The influence of Y on the structure and growth mechanism of alumina scales

Abstract

Small additions of yttrium to NiAl result in a decreasing oxygen transport through the Al2O3 scale on AiAl at 1270 K, as has been shown by nuclear reaction analysis (18O(p, α)15N). 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‐Al2O3 is formed after oxidation for several minutes at higher temperatures (1470 K). The rate of transformation of transition Al2O3 to the stable α‐Al2O3 modification depends on the oxidation temperature. The transition is shown to be retarded by the presence of Y. The primary transport processes in the Al2O3 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‐Al2O3 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 Al2O3.

Materials Issues Relevant to the Development of Future Metal Foil Automotive Catalytic Converters

“…The role of titanium, a beneficial element or detrimental impurity, is still an open debate. Recent compelling papers by Quadakkers and coworkers [33,39,40] suggest that for small additions it behaves like hafnium and zirconium, but that in combination with other active elements it is easy to overdope the alloy [37,38].…”

“…[29,[37][38][39][40]. This is associated with their ability to tie up carbon and nitrogen impurities as tiny carbo-nitrides [39,40].…”

“…Similarly zirconium may be considered beneficial, when added singly, but in combination with yttrium may be detrimental to thin foil material by increasing the depth of internal oxidation [29,38]. Thus to a Fe-20Cr-5Al alloy containing 0.04wt% Y, the addition of 0.04wt% Zr may be beneficial, however, increasing either the yttrium content or zirconium content above this level can cause extensive internal oxidation, penetrating up to 120mm from each free surface [29].…”

Phase Transformation Behavior of Al2O3 Scale Formed on Pt-Modified γ′-Ni3Al-Based Alloys With and Without Hf Addition

“…However, only a limited amount of attention has been given to elucidating the effects of REs on the phase transformation to -Al 2 O 3 [11][12][13][14][15][16]. From those limited investigations, yttrium has been reported by some to slow the transformation to -Al 2 O 3 on Ni-based alumina-forming alloys during high-temperature oxidation [11,12,16];…”

“…From those limited investigations, yttrium has been reported by some to slow the transformation to -Al 2 O 3 on Ni-based alumina-forming alloys during high-temperature oxidation [11,12,16];…”