| As a new generation of ultra-high temperature structural materials,Al2O3-based eutectic ceramics exhibit remarkable high temperature mechanical properties,microstructural thermal stability,high-temperature oxidation resistance and creep resistance,which are expected to be perspective candidates used for high-temperature structural components in advanced aerospace engines and gas turbines.However,the relationships between the eutectic microstructures and mechanical properties of Al2O3-based eutectic ceramics are still unclear,as well as the scarcity of insights into their fracture behaviors and strengthening-toughening mechanisms.In this work,the theoretical micromechanics models of Al2O3-based eutectic ceramics were established and combined with the analysis of finite element method(FEM).In situ TEM micromechanical testing technique was performed to evaluate the micro-mechanical properties of sub-micron pillars and microcantilever specimens.The effects of eutectic texture,interfacial complexions,sample size and geometry on the microscale deformation and fracture behaviors of Al2O3-based eutectic ceramics were investigated systemically,to reveal the plastic deformation and strengthening-toughening mechanisms of typical Al2O3-based eutectic ceramics.Dense polycrystalline Al2O3/SAP ceramics were fabricated by hot pressing with desired ?-Al2O3 and SmAlO3 phase.The Al2O3/SAP sample with equiaxial grains hot-pressed at 1500? has the optimum mechanical properties at room temperature.Namely,its relative density and average grain size are 99%and 1.3?m,while its flexural strength and SENB fracture toughness are 558MPa and 5.59MPa·m1/2,respectively.The precipitation of nanoscale intragranular particles in Al2O3 grains is attributed to the diffusion enrichment and nucleation process of supersaturated rare-earth elements in the pristine nanoscale coprecipitation powders.Al2O3/SAP eutectic ceramics with different eutectic textures were fabricated by pulse discharge plasma assisted melting.With the increase of processing temperature,the "semi-solid to liquid" coarse eutectic mircostructure gradually evolves into a uniform-refined eutectic microstructure.The Al2O3-SAP eutectic mircrostructure has a low-energy coherent interface,with the orientation relationship namely Al2O3[0001]//SmAlO3[021],Al2O3(1100)//SmAlO3(100);However,the different grain boundaries between equiaxial grains in sintered Al2O3-SAP micro structure exhibit as randomly-oriented incoherent interfaces.In situ TEM microcantilever was used to test the interfacial fracture toughness of different complexions.The average interfacial fracture toughness of eutectic Al2O3-SAP coherent interface is as high as 2.83MPa·m1/2,which is higher than all equiaxial grain boundaries of same composition.The results show that the interfacial fracture toughness depends upon their constituent microstructure and composition.The interfacial energy is determined by the microstructure composition and grain boundary complexions together.The higher the interfacial energy,the lower the fracture toughness.The micro-compression tests combined with FEM analysis show that the Al2O3 matrix and other rare-earth-containing components(e.g.REAG and REAP phase)form synergistic deformation during the micro-compression process,including the active deformation mechanisms of Al2O3 basal slip and eutectic interfacial slip.Strong interfacial bonding in the eutectic ceramics can effectively block the interfacial slip,resulting in a higher compressive strength.The compressive strengths of Al2O3/YAG and Al2O3/SAP eutectic ceramics are 7.2GPa and 5.9GPa,respectively.For the uniaxial micro-compression of rod-like Al2O3/GAP eutectic micropillars,the yield and compressive strengths are 5.3GPa and 6.1GPa,respectively.However,it exhibits a distinct anisotropy in response to the micro-compression plastic deformation.The transverse-sectioned specimens perpendicular to the loading direction mainly exhibits an interfacial slip mechanism.While,the longitudinal-sectioned specimens parallel to the loading direction exhibit interfacial failure of cracking due to the inconsistent stress between constituent phases.Fine ZrO2 phase in Al2O3/EAG/ZrO2 eutectic ceramics is uniformly distributed in the eutectic matrix.In the compression deformation,nanoscale ZrO2 particles significantly impede plastic deformation,which leads to strengthening and toughening of the eutectic ceramic.Based on in situ TEM fracture tests of microcantilever bending,the eutectic samples with different microstructure and orientation are of distinct diversities.Particularly,a distinct R-curve toughening effect occurs in the longitudinal section of rod-like Al2O3/GAP eutectic microcantilevers.Accordingly,micro-fracture FEM simulations verify the significant difference of stress field along crack propagation path due to the distribution of fibrous microstructure.Thus,the normal direction of crack plane deviates from the sample plane.During the process of crack opening,the nanoscale crack-bridging and crack deflection can result in the energy dissipation and toughening effect,which brings the fracture toughness to 3.07MPa·m1/2.The bending fracture tests of Al2O3/YSZ eutectic microcantilevers show that,the stress-induced ZrO2 phase transition effect enhances its fracture toughness to be 5.50MPa·m1/2. |
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