Composites ofγ ⁺ TaC in the Ni−Ta−C ternary system

Abstract

The Ni−Ta−C ternary system has been studied at the Ni-rich end of the phase diagram. The investigation was directed toward developing a detailed picture of γ+TaC composites. Alloys were melted with Ta/C atomic ratios of 0.79 to 3.33 in an attempt to define the composition range that would produce two-phase γ+TaC eutectics. The liquidus trough rises in temperature across the ternary diagram moving away from the Ni−C eutectic (1320°C) and toward the Ni−Ni₃Ta eutectic (1380°C). Bulk chemistries were determined for aligned regions to map the liquidus trough compositions. Ratios of Ta/C atoms varied from 1.3 to 8.8 in the aligned regions. Matrix composition was determined by electron microprobe analysis, and lattice parameters of extracted TaC fibers were measured by X-ray diffraction. Fiber compositions ranged from TaC_0.99 to TaC_0.97 as the Ta/C ratio of the aligned region increased. The matrix compositions and TaC stoichiometries were used to map tie-lines across the ternary diagram. Volume fraction and microstructural features of the TaC phase were also studied. The volume fraction of TaC decreased from 6.7 vol pct to 1.7 vol pct as Ta/C increased from 1.3 to 8.8. The decreasing volume fraction can be explained by a lever-arm rule application for the ternary phase diagram, based on the liquidus trough and tie-line compositions determined in this study. The TaC growth axis was <111> in each case, but the carbide morphology changed progressively across the phase diagram. The change in morphology is primarily a consequence of the change in volume fraction. Implications of the findings of this study for more complex γ+MC eutectics will be discussed.