A Study Of The Superplasticity Of Representative Structural Ceramics

Superplasticity is the general potential property of materials of latticestructure. It's a process that materials establish a stable deformation processafter they are deformed and become instability. It's mostly a boundary behavior,i.e. boundary gliding, transferring and shifting occurred as a microcosmicphysical behavior. Realization of superplasticity is the result of accomodationbetween intrinsic and extrinsic qualification of material. Ceramic andceramic-base-composite, metal-base-composite, intermetallic compounds,mechanical-alloy-compounds, functional materials, geology materials, andcomposite materials et al, are all advanced materials of superplasticity。The structural ceramic has the impotant location and great scale in modernceramics. Because of its low density, abundant raw material, it has the greatusefulness potence. In all of them, the price of Al2O3 ceramic is low, which issuperior than others. Its development is early, its cost is low, its usefulness isextensive. While ZrO2 ceramic is late-model ceramic, because it has theexcellent physical and chemic property, It has been the hotspot not only instudy field, but also in industry. It has become the favourite plastic materialbecause of its Phase transition toughness.But the brittleness of ceramic is the problem which makes it to be themachining parts, which limit the use and spread of ceramic. The brittleness isthe character of ceramic, also its deadly defect. This reflects on indirectly itslower anti-mechanical impact strength and inferior anti-temperature variabilityunexpectedly. Manifest right in that its rupture take on the fulminic characterand catastrophic result when it is loaded critically. The brittleness essence ofceramic crystal structure lies in lacking five unattached slip system. It'sdifficult for it to occur to plastic deformation arosed by sliding to slack thestress. The brittleness essence of ceramic in microstructure is the crack existing;furthermore, it tends to induce the stress concentration. thereby it has agesignificance and practical use value studying the superplasticity about ZrO2,Al2O3 and their toughed material.This dissertation make the summary about the toughness meathod oftypical structural ceramics: ZrO2 and Al2O3 ceramic composites, moreover,make the study on corresponding toughening mechanism.In typical structure of ZrO2(Y-TZP), We can obtain the toughness ceramicswith superior mechanical property when the content of Y2O3 is 2.5mol%～3.0mol%. The effect phase transition will fall and the rupture intensity andtoughness will drop too when Y2O3 is more. The grain size is controlled by thehot press temperature and heat preservation time. While depth phase transitionis affect by the grain size. Rupture toughness will improve with increasing ofdepth phase transition. The way of crackle spread chiefly is along grain, aidedin throughing grain.The toughening way of stress inducing phase transition and crackle aremain toughening mechanism of ZTA. If ZTA is compounded withSiCpWhiskers or SiCw grain, the toughness of material will be better than pureZrO2.This dissertation put emphasis on summary and analysis to superplastictest of typical ZrO2 and Al2O3 ceramic composites.The plastic deformation of Ceramic material in high temperature followthe definite rule: The plastic flow stress decreases with initial strain rate. Stressincreases with the strain rate which is the primary character of plasticdeformation. Strain hardening will occur for the fine-grained Y-TZP. This iscommon to fine-grained Ceramic at high temperature plastic deformation.Strain hardening occur to other Ceramics besides Y-TZP,Al2O3,The differenceis the deformation mechanism. Grain size has the obvious influence, thehigherof the plastic stress at costant strain rate, the bigger of grain size.Presently, It's not clear whether the glassy phase lies in superplastic Y-TZPand Al2O3-ZrO2 ceramic composites, Because all microstructure is studied inroom temperature, Now, we can't give creditable proof to prove that glassyphase lies in grain boundary at high temperature. Although the glassy phasebetween is not the necessary condition which makes ceramics superplastic, theexistence of glassy phase can improve the superplasticity of structural ceramicsremarkably, such as reduce superplastic temperature and deformation stress,increase strain rate, enhance diffuse course of superplastic ceramics anddeformation ratio.Though Y-TZP bear the large extension ratio in the course of ceramicssuperplastic tension, it tend to occur cavitation. When coarser-grainedspecimens deform at high enough strain rate level, creep cavitation is observedin grain intersection. The study showed the same thing occurred to Al2O3-ZrO2ceramic composites. The cavitation is the crucial organization feature tosuperplastic deformation.In the process of superplastic deformation, grain boundary sliding is aaccepted deformation mechanism. The difference between all kinds ofsuperplastic theory model mainly lies with the different detail of stresscentralize caused by corresponding grain boundary sliding. Owing to immanentstrong covalent bond and the orientation of bond in ceramics, it could bepredicted that, dislocation sliding with relationship with accommodation inceramic is less effective than diffusion stress.This dissertation make the summary about typical structural ceramics:summarizes the research actuality and developing tends of structural ceramicsdomestic and overseas, describe roundly physical and chemic property and usefield to ZrO2, Al2O3 ceramic composites, analyse contrastively the testmethod and result of two typical ceramic composites, obtain the difference onsuperplasticity between these ceramic composites, and study deeply the testtechnique, test rule and test result, probe into the macroscopical mechanicalrule, microcosmic organization and deformation mechanism. The study is thecontinuum of superplasticity for crisp material, aimed to bring forward thedirection for superplastic research for the future. It can be regarded as abackground and reference for the following study.