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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 (32 2 ¯) R //(200) P , (4 4 ¯ 2) R ∼3.2° from (1 1 ¯ 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. [Display omitted] • TCP phase with R/P intergrowth structure was found in an as-cast Ru-containing Ni-based single crystal superalloy. • The microstructural characteristics and the formation mechanism of TCP phase with R/P intergrowth structure were studied. • The accumulation of TCP phase formers in the residual liquid near the β-NiAl phases led to the precipitation of TCP phase. • Homologous crystal structure, coherent R/P interface, and similar composition promoted the nucleation of P phase on R phase. [ABSTRACT FROM AUTHOR]

• The peritectic reaction was investigated by the planar interface solidification. • The phase transformation process of the peritectic reaction was clarified. • The peritectic reaction mechanism was studied from the perspective of crystallography. • The relaxation of some atoms on the matching planes maximized the atomic matching. • Good interfacial atomic matching promoted the nucleation of the peritectic phase. [Display omitted] The peritectic reaction L + β-NiAl → γ′ during directional solidification in a Ru-containing nickel-based single crystal superalloy was investigated by the planar interface solidification and the phase transformation process was clarified in detail. The phase transformation mechanism of the peritectic reaction was further studied in the combination of the transmission electron microscope and the edge-to-edge matching model from the crystallographic point of view. The results indicated that the relaxation of some atoms on the matching planes (1 1 ̅ 0) β and (1 1 ̅ 1) γ′ maximized the atomic matching at the interface, increasing the possibility of peritectic γ′ phase nucleated on primary β-NiAl phase in the peritectic reaction. [ABSTRACT FROM AUTHOR]

Two Nickel-based single crystal superalloys (one Ru-free and one containing 3 wt.% Ru) were comparatively investigated by the three-dimensional atom probe tomography and the nanoindentation atomic force microscope to reveal the effect of Ru addition on the microstructure and the mechanical properties of γ' phases after standard heat treatment. The results showed that the Ru addition neither altered the morphology and the volume fraction of γ' phases, nor changed the interface structure and the partition coefficients of alloying elements between γ and γ' phases, however, it enhanced the hardness and elastic modulus of γ' phases. First-principles calculation indicated that no matter whether the Ni site or Al site was occupied by Ru, the elastic modulus of γ' phases increased, which might be ascribed to the solution strengthening effect of Ru on γ' phases. [Display omitted] • The effect of 3 wt.% Ru addition on elemental partitioning coefficients was determined by 3D-APT. • The effect of 3 wt.% Ru addition on on the elastic modulus of γ′ precipitates was studied by NI-AFM. • The strengthening mechanism resulted from Ru element partition on γ' phases was clarified. [ABSTRACT FROM AUTHOR]

The distribution of Re in a Ni-based single crystal superalloy is specified by atom probe tomography after different heat treatment. At higher temperatures (950-1100°C), the Re diffusivity in γ matrix is becoming smaller than the actual effective diffusion for γ΄ growth, during the subsequent cooling process and the interface migration, consequently Re atoms are insufficient to diffuse away and keep accumulating, leading to forming segregation at γ/γ′ interface. While at low temperatures (800-900°C), the Re diffusion is capable of balancing the effective diffusion for γ΄ growth. After introducing more alloys with different Re content and different aging temperatures, Re segregation is irrelevant to the Re content but highly temperature-dependent. These results provide a novel perspective on the comprehension of the Re effect on γ΄ growth in Ni-based superalloy and are valuable to both improving the γ/γ′ microstructure stability and designing high-performance superalloys. [ABSTRACT FROM AUTHOR]

• Re addition significantly enhanced the γ′ precipitates stability: higher volume fraction and smaller coarsening rate. • Re addition reduced interfacial energy, effective diffusion and sensitivity of lattice misfit to the temperature. • The γ ′ morphological transition mechanism during aging process was discussed. The effect of 2 wt% Re addition on the microstructure stability and γ/γ′ interfacial characteristics of designed Ni-based single crystal (Ni-SX) superalloys during long-term aging up to 2000 h under 900 °C and 1000 °C was investigated. Re addition significantly affected the γ′ precipitate morphology, γ/γ′ lattice misfit and microstructural stability during long-term aging. Results showed that Re addition significantly enhanced the γ′ precipitates stability: higher volume fraction and smaller coarsening rate. Adding Re delayed the dissolution of the γ′ precipitates at high temperature, suppressed the variation of composition in the γ matrix and attenuated temperature sensitivity of the lattice misfit. Thus, this further reduced the interfacial energy of thermodynamic driving force for coarsening. Through the comparison with available commercial alloys, the Re-containing Ni-SX superalloys possessed minimizing lattice misfit and interfacial energy. More importantly, as the slowest alloying element in terms of diffusion coefficient, Re reduced the effective diffusion coefficient of the alloy, effectively hindering the matrix-diffusion-controlled coarsening of γ′ precipitates. Finally, the γ ′ morphological evolution during aging process was discussed, the bifurcation point of the morphological transitions, spheroid-to-cuboid transition and cuboid-to-rod-like transition, were determined based on dimensionless parameter L and shape parameter η. [ABSTRACT FROM AUTHOR]

Three-dimensional atom-probe tomography and scanning electron microscopy were utilized for an in-depth examination of the formation of secondary γ′ precipitates and the composition difference between secondary γ′ precipitates and primary γ′ precipitates in a Re-containing Ni-based single crystal superalloy. It was found that the secondary γ′ precipitates was formed during the air cooling after primary aging of 1180 °C/4h and would grow up during the secondary aging at 870 °C. Al and Ta were mainly distributed in γ′ precipitates (including the primary and the secondary γ′ precipitates) and Mo, Cr, Co and Re were more distributed in γ matrix. Compared with the primary γ′ precipitates, the secondary γ′ precipitates contained more γ-forming elements, such as Re, Cr, W and Mo, while the content of Ta was less. The distribution of W in the secondary γ′ precipitates was much higher than that in the primary γ′ precipitates and was equal to that in γ matrix. Based on the diffusion constant of alloying elements in Ni, the diffusion distance of various elements was as followed during the air cooling after primary aging: Re < W < Co < Mo < Ni < Ta < Cr < Al. The insufficient diffusion distance of γ-forming elements was an important factor leading to their enrichment in the secondary γ′ precipitates. And the Re element might hinder the diffusion of other γ-forming elements, such as Cr and Mo. • 3D-APT was utilized for an in-depth examination for secondary γ′ precipitates. • Secondary γ′ precipitates were formed and grown during air cooling. • The chemical composition of secondary γ′ precipitates was clarified. • Secondary γ′ precipitates contained more γ-forming elements and W element. • The diffusion distance of elements in γ matrix was calculated during air cooling. [ABSTRACT FROM AUTHOR]