High and Low Temperature Superplastic ity in Nanocrystalline Materials
Dept. Materials Science & Engineering Pennsylvania State University, University Park PA 16802 U.S.A. Nanostructured Materials
01/1997; 9(1):717-726. DOI: 10.1016/S0965-9773(97)00158-X
Superplasticity, the ability of a crystalline material to deform to hundreds of percent strain, has been demonstrated at elevated temperatures for several nanocrystalline metal and ceramic systems. Nanocrystalline materials manifest superplasticity at lower temperatures and faster strain rates than their larger-grained counterparts; however, their enhanced superplasticity can easily disappear during deformation due to a combination of static and dynamic grain growth. Despite this limitation, applications such as near net shape forming, diffusion bonding, thermally mismatched composite structures, and flaw-free processing are already under development. In contrast to conventional superplasticity, low (room temperature) superplasticity has yet to be demonstrated conclusively in nanocrystalline materials. Early measurements of a room temperature ductility/superplasticity effect can be largely attributed to the presence of porosity. Unusual trends in room temperature strain rate sensitivity may reflect thermally activated dislocation glide past synthesis-generated defects, rather than a true change in deformation mechanism at ultrafine grain sizes.
Available from: Ozlem Caglar Duvarci
- "Nanocrystalline ceramics can also have cost reducing properties such as low temperature sintering and superplasticity. Superplasticity can be loosely defined as the ability of a crystalline material to undergo tremendous elongations prior to failure, on the order of hundreds or thousands of percent . Therefore, manufacturing near-net shaped pieces without machining is possible with nanocrystalline ceramics  . "
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ABSTRACT: The preparation of nanocrystalline titania and alumina was investigated by sol-gel methods using titanium isopropoxide, boehmite and aluminum isopropoxide. Various drying control chemical additives like oxalic acid, acetic acid and polyacrylic acid were used for modifying the drying behaviour and shrinkage of the gels. The sintered densities of the ceramics prepared by sol-gel processing and the dried gels were in the 79-99% of theoretical density for rutile. The green and sintered densities of the pellets prepared by uniaxial pressing of powders derived from sols, gels and precipitation techniques for titania were in the 40-52% and 55-83% respectively. The titania ceramics were observed to experience anatase-rutile phase transformation upon heat treatment at 650degreesC. The grain size of the sintered ceramics at 650degreesC was determined to be about 26 nm. Grain size of titania increased to 213 nm. at 850degreesC. The mechanical properties of these nanocrystalline ceramics were investigated by using microhardness testing.
Available from: Renaud Bouchet
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ABSTRACT: The preparation conditions of nanocrystalline phase-pure TiO2 anatase ceramics by hot pressing are described. Density, surface area, pore size distribution and grain size are determined by various techniques, including gas adsorption, mercury porosimetry, transmission electron microscopy (TEM) and X-ray diffraction (XRD). The evolution of the structural parameters is followed as function of temperature and pressure programme. It is shown that the porosity, grain and pore size of the ceramics can be controlled by a suitable choice of experimental conditions. Ceramics with densities higher than 90% of the theoretical limit with a mean grain size of 30 nm can be obtained at temperatures as low as 490 ◦C under 0.45 GPa for 2 h. The experimental results are discussed in view of the sintering theory.
Available from: Tomas P. Raming
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