Observation and modeling of α-NiPtAl and Kirkendall void formations during interdiffusion of a Pt coating with a γ-(Ni-13Al) alloy at high temperature

During the last 15 years, Pt-rich γ-γ′ bond-coatings have been studied extensively for their corrosion and oxidation resistance, and as a lower cost alternative to β-(Ni,Pt)Al bond-coatings in thermal barrier coating systems. To optimize their fabrication and durability, it is essential to investigate their interdiffusion with Ni-based superalloys. This study reports on experimental results and modeling of the interdiffusion of the model Pt/γ-(Ni-13Al) alloy system. Pt coatings were deposited either by electroplating or by spark plasma sintering using a Pt foil. Heat treatments at 1100 °C for 15 min to 10 h were performed either in a high-temperature X-ray diffraction device under primary vacuum or in a furnace under argon secondary vacuum. The α-NiPtAl phase with L10 crystal structure formed very rapidly, implying fast uphill Al diffusion toward the surface. For Pt electroplating, α-phase transformed to γ′-(Ni,Pt)3Al after only 45 min-1 h at 1100 °C. The resulting two-phased γ-γ′ microstructure remained up to 10 h. When using a Pt foil coating, the continuous layer of α-NiPtAl phase disappeared after 10 h and the γ′-(Ni,Pt)3Al or γ-(Ni,Pt,Al) phase appeared, resulting in two different diffusion paths in the Ni-Pt-Al phase diagram. Voids also formed at the interdiffusion zone/substrate interface for both systems after 1 h or more. Composition analyses confirmed that voids were located at the Pt diffusion front corresponding to the Al-depleted zone. Experiments performed with the samples coated with a Pt foil confirmed that voids are due to a Kirkendall effect and not to the Pt deposition process. Numerical simulations including the cross-term diffusion coefficients in the diffusion flux equations reproduced the experimental concentration profiles for the γ-phased systems.