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Mechanically alloyed, Fe3Al-based, oxide dispersion strengthened alloys form a surface oxide scale during powder processing. This scale becomes entrained in the consolidated alloy, and may have a significant effect on subsequent recrystallisation behaviour. The high oxidation rates found in these alloys are mainly due to the bulk alloy composition. However, batch-to-batch differences in oxidation mass gain occur in powders with ostensibly identical compositions. Batches PMWY2 and PMWY3 were studied and parameters such as alloy composition and homogeneity, powder surface area to volume ratios and scale thickening rates considered. Batch PMWY2 showed 20-90% faster weight gain than PMWY3 and reached the onset of breakaway oxidation approximately twice as quickly. PMWY2 was found to contain aluminium-depleted regions, whereas PMWY3 is much more homogeneous. The surface area to volume ratio for PMWY2 was 44% higher than that of PMWY3, and batch PMWY2 was found to contain extremely fine powder particles. The scale on batch PMWY2 thickened more quickly than that on batch PMWY3, with rates 20-40% higher at different stages in the oxidation. The major contributory factor to the difference in oxidation mass gain between the two alloy batches is scale thickening rate and factors influencing thickening rates are discussed.

AbstractThe short term oxidation behaviour of an Fe,Al,Si alloy, oxidised at 1000°C in unpurified air, has been characterised using a range of techniques including transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was observed that this alloy formed a flat adherent alumina scale, even though analysis did not detect any conventional reactive elements. The research showed that the scale developed during the early stages of oxidation was similar to that formed at longer times (up to 24 hours) and that the tendency for the scale to remain flat and adherent was due to the presence of an impurity, uranium. Upon initial oxidation, uranium was found to segregate strongly to the metal/oxide interface, but was not detected at the oxide grain boundaries until oxidation times of approximately 20 minutes. Sulphur was observed within the scale during the initial stages of oxidation but was not associated with the uranium, the oxide grain boundaries or the metal/oxide interface. Therefore, uranium is not acting as a sulphur getter, reducing sulphur activity in the scale, but instead it appears to either block the sites at the oxide/metal interface where sulphur would normally segregate or change the chemistry of the interface in such a way as to make the segregation of sulphur thermodynamically unfavourable.

Specimens of polycrystalline nickel have been annealed at ~700°C and oxidised in situ within a scanning Auger microscope. Auger spectroscopy has been used to monitor the chemical composition of the free surface, and real time secondary electron imaging used to observe the specimen surface. A novel segregation effect has been recorded whereby sulphur emerges on to the free surface via emergent grain boundaries in the form of sulphurous bands, displacing the established oxide overlayer. The kinetics of the broadening of these bands depends on the tenacity of the oxide which, in turn, depends on the levels of sulphur present on the specimen surface before oxidation. Qualitative mechanisms are proposed for the observed effects.