IMPROVEMENT OF OXIDATION RESISTANCE FOR TIAL BY SURFACE TREATMENTS UNDER A LOW PARTIAL-PRESSURE OXYGEN ATMOSPHERE AND ALUMINUM DIFFUSION COATING

The improvement of oxidation resistance for intermetallic compound TiAl and TiAl containing 1.5 mass% manganese was investigated by means of surface treatments; heat treatment under a low partial pressure oxygen atmosphere, aluminum diffusion coating, and combined surface treatment composed of two stated above. The effect of the surface treatments was evaluated from the results of the cyclic oxidation tests carried out at 900-degrees-C and 950-degrees-C in static air. Heat treatment under the low partial pressure oxygen atmosphere was very effective to improve the resistance for the cyclic oxidation of TiAl at 900-degrees-C, but not sufficient for TiAl at 950-degrees-C or for TiAl-1.5%Mn alloy at 900-degrees-C and 950-degrees-C. Aluminum diffusion coating was effective for both the specimens at 900-degrees-C and 950-degrees-C, but large cracks were found in every specimen at the edges. The specimens with combined surface treatment showed a superior oxidation resistance to the nickel-base superalloy Inconel 713C. Especially, specimens heat treated under the low partial pressure oxygen atmosphere followed by aluminum diffusion coating were found to have a tight diffused layer with smaller cracks, compared to ones with diffusion coating only.