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Abstract
This paper reports on the comparison of scale development on unmodified β-NiAl which contains volume and surface additions of yttrium. This reactive element was incorporated using conventional alloying (0.05 or 0.1 wt%) and ion implantation. The materials containing alloyed yttrium were also implanted with yttrium to study the combined influence of volume and surface additions. A two-stage oxidation approach was applied with the use of 18O2 as a tracer. The reaction temperature was 1100°C and the samples were oxidised for up to 64 h.
The scale growth mechanism was followed with the aid of in-depth distributions of the oxygen isotopes across the scale using secondary ion mass spectrometry, the surface morphology of the scales was observed using scanning electron microscopy, while their phase composition was determined using photoluminescence spectroscopy.
Similar stages of the scale evolution were observed for the growing scales. Initially the oxide layer consisted of transient oxides, and at its surface more or less developed blade-like surface grains were observed. Subsequently, phase-transformation-related cracks and round patches appeared, and finally ridges which successively covered the outer surface of the scale and constituted the outer layer of a duplex scale developed. The mechanism of scale growth was a mixed outward and inward, and the relative contribution of both mechanisms varied depending on the analysed scale region and on the oxidation stage. The cracks and patches were regions where the transient aluminas were preferentially transformed into α-Al2O3. The ridges formed in cracks essentially consisted of α-Al2O3. For the initial stages of oxidation, the transient aluminas and α-Al2O3 co-existed in the scales, while subsequently the latter prevailed and finally, became the only phase found in the oxide layers.
The evolution rate of the scale was affected by alloyed and implanted yttrium additions: the former exhibited minor and content-dependent effect while the latter significantly retarded it.