Chemical Reaction of TiAl Intermetallics with a Nitrogen Plasma

Direct surface nitridation of TiAl alloy was successfully performed using a reactive plasma process. The modified layer, composed of a double-layer structure, was fabricated using this plasma process. The top surface layer mainly consisted of a ternary nitride, Ti2AlN. The reaction mechanism in this plasma process is divided into three stages: surface reaction, diffusion of atomic nitrogen, and rearrangement of atomic configuration. The driving force to form the ternary nitride from TiAl was considered by a quantum chemical simulation. As a result of the simulation, the strong Coulomb attractive interaction between Ti and interstitial N atoms in the TiAl alloy is inferred to be predominate in the rearrangement of the atomic configuration, rather than the covalency between Al and N atoms. The surface modified layer significantly exhibits a higher hardness than the TiAl alloy. The subsurface layer retains sufficient ductility, although solution hardening was slightly achieved.