Environmental Engineering Reference
In-Depth Information
The alumina scale generally consists of two zones comprising equiaxed and co-
lumnar grains, where the former are localized to the oxide-gas interface and the
latter to the inner, alloy-scale interface. Even though the presence of reactive
element in alumina-forming alloys does not exactly reverse the scale growth
mechanism as observed in Cr
2
O
3
-forming alloys, it is often proposed that the
outward transport of aluminum is suppressed. But the most spectacular effect
that is brought about by the presence of a reactive element in alumina-forming
alloys is a change in the microstructure and morphology of the oxide scale. Oxida-
tion of yttrium-free
-NiAl produced a convoluted scale surface with large ridges
and whiskers, whereas Y-implanted
β
-NiAl gave rise to a smooth scale surface
without any whiskers. Studies on FeCrAl alloys showed that Y-doped samples
developed a distinct columnar grain morphology (average width 0.5
β
µ
m) in con-
trast to equiaxed grains (1
m) formed on Y-free samples [52]. So the presence
of a reactive element in alumina-forming alloys decreases scale convolutions,
induces oriented rather than randomly distributed oxide grains, and decreases the
oxide grain size, favoring oxygen transport over aluminum diffusion. The reactive
elements probably exert their influence by segregating to the grain boundaries
in the oxide scale and thereby cause significant changes in the grain boundary
transport. An enhanced oxygen transport and a reduced aluminum transport favor
oxide growth at the alloy-scale interface and reduce the formation of voids and
cavities at this interface. This, in turn, possibly reduces exfoliation and spallation
of the alumina scales in RE-containing alloys.
Buckling of the Cr
2
O
3
scale on pure chromium is also thought to be due to
the formation of new oxide within the existing layer, and the role of active ele-
ment addition is to modify this.
During high-temperature performance of either Cr
2
O
3
-orAl
2
O
3
-forming
alloys, improvement in scale adherence is possibly the major beneficial effect
contributed by the reactive element/oxide dispersion addition, particularly during
thermal cyclings. Of the mechanisms suggested, elimination of interfacial voids
and mechanical keying or peg formation seem to be of most significance. The
formation of oxide pegs is perhaps the most decisive factor for oxide adhesion
during the latter stages of growth, whereas vacancy sink processes are important
in the early stages of oxidation. However, a fine, uniform distribution of tiny
oxide pegs will produce the best effects. For Al
2
O
3
-forming alloys, there exists
controversy about the beneficial or detrimental effects of certain active elements
even though Y has shown beneficial effects in terms of both reduction of oxida-
tion rate (marginally) and improved scale adhesion (most recognized). Reactive
elements do modify the microstructure and morphology of the alumina scales
not only by decreasing the grain size but by inducing intergranular segregation
or second-phase precipitation, which influences both transport processes and the
plasticity of the scales. The most recognized effect of the reactive element addi-
tions in Al
2
O
3
-forming alloys is the improvement in scale adhesion. It generally
µ
