Oxide scale microstructure

Metal-metal eutectics have been studied for many years due to their excellent mechanical properties. Recently, oxide-oxide eutectics were identified as materials with potential use in photonic crystals. For example, rodlike micrometer-scaled microstructures of terbium-scandium-aluminum garnet terbium-scandium per-ovskite eutectics have been solidified by the micro-pulling-down method (Pawlak et al., 2006). If the phases are etched away, a pseudohexagonally packed dielectric periodic array of pillars or periodic array of pseudohexagonally packed holes in the dielectric material is left. 

Fig. 14. Schematic illustration of the stages ofTiAl oxidation by means of the weight gain curve and the corresponding microstructures of the oxide scale.

The observed differences in scale thickness and microstructure between the oxide scales and subsurface zones at the various oxidation temperatures seem to be mainly attributed to the different diffusion rates at the respective temperatures. Since the oxidation products formed do not show any differences in the temperature range of 800°C to 1000°C it is concluded that no significant effect of the thermodynamic stability on the composition and structure of the oxidation products occurs. From the calculations of Rahmel and Spencer [21] it is known, however, that the activity of A1 and Ti in the system Ti-Al varies depending on the temperature. Thus it has to be taken into account that the temperature may have an influence on the expansion of the phase fields of some important phases in the system Ti-Al-N-O. Nevertheless it is evidently the temperature which mainly influences the kinetics because the structure of the metal/oxide interface, the formation of titanium nitrides, A127039N and an aluminium depleted metal phase is on principle always very similar. In this way the effect of different temperatures can, to a certain degree, be interpreted as that of a shift in the different stages of the oxidation process. 

Figure 8.32 Comparison of the microstructure of Co-Cr-Al-Y specimens after isothermal oxidation at 1000 °C in 1 atm of oxygen where (a) the specimens were coated with 5 mg cm of Na2S04, or (b) pre-sulphidized for 20 s in an H2S-H2 gas mixture with H2S/H2 = 0.2 prior to oxidation. Protective oxide scales have been formed on both specimens.

SEM micrographs of the surfaces after oxidation at 1000, 1200, and 1400 C (represented as Z-1000, Z-1200, and Z-1400, respectively) are shown in Figure 4.12. The microstructures of the oxide scale surfaces varied considerably with oxidation temperature. After 1000 C oxidation (see Z-1000), no visible oxide crystals could be seen except for some bubbles on the surface of Zl-1000. 

Mechanical properties, microstructure, adhesion and growth of oxide scale has been reviewed recently [105,106] and it has been found that oxidation resistance of a metal largely... 

S. Shimada, M. Nishisako, M. Inagaki, and K. Yamamoto, Formation and microstructure of carbon containing oxide scales by oxidation of single crystals of zirconium carbide. J. Am. Ceram. Soc., IS, 41-48, 1995. 

Very few studies have been conducted on the oxidation behavior of CoAl (Hutchings andLoretto, 1978 Barrett and Titran, 1992). It seems to form an AI2O3 scale similar to that of NiAl and spall in a similar manner. No careful study has been made of AI2O3 scale microstructure or growth rates, nor is information available about the effect of adding a RE. 

Addition of Hf has a similar effect to that of Zr with respect to increasing the scale resistance to spallation and suppressing the broccoli effect . However, the effect of Hf on the scale microstructure is somewhat different from that of Zr resulting in slower oxidation kinetics. 

Comparison of the surface morphologies of the samples after exposure showed two different microstructures when viewed under a low power optical microscope. In some samples the oxide scale had spalled randomly across the whole surface of the coupon, in small discrete regions, while in other samples the surface was criss-crossed with a series of cracks leaving the... 

The matrix becomes depleted in those elements that are selectively oxidised to form the thick oxide scales, leading to microstructural changes and associated potential effects on the long-term strength. This effect has been investigated in this work for AUoy 800 and P92. 

U. Krupp, S. Y. Chang and H. J. Christ, Microstructural changes in the sub smface area of Ni-base superalloys as a consequence of oxide scale failure. Materials Science Forum, 2001, 369-372, 287. 

The examples presented above demonstrate the applicability of the combination of the finite-difference technique and computational thermodynamics to complex corrosion processes which depend substantially on the material s microstructure. Of course, much more experimental work is required to describe in detail the variety of possible transport processes, e.g., taking place within the porous inner oxides scale and along the substrate grain boundaries. Strictly speaking, the structure of the interfaces should change as soon as the grain boundaries are covered by an oxide phase which is in contact with the matrix on both sides. Also, the actual value of the grain boundary thickness and its implementation in the finite-difference approach... 

Shimada, S., Nishisako, M., Inagaki, M. (1995). Formation and Microstructure of Carbon-Containing Oxide Scales by Oxidation of Single Crystals of Zirconium Carbide. Journal of the American Ceramic Society, 78( ), 41-48. doi 10.1111/j. 1151 -2916.1995. tb0835 8.x. 

Fig. 4 Schematic ideal microstructure in cross-section view of porous supports of MS-SOFC showing metallic bridges between grains (necks) where the oxide scale Ct203 (in blue) does not disrupt the necks between the alloy particles. This enables fast thermal and eleetrieal conductivity. Thick oxide scale contributes to corrode eompletely the neeks and is thus not favorable...

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