Microstructures and mechanical properties of Al₂O₃-basic eutectic in situ composites directionally solidified by laser floating zone remelting

Directionally solidified oxide eutectic in situ composites have been attracting increasing interest in recent years for use as the next generation of ultra-high-temperature structural materials because of their excellent high-temperature strength, oxidation and creep resistance, as well as outstanding microstructural stability. Al₂O₃/YAG/ZrO₂ ternary eutectic in situ composites with high density are prepared by laser floating zone remelting technique. The microstructure evolution of Al₂O₃/YAG/ZrO₂ ternary eutectic under high temperature gradient and different growth rates is investigated. The relationship between solidification rate and eutectic spacing for the ternary oxide eutectic is quantificationally characterized. On this basis, the mechanical properties and relationship between microstructure and fracture toughness are analysed. The results show that the directionally solidified Al₂O₃/YAG/ZrO₂ ternary eutectic in situ composite belongs to typical irregular lamellar eutectic structure. The microstructure is rapidly refined with the increase of the solidification rate V. The minimal eutectic spacing observed is as fine as 0.46 μm when the solidification rate is 200 μm/s. The relationship between the average eutectic spacing (λ_av) and V is determined to be λ_avV^0.5=12.4 μm^1.5·s^-0.5. Moreover, the ternary eutectic lamellar spacing is much smaller than the binary one at the same solidification condition. The average hardness and room-temperature fracture toughness of the ternary eutectic are (19.0±1.0) GPa and (3.31±0.2) MPa·m^1/2, respectively. As compared with the binary eutectic, the crack arrest, deflection and mismatch of thermal expansion coefficient of eutectic phases are the predominant toughening mechanisms of ternary eutectic composite.