Mechanism of Improvement in Oxidation Resistance of TiAl-Based Alloys by Heat Treatment under a Low Partial Pressure Oxygen Atmosphere

研究了TiAl基合金在静态空气中的抗氧化性以及低分压氧气氛下热处理对TiAl基合金抗氧化性的改善作用，以阐明热处理机理。用X射线分析、电子探针或俄歇电子能谱对氧化或处理过程中形成的表面层进行了研究。铝含量高的试件在空气中的抗氧化性好于铝含量低的试件，且处理效果持续时间较长。推测TiAl中较高的铝含量促进了Al_2O_3的形成。在800ﾟC空气中氧化的样品的衍射图与处理前的结果基本相同，但前者的衍射图中出现了TiO_2。在空气中氧化时，由于TiAl中Al在TiAl中的扩散系数很小，而TiAl中的Al在TiAl中的扩散系数很小，所以在空气中氧化时，不能形成连续的Al_2O_3层。另一方面，在低分压氧气氛下，虽然生成了二氧化钛和三氧化二铝，但其生长速度很慢。由于氧在Ti3Al中的溶解作用，在贫铝区Ti3Al中很难形成TiO2，而富铝Ti3Al中可能会生成Al_2O_3。因此，在低分压氧气氛下铝的选择性氧化生成了致密而连续的Al_2O_3层。这种处理效果是由于处理过程中形成了Al_2O_3层所致。

Oxidation resistance of TiAl-based alloys in static air and its improvement by heat treatment under a low partial pressure oxygen atmosphere was studied to elucidate the mechanism of the treatment. Surface layer formed on the specimen during oxidation or the treatment was investigated by means of X-ray analysis, EPMA or AES. The oxidation resistance in air was better and the effect of the treatment was maintained longer in the specimen with higher aluminum content than that in the case with less aluminum content. It is presumed that the higher aluminum content of the TiAl is considered to promote the formation of Al_2O_3. The diffraction patterns obtained from the specimen oxidized in air at 800ﾟC were almost the same to that with the treatment, except that TiO_2 is identified in the former case. In the case of oxidation in air, since TiO_2 has a large growth compared to Al_2O_3 and diffusion coefficient of aluminum in TiAl is very small, continuous Al_2O_3 layer can not be formed during oxidation. On the other hand, though TiO_2 is formed as well as Al_2O_3, it grows very slowly under a low partial pressure oxygen atmosphere. TiO_2 is hardly formed in aluminum depletion region, Ti_3Al, because of the solubility of oxygen in Ti_3Al, however, Al_2O_3 is expected to be formed in aluminum rich Ti_3Al. Thus, dense and continuous Al_2O_3 layer grows as a result of selective oxidation of aluminum under a low partial pressure oxgen atmosphere. The effect of the treatment is attributed to the Al_2O_3 layer formed during the treatment.