Study on Stress-Induced Phase Transformation of Rare Earth Aluminates

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Transformation toughening is one effective approach to improving the fracture toughness and fracture strength of brittle ceramics. Transformation toughening requires the stress-induced martensitic transformation of particles, as is well known in tetragonal zirconia ceramics. The main purpose of our study is to find new oxide ceramics with stress-induced phase transformation. In the present study, the relationship between orthorhombic and monoclinic phase, the possible factors for metastabilization of orthorthombic phase at room temperature and thermal and mechanical properties in (R_<1-x>La_x)_4Al_2O_9 and Gd_4Al_2O_9/MgO composites are examined. Single phase solid solutions with orthorhombic structure were synthesized by reaction sintering at 1700ﾟC for 30h in air within the composition range of 0*x<0.5 for (Ho_<1-x>La_x)_4Al_2O_9 and 0*x<0.3 for (Gd_<1-x>La_x)_4Al_2O_9 system. Above these composition ranges, perovskite-type LaAlO_3 coexisted as second phase. Metastable orthorhombic p … More hase was capable of quenching to room temperature under the conditions of grain growth above the critical particle size of 6.2mum for x=0, and 3.3mum for x=0.2 in (Ho_<1-x>La_x)_4Al_2O_9 and 2.7mum for x=0 and 2.4mum for x=0.2 in (Gd_<1-x>La_x)_4Al_2O_9. After polishing the surface of all sintered bodies with orthorhombic phase at room temperature, the crystal structure of samples changed to a monoclinic structure ; i.e.a stress-induced phase transformation occurred during the polishing process. This stress-induced phase transformation from orthorthombic to monoclinic structure was accompained by an increase of volume of about 0.5%. From the results of DSC measurements, the thermal phase transformation of Gd_4Al_2O_9 is reversible and has hysteresis. The transition temperature, defined as the average temprarure of exothermic and endothermic peaks, was 1110ﾟC,which correspond to the phase transformation from low temperature monoclinic to high temperature monoclinic structure. On the other hand, a second transition temperature was observed at 1430ﾟC on heating and at 1420ﾟC on cooling, which would correspond to the phase transformation from monoclinic to orthorhombic structure. Less