James L. Smialek

Effect of the /theta/-/alpha/-Al/sub 2/O/sub 3/ transformation on the oxidation behavior of /beta/-NiAl + Zr

Isothermal oxidation of NiAl + Zr has been performed over the temperature range of 800–1200°C and studied by TGA, XRD, and SEM. A discontinuous decrease in growth rate of two orders of magnitude was observed at 1000° C due to the formation of α-Al2O3 from θ-Al2O3. This transformation also resulted in a dramatic change in the surface morphology of the scales, as a whisker topography was changed into a weblike network of oxide ridges and radial transformation cracks. It is believed that the ridges are evidence for a shortcircuit outward aluminum diffusion growth mechanism that has been documented in a number of18O tracer studies.

SiC Recession Caused by SiO2 Scale Volatility under Combustion Conditions: II, Thermodynamics and Gaseous-Diffusion Model

In combustion environments, volatilization of SiO2 to Si-O-H(g) species is a critical issue. Available thermochemical data for Si-O-H(g) species were used in the present study to calculate boundary-layer-controlled fluxes from SiO2. Calculated fluxes were compared to volatilization rates of SiO2 scales grown on SiC, which were measured in a high-pressure burner rig, as reported in Part I of this paper. Calculated volatilization rates also were compared to those measured in synthetic combustion gas furnace tests. Probable vapor species were identified in both fuel-lean and fuel-rich combustion environments, based on the observed pressure, temperature, and velocity dependencies, as well as on the magnitude of the volatility rate. Water vapor was responsible for the degradation of SiO2 in the fuel-lean environment. SiO2 volatility in fuel-lean combustion environments was attributed primarily to the formation of Si(OH)4(g), with a small contribution of SiO(OH)2(g). Reducing gases such as H2 and/or CO, in combination with water vapor, contributed to the degradation of SiO2 in the fuel-rich environment. The model to describe SiO2 volatility in a fuel-rich combustion environment gave a less satisfactory fit to the observed results. Nevertheless, it was concluded-given the known thermochemical data-that SiO2 volatility in a fuel-rich combustion environment is best described by the formation of SiO(g) at 1 atm total pressure and the formation of Si(OH)4(g), SiO(OH)2(g), and SiO(OH)(g) at higher pressures. Other Si-O-H(g) species, such as Si2(OH)6, may contribute to the volatility of SiO2 under fuel-rich conditions; however, complete thermochemical data are unavailable at this time.

Transient oxidation of Single-Crystal β-NiAl

The transient oxidation of β-NiAl in air at 800 °C and 1100 °C has been studied using electron microscopy. The oxide scale consists predominatly of metastable Al2O3 phases. θ-Al2O3 is the major oxide phase within 10.0 hr of oxidation at 800 °C and 0.1 hr at 1100 °C. The scales form epitaxially on (001)β and (012)β specimens throughout the transient stage, whereas the degree of preferred oxide orientation decreases with oxidation time on (011)β and (111)β specimens. The orientation relationships reflect the small mismatch between parallel close-packed directions in the metal and in the cation sublattice of the oxides. The correlation of distinctive oxide surface morphologies with internal structural defects indicates the strong tendency of the Al2O3 scale to growvia short-circuit diffusion paths.

Oxide morphology and spalling model for NiAl

The Rinetics of A12O3 growth and spalling were observed for Ni-42 at. pct Al oxidized in air at 1100°C up to 1500 h. While crystallographic voids formed at the oxide-metal interface due to the oxidation process, the oxidation resistance was good in 1 h cycle tests. Spalling to bare metal was the predominant mode of oxide loss. The A12O3 grain size at the metal interface varied with a time-exponent of 0.2. In isothermal tests the oxide thicRness was nearly parabolic with time, having a time-exponent of 0.40, while the volume per void varied directly with residence time underneath intact oxide. The total void volume accounted for ~1/2 the aluminum needed to form the AI2O3 scale. In 1 h cycle tests the oxide thicRness and volume per void reached a plateau at ~150 h due to the spalling process. The correlation between total void volume and oxide volume was eventually obliterated by extensive cycling. A cyclic step-process spall model was used to predict the parabolic rate constant, Rp, and the oxide spall fraction, Rs, from gravimetric curves. Predicted values of Rp agreed well with experimental values, while predicted Rs values were often less than measured values. According to this model the severity of a long time test can be rated according to the factor fMe√ Rs · Rp · Δt mg/cm2 ·cycle, where fMe is the ratio of metal-to-oxygen in the oxide and Δt is the cycle time. Measured values of Rs in isothermal tests varied linearly with exposure time or approximately with (oxide thicRness).2 Cyclic tests showed more scatter and less dependence of Rs on oxide thicRness, presumably due to the complex oxide topography and relaxed stress states.

