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The Influence of Yttrium on the Oxidation Behaviour of Fe-19Cr-5Al Alloy at High Temperatures: I. Oxidation Resistance

The oxidation resistance of Fe-19Cr-5Al alloys (concentrations in wt. %) was studied in air under isothermal and thermal cycling conditions at temperatures ranging from 1173 to 1673 K. Three sets of samples were investigated: unmodified (i.e. 'pure' alloy), yttrium alloyed material (0.27 wt. % Y), yttrium implanted samples (fluence: 2xl0 ions/cm, energy: 70 keV, projected mean range: 170 Ä). In general, the reaction obeyed the parabolic oxidation rate law which suggests that the oxidation was diffusion controlled. However, for a given temperature, different regimes were found which could be described by different values of the (parabolic) oxidation rate constant. Five degradation modes were observed: (I) cracking of the oxide scale, (II) spalling of the protective alumina scale, (III) formation of less protective Fecontaining oxides, (IV) development of fast growing nodules of the iron oxides, (V) macroscopic disintegration of the samples. The main role of the alloyed yttrium was to improve the resistance to spalling of the oxide scale. However, it increased the oxidation rate. Implanted yttrium only little affected the oxidation rate and the resistance to spalling of the scale for < 1623 K. Under the studied conditions, the scale adhered fairly well to the substrate even on the unmodified alloy.