Superplastic behaviour in nano Superplastic behaviour in nano ceramicsceramics

A. Domínguez-RodríguezA. Domínguez-Rodríguez

University of Seville (Spain)University of Seville (Spain)

*Definition macro and micro of *Definition macro and micro of superplasticitysuperplasticity

*Equation of superplasticity*Equation of superplasticity

*How to improve superplasticity *How to improve superplasticity

*Superplasticity in nano-ceramics:*Superplasticity in nano-ceramics:nano-MgOnano-MgOnano-YTZPnano-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 In a material superplastically deformed:superplastically deformed:

*The deformation is due to *The deformation is due to grain boundary slidinggrain boundary sliding*The strain rate is *The strain rate is controlled by the controlled by the accomodation process:accomodation process:

-Diffusion of point -Diffusion of point defectsdefects

-Activity of dislocations-Activity of dislocations-Cavities-Cavities

kTQ

Dd

A op

n

exp0

is the strain rateσ is the applied stressσ0 is the threshold stressn and p the stress and grain size exponentsQ is an activation energy

Equation of Equation of superplasticitysuperplasticity

How to improve How to improve superplasticitysuperplasticity

The strategy to enhance superplasticity The strategy to enhance superplasticity is twofold:is twofold:

*Refinement of the microstructure*Refinement of the microstructure

*Improvement of the accommodation *Improvement of the accommodation

processprocess

Although both processes are Although both processes are independent each other, in many cases independent each other, in many cases are connected.are connected.

Superplasticity in nano-MgOSuperplasticity in nano-MgO

Grain size distribution from the nc-MgO showing Grain size distribution from the nc-MgO showing the log-normal distribution with mean grain the log-normal distribution with mean grain

diameter of 37 nmdiameter of 37 nm

Nano-MgO superplastically deformedNano-MgO superplastically deformed

0 10 20 30 400

100

200

300

400

500

600

700

796ºC

756ºC

696ºC

Str

ess

(MP

a)

Strain (%)

These nano-MgO could be These nano-MgO could be deformed deformed in compression, at temperatures in compression, at temperatures between 700 and 800 ºC between 700 and 800 ºC at strain at strain rates around 10rates around 10-5-5 s s-1-1 and strains and strains around 40 %. around 40 %.

Values of the stress exponent, n = Values of the stress exponent, n = 2, and the activation energy 2, and the activation energy of 200 of 200 kJ/molkJ/mol were obtained for all test were obtained for all test conditions.conditions. Very small grain sizes permit Very small grain sizes permit diffusional processes to vary from diffusional processes to vary from slow lattice diffusion to a much slow lattice diffusion to a much faster grain boundary one and to faster grain boundary one and to allow grains to reach a significant allow grains to reach a significant mobility.mobility.

Superplasticity in nano-YTZPSuperplasticity in nano-YTZP

In the case of YTZP, it has been successively In the case of YTZP, it has been successively shown that Yshown that Y3+3+ segregates at grain boundaries, segregates at grain boundaries, inducing a local electric field which is screened inducing a local electric field which is screened by the gradient of oxygen vacancies between by the gradient of oxygen vacancies between the bulk and the boundaries.the bulk and the boundaries.When the grain size of the polycrystal becomes When the grain size of the polycrystal becomes close to the screening length (nanoscale close to the screening length (nanoscale length), this electric field can influence the length), this electric field can influence the diffusional processes and in consequence the diffusional processes and in consequence the creep behavior of the nano YTZP.creep behavior of the nano YTZP.

----------

Posición (nm)

C(T)

80 60 40 20 0

300

250

200

150

100

50

Y-k

Position (nm)

Yttrium segregation assessedYttrium segregation assessed

Constitutive equation for nano YTZP

ZrlattD

d

b

GkT

Gbx

227102

13

exp41

1

dkT

ReVz

d r

D

Where V(R) is the electric potential at the Where V(R) is the electric potential at the grain boundaries, zgrain boundaries, zDD is the effective electric is the effective electric charge of the diffusing cations, and charge of the diffusing cations, and is the is the screening lenght (Debye length) and screening lenght (Debye length) and εεr r is theis the

dielectricdielectric constant of the material.constant of the material.

Plot of Plot of versus grain size for versus grain size for

different different values values

0 4 0 8 0 1 2 0 1 6 0 2 0 0G r ain siz e (nm )

1 0-1

1

n m

n mn mn m

1 0-2

Final remarksFinal remarks

It is clear that the refinement of the It is clear that the refinement of the microstructure can improve superplasticity microstructure can improve superplasticity in nano-MgO but not in nano-YTZP due to in nano-MgO but not in nano-YTZP due to the nature of the grain boundary in this the nature of the grain boundary in this ceramics.ceramics.

In conclusion: to improve superplasticity it In conclusion: to improve superplasticity it is more important to control the nature of is more important to control the nature of the grain boundaries that the grain size the grain boundaries that the grain size itself.itself.