Abstract
Gamma TiAl based alloys are considered as alternative materials to Ni based superalloys for applications in gas turbine engines. However, they exhibit low oxidation resistance above 700 °C. The need to develop oxidation resistant coatings for gamma-TiAl based alloys was the motivation for this project. Three TiAlCr-X coatings were studied: These were the Ti-50Al-10Cr (TiAlCr) and Ti-48Al-9Cr-4B (TiAlCrB) coatings and the Ti-48Al-9Cr-4B with Ag interlayers multilayer (TiAlCrB/Ag) coating. The coatings were deposited on Ti-48Al-2Nb-2Mn alloy and Si wafer substrates by RF sputtering using optimised deposition parameters. A pure Ag and two alloy (Ti-50Al-10Cr and Ti-48Al-9Cr-4B) targets were used for sputtering. The coatings were studied by SEM, EPMA, XRD, XPS, TEM, DSC, DTA and TG. The morphology and the microstructure of the as-deposited coatings on Si and Ti- 48Al-2Nb-2Mn substrates were the same. All the coatings exhibited dense columnar morphology with ~1 mum thick columns for the TiAlCr and TiAlCrB and coarser columns for the TiAlCrB/Ag. The TiAlCr and TiAlCrB coatings were amorphous with randomly dispersed alpha-Ti nanocrystals. In the TiAlCrB the nanocrystals were smaller (<15nm) and their volume fraction slightly higher than in the TiAlCr. Thermodynamic calculations confirmed that alpha-Ti is the second favourable phase to form after the amorphous phase at the Tdeposition used. For 1 mum/h deposition rate the amorphous phase formation in the as-deposited microstructure was possible for Tdeposition up to 208 and 223 °C for TiAlCr and TiAlCrB respectively. This variation in Tdeposition is attributed to the higher melting point of the Ti-48Al-9Cr-4B alloy (1123 °C compared to 1080 °C for TiAlCr), which affects the mean surface diffusion length, hi the TiAlCrB/Ag coating the microstructure consisted of crystalline Ag and amorphous TiAlCrB layers. The thermal stability of the as-deposited coatings was assessed by thermal analysis of freestanding deposits and the phase evolution path from the as-deposited metastable to the equilibrium microstructure for TiAlCr and TiAlCrB was defined by XRD and TEM. The TiAlCr crystallized at 657, 690 and 714 °C when heated at 5, 20 and 50 K/min heating rates, respectively. The activation energy for crystallization was 293 KJ/mol, which is very close to the activation energy for volume diffusion of Ti in the gamma phase. The amorphous phase transformed first to the gamma-TiAl and alpha2-Ti3Al phases and then the equilibrium Ti(Cr,Al)2 Laves phase formed very slowly at the expense of the alpha2. The TiAlCrB crystallized at 685, 686 and 690 °C when heated at 50, 65 and 80 K/min heating rates, respectively, with activation energy for crystallization 613 KJ/mol, which indicates the enhanced stability of the TiAlCrB. The phase evolution path of the TiAlCrB was the same with TiAlCr, with the addition of fine borides from the early stages of crystallization. The microstructure of the TiAlCrB was finer than TiAlCr after crystallization. Formation of alpha-Ti nanocrystals (> 50nm), was noticed when the amorphous TiAlCr was heated at 550 °C, but not in the TiAlCrB after heating at 600 °C, which confirmed the enhanced stability of the TiAlCrB. The oxidation of the coatings, the substrate and target alloys was assessed by TG at 800 °C for 200 hours. The weight change per unit area and the parabolic rate constants were determined for each material. Mixed alumina and titania scales were formed on the specimens. Clusters of coarse titania crystals tended to form on the surface of the scales. Their formation was favoured by the surface roughness of the specimens. The TiAlCrB exhibited the best oxidation resistance of the three coatings, followed by the TiAlCrB/Ag and the TiAlCr.