A study of aluminide coatings on TiAl alloys by the pack cementation method

The lower case halide-activated pack cementation method was utilized to deposit aluminide coatings on TiAl alloys. Emphasis was placed on the effect of alloying elements on the aluminizing behavior of TiAl alloy. The addition of a small amount of Nb or Cr in the TiAl improved significantly the aluminizing kinetics of TiAl alloys by increasing the solid-state diffusion of Al through the formation of stable TiAl3 layer. The TiAl3 layer formed on the TiAl alloyed with Nb or Cr had better toughness than the TiAl3 formed on the non-alloyed TiAl. The reason for better toughness of the coating formed on TiAl was that partial TiAl3 with tetragonal structure was changed to high symmetry cubic L12 structure since Nb or Cr was dissolved into TiAl3. The TiAl3 layer formed on the TiAl alloyed with Nb or Cr had much better oxidation resistance than the TiAl3 layer formed on the non-alloyed TiAl. It was attributed to change in the crystal structure of TiAl3 from the brittle tetragonal DO22 to the ductile cubic L12 by addition of small amount of Nb or Cr.