Abstract: The isothermal and cyclic oxidation behaviors in air and hot corrosion behaviors in Na₂SO₄ + 25 wt% K₂SO₄ salt of M951 cast superalloy and a sputtered nanocrystalline coating of the same material were studied. Scanning electron microscopy, energy dispersive X-ray spectroscope, X-ray diffraction, and transmission electron microscopy were employed to examine the morphologies and phase composition of the M951 alloy and nanocrystalline coating before and after oxidation and hot corrosion. The as-sputtered nanocrystalline layer has a homogeneous γ phase structure of very fine grain size (30-200 nm) with the preferential growth texture of (111) parallel to the interface. Adherent Al₂O₃ rich oxide scale formed on the cast M951 alloy and its sputtered coating after isothermal oxidation at 900 and 1000 °C. However, when being isothermal oxidized at 1100 °C and cyclic oxidized at 1000 °C, the oxide scale formed on the cast alloy was a mixture of NiO, NiAl₂O₄, Al₂O₃ and Nb₂O₅ and spalled seriously, while that formed on the sputtered coating mainly consisted of Al₂O₃ and was very adherent. Nanocrystallization promoted rapid formation of Al₂O₃ scale during the early stage of oxidation and enhanced the adhesion of the oxide scale, thus improved the oxidation resistance of the substrate alloy. Serious corrosion occurred for the cast alloy. The sputtered nanocrystalline coating apparently improved the hot corrosion resistance of the cast alloy in the mixed sulfate by the formation of a continuous Al₂O₃ and Cr₂O₃ mixed oxide layer on the surface of the coating, and the pre-oxidation treatment of the coating led to an even better effect.

Key words: M951 alloy, Sputtered nanocrystalline coating, Oxidation, Corrosion, Pre-oxidation