Oxidation Behavior of Intermetallic Compound TiAl under Low Oxygen Partial Pressure

本文研究了金属间化合物TiAl在约10^<-3> Pa的低氧分压下的氧化行为。TiAl在空气中氧化性能变差，因为它不能在表面形成连续致密的A1208层。而当TiAl在低氧分压下氧化时，在表面形成一层连续的Al_2O_3保护层，抗氧化性能得到了显著提高。形成连续Al_2O_3层的机制是由于z相作为Al耗尽层的形成，可以溶解大量的氧。这可以延缓TiO_2的形成，有利于形成连续的Al_2O_3层。对于Al含量较高的金属间化合物TiAl_2和TiAl_3，它们在氧化过程中不形成具有较高氧溶解度的相作为Al耗尽层。因此，这些化合物在低氧气压下的热处理没有表现出任何改善。这表明，在低气压热处理过程中，z相对形成连续的Al_2O_3保护层起着重要作用。为了确认在低氧分压氧化过程中形成的保护层的稳定性，用维氏硬度计进行了压痕试验，并进行了扫描电镜观察。因此，当保护面层受到损伤时，表层会产生一些裂纹，但低氧分压下的热处理效果得以保持。这可能是由于由于损伤附近的应变引起的位错导致的Al扩散增强而产生的保护尺度的自愈。

Oxidation behavior of intermetallic compound TiAl has been studied under low oxygen partial pressure of about 10^<-3> Pa using a After oxidation, the phases formed during oxidation were detected by X-ray diffraction. TiAl shows deteriorated oxidation behavior in air, because it dose not form a continuous and dense A1208 layer on the surface. However, when TiAl is oxidized under low oxygen partial pressure, a protective and continuous Al_2O_3 layer is formed on the surface resulting in superior improvement in oxidation resistance. The mechanism to form a continuous Al_2O_3 layer is resulted from the formation of Z-phase as an Al depletion layer which can dissolve a large amount of oxygen. This may retard the formation of TiO_2 and aid to form a continuous Al_2O_3 layer. In the case of the intermetallic compounds with higher Al content, TiAl_2 and TiAl_3, they do not form a phase with higher solubility of oxygen as an Al depletion layer during oxidation. As a resu1t, these compounds do not show any improvement by the heat treatment under low oxygen air pressure. This suggests strongly that Z-phase plays an important role to form a continuous and protective Al_2O_3 layer during the heat treatment in a low air pressure. To confirm a stability of the protective surface layer formed during the oxidation under low partial pressure of oxygen, indentation tests using Vicker's hardness tester and SEM observation were done. As a result, when the protective surface layer is suffered damages, some cracks are induced in the surface layer, however, the effect of the heat treatment under low partial pressure of oxygen is maintained. This is probably due to self-healing of the protective scale yielded from enhanced Al diffusion through dislocations induced by the strains in the vicinity of the damages.