Oxidation and intergranular disintegration of the aluminides NiAl and NbAl3 and phases in the system Nb-Ni-Al |
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Authors: | H J Grabke M Steinhorst M Brumm D Wiemer |
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Affiliation: | (1) Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Deutschland;(2) Present address: Elektro-Thermit GmbH, Essen, Deutschland;(3) Present address: Krupp Stahl AG, Düsseldorf-Benrath, Deutschland |
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Abstract: | The oxidation behavior is very different for an aluminide with a wide homogeneity range such as -NiAl than for a line compound such as NbAl
3.Oxidation of -NiAl at temperatures 1273 K leads to a slow-growing -alumina layer. The metal phase beneath the scale remains as -NiAl; however, cavity formation is observed. The cavity formation may be favored by sulphur surface segregation. Oxidation of NbAl
3
at temperatures 1273 K initially leads to -Al
2O3,but the Al depletion causes the formation of Nb
2
Al beneath the oxide layer. Cracking of the Al
2O3
layer opens Nb
2
Al to the atmosphere, which oxidizes rapidly to Nb
2O5 and NbAlO4.After consumption of the Nb
2
Al, a layer of Al
2O3
formed again on the NbAl
3
phase, but failure of the alumina and the fast growth of the other oxides occur as a repeated process. Thus, NbAl
3
exhibited rapid linear oxidation kinetics. Multiphase alloys in the system Nb-Ni-Al generally behave better than NbAl
3,and the low oxidation rates of -NiAl can be approached. In the temperature range below 1273 K, with a maximum at 1000 K, both NiAl and NbAl
3
show the pest phenomenon, an intergranular disintegration. Preceding the disintegration, oxygen diffuses into the grain boundaries of the material and Al
2O3
is formed at the grain boundaries, beginning from the surface region. NiAl is susceptible only in a very limited range of oxygen pressures and temperatures, whereas NbAl
3
is much more susceptible. |
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Keywords: | Oxidation high-temperature intermetallics nickel and niobium base alloys intergranular disintegration pest |
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