| Title |
Freckle Formation with Convection in the Mushy Zone of a Single Crystal Superalloy |
| Authors |
조대현(Dae-Hyun Jo) ; 도정현(Jung-Hyun Do) ; 서성문(Seong-Moon Seo) ; 권석환(Suk-Hwan Kwon) ; 안성철(Seung-Cheol Ahn) ; 구지호(Ji-Ho Gu) ; 이재현(Jehyun Lee) |
| DOI |
https://doi.org/10.3365/KJMM.2026.64.1.39 |
| ISSN |
1738-8228(ISSN), 2288-8241(eISSN) |
| Keywords |
Single crystal superalloy; Freckle defect; Microsegregation; Rayleigh number; Density inversion |
| Abstract |
Nickel-based single-crystal superalloys are widely used in turbine blades for aerospace and power
generation applications due to their excellent high-temperature properties including superior creep resistance
and microstructural stability. However, complex compositions increase susceptibility to freckle defects that
severely degrade mechanical performance by disrupting single-crystal structure and forming detrimental
grain boundaries. This study compares conventional CMSX-4 SLS with newly developed NASX (New Alloy
Single Crystal superalloy) under identical directional solidification conditions. CMSX-4 SLS exhibits freckles
in thick sections due to density inversions from the segregation of refractory elements (Re, W) to solid phases
and light elements (Al) to liquid phases, causing buoyancy-driven convection that disrupts dendritic structure
and promotes freckle chain formation. NASX was strategically designed with reduced Re and Al contents and
increased Ta and Mo to minimize liquid density gradients during solidification. High-density Ta concentrates
in the interdendritic liquid to compensate for the density reduction caused by Al segregation and Re/W
depletion. Mo mitigates Re segregation through favorable elemental interactions. Thermo-Calc calculations
confirmed NASX exhibits a significant reduction in density variations between interdendritic and bulk liquids
compared to CMSX-4 SLS. Rayleigh number analysis established critical thresholds, with NASX remaining
below this limit. Experimental results confirmed complete freckle suppression in NASX castings under
conditions where CMSX-4 SLS developed extensive freckles, demonstrating effective composition-controlled
alloy design for improved castability and manufacturing yield. |