Abstract
The effect of Y content on the oxidation behavior of Ni3Al alloys in air has been studied in the cyclic oxidation between 1273, 1373 or 1473 K and room temperature as well as in the isothermal oxidation at 1373 K by means of mass gain measurements, optical micrography, scanning electron microscopy, X-ray diffraction, and electron pyobe microanalysis.
In the cyclic oxidation, the scales formed on the Ni3Al base alloy spalled markedly during cooling, resulting in a very complicated construction of oxides consisting of NiO, NiAl2O4 and Al2O3. On the other hand, the alloys containing 0.03 and 0.5 at%Y showed no spalling of the surface oxides in any of the oxidation conditions studied. In the isothermal oxidation, the surface oxide on the base and Y-added alloys was predominantly Al2O3 and the mass gain slightly increased with increasing Y content.
The scales were very adherent on the Y-added alloys. This was attributed to the annihilation of Kirkendall voids formed at the scale/alloy interface. The deeply penetrating Al2O3 layer along the alloy grain boundaries was observed in the 0.5 at%Y alloy. There is a possibility that this protruding oxide leads to the operation of the so-called keying mechanism. But less adhesion of the scale on the 0.5%Y alloy than on the 0.03%Y alloy may be associated with the formation of YAG particles in the vicinity of the scale/alloy interface on the former alloy. The present results indicate the existence of optimum Y content for the best oxidation resistance.
