HIGH-TEMPERATURE OXIDATION BEHAVIOR OF MECHANICALLY ALLOYED NI-BASE SUPERALLOYS

Ni-base superalloys were prepared by machanical alloying of Ni, Cr powders with a few minor elements in a planetary ball mill in order to examine the effect of an addition of minor elements. Milled powders were CIPed, sintered and HIPed, and then they were oxidized by cyclic heating to 1273 K from room temperature. Oxidation behavior of the specimens has been studied by means of mass gain measurements, scanning electron microscopy, X-ray diffraction, grazing incidence X-ray diffraction, electron probe microanalysis and energy dispersive X-ray spectroscopy. In the process of cyclic oxidation, scales formed on the NNi-20Cr alloy were spalled markedly; here NNi indicates Ni powders containing 11 mass%Fe in each particle. On the other hand, the alloy containing 1.0 mass%Y2O3 showed no spalling of scales, the surface oxides being spinel, Cr2O3 and YCrO3. The outer layer of the oxides consists mainly of spinels, and Cr2O3 and YCrO3 are located in the inner layer. The other alloys containing Al, Ti and Y2O3 or specimens in which NNi is replaced by pure Ni(INi) were investigated in the same way as described above. It is concluded that the Ni-Cr superalloys which contain NNi powders with the addition of 1.0 mass%Y2O3 improve the high-temperature oxidation resistance considerably.