![]() Densification in both cases depended on creep of the beads and densification rates at 1350✬ were predicted well both by equations from Helle et al. The second were Al 2 O 3 -ZrO 2 (1%Y 2 O 3 ) synthesized by plasma spraying spray dried powders into water to quench-in a partially amorphous, partially nanocrystalline structure. The average bead diameter was 51 μm and contained 0.3 μm grains. The first were spherical ZrO 2 beads synthesized by sintering TOSOH spray dried ZrO 2 powder loose in a crucible. The kinetics of hot pressing and the resulting microstructure of two types of polycrystalline spherical powders (beads) were studied. Acknowledgement Financial support for this work came from Basic Energy Sciences Division of the Department of Energy Grant Number DE-FG02-O2ER46010. In this case there was apparently a higher rate of diffusion allowing grain growth to occur. Behavior was only slightly different in the spinel-ZrO 2 binary even though there was no transformation similar to γ-Al 2 O 3 to α-Al 2 O 3 one. ![]() Above 1250☌ ZrO 2 particles or polycrystalline phase decorated the grain boundaries of the Al 2 O 3. As described in the microstructural section, although some aspects of the microstructure were in the nanocrystalline regime (99% density but as γ-Al 2 O 3 converted to α-Al 2 O 3 grains grew to 1-2 μm diameter while still maintaining the ~50 nm spacing of ZrO 2 phase within the interior of grains. Hot pressing alone in an alumina die at 1250✬ was not successful in achieving >80% of theoretical density. The resulting curves comparing experimental results of hot pressing AZY and sintered ZrO 2 spray dried spheres (described in paper II) in a silicon carbide die at 1350✬ are shown in Tm is the melting temperature 1850✬ Forming Methods-Prospective: Table II below lists the final densities achieved by the two step process of hot pressing then forging. This analysis, therefore, assumed that the strain rate is independent of the load-strain rate history. density was plotted for a pressure of interest. The density at the same stress was picked from every curve and then the densification rate vs. A series of stress-density curves are generated at different displacement rates. The sequence is repeated in order to obtain a stress-density curve. Then each element is allowed to creep for a time period, dt and then the stresses are recalculated. In this method the distribution of elastic stresses within a 3-D spherical particle under diametrical z axis loading with lateral confinement is determined after a displacement, dz. ρ is by a finite element technique developed with partial support from this program. ![]() At ρ >90% ( ) The second method of predicting ρ vs. B is the constant in the Norton equation and σ a is the applied stress. From the creep results the stress and temperature dependence could be derived. In order to estimate the creep rate of the quenched spherical particles, which are fully dense, the strain rate for the last 5 minutes of forging was used with a correction for porosity, by multiplying the creep rate by the factor k p where β=-4 and P is the fraction of porosity. Also a temperature dependence study of densification rates was performed on specimens in alumina dies. In the second set of experiments, the kinetics of hot pressing in a silicon carbide dies were studied to compare experimental rates with rates predicted by equations for hot pressing and HIPping. The kinetics were routinely measured only during forging. However, not all features of the microstructure were 80%) could not be reached by hot pressing in a die whereas rapid densification to near theoretical density could be reached for all specimens by hot forging. Accomplishments: The first goal of fabricating nanocrystalline ceramics from superplastically deforming powder was accomplished. The second goal was to achieve transparent nanocrystalline ceramics. The major challenge was to hot press or to hot forge the powders to full density while maintaining the nanostructure. Rather than starting with nano-sized powders, we used powders 2-25 μm in diameter (average 16 μm) having a nanoscale microstructure. ![]() Summary: The goal of the program was to fabricate nanocrystalline ceramics from superplastically deforming powder with nanocrystalline microstructures using a new approach.
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