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The solidification reaction sequences of experimental superalloys containing systematic variations in Fe, Nb, Si, and C were studied using differential thermal analysis (DTA) and microstructural characterization techniques. The reaction sequences responsible for microstructural development were found to be similar to those expected in the Ni-Nb-C ternary system and commercial superalloys of comparable composition. The solute-rich interdendritic liquid generally exhibited two eutectic-type reactions at the terminal stages of solidification: L → (␥ ϩ NbC) and L → (␥ ϩ Laves). The Nibase alloys with a high C/Nb ratio represented the only exception to this general solidification sequence. This group of alloys terminated solidification with the L → (␥ ϩ NbC) reaction and did not exhibit the ␥/Laves constituent. At similar levels of solute elements (Nb, Si, and C), the Fe-base alloys always formed more of the ␥/Laves eutectic-type constituent than the corresponding Ni-base alloys. Silicon additions also increased the amount of the ␥/Laves constituent that formed in the assolidified microstructure, while C additions promoted formation of ␥/NbC. The influence of Nb was dependent on the C content of the alloy. When the C content was low, Nb additions generally promoted formation of ␥/Laves, while Nb additions to alloys with high C led to formation of the ␥/NbC constituent. The results of this work are combined with quantitative analyses for developing ␥-Nb-C pseudoternary solidification diagrams in a companion article.

Laser Engineered Net Shaping (LENS) is a novel manufacturing process for fabricating metaI parts directly from Computer Aided Design (CAD) solid models. The process is similar to rapid prototyping technologies in its approach to fabricate a solid component by layer additive methods. However, the LENS technology is unique in that fully dense metal components with material properties that are similar to that of wrought materials can be fabricated. The LENS process has the potential to dramatically reduce the time and cost required realizing functional metal parts. In addition, the process can fabricate complex internal features not possible using existing manufacturing processes. The real promise of the technology is the potential to manipulate the material fabrication and properties through precision deposition of the material, which includes thermal behavior control, layered or graded deposition of multi-materials, and process parameter selection.

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 Ϸ 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 Ϸ2 wt pct to Ϸ47 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 ␥-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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