Oxidation of Fe-Cr-Al and Fe-Cr-Al-Y Single Crystals

Abstract

Single crystal samples of the alloy Fe-20%Cr-5%Al with and without Y-doping were used to study the "reactive element" (RE) effect, which causes improved oxidation behaviour and formation of a protective Al2O3 layer on this alloy. The oxidation was followed by AES at 10-7 mbar O2 up to about 1000 °C. Most observations were peculiar for this low pO2 environment, but yttrium clearly favors the formation of Al-oxide and stabilizes it also under these conditions, probably by favoring its nucleation. The oxides formed are surface compounds of about monolayer thickness, not clearly related to bulk oxides. Furthermore, the morphologies of oxide scales were investigated by SEM, after oxidation at 1000°C for 100 h at 133 mbar O2. On Fe-Cr-Al the scale is strongly convoluted and tends to spalling, whereas the presence of Y leads to flat scales which are well adherent. This difference is explained by a change in growth mechanism. The tendency for separation of oxide and metal was highest for the samples with low energy metal surface, i.e. (100) and (110), the scale was better adherent on the (111) oriented surface and on the polycrystalline specimen, since in the latter cases the overall energy for scale/metal separation is higher. All observations, from the low and from the high pO2 experiments, are discussed in relation to the approximately ten mechanisms proposed in the literature for explanation of the RE effects.

Influence of minor additions on the emissivity of the oxide scale of FeCrAlY alloys during resistance heating

“…During service, the aluminum reservoir of the alloy is consumed down to a critical level, when formation of oxides other than alumina occurs, causing failure of the component [2,3]. But the consumption rate is also determined by the minor alloying elements such as additions of reactive elements e.g., yttrium, zirconium, hafnium, titanium, by an improvement of scale adherence, the prevention of the deleterious effects of sulfur, the change of the scale growth mechanism, and/or an improvement of scale plasticity by decreasing oxide grain size [4][5][6][7][8][9].…”

Surface and interface segregation in the oxidation of metals

“…Nucleation is strongly favoured by the presence of a very low yttrium content, 9,19 as demonstrated by AES observation of the events during heating of Fe-Cr-Al and Fe-Cr-Al-0.003Y single crystals (Fig. 4).…”

“…is obtained, 24 whereas otherwise, especially at temperatures <1000°C, metastable aluminas generally are formed on Fe-Cr-Al alloys and aluminides such as NiAl and FeAl. 9,25 For the initial growth of protective oxide scales, the supply of Cr or Al from the alloy is important, especially at lower temperatures where diffusion is slow. Owing to insufficient supply of Cr at temperatures of <600°C, chromia-forming steels tend to form unprotective Fe-rich scales.…”

“…Oxidation of Fe-Cr-Al single crystals has demonstrated much more cavity formation on the single crystals with (100) and (110) orientations of low surface energy, whereas less cavities were observed on the single crystals with (111) orientation and on polycrystalline material. 9 Generally, at the oxide/metal interface there should be strong bonds between the oxygen ions in the oxide and the metal atoms, corresponding to the strong bonds in the chemisorption of oxygen. With continued growth, however, more and more defects will arise at the interface, caused by: growth of oxides by outward cation diffusion (FeO, Fe 3 O 4 , NiO .…”

Pengaruh Penambahan Suatu Elemen Reaktif Pada Pertumbuhan Kerak Alumina Dalam Material Paduan Suhu Tinggi

“…Dalam paduan suhu tinggi FeAl dan FeCrAl tanpa penambahan elemen reaktif, kerak Al2O3 tumbuh karena terjadinya difusi oksigen ke arah dalam dan difusi aluminium ke arah luar paduan melewati batas-batas butir, sehingga sangat mudah terjadi pengelupasan kerak dari permukaan paduan. Sebaliknya dalam paduan suhu tinggi FeAl atau FeCrAl yang mengandung elemen reaktif, difusi aluminium ke arah luar ditekan dan pertumbuhan kerak Al2O3 terjadi terutama oleh difusi oksigen ke dalam paduan sepanjang batas-batas butir, sehingga adesinya lebih baik dan laju pertumbuhannya berkurang (7,8) . Penambahan elemen reaktif tersebut bertujuan untuk menghasilkan struktur-mikro dengan butiran-butiran yang halus dan untuk memperbaiki sifat oksidasi, sedangkan sifat mekanik yang menguntungkan masih dapat dipertahankan (9) .…”