Effect of Ti, Zr and Hf on high-temperature oxidation resistance of Fe-20Cr-5Al-0.1La alloy foils

The oxidation behavior of 50 mu m thick Fe-20 mass%Cr-5 mass%Al-0.1 mass%La alloy foils containing Ti, Zr and Hf was examined by a cyclic oxidation test at 1373 and 1473 K in air. The small addition of less than 0.08 mass%Zr and 0.25 mass%Hf decreased the growth rates of both the Al2O3 scale, which grew until the Al in the foil had been depleted, and the Cr2O3 scale, which was formed between the Al2O3 scale and the substrate after the depletion of Al. In particular, the reduction in the growth rate of the Al2O3 scale by the addition of Hf was remarkable. On the other hand, the addition of Ti increased the growth rate of the Cr2O3 scale. The segregation of Zr or Hf at the Al2O3 grain boundaries was observed using a transmission electron microscope. However, no significant Ti segregation was detected. The reduction in the growth rate of the Cr2O3 scale indicates that the oxygen diffusion rate in the Al2O3 scale is reduced by adding a small amount of Zr or Hf. Therefore, it is presumed that the segregation of Zr or Hf suppresses oxygen diffusion along the Al2O3 grain boundaries, resulting in a decrease in the growth rate of the Al2O3 scale.