SOLIDIFICATION OF NB-BEARING SUPERALLOYS - PART II - PSEUDOTERNARY SOLIDIFICATION SURFACES

Equilibrium distribution coefficients and pseudoternary solidification surfaces for experimental superalloys containing systematic variation s in Fe, Nb, Si, and C were determined using quenching experiments and microstructural characterization techniques. In agreement with previo us results, the distribution coefficient, k, for Nb and Si was less th an unity, while the ''solvent'' elements (Fe, Ni, and Cr) exhibited li ttle tendency for segregation (k approximate to 1). The current data w ere combined with previous results to show that an interactive effect between k,, and nominal Fe content exists, where the value of k(Nb) de creases from 0.54 to 0.25 as the Fe content is increased from approxim ate to 2 wt pet to approximate to 47 wt pet. This behavior is the majo r factor contributing to formation of relatively high amounts of eutec tic-type constituents observed in Fe-rich alloys. Pseudoternary gamma- 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 calcula ted amounts of eutectic-type constituents (via the Scheil equation) an d those measured experimentally, and good agreement was found. The cor responding C contents needed to fully define the diagrams were estimat ed from knowledge of the primary solidification path and k values for Nb and C.