Composites ofγ
+
TaC in the Ni−Ta−C ternary system |
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Authors: | M R Jackson |
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Affiliation: | (1) General Electric Corporate Research and Development, 12301 Schenectady, NY |
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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 TaC0.99 to TaC0.97 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. |
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