Preparation of Ni Aluminide/Ni or TiAl(3) Bilayer Coating on TiAl by Molten Salt Electrodeposition and Cyclic Oxidation Resistance

A coating of Ni aluminide/Ni or TiAl(3) bilayer on a TiAl substrate was formed by the electrodeposition of Ni with a different deposition time and the electrodeposition of Al using a molten salt electrolyte. The electrodeposition of Al was carried out using a potentiostatic polarization method in molten NaCl-KCl-AlF(3) at 1023 K. By this electrodeposition treatment, surface layers mainly consisting of Ni(2)Al(3) and intermediate layers consisting of Ni or TiAl(3) were formed. The cyclic oxidation resistance of these specimens was examined in air at 1323 K. As a result, for the specimens with a thick Ni layer as the intermediate layer, an exfoliation of the coating was observed and the mass reduction due to spalling of an oxide scale was observed. On the other hand, for the specimens with TiAl(3) or thin Ni layer as the intermediate layer, the exfoliation of the coating was not observed and a high-cyclic oxidation resistance was observed. For the specimens showing a high-cyclic oxidation resistance after the cyclic oxidation test, a surface layer consisting of NiAl with Al(2)O(3) scale and an intermediate layer consisting of TiAl(2) containing a small amount of Ni were formed. For the specimens showing exfoliation of the coating, the formation of the intermediate layer consisting of TiAl(2) was not observed.