Microstructural evolution of Pt-aluminide coating influenced by cycle oxidation service conditions

This work investigated the degradation of a platinum-modified aluminide diffusion coating on GTD 111 SC Ni-base superalloy turbine blades after ∼16,000 h of exposure to different thermal cycles (critical heating temperatures reported as ∼1000 °C and 1120 °C). The initial coating condition and the evolution of degradation were characterized with conventional microscopy and backscatter scanning electron microscopy. The initial microstructure consisted of a two-phase coating (intermetallic PtAl2 dispersed in a matrix of β-(Ni,Pt)Al). The major microstructure degradation was associated with the formation of Kirkendall voids, the partial transformation of β-(Ni,Pt)Al to γ′-Ni3Al, and the dissolution of the intermetallic PtAl2, resulting in a more brittle, single-phase coating. This type of degradation facilitates the spallation of the coating and crack initiation, resulting in the loss of the coating and eventual blade failure.

► Severe Pt-aluminide coat degradation after cycle oxidation (∼1120 °C/24 h) service.
► Complete dissolution of the precipitates PtAl2 of the initial single phase coat.
► Intermediate microstructural transformation of β-(Ni,Pt)Al to γ′-(Ni3Al) phase.
► High void formation by higher diffusion of Ni and Al across the outer zone.
► Inward and outward (Al, Pt and Ni) diffusion favor the spalling and cracking coat.