Microstructural characterization of a platinum-modified diffusion aluminide bond coat for thermal barrier coatings

Microstructural and chemical evolution induced by thermal cycling of a platinum-modified diffusion aluminide bond coat was investigated with transmission electron microscopy (TEM), X-ray diffraction, (XRD) and electron microprobe analysis. As-fabricated, the bond coat was confirmed to be an ordered B2 structure, but the underlying microstructure was found to be modulated. Thermal cycling resulted in a primarily outward diffusion of Ni and the formation of a Ni-rich bond coat containing secondary L1(2) precipitates. Closer inspection of the bond coat revealed a transformation from its original B2 structure to a L1(0) martensite. In-situ TEM observations indicated that the martensite is stable at lower temperatures and that the parent B2 structure reappears at higher temperatures. These observations can be used to explain the variations in strength that have recently been measured in thermally cycled bond coats. The resulting transformation strain is also argued to play an important role in determining the accumulation of stress and strain in thermally cycled thermal barrier coatings (TBCs).