Nucleation and transition sequences of TCP phases during heat-exposure in a Re-containing Ni-based single crystal superalloy

The nucleation and transition sequences of topologically close-packed (TCP) phases in a Re-containing Ni-based single crystal superalloy were systematically investigated using in-situ transmission electron microscopy (TEM) and three-dimensional atom probe technology (3D-APT). During the initial stage of heat-exposure at 1100 °C, the TCP phase forming elements (Re, Co, Cr, etc.) segregated at the γ/γ′ interface near the γ matrix side to provide the concentration undulations for the nucleation sites of TCP phases, following which the σ and P phase coherently nucleated along the (111‾)_γ and (022)_γ planes from the γ/γ′ interface near the γ matrix side, respectively. With prolonged heat-exposure time, transitions from σ phase to P phase, σ phase to μ phase, and P phase to μ phase occurred. Besides, the orientation relationships of TCP phase intergrowth structures indicated that the P phase grew along the (1¯01)_σ plane of the σ phase by co-lattice precipitation, meanwhile, the µ phase grew with smaller lattice misfits along the (04¯0)_σ plane of the σ phase and the (400)_P plane of the P phase. Additionally, the result by first-principles calculation evidenced that the μ phase had the lowest system energy to make the transition of σ phase and P phase to μ phases inevitable, therefore, the TCP phase ultimately existed as the most stable μ phase. Finally, the transition sequences of TCP phase during heat-exposure could be summarized into three types: γ matrix→σ→μ, γ matrix→P→μ, and γ matrix→σ→P→μ.