Solidification Behavior and Microstructural Characteristics of Al2O3/ZrO2(Y2O3) Eutectic Ceramics

Author:Fu Lian Sheng

Supervisor:zhou wen long chen guo qing


Degree Year:2019





A1203-based in-situ eutectic composite ceramics possess outstanding high-temperature mechanical properties,excellent thermal stability,and chemical resistance at ambient and elevated temperature.They have been considered as the most promising candidates applied in high-temperature oxygen-enriched environment above 1600 K for long periods,which in recent years has attracted ever-increasing research attentions.In the past,several new processing techniques had been developed for the eutectic oxide ceramic(such as the micro-pulling down,edge-defined film-fed growth,and laser-heated floating zone methods,etc.).Considerable attention was mainly focused on the relationship between the processing parameters,microstructural characterization,and mechanical properties.Nevertheless,less effort is devoted to investigate its solidification characteristics and microstructural evolution,which plays a crucial role to achieve the microstructural control and further improve the properties for the oxide eutectic composites.Thus,a deeper understanding for the solidification behavior is of great importance to promote further development of the oxide eutectic composites and realize its engineering application in components.In this study,the Al2O3/ZrO2(Y2O3)pseudo-binary eutectic system is considered as a research object,which has a more complex eutectic solidification behavior due to the third component addition(Y2O3 addition).High temperature melt-grown method with a modest thermal gradient was adopted to prepare the large-dimension eutectic bulks,in which a low G/v ratio(thermal gradient G divided by growth rate v)and a small undercooling can be obtained.It is found that along the solidification structure consists of fine equiaxed grain zone,columnargrain zone and coarse equiaxed grain zone.The microstructure characteristic,evolution rule and growth behavior of the melt-grown Al2O3/ZrO2 eutectic bulks doped with different amounts of Y2O3 in a concentration range of 0-9.2 mol%were systematically investigated.In addition,some mechanical properties were also studied.The main research achievements are as follows:Microstructural controls in the melt-grown A12O3/ZrO2 eutectic can be achieved by the Y2O3 addition,which may significantly affect its growth behavior.The microstructure of all Y2O3 doped Al2O3/ZrO2 eutectic samples was composed of a typical colony structure.The geometrical shapes of the colony morphology change from a sub-rounded cellular to ellipsoidal cellular with protrusions or pits,the serrated cellular,and then to an abnormal dendritic form with the increase of Y2O3 content.It indicates that the Y2O3 addition remarkably changes the growth interface morphology of A12O3/ZrO2 eutectic melt at a small supercooling.This is because that the addition of Y2O3 into the A12O3/ZrO2 eutectic can create an additional constitutional undercooling ahead of the solid/liquid interface,and the supercooled zone length will become much larger with the increase of Y2O3 content.The colony size decreases and intercolony area increases as the Y2O3 content increases.The size and severity of pores and cavities within the colony boundaries increases,simultaneously.As the Y2O3 content increased up to a certain value(≥4.5 mol%),the Al2O3/ZrO2/Y3Al5O12]ternary eutectic in-situ microstructure may appear in the inter-colony region.The volume fraction of the ternary eutectic will increase with the increase of Y2O3 content.The Scheil equation(or nonequilibrium lever rule)can be applied to predict the amount of ternary eutectic well.The center of each colony is composed of an Al2O3 matrix(dark areas)containing a regular distribution of white ZrO2 fibers or a disordered interpenetrating network of fine ZrO2 lamellae.The average interphase spacing tends to decrease from 1.8 um to a minimum of 1.1 um with increasing Y2O3 content up to 1.1 mol%,and then increases slightly at higher Y2O3 contents.According to the J-H model(λ2v≈Constants pm3/s,v is the growth rate and λ the interphase spacing),the eutectic growth velocity changes in an opposite way of the eutectic interphase spacing.The dependence of the interphase spacing on the Y2O3 content indicates that the eutectic growth rate is affected by the amount of Y2O3 during the solidification.When the Y2O3 content is smaller(<1.1 mol%),the effect resulting from the decrease of the dendrite tip is dominant.The groMwth therefore speeds up.If the Y2O3 content is large enough,too much the Y2O3 solute piles up in front of the interface,and cannot be diffused away quickly during the solidification.Consequently,the growth velocity declines.Besides,the growth rate v and interface undercooling {AT)can be determined from the J-H model based on the interphase spacing(λ=1~2μm)are~ll um/s and~0.53 K,respectively.The XRD results show that the specimens with different Y2O3 contents were composed of different phases.The monoclinic ZrO2 was present up to contents of 1.1 mol%Y2O3,while the amount of tetragonal ZrO2 gradually increased with yttria content up to 1.1 mol%.Cubic ZrO2 was the only phase detected when the Y2O3 content reached 1.7 mol%.However,further increasing Y2O3 content up to 4.5 mol%,except for the two major phases(a-Al2O3 and cubic ZrO2)existing,a few weaker diffraction peaks of Y3Al5O12 appeared.Factors expected to affect the mechanical properties in the ceramic are mainly depending on the component phases,voids and growth defects.In the Al2O3/ZrO2(Y2O3)eutectic system,when Y2O3 is added,the hardness increased,reached the maximum at 1.1 mol%addition of Y2O3.It derives from that Y2O3 stabilizer addition can substantially reduce the volume fraction of m-ZrO2 phase with low hardness and the magnitudes of transformation microcracks.As Y2O3 content is further increased,the hardness decreases slightly.It results from the fact that the size and severity of pores and cavities located the intercolony area increases with the increase of Y2O3 content.The fracture toughness Klc estimated from the indentation method decreases with increase of Y2O3 content.These samples without or with low Y2O3 addition(<1.1 mol%)possessing high fracture toughness are attributed to microcrack toughening.Higher Y2O3 addition may trigger the increase of defects and coarse particles within intercolony regions.As a result,the fracture toughness declines finally.