Growth And Mechanical Properties Of Alumina-based Eutectic Ceramics By Directional Solidification Method

Aircraft engine with high thrust ratio,high efficiency and high reliability is urgently needed and developed quickly in the fields of aerospace and aeronautics technology.The higher thrust and efficiency could be achieved by increasing the temperatures of front import in the turbines,so the high-temperature structural materials would suffer severer application conditions?under higher temperatures,complex loads,thermal shock,etc?and harsher performance requirements.Alumina-based eutectic ceramics have been widely studied over recent decades owing to its high melting point,low density,outstanding mechanical properties,superior microstructural and chemical stability in oxidizing atmospheres for a long time,which could reduce the weight of turbine blades and usage volume of cooling gas.Compared to the powder sintering method,eutectics grown by directional solidification techniques have clean interfaces between the two phases,free of glassy phases and grain boundaries,and strength retention up to near the melting point.Eutectics with a fine microstructure on the?m scale are sequentially solidified from over-heated homogeneous melt at a certain speed along a specific temperature gradient.However,the produced plate or rod samples always exhibit the weaknesses of smaller dimensions and lower utilization,which are difficult to meet the practical requirements.Horizontal Directional Solidification?HDS?method was firstly introduced into the fabrication of large size Al₂O₃/YAG eutectic ceramics based on the skillful growth experiences of single crystals.The quality of eutectics was improved by modifying the design of temperature field and seeking the optimal growth parameters.The influence of growth rates and seed types on microstructure,preferred orientations,thermal and mechanical properties was studied in this paper.Meanwhile,thermal shock conditions would happen by rapid temperature changes during startup and shutdown of turbines.The rods of Al₂O₃/YAG,Al₂O₃/EAG and Al₂O₃/EAG/ZrO₂ eutectic ceramics were grown by Laser Floating Zone?LFZ?method with different flexural strengths to analyze the dominant factors on thermal shock resistance temperatures in detail,providing references for further investigation.Large size HDS-Al₂O₃/YAG eutectic bulks in boat shape with dimensions of?145±5?mm×?90±5?mm×?25±5?mm were grown in vacuum at 10mm/h and 20mm/h.The slice seeds were extracted along the directions of perpendicular?vertical seed?and parallel?parallel seed?to the solidification direction from the 50mm/h-Al₂O₃/YAG eutectic,respectively.The annealing treatment?1500?×25h in air?is necessary for HDS-Al₂O₃/YAG eutectics to remove oxygen vacancies and release residual thermal stress,thereby enhancing the structural integrity and mechanical strengths.There is no grain growth after 1700? for 100 h,revealing excellent thermal stability.Then the two directions of specimens,transverse?T-specimen?and longitudinal?L-specimen?to the solidification direction,were cut from every eutectic.All the HDS-Al₂O₃/YAG eutectics show the microstructure of an interpenetrating three-dimensional network without grain boundaries.The interspace between 10mm/h and 20mm/h eutectics is similar,an obvious decrease in 50mm/h eutectic.Several preferred orientations of Al₂O₃ and YAG phases could inherit from the crystal planes of seed,but the orientation relationships on the cleave surfaces are not simple due to the faceted-faceted growth manner,lower solidification rate and vigorous convection in our large melting zone.The preferred peaks of YAG phase were more complicated,while it is relatively clear in parallel-seed eutectics and L samples.The Vickers hardness H_v,fracture toughness K_ICand Young's modulus E of HDS-Al₂O₃/YAG eutectics were fairly constant and independent of other parameters.Crack arrest by interface,crack deflection and crack branching consume more energy,and promote the fracture toughness.The flexural strengths maintained the approximately isotropic until to 1200? or to 1500?,and the compressive strengths decreased with increasing temperatures.The strengths and Weibull modulus of no-seed and parallel-seed samples were better than those of vertical-seed ones.The strengths of20mm/h eutectics were slightly higher than those of 10mm/h eutectics in same seed condition,and the values of L-specimens were also slightly higher than those of T-ones.The fracture was in a brittle manner.In HDS method,seed plays a favorable role in crystallization to serve as a nucleation center,but it sometimes might bring the adverse effect on the mechanical properties because the new eutectics might replicate and magnify some intrinsic defects in the seed inevitably,especially in vertical seeds.In order to research the critical thermal shock resistance temperature??T_c?,the rod Al₂O₃/YAG,Al₂O₃/EAG and Al₂O₃/EAG/ZrO₂ eutectic ceramics with the size of??1.11¹.7?mm×?85¹85?mm were grown by LFZ method in nitrogen atmosphere.Two processing rates?25mm/h and 750mm/h?were used to obtain eutectics with different microstructure,i.e.the original flexural strengths of 750mm/h samples were more than two times than those of 25mm/h ones.Quenching in room-temperature water?severe thermal shock?,the?T_c is similar?255?²82??in same composition.The?T_c of HDS-Al₂O₃/YAG eutectics is 178?²28?,as better than calculated results.The interspace?is inversely proportional to the flexural strengths.The interspace of LFZ-Al₂O₃/YAG eutectics is only 1/3 that of HDS-Al₂O₃/YAG ones due to greater thermal gradient.Quenching in boiling water?mild thermal shock?,the?T_c of LFZ-Al₂O₃/EAG eutectics increased to 655? owing to the deeply decrease in heat transfer coefficient h,but that of HDS-Al₂O₃/YAG samples was still 175?²05?.After thermal shock tests,the cracks initiated from eutectic surfaces and then propagated to the interior area.When?T<?T_c,the increase rate of crack propagation were almost same in all samples.The critical flaw size is controlled by fracture toughness and flexural strengths,at a rate proportional to ?.