TEM study of scale microstructures formed on Ni-10Cr and Ni-10Cr-5Al alloys with and without Y addition

A study was conducted to investigate the effect of Y addition on the isothermal-oxidation behaviors of Ni-10Cr, Ni-10Cr-0.5Y, Ni-10Cr-5Al, and Ni-10Cr-5Al-0.5Y alloys. The alloys were oxidized in air for 50 hr at 1000degreesC. The oxides formed on the alloys were characterized using primarily cross-sectional transmission-electron microscopy techniques along with light microscopy, scanning-electron microscopy, and X-ray diffraction. Although the Al-containing alloys showed comparatively better oxidation behavior, all alloys exhibited "nonprotective" scaling, as suggested by the thick oxides formed. The major component of the outer oxide was NiO. However, modified Y-containing alloys formed protective layers (i.e., alpha-Cr2O3 for NiCrY and alpha-Al2O3 for NiCrAlY) at the scale-alloy interface following the nonprotective scaling. The spalling resistance of the modified Y-containing alloys was better than their counterpart unmodified Y-free alloys, while their overall oxidation mechanism remained unchanged after Y addition.