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.