Abstract
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 ℃,the TCP phase forming elements(Re,Co,Cr,etc.)segregated at the γ/γ'in-terface near the y 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(1(11))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(101)σ plane of the σ phase by co-lattice precipitation,meanwhile,the μ phase grew with smaller lattice misfits along the(040)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.
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