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

制备了Ni_3Al、Ni_3Al-0.1 b和Ni_3Al-1b-0.42 hf试样，并进行了以下表面处理：(1)用CO_2激光器进行激光上光，(2)在极低的氧气压力下或在氢气中进行退火，(3)用离子束技术进行表面处理。每次处理后，试样在常压下的纯净氧气流中在1100至1400 K的温度范围内氧化。在规定的条件下，对处理前后和氧化后的试样进行金相检查。490 ~ 2000 W的激光表面处理表明，随着激光功率的增加，形成的氧化铝保护层增多，抗氧化能力增强。这可能是由于处理后的晶粒细化作用增强了al的扩散，真空退火后形成了较薄的Al_2O_3层，对抑制后续氧化有很好的保护作用。在氢气气氛中退火的结果与在真空中退火的结果非常相似。在氧化初期，将Mg或Ta离子注入试样表面，在不消耗基体Ni的情况下形成极具保护作用的Al_2O_3鳞片。离子混合处理后合金表面形成含硅层。该处理还促进了极具保护性的Al_2O_3鳞片的生长。离子束增强沉积形成厚度为0.5微米的Cr_2N膜和厚度为（1微米）的Si_3N_4膜显著降低了氧化速率。

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.