Microstructures And Properties Of Al₂O₃-based Eutectic Ceramics Prepared By Laser Floating Zone Melting With High Temperature Gradient

Al₂O₃-based eutectic in situ composite ceramics can form through eutectic reaction during directional solidification(DS)processing with the major phase and minor phase in situ formed from the molten liquid.The Al₂O₃-based eutectic in situ composite ceramics possess excellent mechanical properties and thermal-chemical stabilities especially at elevated temperatures even closing to their melting points.Moreover,some of the Al₂O₃-based eutectic in situ composite ceramics also possess exceptional functional properties.Therefore,it is noticeable that structure and function integration can be achieved in these oxide eutectic systems.Furthermore,the Al₂O₃-based eutectic in situ composite ceramics present unique faceted growth behavior.Hence,it is a group of ideal material systems for investigation of irregular eutectic growth mechanism.In this thesis,directionally solidified Al₂O₃/EAG(Er₃Al₅O₁₂)binary and Al₂O₃/EAG/ZrO₂ternary eutectic in situ composite ceramics were fabricated by laser floating zone melting(LFZM)technique with high temperature gradient.The processing parameters,molten zone stabilities and temperature gradient were first explored.Then coupled eutectic growth,precipitation rule,microstructure evolution,interface structure and irregular-regular eutectic transformation mechanism of the faceted eutectic ceramics were analyzed.Lastly,the influences of compositions and solidification parameters on the mechanical properties,microstructure stabilities,and optical absorption performances were analyzed.The main results are shown as follows:(1)Variation of temperature gradient with changing the solidification rate and time are analyzed.At low solidification rate of 4?m/s,the periodical oscillations of the molten zone shape and size,temperature distribution,temperature gradient,and rod diameter were generally detected with almost the same period(?275s).The time-dependent temperature gradient oscillated between 3.6×10³K/cm and 6.9×10³K/cm and its average value is about 5.3×10³ K/cm at low solidification rate of 4?m/s.As increasing the solidification rate,the oscillation phenomenon weakened and the temperature gradient decreased.When the solidification rate increased up to 200?m/s,the average value of temperature gradient decreased down to 4.1×10³K/cm.The result of temperature gradient evolution obtained from the experiment fits the variation tendency of the calculation result.(2)The Al₂O₃/EAG binary eutectic in situ composite ceramics present“Chinese script” irregular eutectic morphologies.The eutectic spacings decreased gradually with increasing the solidification rate.At low solidification rate of 4?m/s,the eutectic spacing is about 6.43?m.When the solidification rate increased up to 400?m/s,the eutectic spacing decreased down to about 570nm.The relationship between square root of solidification rate V^0.5 and eutectic spacing?deviates from linear relationship.The slope value of?V^0.5 in the lower solidification rate range(4?25?m/s)and higher solidification rate range(50?400?m/s)is about 8.5 and 13.1.(3)The couple growth range of Al₂O₃/EAG binary eutectic at low solidification rate is given in a composition range of 15.5?22.5mol%Er₂O₃.A symmetric coupled zone is obtained by comparing interface temperature of eutectic and dendrite.Oscillation of regular eutectic and lamellar-rod transition happens at hypoeutectic composition and the enlarged coexistence region of lamellar and rod-like eutectic structure is evaluated.As deviating from the eutectic point(hypoeutectic composition),regular eutectic can form at lower solidification rate.The weak-faceted EAG phase nucleated on the Al₂O₃ substrate is considered to be a probable reason of regular arrangement structure formation under low solidification rate condition.(4)Al₂O₃/EAG/ZrO₂ ternary eutectic ceramic is constituted by Al₂O₃,EAG,and ESZ phase.The relationship between eutectic spacing and solidification rate follows ?V^0.5=11.14?m^1.5s^-0.5.Formation of irregular eutectic structure with ZrO₂ phase dispersed inside the Al₂O₃ matrix or attached to the edge of the EAG phase is analyzed:(1)ZrO₂ phase embedded in the Al₂O₃ phase is considered to form by the depression of Al₂O₃ phase and engulfment of ZrO₂;(2)ZrO₂ phase attached to the edge of the EAG phase is considered to form through volume fraction adjustment.(5)Based on LFZM and liquid-metal forced cooling,a method called MFQ method with high cooling rate is designed for fabrication of Al₂O₃-based eutectic ceramic.The eutectic spacings of Al₂O₃/EAG binary eutectic in situ composite ceramic prepared by this method ranges from 2.28?m to 30nm.As the eutectic spacings decreasing from submicronscale to nanoscale,different kinds of microstructure morphology or phenomenon can be detected:"Chinese script"irregular eutectic;"Chinese script"irregular eutectic+eutectic colony structure(>285nm); irregular-regular eutectic transformation(?285nm);irregular eutectic+regular eutectic(285nm?50nm);regular eutectic(<50nm).Solidification rate of 8.65?m/s?5.32×10⁴?m/s is estimated according to the eutectic model.The irregular eutectic growth and transformation is evaluated through analyzation of interface growth mode,leading phase growth,and spacing adjustment.(6)The hardness and fracture toughness of Al₂O₃/EAG binary eutectic in situ composite ceramics both increased with increasing the solidification rate.The maximum value of hardness and fracture toughness is about 17.8GPa and 3.1MPa·m^1/2.The fracture toughness improved with increasing the ZrO₂ content.When the ZrO₂ content increased up to 20.6mol%,the fracture toughness can reach up to 3.5MPa·m^1/2.The room temperature bending strength of the Al₂O₃/EAG binary eutectic in situ composite ceramic also improved with increasing the solidification rate.The bending strength value is about 500MPa for the specimen grown at solidification rate of 4?m/s and its value improved up to 1.8GPa for the specimen grown at 200?m/s.(7)Al₂O₃-based eutectic in situ composite ceramics exhibit excellent microstructure stability.Result of the heat exposure experiment at 1500? for 200h exhibits that the Al₂O₃/EAG/ZrO₂ ternary eutectic ceramics obtained at lower solidification rates possess better microstructure stability.The microstructure shows rounding and coarsening phenomenon after the heat exposure but eutectic spacing restricted to about 1?m.The fracture toughness does not exhibit severe variation after the heat exposure process.(8)Strong absorption peaks near wavelength of 1.5?m(near-infrared)can be detected both in Al₂O₃/EAG binary and Al₂O₃/EAG/ZrO₂ ternary eutectic in situ composite ceramics.As increasing the solidification rate,the intensity of absorption peak in Al₂O₃/EAG increased and red shift of 1?3nm is detected.No obvious difference of optical absorption is found with decreasing the Er₂O₃ content besides a red shift of 2nm in hypoeutectic specimen.The absorption intensity of Al₂O₃/EAG/ZrO₂ specimen is generally stronger than that of Al₂O₃/EAG specimen.