Influence of Ru on the solidification and microstructure characteristics of Co-free nickel-based single crystal superalloys

The solidification and microstructural features of Co-free nickel-based single crystal superalloys with different amounts of Ru were examined through OM, EDS, EMPA, and TEM. The findings reveal that the introduction of Ru into the alloy has a negligible effect on the alloy's liquidus temperature. However, Ru expands the alloy's solidification temperature range to a certain extent and complicates the feeding process during solidification, thereby promoting the formation of microporosity in directional solidification. Moreover, although Ru exhibits negligible influence on dendritic morphology, it intensifies the segregation propensity of Al, Ti, and Ta, consequently elevating the volumetric fraction of γ-γ' eutectic. Compared with alloys without Ru, the incorporation of Ru decreases the segregation degree of Re and W. An increment in Ru content curtails the γ' phase content and size in the alloy. In Co-free single-crystal superalloys, over 80 % of Ru preferentially partitions into the γ matrix, thereby significantly reducing the Ni content in the γ phase. This partitioning proportion demonstrates a marked increase compared to previous observations in Co-containing single-crystal superalloys with Ru additions. The addition of Ru promotes the partitioning of a higher fraction of Re into the γ matrix compared to Ru-free counterparts, consequently, the misfit between the γ′ phase and the matrix becomes more negative, and improving the cubic degree of the γ′ phase.