Microstructure of Al ₂O ₃/YAG/ZrO ₂ hypereutectic alloy directionally solidified by laser floating zone method

Due to the excellent high temperature mechanical properties, Al ₂O ₃/YAG/ZrO ₂ ternary eutectic in situ composite is considered to be a promising candidate for the material, replacement for nickel based superalloy, of new generation aero space engine turbine blade. The directionally solidified Al ₂O ₃/YAG/ZrO ₂ hypereutectic ceramics are prepared with recently developed laser floating zone melting (LFZM) apparatus. Full eutectic lamellar microstructure, free of primary phase, was obtained with hypereutectic composition. The formation of solid/liquid interface morphology was analyzed in detail. The microstructure texture tendency was explained by combination with interface morphology. The experimental result indicates that, just as the prediction of JH model, average spacing of hypereutectic (λ _av) agrees with the inverse-square-root dependence on solidification rate (V) according to λ _avV ^0.5=14.7 μm ^1.5 ·s ^-0.5. In lower solidification rate, the lamellar spacing of hypereutectic is higher than that of eutectic composition, but the situation reverses in higher rate. The main reason of such phenomenon is that the addition of ZrO ₂ effects the thermal and solute transformation in the melt. The influence of transformation condition on lamellar spacing was analyzed synthetically by using classical irregular growth model. The formation mechanism of banded microstructure, often observed in laser zone melted solidification processing, was also discussed.