Superplastic forming of ceramic insulation

Superplasticity has been demonstrated in many fine-grained structural ceramics and ceramic composites, including yttria-stabilized tetragonal zirconia polycrystal (YTZP), alumina, and Al2O3-reinforced zirconia (Al2O3/YTZ) duplex composites and SiC-reinforced Si3N4. These superplastic ceramics obviously offer the potential benefit of forming net shape or near net shape parts. This could be particularly useful for forming complicated shapes that are difficult to achieve using conventional forming techniques, or require elaborate, subsequent machining. In the present study, we successfully demonstrated the following: (1) superplastic 3Y-TXP and 20 percent Al2O3/YTZ composite have for the first time been successfully deformed into hemispherical caps via a biaxial gas-pressure forming technique; (2) no experimental difficulty was encountered in applying the required gas pressures and temperatures to achieve the results, thus, it is certain that higher rates of deformation than those presented in this study will be possible by using the current test apparatus at higher temperatures and pressures; and (3) an analytical model incorporating material parameters, such as variations during forming in the strain rate sensitivity exponent and grain growth-induced strain hardening, is needed to model accurately and therefore precisely control the biaxial gas-pressure forming of superplastic ceramics. Based on the results of this study, we propose to fabricate zirconia insulation tubes by superplastic extrusion of zirconia polycrystal. This would not only reduce the cost, but also improve the reliability of the tube products

Superplasticity in aluminum alloys

We have characterized in the Al-Mg system the microstructure and mechanical properties of a cold-rolled Al-6Mg-0.3Sc alloy. The alloy exhibited superplasticity at relatively high strain rates (about 10-2 s-1). At a strain rate of 10-2 s-1 there exists a wide temperature range (475-520`C) within which the tensile elongation is over 1000%. There also exists a wide strain rate range (10-3 - 10-1 s-1) within which the tensile elongation is over 500%. The presence of Sc in the alloy results in a uniform distribution of fine coherent Al3SC precipitates which effectively pin grain and subgrain boundaries during static and continuous recrystallization. As a result, the alloy retains its fine grain size (about 7 micron), even after extensive superplastic deformation (>1000%). During deformation, dislocations Mg with a high Schmidt factor slip across subgrains but are trapped by subgrain boundaries, as a result of the strong pining of Al3Sc. This process leads to the conversion of low-angled subgrain boundaries to high-angled grain boundaries and the subsequent grain boundary sliding, which produces superelasticity. A model is proposed to describe grain boundary sliding accommodated by dislocation glide across grains with a uniform distribution of coherent precipitates. The model predictions is consistent with experimental observations

Off-Diagonal Long-Range Order: Meissner Effect and Flux Quantization

There has been a proof by Sewell that the hypothesis of off-diagonal long-range order in the reduced density matrix $\rho _2$ implies the Meissner effect. We present in this note an elementary and straightforward proof that not only the Meissner effect but also the property of magnetic flux quantization follows from the hypothesis. It is explicitly shown that the two phenomena are closely related, and phase coherence is the origin for both.Comment: 11 pages, Latex fil

Role of interfacial dislocations on creep of a fully lamellar tial

Deformation mechanisms of a fully lamellar TiAl ({gamma} lamellae: 100 {approximately} 300 nm thick, {alpha}{sub 2} lamellae: 10 {approximately} 50 nm thick) crept at 760 C have been investigated. It was found that, as a result of a fine structure, the motion and multiplication of dislocations within both {gamma} and {alpha}{sub 2} lamellae are limited at low creep stresses (< 400 MPa). Thus, the glide and climb of lattice dislocations have insignificant contribution to creep deformation. In contrast, the motion of interfacial dislocations on {gamma}{alpha}{sub 2} and {gamma}{gamma} interfaces (i.e. interface sliding) dominates the deformation at low stresses. The major obstacles impeding the motion of interfacial dislocations was found to be lattice dislocations impinging on lamellar interfaces. The number of impinging lattice dislocations increases as the applied stress increases and, subsequently, causes the pileup of interfacial dislocations on the interfaces. The pileup further leads to the formation of deformation twins. Deformation twinning activated by the pileup of interfacial dislocations is suggested to be the dominant deformation mechanism at high stresses (> 400 MPa)

Optical-conductivity sum rule in cuprates and unconventional charge density waves: a short review

We begin with an overview of the experimental results for the temperature and doping dependences of the optical-conductivity spectral weight in cuprate superconductors across the whole phase diagram. Then we discuss recent attempts to explain the observed behavior of the spectral weight using reduced and full models with unconventional $d_{x^2-y^2}$ charge-density waves.Comment: 17 pages, RevTeX4, 4 EPS figures; Invited paper for a special issue of Low Temperature Physics dedicated to the 20th anniversary of HTS

Does the 2D Hubbard Model Really Show d-Wave Superconductivity?

Some issues concerning the question if the two-dimensional Hubbard model really show d-wave superconductivity are briefly discussed.Comment: Revtex, no figure

Electron-positron annihilation: Statistical and thermodynamic considerations

Lightcone considerations suggest that, in e+e- annihilation, the hadronic matter is initially created within a spatial volume in the c.m. frame of the order of E-3, where E/2 is the colliding beam energy. Its implications, within the statistical thermodynamic descriptions of Fermi and Landau, are conjectured. A distinctive result is that the multiplicity for e+e- annihilation is a constant.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22038/1/0000456.pd