensitivity 10−6 g) was set.
The oxidized samples were analyzed in a scanning electron microscopy (SEM; FEI Nova NanoSEM 200) coupled with an energy-dispersive X-ray spectroscopy (EDS) system, in order to understand the oxidation phenomena in terms of scale morphology and oxidation product distribution.
3. Results and discussion
3.1. Chemical analyzer
The chemical composition of superalloy C-263, used in the present study, is given in Table 1. The chemical analysis was performed using the technique of X-ray fluorescence.
|Chemical composition (% wt.)|
3.2. Alloy microstructure
The microstructure of the alloys in the study (Figure 3) is composed of an austenitic matrix gamma (γ) and twins that are typical in this material, with intermetallic precipitates of gamma prima (γʹ). The precipitates in the microstructure are in the form of carbides and some borides, and the grains are regular.
3.3. Thermogravimetric analyzer
The kinetic measurements indicated that, in general, the alloys presented a parabolic behavior, according to
where (∆m/A) is the weight gain per unit area; t and to are the final and initial time, respectively; and Kp is the parabolic rate constant .
The increase in mass gain, higher as temperature increases, is significantly lower as the exposure time increases. This progressive decrease in the rate of oxidation indicates the formation of a compact and well-adhered oxide. According to Wagner’s  theory, systems subjected to high temperature and controlled by diffusion processes, through the network of the oxide layer, follow an oxidation kinetics described by a parabolic-type equation.
Oxidation kinetics of superalloy C-263 was studied at 700, 800, 900, and 1000°C; Figure 4 shows the weight change per unit area versus time. Evidently, the oxidation rate increases as the temperature rises. A parabolic behavior was found at all testing temperatures. The kinetic results are in agreement with those reported in the literature in the sense that the kinetic behavior is parabolic [13, 14].
In order to better visualize this behavior, the square of the mass increase per unit area for superalloy C-263 at the temperatures mentioned above has been plotted in Figure 5. For all the cases studied, there is a linear relationship between both variables. This fact confirms that the oxidation of superalloy at the four test temperatures follows a parabolic law. The indicated graphs also allow to calculate the parabolic oxidation rate constants (see Table 2).
|Temperature (°C)||Parabolic oxidation rate constants (mg2/cm4 h)|
|700||4.18 × 10−8|
|800||7.21 × 10−8|
|900||7.66 × 10−6|
|1000||7.16 × 10−5|
Knowing the parabolic rate constant for each temperature, the activation energy of the oxidation reaction can be calculated from the Arrhenius equation, which relates Kp to the temperature by the expression:
where Q is the activation energy; R is the gas constant, in joules or calories; Ko is the rate constant at room temperature; and T is the absolute temperature.
In Figure 6 the parabolic oxidation rate constant is obtained against the inverse of the temperature. The value of the activation energy Q is 28.37 kcal/mol and is indicative that the oxidation process is diffusion controlled through the layer of oxides formed.
The oxide layer in the superalloy samples at 700, 800, and 1000°C remained adherent to the substrate, but not at 900°C, which, after finishing the test, was probably due to the stresses generated with the thermal contraction.
Garcia-Alonso et al. studied the oxidation kinetics of the MA 956 superalloy in the temperature range of 800–1200°C for up to 200 h exposure. During oxidation the alloy develops a fine, compact, and very well-adhered alpha-alumina layer, the thickness of which increases with increasing time and temperature. The oxidation kinetics obeys a subparabolic-type behavior, and the oxidation process would be controlled by alpha-alumina and below 900°C by 7-alumina.
Other authors have studied the behavior of high-temperature oxidation of nickel-based superalloys in different temperature ranges, where the kinetics of oxidation obeys the parabolic law by the addition of elements such as aluminum, titanium, niobium, or yttrium [15, 16].
The Encinas-Oropesa study targeted at characterizing the oxidation behavior of a new nickel-based disk alloy (RR1000) at intermediate temperatures (700–800°C for exposures up to 200 h). The mass gain data obtained have been used to derive oxidation reaction rate parameters, using established methodologies, with parabolic rate constants varying between 1.4 × 10−5 mg2/cm4/h at 700°C and 8.4 × 10−4 mg2/cm4/h at 800°C .
3.4. Scanning electron microscopy (SEM)
The results obtained by SEM on the oxidized specimens revealed different morphologies depending upon the temperature of testing.
The superficial morphology at 700°C presents fine-grained oxides and brittle banded agglomerates (Figure 7). Cross-sectional analysis of the samples by SEM/EDS on the layer formed on all alloys suggested the formation of titanium- and chromium-rich oxides.
At 800°C morphology with banded agglomerates, rich in chromium and titanium, is observed; these same elements develop a thin and not very homogeneous layer of chromium oxide and titanium (see X-ray map; Figure 8).
Observing the morphology obtained from the superalloy C-263 heat treated at 900°C for 48 h (1000 X), a surface totally covered by a great amount of particles with different sizes and defined morphologies can be noted, although they are more evident those that have form of bars, with larger particle size. Figure 9 shows the formation of an abundant oxide layer, in which two layers of oxide, internal and external, can be observed. The inner layer consists mainly of chromium oxide, and the outer layer is formed by an oxide of cobalt and nickel. Between both layers there is an oxide of titanium.
The presence of chromium in this type of superalloys not only improves the mechanical properties by hardening at high temperatures but also stabilizes the phase. Oxidation studies in superalloys with the presence of aluminum and chromium have shown a high tendency of formation of continuous and protective films above 900°C [18, 19, 20].
When this superalloy C-263 was treated at 1000°C for 48 h, different morphologies and particle sizes are observed. On the one hand, defined grains with particle sizes between 5 and 10 μm and in some areas the formation of smaller particle agglomerates covering the grains are observed. In X-ray map can be observed, the formation of an abundant oxide layer, in which two layers of oxide, internal and external, is observed; the inner layer is made up mostly of chromium oxide, and the outer layer is formed by a cobalt and nickel oxide (see Figure 10; 1000 X).
By SEM-EDS at 800, 900, and 1000°C, the formation of a larger amount of Cr oxide, Ti, and Ni on the surface of superalloy was observed. It was determined that while the temperature of the thermal treatment increased, the amount of the oxide is increased.
The growth rate of Cr2O3 scales at high temperatures is greatly reduced by the presence of elements as nickel, titanium, and cobalt .
The kinetics of high-temperature oxidation of the superalloy C-263 obeys the parabolic rate law for temperatures 700, 800, and 1000°C.
Cross-sectional analysis, by SEM/EDS, of the layer formed on all samples suggested the formation of titanium- and chromium-rich oxides and in some cases cobalt and nickel oxide after 900°C.
The present investigation found that at 700°C, there is almost no oxide on the surface of the superalloy, only seen a little agglomerates of Mn, Cr, and Ni as major components of the superalloy matrix.