Study of Ti-Al-Cr oxidation resistant coatings for gamma-TiAl based intermetallic alloys.

Magnetron sputter deposition with single target materials was used to produce amorphous and crystalline Ti-A1-(Cr) alloy coatings on a Ti-50Al substrate. The following coatings were studied: Ti-50Al-10Cr, Ti-53Al-15Cr, Ti-50Al-20Cr and Ti-48Al. The microstructures of the coatings were studied in the as deposited condition and after devitrification and heat treatment. A random distribution of nano-precipitates was formed in amorphous as-deposited coatings. Columnar features were present when the as-received deposit had crystallised during sputter deposition. If crystallization occurred during deposition, a columnar microstructure formed with the columnar fibres being parallel to the deposition direction. For the Ti-50Al-10Cr and Ti-53Al-15Cr deposits, the crystalline domains consisted of lamellar gamma // alpha, i.e. the crystalline deposits had a gamma // alpha texture. The alpha → alpha transformation occurred during deposition. Phase competition in the alloys was studied by combining thermodynamic modelling and transformation kinetics. At 1173K, the Ti-50Al-10Cr alloy transformed to a two-phase microstructure, consisting of the gamma and C14 Ti(Al, Cr)2 phases. The Ti-53Al-15Cr alloy transformed to a three-phase microstructure, consisting of the gamma, tau and the C14 Laves phase. The gamma and tau phases were mixed finely, with gamma // tau. The Ti-50Al-20Cr alloy transformed to a two-phase microstructure, consisting of the gamma and the Cl4 Laves phase. No orientation relationship between the gamma and the C14 Laves phase was observed. Phase evolution studies at lower temperatures in the range 773K to 973K indicated that for the amorphous Ti-48Al alloy, the phase transformation path is: the amorphous phase → alpha → gamma + alpha/alpha2. A fine lamellar structure was formed, with gamma being the dominant phase. For the Ti-50Al-20Cr alloy, the phase transformation path was: the amorphous phase → gamma → gamma + Ti(Al, Cr)2. The experimental observations and the modelling results have clearly suggested a tendency of amorphous phase stabilisation via Cr addition. Thermodynamic modelling also indicated that the driving force for amorphous alloy formation is not much less than that for the precipitation of disordered solution phases. Kinetically, the amorphous phase formation during sputter deposition is related to the suppression of surface diffusion at low substrate temperatures. The temperature processing window for ordered phase formation in the Ti-Al(-Cr) alloys during magnetron sputter deposition was evaluated by the effective diffusion distance. Time dependent nucleation calculations showed that in the Ti-48Al amorphous alloy, it would be kinetically easier to precipitate the alpha phase than the gamma phase. In the case of the Ti-50Al-20Cr alloy, the gamma phase forms in preference of alpha, which is consistent with experimental observations. Diffusion phenomena at the coating/substrate interface and the oxidisation of the coatings were also studied. The experimental Cr diffusion profiles and the simulations for the Ti-Al-Cr coatings and the Ti-50Al substrate indicated that diffusion at 1173K is reasonably slow. The Ti-50Al-10Cr, Ti-53Al-15Cr and the Ti-50Al-20Cr coatings could form protective oxide scales at 1173K. When a columnar structure was present in the crystalline deposit, cracking of the coating was observed when the coating was subject to thermal cycling from elevated temperatures. It is concluded that if cracking of the coatings was to be avoided, amorphous deposits should be preferred