UP DATE May 16, 2008
Japanese
Outline of Current Research
1. Development of high-strain-rate superplastic oxide ceramics
2. Theoretical analysis and simulation of high-temperature deformation and relating phenomena in polycrystalline solids
3. Analysis of superplastic deformation behavior in fine-grained oxide ceramics
4. Analysis of intergranular cavitation and cracking in superplastic ceramics
5. Synthesis of nano-structured oxide composites
6. Analysis of the relationship between grain boundary chemistry and transport phenomena
7. Synthesis of transparent oxides by spark plasma sintering
8. Analysis of high-temperature flow behavior in textured α-alumina ceramics
This reseach has been conducted as a major project (environment and energy materials) of NIMS.
High-strain-rate superplasticity (HSRS) has been attained at 1 s-1
in Al2O3-ZrO2-spinel (Fig. 2) and at 0.01-0.3 s-1
in ZrO2 dispersed with MgAl2O4.
The research has also attained a highly enhanced tensile ductility of 2500% at 0.09 s-1
in the tri-phase composite and HSRS at a relatively low temperature around 1350oC
in monolithic tetragonal zirconia. Inspection of deformation behavior and deformed microstructure
has revealed the following new aspects.
- yield-drop appearing at strain rates higher than about 0.1 s-1 (Fig. 2(c))
- intragranular substructures such as tangled dislocations and sub-boundaries (Fig. 2(d)) indicating that the grains are not perfectly rigid during deformation
- strongly suppressed cavitation damage
The aspects have been found to appear in ZrO2-MgAl2O4
and Al2O3-ZrO2-spinel and can be related to
mechanisms that enable the material to exhibit HSRS.
Fig. 2 HSRS attained at 1 s-1 in Al2O3-ZrO2(3Y)-MgO/nAl2O3
(Nature 413 (2001), 288-291): (a) specimens before and after deformation at 1650oC, (b)scanning electron micrograph of the as-synthesized material,
(c) stress-strain curves accompanied by yield-drop and (d) dark-field transmission electron micrograph of a ZrO2
grain in which tangled dislocations and sub-boundaries were formed during deformation.
This project includes a basic study on the net-shaping of superplastic oxide ceramics.
Figure 3 shows an example, where a flat plate of ZrO2 (3Y) doped with Al2O3 was shaped to a hemisphre by using a SiC die.
Fig. 3 Net-shaping of tetragonal ZrO2: before (left) and after (right) shaping in a vacuum furnace.
This research aims at obtaining basic information on the dynamic aspects of high-temperature deformation and relating phenomena
such as grain-boundary migration, grain growth, and grain-boundary sliding accommodated by diffusion.
The information gives theoretical bases for predicting deformation behavior and microstructural design.
Fig. 4 Microstructural change accompanying diffusional creep: (a) before and (a) after constant-stress creep deformation.
In (b), there appear grain growth, grain annihilation, grain elongation, grain impingement and a change in the relative position of the grains.
This is a basic research that places special attention on the relationship between superplastic deformation and nanometer- or
submicrometer-sized structures. Analysis of stress-strain rate relationship (Fig.5(a)) and structural changes during deformation (Fig. 5(b))
has revealed new aspects of superplastic deformation in ZrO2-base materials and gives a guide for attaining HSRS in oxide ceramics.
Fig. 5 Superplastic behavior of fine-grained tetragonal ZrO2: (a) stress-strain rate relationship and (b)
dark-field transmission electron micrograph of a ZrO2 grain showing the formation of densely aligned dislocations (P)
and sub-boundaries (S) during deformation.
This research aims at obtaining quantitative information on cavity nucleation and growth
during superplastic deformation. Analysis of cavity- or crack-size distribution (Fig. 6)
has revealed the basic aspects of cavity-damage accumulation, cracking and failure
in superplastic ceramics.
The information gives a guide for enhancing superplastic ductility, which depends strongly
on the rates of cavity nucleation and growth.
Fig. 6 Analysis of cavity-size distribution in a superplastic Al2O3-10%ZrO2.
The bimodal size distribution indicates a change in cavity-growth mechanisms.
This research addresses powder processing for synthesizing nano-composites with a grain size of < 100nm (Fig. 7),
for which we can expect enhanced superplasticity at high-temperatures and enhanced strength around room temperature.
To obtain dense nano-structured composites, we have studied a process consisting of high-energy ball-milling and
spark plasma sintering. In a recent trial, we succeeded in synthesizing a nano-structured composite with a flexural strength higher than 2000 MPa.
Fig. 7 Transmission electron micrograph of nano-structured ZrO2(3Y)-30vol%MgAl2O3
having an average grain size of about 90 nm.
Physical and chemical properties in ceramic polycrystal strongly depend on nanostructure of grain boundaries.
This research aims at obtaining basic information on the relationship between the nano-scale structure
of grain-boundaries, which is investigated by using a first-principle molecular-orbital calculation,
and matter transport through grain-boundaries, which is examined by experiments.
The information is expected to give an essential guide for designing multi-functional ceramic materials
where excellent mechanical properties are integrated with optimized physical properties
or chemical properties.
Fig. 8 Schematic illustration of dopant cations' grain-boundary segregation.
Using a spark-plasma-sintering (SPS) technique, we are aiming to make highly transparent polycrystalline alumina,
which have usually been sintered by hot-isostatic-pressing (HIP) techniques.
We have found that fine-grained transparent α-Al2O3 can be obtained
when the heating rate is controlled in a range lower than that applied to usual SPS (Fig. 9).
This technique is also effective in fabricating transparent MgAl2O4-spinel polycrystals and
in low-temperature densification of undoped polycrystalline Y2O3.
In the latter, we have attained a relative density of 97.3% with a grain size of about 500 nm
at 850o.
Fig. 9 Alumina sintered by SPS at a heating rate of 100oC/min (left) and
8oC/min (right). Both samples are 0.88 mm thick, and 3 mm above the text.
Under collaboration with Fine Particle Processing Group, this research aims to elucidate the effects of textured
grain microstructure on high temperature deformation in α-Alumina (Al2O3) ceramics.
The following was found by using a textured α-Al2O3 polycrystal (Fig. 10(a))
prepared by colloidal processing in a strong magnetic field. When the tensile stress is applied at 45o
against the c-axis of the textured α-Al2O3, the material can be deformed
at a strain rate several times higher than that for a non-textured α-Al2O3 (Fig. 10(b)).
This texture-dependence becomes pronounced with increasing stress.
The aspect found in this research gives a guide for microstructural design toward
enhanced high-temperature formability in ceramics.
Fig. 10 High-temperature flow behavior of textured and non-textured α-Al2O3 F
(a) EBSP map of textured α-Al2O3 and (b) tensile creep curves.
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