Interdiffusion mechanism at the interface between TiAl alloy and NiCoCrAlY bond coating

Abstract

Surface coatings are the best ways to protect materials from deterioration due to various aspects including corrosion and wear. However, after long-term oxidation, the high temperature oxidation resistance of TiAl-4Nb-1.5Mo (B, Y) alloy coated with NiCoCrAlY bond coating is weakened by the interfacial interdiffusion. Herein, the microstructure evolution, elements interdiffusion, and mechanical properties at the interface between TiAl alloy and NiCoCrAlY bond coating are analyzed through an isothermal oxidation experiment at 950 °C. Results show that the coarsening and coalescence of the β phase occur in the bond coating during high temperature oxidation. Meanwhile, the thickness of the bond coating decreases linearly before oxidation for 100 h. As the oxidation time increases, the consumption rate of the bond coating slows down. Based on the Layer-by-layer phase composition analysis, it is found that the interdiffusion zone forms three layers in sequence: NiAl and Al (Ni, Co)2Ti, AlNi2Ti(Nb, Mo) and Ti3Al, and Al3NiTi2 and TiAl. The TiN phase segregated around the α-Cr as well as the enriched Y element led to generation of microcracks. The TiN phase forms at the phase boundary between the β and γ phases after 250 h of oxidation. The layer composed of AlNi2Ti(Nb, Mo) and Ti3Al phase exhibits a severe hardening phenomenon, which may reduce the long-term service life of the TiAl alloy coated with NiCoCrAlY bond coating.