Thermal shock cycling behavior of NiCoCrAlYSiB coatings on Ni-base superalloys: II. Microstructure evolution

NiCoCrAlYSiB coatings were obtained using an arc ion plating technique on a directionally solidified Ni-base superalloy. Thermal shock resistance of the bare and coated superalloys were investigated by cycling between high temperature (900, 1000 and 1050 °C) and room temperature (forced water quenching). The microstructure evolution in coatings and the underneath substrates was studied. The results showed that phase transformation from γ′ to γ occurred at the surface of the coated and bare specimens due to the consumption of Al and Ti by oxidation. A directional coarsening of the γ′ precipitates (γ′ rafting) just beneath the γ′-impoverishing zone (or interdiffusion zone) is observed. This kind of rafting is believed to come into being from the compressive stresses by temperature gradients due to high cooling rate during cooldown. The relationship between microstructure evolution and cracks formation and propagation is also discussed.