Phase equilibria in Ir-rich portion of Ir-Al-X (X: V, Nb and Ta) ternary systems

Isotherms of the Ir-rich portion of the Ir-Al-X (X: V, Nb, Ta) systems composed of fcc-Ir solid solution ((Ir)ss), Ir-based L12 intermetallic compounds Ir3X and B2-IrAl are presented as a second report of a series of investigations on the phase equilibria in the (Ir or Rh)-Al-X (X: Ti, Zr and Hf, or, V, Nb and Ta) ternary systems. In both Ir-Al-Nb and Ir-Al-Ta systems fcc/L12/B2 three-phase equilibrium is found. In Ir-Al-V system, Ir solid solution and Ir3V have close compositions each other and fcc/L12/B2 three-phase equilibrium is expected. No ternary compound is observed in all of the isotherms. Solid solubility of each phase is confirmed by chemical analysis by wavelength dispersive X-ray spectroscopy (WDS). The direction of the solubility lobe of L12-Ir3X with Al addition is explained in terms of the nearest neighbor interactions. Liquidus surfaces are also established by taking into account the microstructure of as-cast alloys, differential thermal analysis (DTA) data, and the Ir-Al and the Ir-X binary phase diagrams reported previously.