Superalloys used in hot sections of gas turbines must be protected to prevent high-temperature corrosion. MCr Al Y alloys (M = Ni or Co or both) deposited by means of low pressure plasma spraying are known to provide efficient protection. However, the differential changes with temperature of the mechanical properties of the substrate and protection can alter the behavior of the whole. Tension tests performed in NiCoCrAlYTa alloy have shown a fast decrease in elastic modulus, above 870 K, accompanied by an increase in ductility.
Internal friction measurements have been carried out in a variable frequency pendulum in the 10-4-30 Hz frequency range, between 300 and 1150 K. The Q-1 versus vibration frequency diagrams revealed two relaxation peaks superimposed onto an exponential low-frequency background. The elastic modulus drop has also been proved to be thermally activated.
The parameters associated with the main relaxation peak (Hp = 272 kJ/mole and ν0 = 5 × 1015 s-1) indicate the presence of a diffusional process in the temperature range corresponding to the drop in elastic modulus and the increase in ductility. Measurements in β and γ phases, which are the two phases present in NiCoCrAlYTa, have also been carried out to determine precisely the exact diffusion mechanism.
The general aspect of the damping spectra in these two phases is quite similar to that observed in the alloy. In the β phase, the relaxation peak obtained after removal of the low-frequency background is characterized by an energy of 250 kJ/mole and a frequency constant of 6 × 1014s-1. This peak is associated with relaxation of nickel atoms in the β phase. In the γ phase, we obtain a relaxation peak whose parameters change during thermal cyclings and finally reach the values obtained in the alloy. That peak is again due to the relaxation of nickel atoms associated with the progressive dissolution of γ precipitates.
In conclusion, the relaxation effect responsible for the ductility is the diffusion of nickel in the γ phase, but the role of the interface and in particular of the interdiffusion has to be taken into account.