A Stress-Induced Phase Transformation of High Temperature Orthorhombic Phase of (R_1-xLa_x)₄Al₂O₉ (R=Gd,Ho)

Abstract

High temperature orthorhombic phasesof (R_1-xLa_x)₄Al₂O₉ (R=Gd,Ho) were synthesized by reaction sintering at 1700°C for 30–50hr in air. The metastable orthorhombic phase was possible to quench to room temperature under the conditions of grain growth above the critical grain size of 6.0μm for X=0 and 3.3 μm for X=0.2 in (Ho _1-xLa_x)₄Al ₂O₉ system and of 2.7μm for X=0 and 2.4μm for X=0.2 in the system of (Gd_1-xLa_x)₄Al₂O₉ From the results of differential scanning calorimetry measurements, phase transformation from low temperature monoclinic to high temperature monoclinic phase was observed at about 1100°C in both solid solution systems, and transition temperature was observed at about 1400°C in (Gd_l-xLa_x)₄Al₂O₉, which would correspond to the phase transformation from monoclinic to orthorhombic phase. This orthorhombic phase was transformed into monoclinic phase by polishing on the surface of sintered body at room temperature.