Influence of the mode of introduction of a reactive element on the high temperature oxidation behavior of an alumina-forming alloy. Part III: The use of two stage oxidation experiments and in situ X-ray diffraction to understand the oxidation mechanisms

The aim of this work was to investigate several different yttrium introduction routes to improve the high temperature oxidation resistance of a Fe-20Cr-5Al model alloy. Y2O3 sol-gel coatings, Y2O3 metal-organic chemical vapor deposition (MOCVD) coatings, yttrium. ion implantation and yttrium as alloying element (0.1 wt.%) were the different methods of introduction of the reactive element. Both isothermal and cyclic oxidation tests showed that the surface introduction of yttrium or yttrium oxide did not drastically improve the oxidation behavior of the steel. Complementary experiments were performed to understand the lack of major beneficial effects of the so-treated samples. Two stage oxidation experiments under 200 mbar O-16(2) and O-18(2) followed by secondary neutral mass spectrometry (SNMS) were performed to understand the alumina scale growth mechanisms, according to the introduction route of the reactive element. The results exhibited that the yttrium induced an increase of the inward transport of oxygen through the alumina scale compared to the untreated specimen. Nevertheless, the outward transport of aluminum was generally observed, except for the specimen containing Y as alloying element, which exhibited only a single O-18 peak close to the metal/oxide interface. Phase transformations during the oxidation at 1100 degrees C were registered by in-situ X-ray diffraction (XRD). The untreated alloy was only covered by a thin layer of alpha-Al2O3. For implanted specimens, yttrium was incorporated in Y3Al5O12 and YAlO3 phases. All the YAlO3 is transformed into Y3Al5O12 after less than 10 h. For the MOCVD or the sol-gel coated samples, the primary formed YAlO3 phase was progressively transformed into Y3Al5O12. For the Fe-20Cr-5Al-0.1Y alloy, no yttrium containing phases could be detected, even after 40 h of oxidation test at 1100 degrees C.