| Directionally solidified Al2O3 based eutectic composite ceramics have excellent high temperature strength,high temperature structural stability,high temperature oxidation resistance and corrosion resistance,and are expected to become a new generation of ultrahigh temperature structural materials that are long-term service in high temperature oxidizing atmosphere.In this paper,the directionally solidified Al2O3/GdAlO3(GAP)binary eutectic in-situ composite ceramics prepared by the optical floating zone method was used as the research object.Firstly,the preparation process was explored and optimized,the effects of precursors and seed crystals on the preparation of Al2O3/GAP eutectic ceramics were analyzed.The relationship between solidification rate and eutectic structure was studied.The microstructure,phase composition,preferred orientation and phase interface structure of Al2O3/GAP eutectic ceramics were studied.The mechanical properties of Al2O3/GAP eutectic ceramics at different temperatures were measured,including hardness,fracture toughness,flexural strength and compressive strength,and their toughening mechanisms were analyzed.Research indicates:As the withdrawal rate increases,the lamellar spacing of Al2O3/GAP eutectic structure gradually decreases.When it is too large(>60mm/h),macro cracks will appear on the surface of the sample,and cellular and strations will appear in the structure,both of which will reduce the mechanical properties of the material.The Al2O3/GAP eutectic ceramic samples are milky white rods with no cracks and holes on the surface,and the density is high.There are only two phases of Al2O3 and GAP in the sample,and there is no amorphous phase.The volume ratio of the two phases is Al2O3:GAP=48:52,close to its theoretical value(45:55).The Al2O3 phase and the GAP phase are connected to each other and coupled to form a network structure,which is a typical irregular eutectic structure.Compared with the polycrystalline structure of sintered m aterials with the same composition,Al2O3/GAP eutectic ceramics are denser and have no obvious defects.In Al2O3/GAP eutectic ceramics,the preferred growth direction of the two phases is<1010>Al2O3//<001>GAP-1,<1010>Al2O3//<110>GAP-2,<001>GAP-1//<110>GAP-2.The interface orientation relationship between Al2O3 phase and GAP phase is:[1010]Al2O3//[001]GAP-1,(1120)Al2O3//(200)GAP-1;[1010]Al2O3//[110]GAP-2,(1120)Al2O3//(112)GAP-2;[001]GAP-1//[110]GAP-2,(110)GAP-1//(110)GAP-2.The two phases form a low energy interface with good interface matching.At room temperature,the hardness of Al2O3/GAP eutectic ceramics is 16.1±0.3 GPa,the fracture toughness is 9.3±0.6 MPa·m1/2,and the longitudinal section hardness and fracture toughness are slightly larger than the transversal section,showing anisotropy.In high temperature environment,the hardness is 12.3±0.3 GPa,and the fracture toughness is 10.2±0.1 MPa·m1/2.The hardness decreases with the increase of temperature,and the fracture toughness increases slowly.The Vickers indentation crack reduces the energy of the main crack propagation by micro-crack proliferation,bridging,bifurcation and deflection.The barrier effect of the phase interface on crack propagation increases the fracture toughness of the specimen.The three-point flexural strength of Al2O3/GAP eutectic ceramics is 533±28 MPa.The cleavage crack greatly consumes the expansion energy of the main crack through bifurcation and deflection,and improves the strength of Al2O3/GAP eutectic ceramics.The compressive strength of Al2O3/GAP eutectic ceramics at room temperature is 865 MPa.During the compression process,cracks in the sample expand and meet,resulting in cleavage fracture dominated by shear failure.High-temperature Hertz indentation experiment shows that under the action of external force,a large number of high density dislocations in Al2O3 phase and GAP phase will terminate when they meet the phase interface during the movement,and the firmly combined phase interface will hinder the dislocation movement. |
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