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能，并通过低分压氧气气氛热处理提高了其抗氧化性能，以阐明热处理的机理。采用x射线分析、电子能谱分析和原子发射光谱分析等方法研究了氧化或处理过程中试样表面形成的层状结构。铝含量高的试样比铝含量低的试样在空气中的抗氧化性能更好，处理效果维持时间更长。推测TiAl中较高的铝含量促进了Al_2O_3的形成。在800℃的空气中氧化得到的样品的衍射图与处理后的样品几乎相同，只是在前者中发现了TiO_2。在空气中氧化时，由于TiO_2比Al_2O_3有较大的生长，而铝在TiAl中的扩散系数很小，氧化时不能形成连续的Al_2O_3层。另一方面，虽然TiO_2和Al_2O_3都能形成，但在低分压的氧气环境下，TiO_2的生长非常缓慢。由于氧在Ti_3Al中的溶解度，Ti_3Al中几乎不形成TiO_2，而在富铝Ti_3Al中则有望形成Al_2O_3。因此，在低分压的氧气气氛下，铝的选择性氧化使致密连续的Al_2O_3层生长。处理的效果主要归因于处理过程中形成的Al_2O_3层。

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.