Transcript Superplastic behaviour in nano ceramics

Superplastic behaviour in nano ceramics
A. Domínguez-Rodríguez
University of Seville (Spain)
*Definition macro and micro of
superplasticity
*Equation of superplasticity
*How to improve superplasticity
*Superplasticity in nano-ceramics:
nano-MgO
nano-YTZP
3YTZP deformed at 1450 ºC and 3x10-4 s-1
(Courtesy to Prof. F. Wakai)
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)
In a material superplastically
deformed:
*The deformation is due to
grain boundary sliding
*The strain rate is controlled
by the accomodation process:
-Diffusion of point defects
-Activity of dislocations
-Cavities
Equation of superplasticity

  0 
A
n

d
p
 is the strain rate
 Q 
Do exp 

 kT 
σ is the applied stress
σ0 is the threshold stress
n and p the stress and grain size exponents
Q is an activation energy
How to improve superplasticity
The strategy to enhance superplasticity is
twofold:
*Refinement of the microstructure
*Improvement of the accommodation process
Although both processes are independent
each other, in many cases are connected.
Superplasticity in nano-MgO
250
Mean 37 nm
s.d. 17 nm
Number of Grains
200
150
100
50
0
0
20
40
60
80
100
120
Grain Size (nm)
Grain size distribution from the nc-MgO showing the lognormal distribution with mean grain diameter of 37 nm
700
Stress (MPa)
600
696ºC
500
400
300
756ºC
200
796ºC
100
0
0
10
20
Strain (%)
30
40
Nano-MgO superplastically deformed
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.
Superplasticity in nano-YTZP
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.
Yttrium segregation assessed
----Position (nm)
Constitutive equation for nano YTZP
Gb     b 
Zr
  2 x10 
    Dlatt
kT  G   d 

2
7
2

1
    z D eV R    
  1
1  4 exp 
d   3 r kT d  
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 εr is the dielectric constant of
the material.

1
-1
10
 nm
 nm
 nm
 nm
-2
10
0
40
80
120
Grain size (nm)
160
200
Plot of  versus grain size for different 
values
Final remarks
It 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.