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

The improvement of oxidation resistance for the intermetallic compound TiAl and TiAl containing 1.5 mass% Mn was investigated using surface treatments: heat treatment under a low partial pressure oxygen atmosphere, Al diffusion coating and combined treatments utilizing both these processes. The effect of the surface treatments was evaluated by cyclic oxidation tests carried out at 900 and 950-degrees-C in static air. Heat treatment under a low partial pressure oxygen atmosphere was very effective in improving 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 either 900 or 950-degrees-C. Al diffusion coating was effective for both TiAl and TiAl-1.5%Mn at 900 and 950-degrees-C, but large cracks were found at the edges of all treated specimens. Specimens subjected to the combined surface treatment showed superior oxidation resistance to the Ni-base superalloy Inconel 713C. Specimens heat treated under the low partial pressure oxygen atmosphere followed by Al diffusion coating were found to have a tightly diffused layer with smaller cracks than those treated by diffusion coating alone.