Experimental Investigation of Phase Equilibria of the Ho-Ir-O Ternary System at 1073 K.

An experimental study of the phase equilibria of the Ho-rich part of the Ho-Ir-O ternary system at 1073 K by means of x-ray diffraction, differential scanning calorimetry, and scanning electron microscopy has been carried out. Ho-hcp and four binary compounds, namely Ho 3 Ir, Ho 5 Ir 2 , Ho 5 Ir 3 , and Ho 2 O 3 , were identified in the Ho-Ir-O model alloys after long-term annealing (350-1220 h). No solubility of iridium in Ho 2 O 3 oxide and Ho-hcp was observed. No ternary phase was found. Based on the experimental results, an isothermal section of the Ho-rich part of the Ho-Ir-O system at 1073 K was constructed. In addition, the microstructure of as-cast alloys was studied. An irregular eutectic consisting of faceted Ho-phase in Ho 3 Ir phase was observed in the alloys with Ho-hcp + Ho 3 Ir + Ho 2 O 3 phase composition, and the temperature of the eutectic reaction Ho-hcp + Ho 3 Ir ↔ liquid was determined.