Lou Hanyi

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 1000°C 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 micro crystallization reduced the critical aluminum content necessary to form alumina on the surface of this superalloy. No oxide spoliation, 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 spoliation 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.

Effect of Yttrium on the hot-corrosion resistance of sputtered CoCrAl coatings

Abstract The effects of the active element yttrium (0.5 wt.%) on hot-corrosion resistance of sputtered Co30Cr6Al coatings (where the composition is in approximate weight per cent) on superalloy IN-738 have been studied. The results showed that the addition of yttrium obviously enhanced the resistance against the hot corrosion of the CoCrAl coating. The Cr 2 O 3 scale formed on yttrium-free coating was prone to spalling in NaCl-containing molten salts. On the contrary, the Al 2 O 3 scale formed on the CoCrAlY coating was very resistant to hot corrosion. It was suggested that the yttrium may promote the initial process of Al 2 O 3 nucleation to form a protective Al 2 O 3 scale on the CoCrAlY coating. It was shown that the active element yttrium can improve the Al 2 O 3 film adherence remarkably.

Hot corrosion of yttrium-modified aluminide coatings

Abstract The beneficial effect of yttrium on the oxidation and hot corrosion resistance of electron beam-physical vapour deposition and low pressure plasma spraying overlay coatings has been confirmed by many studies. However, these coating processes have two disadvantages: (1) a restriction on the application of the coatings to complex geometries and (2) high equipment investment. In this study, a very simple slurry process was used to introduce yttrium into an aluminide coating. Its hot corrosion was tested in 25%NaCl + 75%Na2SO4 at 1123 K and the result showed that the hot corrosion resistance of the aluminide coating is greatly improved by the addition of yttrium. The internal oxidation and sulphidation of yttrium-free coating is more severe than that of the coating containing yttrium. It is observed that yttrium is enriched in the outer Al2O3 layer which is very adherent to the coating. However, oxide spalling occurred on the surface of the yttrium-free aluminide coating.