A Study Of The Superplasticity Of Structural Ceramics

Superplasticity is the general potential property of materials of lattice structure. It's a process that materials establish a stable deformation process after they are deformed and become instability. It's mostly a boundary behavior, i.e. boundary gliding, transferring and shifting occurred as a microcosmic physical behavior. Realization of superplasticity is the result of accomodation between intrinsic and extrinsic qualification of material. Ceramic and ceramic-base-composite, metal-base-composite, intermetallic compounds, mechanical-alloy-compounds, functional materials, geology materials, and composite materials et al, are all advanced materials of superplasticity。The performance of structural ceramics is excellent. Its intensity, tenacity, rigidity, chemistry stability are very superior and its density is lower. The structural ceramics is also able to endure high temperature, abrasion, corrosion, and anti-creep. It is widely used in the field of electronical, mechanical, metallurgical, chemical. And the foreground of applications in the field of cosmic exploitation, technology of energy sources and ocean, aviation and spaceflight are wider increasingly. Step by step, it become the key-material of high-tech which is indispensable. Using the superplasticity of structural ceramics in high temperature processing deformation is the important measurement in the field of realizing the almost-net-deformation of complex shape part.Classified the structural ceramics by component, it is composed of oxide-ceramics, nitride-ceramics, carbide-ceramics, et al. Oxide-ceramics mainly includes alumina, zirconia, et al. Its atoms are almost linked with ion bond. Therefore it is provided with good mechanism performance in room temperature, high rigidity, anti-chemistry-corrosion. No oxidation is the outstanding merit of oxide-ceramics. So the raw material is cheap, andmanufacture of technics is easy. Since the atoms of nitride-ceramics and carbide-ceramics are linked with covalent bonds, the ceramics has anti-creep, high rigidity and module. And they can hold most of the good performance till high temperature. Compared with oxide-ceramics, this characteristic is very excellent. But the sintering of nitride-ceramics and carbide-ceramics is very difficult. So their yielding costs are higher than oxide-ceramic.Structural ceramics is polycrystal, which is made up of ion bonds or covalent bond with complex structure. With the static action of ion bonds and the obvious orientation nature of covalent bond, ceramics cannot satisfy the qualification of geometry and static which is required in causing slip. The slip system is lack. The creation and movement of dislocation are difficult. And ceramics tends to separate along crystal boundary. All of above induce the inbeing brittleness, which causes the poor processing ability. Superplastic processing, including superplastic forming, juncture, sintering, forging, et al, provides feasible approach to process the structural ceramics. To extending the fresh field of superplastic research in rigidity and brittleness structural material, the aim of this paper is the prophase work of superplastic research in structural ceramics latter.In principle, the superplasticity of structural ceramics can be separated into structure superplasticity and phase transformation superplasticity. In the research of structure superplasticity, fined grain superplasticity has been studied much. Most of the conclusion about ceramics superplasticity is obtained in fined structure, and all the superplasticity, which mentioned in this dissertation belong to this.The primary immanent factors affected the fined grain ceramics superplasticity is the size of grain and property of grain boundary. The finer size of grain, the more phase of grain boundary, the easier sliding of grain boundary, and the higher representation extension ratio. The external factors include deformation temperature and strain speed. When temperature rises up, rheo-stress comes down. But it is wrong that the higher temperature, the large extension ratio. At a certain temperature, the maximum will appearance. The deformation rate and deformation temperature restrict each other. There isdifferent optimal deformation rate at different temperature. Simultaneity, there is different optimal deformation temperature with different deformation rate. Superplastic deformation will produce strain which makes grain grow up. When the deformation increases, the size of grain largen, too.There is following same point in structure superplasticity between structural ceramics and metal alloy: 1) strong ability of crystal lattice strain; 2) equal axes shape of grain with small size; 3) stable structure in the deformation term, that is, the size and shape of grain are stable; 4) behave good superplasticity in certain range of strain rate and temperature; 5) the phenomena of grain boundary sliding, grain growing up with strain reinforcing, cavitation between grain.In the process of superplastic deformation, grain boundary sliding is a accepted deformation mechanism. The difference between all kinds of superplastic theory model mainly lies with the different detail of stress centralize caused by corresponding grain boundary sliding. Owing to immanent strong covalent bond and the orientation of bond in ceramics, it could be predicted that, dislocation sliding with relationship with accommodation in ceramic is less effective than diffusion stress.Although the vitreous phase between is not the necessary condition which makes ceramics superplastic, the existence of vitreous phase can improve the superplasticity of structural ceramics remarkably, such as reduce superplastic temperature and deformation stress, increase strain rate, enhance diffuse course of superplastic ceramics and deformation ratio. Notwithstanding, in the course of ceramics superplastic tension, it gained the large extension ratio yet.This dissertation entirely summarizes the research actuality and developing tends of structural ceramics domestic and overseas, sums up the preparation methods of oxide-ceramics, nitride-ceramics, carbide-ceramics, vitreous-ceramics, and expatiate the applications of structural ceramics. Besides, it contrasts and analyzes the experimental procedures and results of superplasticity on oxide-ceramics, obtaining the difference on superplasticity between these oxide-ceramics, and bringing forward the direction for superplastic research for the future. It can be regarded as a background and...