Microstructural evolution of platinum modified nickel aluminide bond coat during thermal cycling

Microstructural evolution induced by thermal cycling in a platinum modified diffusion aluminide bond coat was investigated with transmission electron microscopy and elevated temperature X-ray diffraction (XRD). Before thermal cycling, the structure of the as-received bond coat was confirmed to be an ordered B2 phase, but significant lattice strains were found which were associated with the formation of a modulated structure. Thermal cycling resulted in significant changes in the microstructure of the bond coat. The compositional development assisted by chemical diffusion during thermal cycling has been related to the transformation of the bond coat from its original B2 structure to a Ni-rich L10 martensite. The L10 martensite was found to be stable at temperatures below approximately 600 °C and the B2 parent phase stable at elevated temperatures. Quantitative XRD measurements indicated that the volume of the B2 phase is approximately 2% larger than that of the martensite, which produces a ∼0.7% linear transformation strain.