Modification of alumina scale formation on FeCrAlY alloys by minor additions of group IVa elements

The effect of Ti, Zr and Hf minor additions on the alumina scale formation on a high-purity, FeCrAlY model alloy has been studied. Thermogravimetry at 1,200–1,300 °C in Ar–20%O2 and two-stage oxidation using 18O-tracer were combined with characterisation by electron microscopy and sputtered neutral mass spectroscopy. After oxidation, the incorporation of Hf and Zr into the scale was far more substantial than that of Ti. This is explained by the higher thermodynamic stability of the Zr- and Hf-based oxides because the incorporation occurred to a large extent via an internal oxidation process. The scale growth kinetics is accelerated by incorporation of zirconia precipitates that provide short-circuit paths for oxygen diffusion, reduce the scale grain size and cause formation of porosity. In contrast, the incorporation of Hf-containing oxides has no such accelerative effect on the scale growth kinetics.