Microstructural observations of as-prepared and thermal cycled NiCoCrAlY bond coats

Microstructures of as-prepared and 1100 °C/100 h isothermally annealed NiCoCrAlY bond coat specimens as well as a bond coat obtained from an end of life turbine blade were characterized with TEM. In all specimens the γ grains were observed to consist of fine γ′ precipitates, which form during cooling and are unstable at the higher operating temperatures. The β grains present in the as-prepared specimens were observed to transform to L10 martensite in the 1100 °C/100 h isothermally annealed specimen. As a result of substrate-bond coat interdiffusion the Ms temperature increases during thermal cycling due to an increase in Ni and decrease in the Cr concentrations of the β-phase. The turbine blade bond coat was also found to contain Cr and Co-rich σ-phase precipitates.