Microstructure And Formation Mechanism Of Al₂O₃–ZrO₂?Y₂O₃?eutectic Layer Fabricated By Laser Surface Melting

Al₂O₃-ZrO₂ eutectic ceramics are considered as potential candidates for ultrahigh–temperature structural application in oxidizing environment such as aerospace field due to their excellent mechanical properties and chemical stability near melting point.For a long time,most researchers concentrated on the influence of processing parameters on microstructure and mechanical properties,but rarely on its solidification mechanism.In this paper,Al₂O₃-ZrO₂?Y₂O₃?pseudo-binary eutectic layers were rapidly remelted by laser on the surface of composite precursors prepared by hot-pressing sintering.Solidification The effects of the solidification conditions and the content of the third component on the microstructure evolution were studied and the correlation between eutectic microstructure and mechanical properties was analyzed by analyzing the microstructure and mechanical properties.The Al₂O₃-ZrO₂?Y₂O₃?composite precursors with a density of more than 98% was successfully prepared by hot pressing sintering.The Al₂O₃-ZrO₂?Y₂O₃?eutectic layer is rapidly solidified on the surface of the composite by laser surface melting.At the solid-liquid interface,Al₂O₃ nucleates first in faceted growth character.The growth direction of eutectic cells gradually deflected from the solid-liquid interface at the bottom to the surface of eutectic layer under the action of both temperature gradient and growth rate,and the microstructure gradually refined.The surface of the eutectic layer is composed of oriented columnar eutectic cells.At the same time,the microhardness of the eutectic layer increases from the bottom to the surface.The temperature gradient is the dominant factor of the eutectic growth at the bottom of the molten pool,while the eutectic growth on the surface of the eutectic layer is mainly governed by the growth rate.The characteristic size of the columnar eutectic cells on the surface decreased with the increase of scanning rate.Regular and irregular eutectic spacing in columnar grains at different growth rates are ?_re=115?150nm and ?ir=143?239nm,respectively.According to JH model,the growth rates of eutectics are 290.6?835.2?m/s,and the undercooling temperature of the eutectic melt are 2.73?4.62 K.The transformation of regular eutectics and irregular eutectics under rapid solidification condition is the result of kinetic undercooling.With the increase of growth rate and undercooling temperature,the growth manner of solid-liquid interface changes from planar characteristics to cellular characteristics under different solidification process.Eutectic growth under rapid solidification condition is mainly controlled by diffusion of the solute.With the increase of Y₂O₃ concentration,crystal structure of ZrO₂ in eutectic layer changed from tetragonal to cubic gradually.At the same time,the solute distribution at the solid-liquid interface and the undercooling temperature change with the increase of Y₂O₃ concentration,resulting that characteristic size of the columnar eutectic cells on the surface decreased first and then increased.The microstructure inside the columnar eutectic cell changed into regular eutectic gradually with the decrease of kinetic undercooling temperature.The microhardness of Al₂O₃-ZrO₂?Y₂O₃?eutectic layer with different Y₂O₃ concentration is significantly higher than that of the composite substrate with the same composition.With the increase of Y₂O₃ concentration,the phase structure and microstructure of eutectic ceramics changed,and the microhardness increased first and then decreased.