Studies on the Improvement of High-Temperature Corrosion Resistance of Ni_3Al Base Alloys

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Ni_3Al, Ni_3Al-0.1B, and Ni_3Al-1b-0.42Hf specimens were prepared and then subjected to the following surface treatments.(1) laser glazing using a CO_2 laser,(2) annealing under a very low oxygen pressure or in hydrogen, and(3) surface treatments using ion beam techniquesAfter each treatment the specimens were oxidized in a flow of purified oxygen under atmospheric pressure in a temperature range 1100 to 1400 K. Metallographic examinations were carried out for the specimens before and after the treatment and also after the oxidation under specified conditions.The laser surface treatments at the powers of 490 to 2000 W showed that the increase in the laser power resulted in the formation of more protective alumina scales leading to enhanced oxidation resistance. This was considered to be due to the grain refining effect brought about by the treatment which would have enhanced the diffusion of Al.The annealing in a vacuum resulted in the formation of thin Al_2O_3 scales, which were very protective to suppress the subsequent oxidation. The annealing in hydrogen atmosphere showed very similar results as those for the annealing in the vacuum.Implantation of Mg or Ta ions to the specimen surfaces resulted in the formation of very protective Al_2O_3 scales during the initial periods of oxidation without consuming Ni of the base alloy. The ion mixing treatment of the alloy formed a surface layer containing Si. This treatment also enhanced the growth of very protective of Al_2O_3 scales. A Cr_2N film of 0.5 micron meter thickness and a Si_3N_4 film of (1 micron meter) formed by ion beam enhanced deposition decreased the oxidation rate significantly.