Precipitation phases in the nickel-based superalloy DZ 125 with YSZ/CoCrAlY thermal barrier coating

Interdiffusion behavior of the thermal barrier coating (TBC) with the CoCrAlY bond coat (BC) and directionally solidified Ni-based superalloy DZ 125 was investigated. Severe inward-diffusion of Al, Co and Cr from the BC to the superalloy and outward diffusion of Ni and refractory elements such as W from the superalloy occur during annealing at 1050 °C in air. After 100 h annealing, a ∼30 μm thick inter-diffusion zone (IDZ) forms between the BC and superalloy, and a ∼35 μm thick secondary reaction zone (SRZ) forms beneath the IDZ. The IDZ mainly consists of β phase and γ matrix. Besides, small amount of Ta and Hf containing carbides are also observed in the IDZ. Needle and fine granular topologically close-packed (TCP) phases, characterized as rhombohedral μ phases, are abundant in the SRZ. The formation of SRZ is mainly due to the precipitation of refractory elements such as W and Mo from the γ matrix and β phase. The formation mechanism of SRZ and μ-TCP phase is discussed.