Evaluation of the invariant reactions in the Ta-rich region of the Ta-B system
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Invariant reactions in the Ta-rich side of the Ta-B system were evaluated. The alloys were arc melted from pure materials. Pellets prepared from powder mixtures of Ta and TaB were heat treated under vacuum at 1900 and 1950 °C. The microstructures of the alloys were characterized through scanning electron microscopy images and x-ray diffraction (XRD). The microstructural analyses confirmed the type of reactions occurring in Ta-rich region; however, the composition of the liquid phase should be altered: (a) L ↔ TaSS+Ta2B from 23 to 18 at.% B and (b) L+TaB↔Ta2B from 27 to 22.5 at.% B. The formation of Ta3B2 was confirmed as the understoichiometry of Ta2B. The decomposition of Ta2B occurs between 1900 and 1950 °C. The XRD data for alloys with composition between 34 and 50 at.% B indicate the existence of a new high-temperature phase.
Keywordsborides phase diagram Ta-B system
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