Investigation on the Secondary Reaction Zone Formation of Metallic Bond Coating Layer Produced by High Velocity Oxygen Fuel Deposition in a Ni-Based Single Crystal Superalloy

The formation of the secondary reaction zone (SRZ) at the interface between the CoNiCrAlY coating and experimental Ni-based single crystal superalloy substrate was investigated. Transmission electron microscope (TEM) observation revealed that the SRZ was composed of recrystallized gamma' grains with a high density of fine topologically close-packed (TOP) precipitates. The TOP precipitate was identified as the faulted and twinned rhombohedral mu phase. TOP precipitation appears to be analogous to the discontinuous precipitation. The gamma' grains nucleated epitaxially with grains in the coated layer. Preferential formation of the SRZ was observed along the < 110 > direction. This anisotropy of SRZ formation may be essentially related to the preferential < 110 > dislocation gliding direction where recrystallization would be most active due to high strain energy and the supply of fast interdiffusion pathways. Based on the suggested SRZ formation mechanism, the intermediate annealing is proposed to suppress SRZ formation in terms of mitigation of strain energy and interdiffusion.