June 22-27, 2014
This research focuses on analysing the interaction between three commercially used diffusion coatings and a nickel based superalloy CMSX-4. The coatings applied are alumide, Pt-alumide and Pt-modifed γ/γ′. The research has two main themes, firstly diffusion and secondly oxidation. Results of quantitative EPMA show that titanium diffusion towards the surface varies between coatings, and has a strong correlation with the use of platinum within coatings. The concentrations of Ti within Pt-aluminide coatings after 100 hours oxidation in air at 1100°C were found to be twice as high as those within the plain aluminised CMSX-4 samples. Furthermore uphill diffusion of Ti was observed to occur within Pt-modifed γ/γ′ after the same exposure. In order to examine the effect of platinum additions on the diffusive behavior of titanium, first principle calculations using the density functional theory implementation CASTEP have been performed. In an ordered lattice structure, diffusion of a solute species must occur via the presence of defects, either anti-site (NiAl) or vacancy (VNi). Moreover clustering of these defects leads to increased opportunities for solute diffusion and hence increased diffusion rates within that structure. We have shown that the use of Pt strengthens defect formation and defect clustering within the Ni3Al L12 phase. By increasing the amount of defect clusters Ti can diffuse more freely from the substrate through the coating to the surface.
Robbie J. Bennett, Ian Edmonds, Neil Jones, and Catherine Rae, "On the behavior of titanium within thermal barrier coatings and its influence on residual stress within the TGO" in "Thermal Barrier Coatings IV", U. Schulz, German Aerospace Center; M. Maloney, Pratt & Whitney; R. Darolia, GE Aviation (retired) Eds, ECI Symposium Series, (2015). http://dc.engconfintl.org/thermal_barrier_iv/11