OXIDATION BEHAVIOR OF SEVERAL CHROMIA-FORMING COMMERCIAL NICKEL-BASE SUPERALLOYS

Several commercially available Ni-base superalloys were exposed isothe rmally in air at temperatures between 750 degrees and 1000 degrees C a nd also under cyclic conditions at 1000 degrees C. The kinetics of oxi dation were determined and the scales were analyzed by electron micros copy and X-ray diffraction. Thin adherent chromia-rich scales for med on the alloys at 750 degrees C after 1000 hr. Although Waspaloy showed rite lowest weight gain in this test, it also showed the deepest inte rnal corrosion due to oxidation of the grain-boundary car bides. Ar te mperatures up to 1000 degrees C the external scales were also chromia- rich but there was greater internal corrosion. Titanium in the alloys oxidized, diffusing through the chromia scale to form faceted rutile ( TiO2) grains at the surface as well as forming TiO2 and TiN internally . The amount of rutile at the oxide surface increased with temperature and alloy Ti concentration. Alumina formed as discrete internal oxide s below the chromia scale, although Astroloy when oxidized isothermall y at 1000 degrees C developed a semicontinuous internal layer of alumi na due to its higher Al content. Under cyclic conditions Astroloy form ed a thicker, less-protective scale of transition oxides probably due to its lower Cr content.