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.

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.

Nanoindentation testing has been used to measure the hardness and elastic modulus of Ag3Sn, Cu6Sn5, and Cu3Sn intermetallics, as well as Sn–Ag–Cu solder and pure Sn and Cu. The intermetallics were fabricated by solid-state annealing of diffusion couples prepared from a substrate (Cu or Ag) and a solder material (Sn or Sn–Ag–Cu solder), providing geometries and length scales as close as possible to a real solder joint. Nanoindentation results for the intermetallics, representing penetration depths of 20–220 nm and loads from 0.7 to 9.5 mN, reveal elastic/plastic deformation without evidence of fracture. Measured hardness values of Cu6Sn5 (6.5 ± 0.3 GPa) and Cu3Sn (6.2 ± 0.4 GPa) indicate a potential for brittle behavior, while Ag3Sn (2.9 ± 0.2 GPa) appears much softer and ductile. Using a bulk Cu6Sn5 sample, Vickers hardness testing revealed an indentation size effect for this compound, with a hardness of 4.3 GPa measured at a load of 9.8 N. An energy balance model is used to explain the dependence of hardness with load or depth, where the observation of an increasing amount of fracture with applied load is identified as the primary mechanism. This result explains discrepancies between nanoindentation and Vickers results previously reported.