A study on aluminide and Cr-modified aluminide coatings on TiAl alloys by pack cementation method

The halide-activated pack cementation method was utilized to deposit a suitable coating on TiAl to ensure its oxidation resistance. TiAl alloys were coated by two different pack cementation processes with simple aluminide coating and Cr-modified aluminide coating. The coatings formed on the TiAl alloyed with Nb and Cr were thicker and tougher than those formed on the non-alloyed TiAl. The TiAl3 layer formed on the TiAl alloyed with Nb and Cr had much better cyclic oxidation resistance than that formed on the non-alloyed TiAl. This is attributed to a change in the crystal structure of the TiAl3 from the brittle tetragonal DO22 to the ductile cubic LI2 by addition of a small amount of Cr and Nb. The Cr-modified aluminide coating on the TiAl alloyed with Nb and Cr exhibited excellent cyclic oxidation resistance due to formation of the Al67Ti25Cr8 phase as the coating layer.