Fabrication, microstructure and high-temperature plastic deformation of three-phase Al2O3/Er3Al5O12/ZrO2 sintered ceramics

The fabrication, microstructure and high-temperature creep behavior of chemically compatible, three-phase alumina/erbium aluminum garnet (Er3Al5O12, EAG)/erbia fully-stabilized cubic ZrO2 (ESZ) particulate composites with the ternary eutectic composition is investigated. The composites were fabricated by a solid-state reaction route of alpha-Al2O3, Er2O3 and monoclinic ZrO2 powders. The final phases alpha-Al2O3, EAG and ESZ were obtained after calcination of the powder mixtures at 1400 degrees C. High dense bulk composites were obtained after sintering at 1500 degrees C in air for 10 h, with a homogeneous microstructure formed by fine and equiaxed grains of the three phases with average sizes of 1 mu m. The composites were tested in compression at temperatures between 1250 and 1450 degrees C in air at constant load and at constant strain rate. As the temperature increases, a gradual brittle-to-ductile transition was found. Extended steady states of deformation were attained without signs of creep damage in the ductile region, characterized by a stress exponent of nearly 2 and by the lack of dislocation activity and modifications in grain size and shape. The main deformation mechanism in steady state is grain boundary sliding, as found in superplastic metals and ceramics. In the semibrittle region, microcavities developed along grain boundaries; these flaws, however, did not grow and coalescence into macrocracks, resulting in a flaw-tolerant material. Alumina is the creep-controlling phase in the composite because of the grain boundary strengthening caused by the (unavoidable) Er3+- and Zr4+-doping provided by the other two phases.