Alumina scales

Cyclic Oxidation In many industrial applications it is particularly important for the component to be resistant to thermal shock for example, resistance-heating wires or blading for gas turbines. Chromia, and especially alumina, scales that form on nickel-base alloys are prone to spalling when thermally cycled as a result of the stress build-up arising from the mismatch in the thermal expansion coefficients of the oxide and the alloy as well as that derived from the growth process. A very useful compilation of data on the cyclic oxidation of about 40 superalloys in the temperature range 1 000-1 I50°C has been made by Barrett et... 

M. Le Gall, A.M. Huntz, B. Lesage, C. Monty, J. Bemardini. Self-diffusion in a-AI2O3 and growth rate of alumina scales formed by oxidation effect of Y203 doping // J.Mater.Sci.- 1995.- V.30, No.l.- P.201-211. 

T. A. Ramanarayanan, M. Raghaven, and R. Petkovic-Luton, The Characteristics of Alumina Scales Formed on Fe-Based Yttria-Dispersed Alloys, J. Electrochem. Soc., 131, 923-931 (1984). 

Fig.4. Binary phase diagram for the Ni-Al system showing the compositions which form protective external alumina scales, after Doychak [5],...

Alloys in the Ti-Al system arc of interest for high temperature systems such as aircraft engines because they have low density and maintain strength at high temperature. However, their resistance to oxidation and interstitial embrittlement is a concern. Those alloys which form alumina scales have excellent resistance to surface re-... 

The oxidation of Ti3A1 alloys would not be expected, in light of the above thermodynamic considerations, to form continuous alumina scales. Instead they form mixed rutile-alumina scales [49].The oxidation kinetics of Ti3Al between 600 and 950°C are reported to be essentially those expected for rutile growth [50,51]. These kinetics result from the development of a complex oxide layer which contains continuous paths of Ti02 through which rapid transport occurs. Typical oxide scales are shown schematically in Figure 14. 

In a study to clarify the nitrogen effect [60] protective alumina scales were observed to form on TiAl (52 at% Al), exposed in Oz, up to 1000°C. However, the same exposures conducted in air resulted in the formation of Ti02-rich scales which grew at rates orders of magnitude faster than pure alumina scales and even trace amounts of N2 influenced the oxidation morphology, Fig. 16. The rate of oxidation increased continually as increasing amounts of N2 were added to pure 02 at 900 °C. The addition of... 

N2 to 02 resulted in the formation of nodules of intermixed Ti02 and AI2O3 interspersed with thin areas of protective A1203 which would cover the entire surface in the absence of N2.The area density of these nodules increased as the concentration of N2 increased until the surface was completely covered with the mixed oxides when the gas contained 90 % N2. One effect of N2 appears to involve the nucleation and initial growth of the scale since preoxidation in a nitrogen-free gas develops an alumina scale which remains protective during subsequent exposures in air [60] even under cyclic conditions [68]. 

Fig. 22. Oxide map showing the regions of Cr and A1 contents which form external alumina scales in air at various temperatures (left) and cyclic oxidation kinetics forTi-Cr-Al alloys over a range of compositions for exposures in air at 1000°C (right).

Fig. 23. Section of the Ti-Cr-Al phase diagram at 1000°C indicating the equilibrium phases for the alloy compositions which form continuous alumina scales (after Brady et al.).

A number of studies have evaluated the effects of alloying elements on the oxidation behaviour of Nb-Al alloys [132,136-141], In some cases the alloys formed alumina scales but none of the alloys were resistant over a broad enough range of exposure conditions to be considered for extensive application. This is particularly the case when the compitions are restricted to those which provide even marginal mechanical... 

In the earlier studies [1] the phase 3-NiAl was found to be very oxidation resistant, forming a protective alumina scale in a wide range of temperature and oxygen pressure. However, in more recent studies [2-5] many critical features were detected in this alumina formation and even accelerated attack by intergranular and internal oxidation was observed under special conditions. 

Failure of Alumina Scales on NiAl Under Graded Scale Loading... 

One of the difficulties of a quantitative analysis of scale failure is the determination of the stress state in the scalers it results from the oxide growth process and from additional substrate deformations. The application of X-ray diffraction provides usually only a limited lateral spatial resolution. For alumina scales consisting of the a-Al20, phase, Lipkin and Clarke [20] have shown recently that stresses can be measured at RT also by means of optical fluorescence spectroscopy (OFS) employing the piezo-spectroscopic effect [21]. This method permits a high spatial resolution of stress measurements in the micrometre region and is therefore especially useful for the analysis... 

The morphology of alumina scales on NiAl depends on the oxidation temperature and oxidation time. At lower temperatures (<1400 K). the metastable oxide phases 7-and 0-Al2O3 are formed initially. These metastable phases transform into a-AFO, during a transient time which decreases with higher temperature (> 1200 K) [24—26J. 

After cooling of the specimens to RT, the alumina scales were mostly well adherent. Spontaneous spallations were observed only occasionally. In those cases, the formation of large interfacial voids or high densities of smaller voids was found to be the reason for the weak adherence. 

The development of stresses in the scale is caused by various mechanisms which are briefly considered in the following. The relation between the stress, elastic strain, 8el, within the alumina scale is given by the Hooke s law. The elastic properties of the polycrystalline scale are assumed to be isotropic with E0 as Young s modulus and i as Poisson s ratio. Because of the free surface of the scale, a plane stress state in the scale is supposed with = 0. z is the direction perpendicular to the film plane, and x,v are the in-plane coordinates. The x-component of the stress tensor is then given by... 

The alumina scale shown in Figs. 7c and d failed certainly by the buckling mechanism. Within the spalled regions on the specimen surface, large interfacial voids were detected which represent interfacial flaws where the scale buckles during scale loading. A mechanical stability analysis yields the critical equi-biaxial stress, orh. when buckling occurs [7]... 

In the present analysis, large-scale spallations of alumina scales on NiAl were always connected with the presence of interfacial voids. In the absence of large voids and for small strain rate, it was found that the scale was well adherent under compressive substrate deformation even for large specimen deformations (cf. Fig. 7a).This suggests the presence of an effective stress relief mechanism by a slight scale wrinkling or by oxide Coble creep. 

The oxidation life of ODS FeCrAl alloys is determined by their ability to form or reform a protective alumina scale, and can be related to the time required for the aluminum content of the alloy to be depleted to some minimum level [2-5]. As a result, the service life is a function of the available aluminum content of the alloys and the minimum aluminum level at which breakaway oxidation occurs. Therefore, there is a minimum cross sectional thickness which can be safely employed at temperatures above 1200°C.The major factors that result in depiction of the reservoir of aluminum in the alloy are the inherent growth rate of the aluminum oxide and the tendency for the scale to spall, which results in a (temporary) increase in oxide growth rate in the area affected by spallation. Because of their significantly higher aluminum content >28 at% compared to 9 at%), alloys based on Fe3Al afford a potentially larger reservoir of aluminum to sustain oxidation resistance at higher temperatures and, therefore, offer a possible improvement over the currently-available ODS FeCrAl alloys [61-... 

Fig. 10. SEM secondary electron image of the alumina scale surface formed on La203-dispersed (0.2 %La) FAS after 50 h at 1200°C in Oz.The highly convoluted scale results in an accelerated oxidation rate.

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