High-temperature oxidation resistance of sputtered micro-grain superalloy K38G

The oxidation of sputtered and cast superalloy K38G specimens was studied. The sputtered alloy was microcrystalline, with an average grain size <0.1 μm. The mass gains of the sputtered alloy were much less than those of the cast alloy at 800, 900, and 1,000C up to 500 hr, and were even less than those of pack aluminide on the cast alloy. K38G is a chromia-forming cast nickel-base superalloy, so the oxide scale formed on it is composed of Cr2O3, TiO2, Al2O3,and a spinel. The oxide scale formed on the sputtered alloy was Al2O3. This scale is thin, compact, and adherent. This result implied that microcrystallization reduced the critical aluminum content necessary to form alumina on the surface of this superalloy. No oxide spallation, as typically observed for cast of aluminized alloys, occurred on the sputtered superalloy. The reduction of the critical aluminum content for the formation of alumina and the improvement of the spallation resistance may be attributed to the microcrystalline structure formed during sputtering. The numerous grain boundaries favor outward aluminum grain-boundary diffusion, provide increased nucleation sites, and reduced stresses in the oxide scales