Superplastic behaviour in nano ceramics A. Dom

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Superplastic behaviour in nano ceramicsA.
Domínguez-RodríguezUniversity of Seville
(Spain)
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Definition macro and micro of superplasticityE
quation of superplasticity How to improve
superplasticitySuperplasticity in
nano-ceramics nano-MgO nano-YTZP
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3YTZP deformed at 1450 ºC and 3x10-4 s-1
(Courtesy to Prof. F. Wakai)
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A grain switching event observed during
superplastic deformation of Y-TZP. A group of
grains exchange their neighbors during
deformation. (Courtesy to Prof. R. Duclos)
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In a material superplastically deformedThe
deformation is due to grain boundary slidingThe
strain rate is controlled by the accomodation
process -Diffusion of point defects -Activity
of dislocations -Cavities
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Equation of superplasticity
is the strain rate s is the applied stress s0
is the threshold stress n and p the stress and
grain size exponents Q is an activation energy
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How to improve superplasticityThe
strategy to enhance superplasticity is
twofoldRefinement of the microstructureImpr
ovement of the accommodation process Although
both processes are independent each other, in
many cases are connected.
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Superplasticity in nano-MgO
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Grain size distribution from the nc-MgO showing
the log-normal distribution with mean grain
diameter of 37 nm
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Nano-MgO superplastically deformed
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These nano-MgO could be deformed in compression,
at temperatures between 700 and 800 ºC at strain
rates around 10-5 s-1 and strains around 40 .
Values of the stress exponent, n 2, and the
activation energy of 200 kJ/mol were obtained for
all test conditions. Very small grain sizes
permit diffusional processes to vary from slow
lattice diffusion to a much faster grain boundary
one and to allow grains to reach a significant
mobility.
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Superplasticity in nano-YTZP
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In the case of YTZP, it has been successively
shown that Y3 segregates at grain boundaries,
inducing a local electric field which is screened
by the gradient of oxygen vacancies between the
bulk and the boundaries.When the grain size of
the polycrystal becomes close to the screening
length (nanoscale length), this electric field
can influence the diffusional processes and in
consequence the creep behavior of the nano YTZP.
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Yttrium segregation assessed
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Constitutive equation for nano YTZP
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Where V(R) is the electric potential at the grain
boundaries, zD is the effective electric charge
of the diffusing cations, and ? is the screening
lenght (Debye length) and er is the dielectric
constant of the material.
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Plot of ? versus grain size for different ?
values
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Final remarksIt is
clear that the refinement of the microstructure
can improve superplasticity in nano-MgO but not
in nano-YTZP due to the nature of the grain
boundary in this ceramics.In conclusion to
improve superplasticity it is more important to
control the nature of the grain boundaries that
the grain size itself.
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