Interface alloy-oxide

In practice, thermal cycling rather than isothermal conditions more frequently occurs, leading to a deviation from steady state thermodynamic conditions and introducing kinetic modifications. Lattice expansion and contraction, the development of stresses and the production of voids at the alloy-oxide interface, as well as temperature-induced compositional changes, can all give rise to further complications. The resulting loss of scale adhesion and spalling may lead to breakaway oxidation " in which linear oxidation replaces parabolic oxidation (see Section 1.10). 

If the alloy is depleted below the concentration that will allow diffusion in the alloy to provide a sufficient fiux of Cr to the alloy-oxide interface to maintain the stability of the chromia layer, relative to Ni-containing oxides, the chromia layer can break down without fracturing. The condition for this is described by Equation (5.2). Because of this depletion effect, oxidation-resistant alloys based on the Ni-Cr system usually contain at least 18-20 wt% Cr. 

One of the most serious effects of water vapour on high-temperature oxidation is the increased spalling tendency of AI2O3 and Cr20s scales. This effect is illustrated for the cyclic oxidation of the alumina-forming superalloy CMSX-4 in Figure 7.4. The water vapour is believed to lower the fracture toughness of the alloy-oxide interface. 

Smialek has summarized the sulfur effect [10], which is generally explained in terms of the sulfur segregating to the alloy/oxide interface and lowering the work of adhesion. Sulfijr has been shown to lower the fracture energy of artificial Ni/alumina interfaces from F 100 J/m to r w 10 J/m [11]- A question remains regarding the mechanism of the... 

Figure 4.18 Comparison between measured (points) and calculated solid lines) fractions of metallic constituents in the oxide formed on MA956 as a function of distance from the alloy—oxide interface after 600 h exposure at 650°C [75]. The fractions of metallic constituents are normalized to the total concentration of metallic constituents.

After treatment at 950°C, on the other hand, Haynes 230 exhibits thicker surface layers. Figure 26.5 shows a cross-section of a 813 h coupon. The treatment has produced a duplex scale with a loose Cr-Mn spinel at the outside and a dense Cr-rich oxide inward. The alloy/oxide interface is rough and metallic islands remain included within the oxide scale. Beneath the surface layer, fine internal oxide precipitated, possibly rich in Cr. Oxidation of Al at the alloy grain boundaries is observed up to 40 pm deep. 

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