Precipitation of TCP phases with R/P intergrowth structure during directional solidification in a Ru-containing nickel-based single crystal superalloy

The topologically close-packed (TCP) phases with R/P intergrowth structure formed at the final stage of directional solidification in a Ru-containing nickel-based single crystal (Ni-SX) superalloy were investigated. The nodular TCP phases were distributed in the interdendritic regions of the as-cast microstructure. Precipitation of the β-NiAl phases and the subsequent incomplete peritectic reaction (Liquid + β-NiAl → γ′ + β-NiAl_residual) resulted in the accumulation of the TCP-forming elements (Cr, Co, Mo, W, Ta, and Re) in the residual liquid around the β-NiAl phase, which provided a favorable nucleation condition, leading to the precipitation of TCP phase in the vicinity of the β-NiAl phases. In addition, the TCP phases were found to be the R/P intergrowth structure with an orientation relationship of [2,7,10]_R//[001]_P and (322¯)_R//(200)_P, (44¯2)_R ∼3.2° from (11¯0)_P. Further, the formation mechanism of these TCP phases with R/P intergrowth structure was systematically discussed in terms of crystal structure, interface characteristics, and chemical composition. The result can provide theoretical guidance and experimental data for the composition design and heat treatment process optimization of the novel Ru-containing high-generation Ni-SX superalloys.