PHASE EQUILIBRIA IN THE ZrO₂–La₂O₃–Gd₂O₃ SYSTEM AT 1600 ºС

Abstract

Phase equilibria and structural transformations in the ternary ZrO₂–La₂O₃–Gd₂O₃ system at 1600 ºС were studied by X-ray diffraction, microstructural, and electron microprobe analyses over the entire composition range. Fields of solid solutions based on the cubic fluorite-type (F) and tetragonal (Т) modifications of ZrО₂, monoclinic (M) and cubic (С) modifications of Gd₂O₃, hexagonal (A) modification of La₂O₃, and an ordered intermediate phase with La₂Zr₂O₇ pyrochlore-type structure (Py) exist in the system. The boundaries of phase fields and lattice parameters of the phases were determined. In the ZrO₂-rich corner, solid solutions based on the tetragonal modification of ZrO₂ are formed. The solubility of La₂O₃ in the T-ZrO₂ is low and reaches ~0.5 mol.%, which is evidenced by X-ray diffraction and microstructural analyses. The solid solutions based on the tetragonal modification of zirconia cannot be quenched when cooled with furnace conditions. The diffraction patterns recorded at room temperatures include peaks of the M-ZrO₂ monoclinic phase. The solid solution based on the ordered Ln₂Zr₂O₇ pyrochlore-type phase (Ру) is in equilibrium with all phases (except for С-Gd₂O₃) existing in the ternary ZrО₂–La₂O₃–Gd₂O₃ system at 1600 °C and forms substitutional solid solutions with phases of binary systems. The greatest solubility in the Ln₂Zr₂O₇ (Py) lattice is shown by Gd₂O₃ along the Gd₂O₃–(67 mol.% ZrO₂–33 mol.% La₂O₃) section. The La₂Zr₂O₇ (Py) lattice parameters change from а = 1.0781 nm for the single-phase (Py) sample containing 67 mol.% ZrO₂–33 mol.% La₂O₃–0 mol.% Gd₂O₃ to а = 1.0741 nm for the three-phase (F + Py + B) sample containing 43.55 mol.% ZrO₂–21.45 mol.% La₂O₃–35 mol.% Gd₂O₃ along the Gd₂О₃–(67 mol.% ZrO₂–33 mol.% La₂O₃) section. The cubic phases are predominantly in equilibrium in the ZrО₂–La₂O₃–Gd₂O₃ system: F-Fm3m, Py-Fd3m, and C-Ia3. The isothermal section of the ZrО₂–La₂O₃–Gd₂O₃ phase diagram at 1600 °C contains four three-phase regions (T + F + Py, A + Py + B, Py + F + B, and F + C + B) and nine two-phase regions (T + Py, F + Py, F + T, F + C, F + B, B + Py, A + Py, B + A, and B + C).