| Ni-based superalloy K38 has been used as materials for blades and vanes in modern gas turbines. Nanocrystalline coatings, deposited by means of sputtering and having the same chemical compositions as the substrates, exhibit excellent resistance against high temperature oxidation. Addition of minor amounts of reactive elements in alloys was reported to be beneficial to the resistance against high temperature oxidation and hot corrosion. In this thesis, reactive element Y with different contents was added in the cast superalloy K38 and the sputtered nanocrystalline coatings of K38. The isothermal oxidation behavior in air at 800, 900 and 1000°C, as well as the cyclic oxidation behavior at 900 and 1000°C, and the hot corrosion behavior either in the mixed molten salt of 75wt%Na2 S O4 +25wt%K2 SO4 or in the air with the salt deposit of mixed 75wt%Na 2 SO4 +25wt%NaCl at 900°C were studied. The specimens prior to and after tests were characterized using SEM/EDS, XRD and EPMA. The synergetic effects of nanocrystallization and Y addition on the oxidation and hot corrosion behavior were discussed.The additions of reactive element Y in the cast K38 were found to be able to promote the selective oxidation of Al, to reduce or even prohibit the occurrence of internal oxidation, and to improve the adherence of the oxide scales. The alumina volume percentage in the external scales increased with the increase of Y contents in the alloy. The addition of 0.1 wt% Y could eliminate the internal oxidation of Al, resulting in the formation of continuous alumina scales. When the Y contents in the alloy reached to 0.5wt%, however, Y-rich grains formed in the alloy, which led to the formation of oxide nodules.Continuous external scales ofα-Al2 O3 formed on the nanocrystalline coatings. On the Y-free coatings, TiO2 nodules were observed on the alumina scales, and local internal oxidation was observed as well. As to the coatings with the additions of 0.05wt% or 0.1wt% Y, neither surface TiO2 nodules nor internal oxides were observed, but the isothermal and cyclic oxidation rates were lowered, the adherence of the oxide scales was enhanced, and the life times were much extended. For the specimens with the addition of Y above 0.5wt%, the isothermal oxidation rates were accelerated, surface TiO2 nodules formed, and the resistance against cyclic oxidation was reduced. During the tests in molten sulphate at 900°C, the hot corrosion rates of the cast K38 were lowered by Y-modification, which may be attributed to the formation of continuous inner alumina layer, hindering the internal sulfidation. Nanocrystallization increased the hot corrosion resistance significantly through the formation of continuous exclusive alumina scales. The addition of Y in the nanocrystalline coatings further improved the hot corrosion resistance. The optimal performance of the coatings was obtained with the addition of 0.5wt%Y, which remained resistant even after 200 hours tests.During more severe tests at 900°C where the specimens were deposited with the mixed salt films of 75% Na 2 SO4+25% NaCl, the coatings of Y-free or with Y-addition of 0.05% or 0.1wt% exhibited severe chlorization-oxidation, and extensive spallation of the oxide scales was observed after 60 hours exposure. The coatings with 0.5% Y, however, behaved much better than the others. On these specimens, the Al-rich oxide scales remained intact, dense and protective even after 120 hours exposure. The beneficial effects of Y on the hot corrosion resistance could be attributed to the competing chlorization. It is proposed that the chlorization of Y reduced the chlorization of the other alloying components at the oxide/salt interface, and therefore reduced the chlorization consumption of the benefit components and prolonged the lifetime of the coatings.
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