Abstract
Equilibrium distribution coefficients and pseudoternary solidification surfaces for experimental superalloys containing systematic variations in Fe, Nb, Si, and C were determined using quenching experiments and microstructural characterization techniques. In agreement with previous results, the distribution coefficient, k, for Nb and Si was less than unity, while the “solvent” elements (Fe, Ni, and Cr) exhibited little tendency for segregation (k ≈ 1). The current data were combined with previous results to show that an interactive effect between k Nb and nominal Fe content exists, where the value of k Nb decreases from 0.54 to 0.25 as the Fe content is increased from ≈2 wt pct to ≈47 wt pct. This behavior is the major factor contributing to formation of relatively high amounts of eutectic-type constituents observed in Fe-rich alloys. Pseudoternary γ-Nb-C solidification surfaces, modeled after the liquidus projection in the Ni-Nb-C ternary system, were proposed. The Nb compositions, which partially define the diagrams, were verified by comparison of calculated amounts of eutectic-type constituents (via the Scheil equation) and those measured experimentally, and good agreement was found. The corresponding C contents needed to fully define the diagrams were estimated from knowledge of the primary solidification path and k values for Nb and C.
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Abbreviations
- C e :
-
eutectic composition
- C 1 :
-
liquid composition
- C 0 :
-
nominal composition
- C s :
-
solid composition
- C* s :
-
solid composition at solid/liquid interface
- D s :
-
diffusivity of solute in solid
- f e :
-
fraction of eutectic
- f 1 :
-
fraction of liquid
- f s :
-
fraction of solid
- k :
-
equilibrium distribution coefficient
- L :
-
one-half dendrite spacing
- ΔT :
-
solidification temperature
- T 1 :
-
liquidus temperature
- T m :
-
melting temperature of pure solvent
- t f :
-
local solidification time
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DuPont, J.N., Marder, A.R., Notis, M.R. et al. Solidification of Nb-bearing superalloys: Part II. Pseudoternary solidification surfaces. Metall Mater Trans A 29, 2797–2806 (1998). https://doi.org/10.1007/s11661-998-0320-x
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DOI: https://doi.org/10.1007/s11661-998-0320-x