Environmental Engineering Reference
In-Depth Information
form or their oxide dispersoids in alloys are quite beneficial with regard to retarda-
tion in degradation rate and improvements in oxide scale adherence.
Development of high-temperature alloys has taken place through the efforts
of two different research groups. The activities of the first group were mainly
devoted to improvement of high-temperature strength, while the second group
focused attention on improving high-temperature corrosion resistance. Therefore,
the present demand for high-temperature alloys requires that the above-mentioned
concurrent approaches be used so that strength property and corrosion resistance
are simultaneously achieved in the alloy component under consideration.
Many of the factors discussed earlier on the oxidation behavior of pure metals
also apply to alloys. However, alloy oxidation is generally much more complex
as a result of some or all of the following reasons:
1.
The individual elements in the alloy will have different affinities for oxygen
governed by the different free energies of formation of the respective oxides,
2.
Ternary or higher oxides may be formed, e.g., the occurrence of various
spinel-type compound formation is a possibility,
3.
A degree of solid solubilities between the oxides may exist; if aliovalent, a
doping effect may be seen.
4.
The various metal ions will have different mobilities in the oxide phase,
5.
The different metals will have different diffusivities in the alloy phase.
6.
Dissolution of different oxidants into the alloy phase may result in subsurface
precipitation of oxides, carbides, nitrides, etc., of one or more alloying ele-
ments (internal oxidation).
Therefore, the primary requirements of high temperature alloys are as follows:
1.
Acceptable high-temperature mechanical properties, i.e., best combination
of strength and toughness along with reasonable ductility,
2.
Ease of fabrication in forming and assembling the system components,
3.
Oxidation and corrosion resistance to the gaseous environment and perhaps
to fused salt condensate (e.g., in gas turbine parts).
Until now, in the development of high-temperature alloys, most efforts had
been directed to the development of materials for gas turbine engines. Over the
past five decades, in the evolution of turbine materials based on Ni and Co alloyed
with Cr, there has been a gradual increase in secondary alloying elements for
strengthening purposes, with a corresponding reduction in Cr%. As pointed out
earlier, situation demands that both oxidation resistance and strength property of
an alternative alloy progress concurrently; however, position is such that strength
development is more satisfactory than oxidation resistance. For high-temperature
use it has become necessary to consider alternatives to presently used alloys.
This has been done in the past by selecting metals on the basis of availability,
melting point, physical properties, and chemical stability, in that order.
