Martensitic transformation in CVD NiAl and (Ni,Pt)Al bond coatings

The martensitic phase transformation in single-phase β-NiAl and (Ni,Pt)Al coatings was investigated. After isothermal exposure to 1150 °C for 100 h, the β phase in both types of coatings was transformed to a martensite phase during cooling to room temperature. Martensitic transformation was also observed in the (Ni,Pt)Al bond coat with and without a YSZ top layer after thermal cycling at 1150 °C (700 1-h cycles). The transformation took place due to Al depletion in the coating from the formation of the Al2O3 scale and interdiffusion between the coating and superalloy substrate. The effects of the martensitic transformation on coating surface stability (‘rumpling’) via volume changes during the phase transformation are discussed with regard to TBC failure.