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The influence of Y on the structure and growth mechanism of alumina scales
Author(s) -
van Manen P. A.,
Young E. W. A.,
Schalkoord D.,
van Der Wekken C. J.,
de Wit J. H. W.
Publication year - 1988
Publication title -
surface and interface analysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.52
H-Index - 90
eISSN - 1096-9918
pISSN - 0142-2421
DOI - 10.1002/sia.740120705
Subject(s) - yttrium , grain boundary , oxide , electron microprobe , oxygen , diffusion , nial , alloy , aluminium , transition metal , chemistry , oxygen transport , materials science , analytical chemistry (journal) , crystallography , metallurgy , mineralogy , catalysis , intermetallic , thermodynamics , microstructure , biochemistry , physics , organic chemistry , chromatography
Small additions of yttrium to NiAl result in a decreasing oxygen transport through the Al 2 O 3 scale on AiAl at 1270 K, as has been shown by nuclear reaction analysis ( 18 O(p, α) 15 N). A higher concentration, e.g. 0.5 wt%, has the opposite effect. Yttrium segregates to the grain boundaries in the alloy, and an enhanced Y concentration has been observed by means of EPMA in the oxide scale above the grain boundaries of the alloy. Transition aluminium oxides are shown to be formed in the initial stage at 1170 K with (glancing angle) x‐ray diffraction. Only α‐Al 2 O 3 is formed after oxidation for several minutes at higher temperatures (1470 K). The rate of transformation of transition Al 2 O 3 to the stable α‐Al 2 O 3 modification depends on the oxidation temperature. The transition is shown to be retarded by the presence of Y. The primary transport processes in the Al 2 O 3 scales are short‐circuit diffusion of oxygen and/or aluminium. At 1170 K, while transition oxides are still present, Al transport predominates. At higher temperatures, when the oxide has the α‐Al 2 O 3 structure, oxygen transport is enhanced. This would also explain the retardation of oxygen transport by the addition of Y, since Y was found to prevent the formation of the most stable form of Al 2 O 3 .