The Oxidation and Protection of Gamma Titanium Aluminides

The excellent density-specific properties of the gamma class of titanium aluminides make them attractive for intermediate-temperature (600–850 °C) aerospace applications. The oxidation and embrittlement resistance of these alloys is superior to that of the α2 and orthorhombic classes of titanium aluminides. However, since gamma alloys form an intermixed Al2O3TiO2 scale in air rather than the desired continuous Al2O3 scale, oxidation resistance is inadequate at the high end of this temperature range (i.e., greater than 750–800°C). For applications at such temperatures, an oxidation-resistant coating will be needed; however, a major drawback of the oxidation-resistant coatings currently available is severe degradation in fatigue life by the coating. A new class of oxidation-resistant coatings based in the Ti-Al-Cr system offers the potential for improved fatigue life.

SiC and Si3N4 Recession Due to SiO2 Scale Volatility Under Combustor Conditions

SiC and Si3N4 materials were tested under various turbine engine combustion environments, chosen to represent either conventional fuel-lean or fuel-rich mixtures proposed for high speed aircraft. Representative CVD, sintered, and composite materials were evaluated in both furnace and high pressure burner rig exposure. While protective SiO2 scales form in all cases, evidence is presented to support paralinear growth kinetics, i.e. parabolic growth moderated simultaneously by linear volatilization. The volatility rate is dependent on temperature, moisture content, system pressure, and gas velocity. The burner tests were used to map SiO2 volatility (and SiC recession) over a range of temperature, pressure, and velocity. The functional dependency of material recession (volatility) that emerged followed the form: exp(-Q/RT) * Px * vy. These empirical relations were compared to rates predicted from the thermodynamics of volatile SiO and SiOxHv reaction products and a kinetic model of diffusion through a moving ...

Adherent Al2O3 scales formed on undoped nicrai alloys

Changes in the spalling behavior of Al2O3 scales formed on an undoped NiCrAl alloy are described. Two samples of Ni-15Cr-13Al (wt pct), one a control and the other sanded, were subjected to 25 oxidation cycles. It is observed that adherent scales formed on the sanded sample; however, the control sample had speckled, spalled scales. The data reveal that the adherent scales are caused by repeated removal of surface layers after each oxidation cycle. It is determined that interfacial segregation of sulfur influences spallation and sulfur removal increases bonding. The effect of moisture on scale adhesions is investigated.

Oxidation behavior of FeAl+Hf, Zr, B

The oxidation behavior of Fe-40Al-1Hf, Fe-40Al-1Hf-0.4B, and Fe-40Al-0.1Zr-0.4B (at.%) alloys was characterized after 900°, 1000°, and 1100°C exposures. Isothermal tests revealed parabolic kinetics after a period of transitional θ-alumina scale growth. The parabolic growth rates for the subsequent α-alumina scales were about five times higher than those for NiAl+O.1Zr alloys. The isothermally grown scales showed a propensity toward massive scale spallation due to both extensive rumpling from growth stresses and to an inner layer of HfO2. Cyclic oxidation for 200 1-hr cycles produced little degradation at 900 or 1000°C, but caused significant spaliation at 1100°C in the form of small segments of the outer scale. The major difference in the cyclic oxidation of the three FeAl alloys was increased initial spallation for FeAl+Zr, B. Although these FeAl alloys showed many similarities to NiAl alloys, they were generally less oxidation-resistant. It is believed that this resulted from nonoptimal levels of dopants and larger thermal-expansion mismatch stresses.

The role of Cr in promoting protective alumina scale formation by γ-based TiAlCr alloys— II. Oxidation behavior in air

Abstract The oxidation behavior of single-phase γ, single-phase Laves, and two-phase γ + Laves TiAlCr alloys was investigated at 1000°C in air. The addition of 4–5 at % Cr to the γ phase resulted in protective alumina scale formation in dry air, but titania-based nodule formation in humid room air. Two-phase γ + Laves alloys exhibited protective alumina scale formation in both dry air and humid room air. The difference in oxidation behavior between γ and γ + Laves alloys was interpreted in terms of compositional changes at the alloy/scale interface during oxidation.

The role of Cr in promoting protective alumina scale formation by γ-based TiAlCr alloys—I. Compatibility with alumina and oxidation behavior in oxygen

Abstract The substitution of greater than 8–10 at.% Cr for Ti in TiAl reduces the level of Al needed for protective alumina scale formation during oxidation, the “Cr effect”. To elucidate the mechanism of the Cr effect, the interaction of binary TiAl and ternary TiAlCr alloys with alumina was examined at 1000°C by hot-pressed diffusion couples. The oxidation behavior of the same alloys was also examined at 1000°C in dry oxygen. The dissolution of alumina in an alloy/alumina diffusion couple correlated with an inability of the alloy to form a protective alumina scale on exposure in dry oxygen because of internal attack. Both of these effects were associated with high oxygen permeability in the alloy. It was postulated that the Cr effect results from the Ti(Cr,Al) 2 Laves phase, which has a low oxygen permeability and is capable of alumina scale formation despite its relatively low Al content of 37–42 at.% Al.