| Budget Amount *help |
¥2,800,000 (Direct Cost: ¥2,800,000)
Fiscal Year 1998: ¥1,400,000 (Direct Cost: ¥1,400,000)
Fiscal Year 1997: ¥1,400,000 (Direct Cost: ¥1,400,000)
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| Research Abstract |
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.
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